Guide

Pulldown-cmark is a CommonMark Markdown parser based on pull parsing, a high-performance, low-memory approach to parsing resursive grammars.

In addition to the base CommonMark language, it supports tables, task lists, strikethrough, footnotes, admonitions, and LaTeX-style math from GFM. It also supports tagging headers with explicit IDs and classes,

Getting started

#![allow(unused)]
fn main() {
// Create parser with example Markdown text.
let markdown_input = "hello world";
let parser = pulldown_cmark::Parser::new(markdown_input);

// Write to a new String buffer.
let mut html_output = String::new();
pulldown_cmark::html::push_html(&mut html_output, parser);
assert_eq!(&html_output, "<p>hello world</p>\n");
}

For more details on the API, see the Rust API docs.

Why a pull parser?

There are many parsers for Markdown and its variants, but to my knowledge none use pull parsing. Pull parsing has become popular for XML, especially for memory-conscious applications, because it uses dramatically less memory than constructing a document tree, but is much easier to use than push parsers. Push parsers are notoriously difficult to use, and also often error-prone because of the need for user to delicately juggle state in a series of callbacks.

In a clean design, the parsing and rendering stages are neatly separated, but this is often sacrificed in the name of performance and expedience. Many Markdown implementations mix parsing and rendering together, and even designs that try to separate them (such as the popular hoedown), make the assumption that the rendering process can be fully represented as a serialized string.

Pull parsing is in some sense the most versatile architecture. It's possible to drive a push interface, also with minimal memory, and quite straightforward to construct an AST. Another advantage is that source-map information (the mapping between parsed blocks and offsets within the source text) is readily available; you can call into_offset_iter() to create an iterator that yields (Event, Range) pairs, where the second element is the event's corresponding range in the source document.

While manipulating ASTs is the most flexible way to transform documents, operating on iterators is surprisingly easy, and quite efficient. Here, for example, is the code to transform soft line breaks into hard breaks:

#![allow(unused)]
fn main() {
let parser = parser.map(|event| match event {
	Event::SoftBreak => Event::HardBreak,
	_ => event
});
}

Or expanding an abbreviation in text:

#![allow(unused)]
fn main() {
let parser = parser.map(|event| match event {
	Event::Text(text) => Event::Text(text.replace("abbr", "abbreviation").into()),
	_ => event
});
}

Another simple example is code to determine the max nesting level:

#![allow(unused)]
fn main() {
let mut max_nesting = 0;
let mut level = 0;
for event in parser {
	match event {
		Event::Start(_) => {
			level += 1;
			max_nesting = std::cmp::max(max_nesting, level);
		}
		Event::End(_) => level -= 1,
		_ => ()
	}
}
}

Note that consecutive text events can happen due to the manner in which the parser evaluates the source. A utility TextMergeStream exists to improve the comfort of iterating the events:

#![allow(unused)]
fn main() {
use pulldown_cmark::{Event, Parser, Options};

let markdown_input = "Hello world, this is a ~~complicated~~ *very simple* example.";

let iterator = TextMergeStream::new(Parser::new(markdown_input));

for event in iterator {
    match event {
        Event::Text(text) => println!("{}", text),
        _ => {}
    }
}
}

More detailed examples can be found in the Rust API docs.

Using Rust idiomatically

A lot of the internal scanning code is written at a pretty low level (it pretty much scans byte patterns for the bits of syntax), but the external interface is designed to be idiomatic Rust.

Pull parsers are at heart an iterator of events (start and end tags, text, and other bits and pieces). The parser data structure implements the Rust Iterator trait directly, and Event is an enum. Thus, you can use the full power and expressivity of Rust's iterator infrastructure, including for loops and map (as in the examples above), collecting the events into a vector (for recording, playback, and manipulation), and more.

Further, the Text event (representing text) is a small copy-on-write string. The vast majority of text fragments are just slices of the source document. For these, copy-on-write gives a convenient representation that requires no allocation or copying, but allocated strings are available when they're needed. Thus, when rendering text to HTML, most text is copied just once, from the source document to the HTML buffer.

When using the pulldown-cmark's own HTML renderer, make sure to write to a buffered target like a Vec<u8> or String. Since it performs many (very) small writes, writing directly to stdout, files, or sockets is detrimental to performance. Such writers can be wrapped in a BufWriter.

Build options

By default, the binary is built as well. If you don't want/need it, then build like this:

> cargo build --no-default-features

Or put in your Cargo.toml file:

pulldown-cmark = { version = "0.10.3", default-features = false }

SIMD accelerated scanners are available for the x64 platform from version 0.5 onwards. To enable them, build with simd feature:

> cargo build --release --features simd

Or add the feature to your project's Cargo.toml:

pulldown-cmark = { version = "0.10.3", default-features = false, features = ["simd"] }

For a higher release performance you may want this configuration in your profile release:

lto = true
codegen-units = 1
panic = "abort"

Cheat sheet

Base syntax

	Markdown	Result
Emphasis	Weak `text _text_` Strong `text __text__`	Weak text text String text text
Code span	`text`	`text`
Paragraph	`first paragraph second paragraph`	first paragraph second paragraph
Autolink	`<https://example.com/destination>`	https://example.com/destination
Link	Inline `[label](https://example.com/destination)` Reference `[label][ref] [ref]: https://example.com/destination`	label
Image	Inline `![rust logo](rust-logo-151179464ae7ed46.svg)` Reference `![rust logo][ref] [ref]: rust-logo-151179464ae7ed46.svg`
Thematic break	`*** --- ___`
Heading	ATX style (h1-h6) `# H1 ## H2 ... ###### H6` Setext style (h1/h2 only) `H1 == H2 --`	H1 H2
Fenced code	```rust fn main() {} ``` ~~~rust fn main() {} ~~~	`fn main() {}`
Indented code	`••••fn main() {}`	`fn main() {}`
Block quote	`> Don't believe everything you > read on the Internet. > > — Abraham Lincoln`	Don't believe everything you read on the Internet. — Abraham Lincoln
Numbered list	`1. first 2. second`	first second
Bulleted list	`- first - second * first * second + first + second`	first second first second first second
Hard line break	`a\ b•• c`	a b c

GitHub-flavored Markdown

Markdown Result

Table

| header | row |
| ------ | --- |
| body   | row |

header	row
body	row

Math

Inline

$2+2=4$

Display

$$x=\frac{-b\pm\sqrt{b^2-4ac} }{2a}$$

Inline: $2+2=4$
Display: \[x=\frac{ -b\pm\sqrt{b^2-4ac} }{2a}\]

Blockquote tags

> [!WARNING]
>
> Huh?

Huh?

Footnote

footnote [^reference]

[^reference]: definition

footnote ^[1]

[1]: definition

Strikethrough

~removed text~

~~also removed text~~

~~removed text~~

~~also removed text~~

Tasklist

- [ ] incomplete
- [x] complete

incomplete
complete

Other Extensions

Markdown Result

Metadata block

---
title: Cheat sheet
description: Sample syntax of all pulldown-cmark markdown extensions
---

title	description
Cheat sheet	Sample syntax of all pulldown-cmark markdown extensions

Heading attributes

# Custom heading {.red #custom-heading}

Custom heading

For more details on the API, see the Rust API docs.

Example	Description
broken-link-callbacks.rs	Dynamically retrieve reference link destinations (e.g. `[text][ref]` / `[ref][]` / `[ref]`)
event-filter.rs	Strip all images and replace text
events.rs	Dump events to the console as tree
footnote-rewrite.rs	Render footnotes to HTML at the bottom, closely matching GitHub
parser-map-event-print.rs	Dump events to the console in flat form
parser-map-tag-print.rs	Dump block-level tag events to the console in descriptive form
string-to-string.rs	Convert markdown to HTML

Dynamically retrieve reference link destinations (e.g. [text][ref] / [ref][] / [ref])

use pulldown_cmark::{html, BrokenLink, Options, Parser};

fn main() {
    let input: &str = "Hello world, check out [my website][].";
    println!("Parsing the following markdown string:\n{}", input);

    // Setup callback that sets the URL and title when it encounters
    // a reference to our home page.
    let callback = |broken_link: BrokenLink| {
        if broken_link.reference.as_ref() == "my website" {
            println!(
                "Replacing the markdown `{}` of type {:?} with a working link",
                &input[broken_link.span], broken_link.link_type,
            );
            Some(("http://example.com".into(), "my example website".into()))
        } else {
            None
        }
    };

    // Create a parser with our callback function for broken links.
    let parser = Parser::new_with_broken_link_callback(input, Options::empty(), Some(callback));

    // Write to String buffer.
    let mut html_output: String = String::with_capacity(input.len() * 3 / 2);
    html::push_html(&mut html_output, parser);

    // Check that the output is what we expected.
    let expected_html: &str =
        "<p>Hello world, check out <a href=\"http://example.com\" title=\"my example website\">my website</a>.</p>\n";
    assert_eq!(expected_html, &html_output);

    // Write result to stdout.
    println!("\nHTML output:\n{}", &html_output);
}

Strip all images and replace text

use std::io::Write as _;

use pulldown_cmark::{html, Event, Options, Parser, Tag, TagEnd};

fn main() {
    let markdown_input: &str = "This is Peter on ![holiday in Greece](pearl_beach.jpg).";
    println!("Parsing the following markdown string:\n{}", markdown_input);

    // Set up parser. We can treat is as any other iterator. We replace Peter by John
    // and image by its alt text.
    let parser = Parser::new_ext(markdown_input, Options::empty())
        .map(|event| match event {
            Event::Text(text) => Event::Text(text.replace("Peter", "John").into()),
            _ => event,
        })
        .filter(|event| match event {
            Event::Start(Tag::Image { .. }) | Event::End(TagEnd::Image) => false,
            _ => true,
        });

    // Write to anything implementing the `Write` trait. This could also be a file
    // or network socket.
    let stdout = std::io::stdout();
    let mut handle = stdout.lock();
    handle.write_all(b"\nHTML output:\n").unwrap();
    html::write_html_io(&mut handle, parser).unwrap();
}

Dump events to the console as tree

use std::io::Read;

use pulldown_cmark::{Event, Parser};

/// Show all events from the text on stdin.
fn main() {
    let mut text = String::new();
    std::io::stdin().read_to_string(&mut text).unwrap();

    eprintln!("{text:?} -> [");
    let mut width = 0;
    for event in Parser::new(&text) {
        if let Event::End(_) = event {
            width -= 2;
        }
        eprintln!("  {:width$}{event:?}", "");
        if let Event::Start(_) = event {
            width += 2;
        }
    }
    eprintln!("]");
}

Render footnotes to HTML at the bottom, closely matching GitHub

use std::collections::HashMap;
use std::fmt::Write as _;
use std::io::Write as _;

use pulldown_cmark::{html, CowStr, Event, Options, Parser, Tag, TagEnd};

/// This example shows how to do footnotes as bottom-notes, in the style of GitHub.
fn main() {
    let markdown_input: &str = "This is an [^a] footnote [^a].\n\n[^a]: footnote contents";
    println!("Parsing the following markdown string:\n{}", markdown_input);

    // To generate this style, you have to collect the footnotes at the end, while parsing.
    // You also need to count usages.
    let mut footnotes = Vec::new();
    let mut in_footnote = Vec::new();
    let mut footnote_numbers = HashMap::new();
    // ENABLE_FOOTNOTES is used in this example, but ENABLE_OLD_FOOTNOTES would work, too.
    let parser = Parser::new_ext(markdown_input, Options::ENABLE_FOOTNOTES)
        .filter_map(|event| {
            match event {
                Event::Start(Tag::FootnoteDefinition(_)) => {
                    in_footnote.push(vec![event]);
                    None
                }
                Event::End(TagEnd::FootnoteDefinition) => {
                    let mut f = in_footnote.pop().unwrap();
                    f.push(event);
                    footnotes.push(f);
                    None
                }
                Event::FootnoteReference(name) => {
                    let n = footnote_numbers.len() + 1;
                    let (n, nr) = footnote_numbers.entry(name.clone()).or_insert((n, 0usize));
                    *nr += 1;
                    let html = Event::Html(format!(r##"<sup class="footnote-reference" id="fr-{name}-{nr}"><a href="#fn-{name}">[{n}]</a></sup>"##).into());
                    if in_footnote.is_empty() {
                        Some(html)
                    } else {
                        in_footnote.last_mut().unwrap().push(html);
                        None
                    }
                }
                _ if !in_footnote.is_empty() => {
                    in_footnote.last_mut().unwrap().push(event);
                    None
                }
                _ => Some(event),
            }
        });

    // Write to anything implementing the `Write` trait. This could also be a file
    // or network socket.
    let stdout = std::io::stdout();
    let mut handle = stdout.lock();
    handle.write_all(b"\nHTML output:\n").unwrap();
    html::write_html_io(&mut handle, parser).unwrap();

    // To make the footnotes look right, we need to sort them by their appearance order, not by
    // the in-tree order of their actual definitions. Unused items are omitted entirely.
    //
    // For example, this code:
    //
    //     test [^1] [^2]
    //     [^2]: second used, first defined
    //     [^1]: test
    //
    // Gets rendered like *this* if you copy it into a GitHub comment box:
    //
    //     <p>test <sup>[1]</sup> <sup>[2]</sup></p>
    //     <hr>
    //     <ol>
    //     <li>test ↩</li>
    //     <li>second used, first defined ↩</li>
    //     </ol>
    if !footnotes.is_empty() {
        footnotes.retain(|f| match f.first() {
            Some(Event::Start(Tag::FootnoteDefinition(name))) => {
                footnote_numbers.get(name).unwrap_or(&(0, 0)).1 != 0
            }
            _ => false,
        });
        footnotes.sort_by_cached_key(|f| match f.first() {
            Some(Event::Start(Tag::FootnoteDefinition(name))) => {
                footnote_numbers.get(name).unwrap_or(&(0, 0)).0
            }
            _ => unreachable!(),
        });
        handle
            .write_all(b"<hr><ol class=\"footnotes-list\">\n")
            .unwrap();
        html::write_html_io(
            &mut handle,
            footnotes.into_iter().flat_map(|fl| {
                // To write backrefs, the name needs kept until the end of the footnote definition.
                let mut name = CowStr::from("");
                // Backrefs are included in the final paragraph of the footnote, if it's normal text.
                // For example, this DOM can be produced:
                //
                // Markdown:
                //
                //     five [^feet].
                //
                //     [^feet]:
                //         A foot is defined, in this case, as 0.3048 m.
                //
                //         Historically, the foot has not been defined this way, corresponding to many
                //         subtly different units depending on the location.
                //
                // HTML:
                //
                //     <p>five <sup class="footnote-reference" id="fr-feet-1"><a href="#fn-feet">[1]</a></sup>.</p>
                //
                //     <ol class="footnotes-list">
                //     <li id="fn-feet">
                //     <p>A foot is defined, in this case, as 0.3048 m.</p>
                //     <p>Historically, the foot has not been defined this way, corresponding to many
                //     subtly different units depending on the location. <a href="#fr-feet-1">↩</a></p>
                //     </li>
                //     </ol>
                //
                // This is mostly a visual hack, so that footnotes use less vertical space.
                //
                // If there is no final paragraph, such as a tabular, list, or image footnote, it gets
                // pushed after the last tag instead.
                let mut has_written_backrefs = false;
                let fl_len = fl.len();
                let footnote_numbers = &footnote_numbers;
                fl.into_iter().enumerate().map(move |(i, f)| match f {
                    Event::Start(Tag::FootnoteDefinition(current_name)) => {
                        name = current_name;
                        has_written_backrefs = false;
                        Event::Html(format!(r##"<li id="fn-{name}">"##).into())
                    }
                    Event::End(TagEnd::FootnoteDefinition) | Event::End(TagEnd::Paragraph)
                        if !has_written_backrefs && i >= fl_len - 2 =>
                    {
                        let usage_count = footnote_numbers.get(&name).unwrap().1;
                        let mut end = String::with_capacity(
                            name.len() + (r##" <a href="#fr--1">↩</a></li>"##.len() * usage_count),
                        );
                        for usage in 1..=usage_count {
                            if usage == 1 {
                                write!(&mut end, r##" <a href="#fr-{name}-{usage}">↩</a>"##)
                                    .unwrap();
                            } else {
                                write!(&mut end, r##" <a href="#fr-{name}-{usage}">↩{usage}</a>"##)
                                    .unwrap();
                            }
                        }
                        has_written_backrefs = true;
                        if f == Event::End(TagEnd::FootnoteDefinition) {
                            end.push_str("</li>\n");
                        } else {
                            end.push_str("</p>\n");
                        }
                        Event::Html(end.into())
                    }
                    Event::End(TagEnd::FootnoteDefinition) => Event::Html("</li>\n".into()),
                    Event::FootnoteReference(_) => unreachable!("converted to HTML earlier"),
                    f => f,
                })
            }),
        )
        .unwrap();
        handle.write_all(b"</ol>\n").unwrap();
    }
}

Dump events to the console in flat form

use pulldown_cmark::{html, Event, Parser};

fn main() {
    let markdown_input = "# Example Heading\nExample paragraph with **lorem** _ipsum_ text.";
    println!(
        "\nParsing the following markdown string:\n{}\n",
        markdown_input
    );

    // Set up the parser. We can treat is as any other iterator.
    // For each event, we print its details, such as the tag or string.
    // This filter simply returns the same event without any changes;
    // you can compare the `event-filter` example which alters the output.
    let parser = Parser::new(markdown_input).map(|event| {
        match &event {
            Event::Start(tag) => println!("Start: {:?}", tag),
            Event::End(tag) => println!("End: {:?}", tag),
            Event::Html(s) => println!("Html: {:?}", s),
            Event::InlineHtml(s) => println!("InlineHtml: {:?}", s),
            Event::Text(s) => println!("Text: {:?}", s),
            Event::Code(s) => println!("Code: {:?}", s),
            Event::DisplayMath(s) => println!("DisplayMath: {:?}", s),
            Event::InlineMath(s) => println!("Math: {:?}", s),
            Event::FootnoteReference(s) => println!("FootnoteReference: {:?}", s),
            Event::TaskListMarker(b) => println!("TaskListMarker: {:?}", b),
            Event::SoftBreak => println!("SoftBreak"),
            Event::HardBreak => println!("HardBreak"),
            Event::Rule => println!("Rule"),
        };
        event
    });

    let mut html_output = String::new();
    html::push_html(&mut html_output, parser);
    println!("\nHTML output:\n{}\n", &html_output);
}

Dump block-level tag events to the console in descriptive form

use pulldown_cmark::{Event, Options, Parser, Tag};

fn main() {
    let markdown_input = concat!(
        "# My Heading\n",
        "\n",
        "My paragraph.\n",
        "\n",
        "* a\n",
        "* b\n",
        "* c\n",
        "\n",
        "1. d\n",
        "2. e\n",
        "3. f\n",
        "\n",
        "> my block quote\n",
        "\n",
        "```\n",
        "my code block\n",
        "```\n",
        "\n",
        "*emphasis*\n",
        "**strong**\n",
        "~~strikethrough~~\n",
        "[My Link](http://example.com)\n",
        "![My Image](http://example.com/image.jpg)\n",
        "\n",
        "| a | b |\n",
        "| - | - |\n",
        "| c | d |\n",
        "\n",
        "hello[^1]\n",
        "[^1]: my footnote\n",
    );
    println!(
        "\nParsing the following markdown string:\n{}\n",
        markdown_input
    );

    // Set up the parser. We can treat is as any other iterator.
    // For each event, we print its details, such as the tag or string.
    // This filter simply returns the same event without any changes;
    // you can compare the `event-filter` example which alters the output.
    let parser = Parser::new_ext(markdown_input, Options::all()).map(|event| {
        match &event {
            Event::Start(tag) => match tag {
                Tag::HtmlBlock => println!("HtmlBlock"),
                Tag::Heading {
                    level,
                    id,
                    classes,
                    attrs,
                } => println!(
                    "Heading heading_level: {} fragment identifier: {:?} classes: {:?} attrs: {:?}",
                    level, id, classes, attrs
                ),
                Tag::Paragraph => println!("Paragraph"),
                Tag::List(ordered_list_first_item_number) => println!(
                    "List ordered_list_first_item_number: {:?}",
                    ordered_list_first_item_number
                ),
                Tag::DefinitionList => println!("Definition list"),
                Tag::DefinitionListTitle => println!("Definition title (definition list item)"),
                Tag::DefinitionListDefinition => println!("Definition (definition list item)"),
                Tag::Item => println!("Item (this is a list item)"),
                Tag::Emphasis => println!("Emphasis (this is a span tag)"),
                Tag::Strong => println!("Strong (this is a span tag)"),
                Tag::Strikethrough => println!("Strikethrough (this is a span tag)"),
                Tag::BlockQuote(kind) => println!("BlockQuote ({:?})", kind),
                Tag::CodeBlock(code_block_kind) => {
                    println!("CodeBlock code_block_kind: {:?}", code_block_kind)
                }
                Tag::Link {
                    link_type,
                    dest_url,
                    title,
                    id,
                } => println!(
                    "Link link_type: {:?} url: {} title: {} id: {}",
                    link_type, dest_url, title, id
                ),
                Tag::Image {
                    link_type,
                    dest_url,
                    title,
                    id,
                } => println!(
                    "Image link_type: {:?} url: {} title: {} id: {}",
                    link_type, dest_url, title, id
                ),
                Tag::Table(column_text_alignment_list) => println!(
                    "Table column_text_alignment_list: {:?}",
                    column_text_alignment_list
                ),
                Tag::TableHead => println!("TableHead (contains TableRow tags"),
                Tag::TableRow => println!("TableRow (contains TableCell tags)"),
                Tag::TableCell => println!("TableCell (contains inline tags)"),
                Tag::FootnoteDefinition(label) => println!("FootnoteDefinition label: {}", label),
                Tag::MetadataBlock(kind) => println!("MetadataBlock: {:?}", kind),
            },
            _ => (),
        };
        event
    });

    let mut html_output = String::new();
    pulldown_cmark::html::push_html(&mut html_output, parser);
    println!("\nHTML output:\n{}\n", &html_output);
}

Convert markdown to HTML

use pulldown_cmark::{html, Options, Parser};

fn main() {
    let markdown_input: &str = "Hello world, this is a ~~complicated~~ *very simple* example.";
    println!("Parsing the following markdown string:\n{}", markdown_input);

    // Set up options and parser. Strikethroughs are not part of the CommonMark standard
    // and we therefore must enable it explicitly.
    let mut options = Options::empty();
    options.insert(Options::ENABLE_STRIKETHROUGH);
    let parser = Parser::new_ext(markdown_input, options);

    // Write to String buffer.
    let mut html_output: String = String::with_capacity(markdown_input.len() * 3 / 2);
    html::push_html(&mut html_output, parser);

    // Check that the output is what we expected.
    let expected_html: &str =
        "<p>Hello world, this is a <del>complicated</del> <em>very simple</em> example.</p>\n";
    assert_eq!(expected_html, &html_output);

    // Write result to stdout.
    println!("\nHTML output:\n{}", &html_output);
}

Detailed Specifications

Pulldown-cmark's test suite follows the convention, set by CommonMark itself, of writing specifications that double as conformance tests.

The CommonMark Specification is copied from https://spec.commonmark.org, but slightly modified because pulldown-cmark and cmark don't escape special characters in the exact same way (though their results are identical when rendered with a web browser).

The "GitHub" specifications have been excerpted from https://github.github.com/gfm/.

title: CommonMark Spec author: John MacFarlane version: '0.31.2' date: '2024-01-28' license: 'CC-BY-SA 4.0' ...

Introduction

What is Markdown?

Markdown is a plain text format for writing structured documents, based on conventions for indicating formatting in email and usenet posts. It was developed by John Gruber (with help from Aaron Swartz) and released in 2004 in the form of a syntax description and a Perl script (Markdown.pl) for converting Markdown to HTML. In the next decade, dozens of implementations were developed in many languages. Some extended the original Markdown syntax with conventions for footnotes, tables, and other document elements. Some allowed Markdown documents to be rendered in formats other than HTML. Websites like Reddit, StackOverflow, and GitHub had millions of people using Markdown. And Markdown started to be used beyond the web, to author books, articles, slide shows, letters, and lecture notes.

What distinguishes Markdown from many other lightweight markup syntaxes, which are often easier to write, is its readability. As Gruber writes:

The overriding design goal for Markdown's formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it's been marked up with tags or formatting instructions. (https://daringfireball.net/projects/markdown/)

The point can be illustrated by comparing a sample of AsciiDoc with an equivalent sample of Markdown. Here is a sample of AsciiDoc from the AsciiDoc manual:

1. List item one.
+
List item one continued with a second paragraph followed by an
Indented block.
+
.................
$ ls *.sh
$ mv *.sh ~/tmp
.................
+
List item continued with a third paragraph.

2. List item two continued with an open block.
+
--
This paragraph is part of the preceding list item.

a. This list is nested and does not require explicit item
continuation.
+
This paragraph is part of the preceding list item.

b. List item b.

This paragraph belongs to item two of the outer list.
--

And here is the equivalent in Markdown:

1.  List item one.

    List item one continued with a second paragraph followed by an
    Indented block.

        $ ls *.sh
        $ mv *.sh ~/tmp

    List item continued with a third paragraph.

2.  List item two continued with an open block.

    This paragraph is part of the preceding list item.

    1. This list is nested and does not require explicit item continuation.

       This paragraph is part of the preceding list item.

    2. List item b.

    This paragraph belongs to item two of the outer list.

The AsciiDoc version is, arguably, easier to write. You don't need to worry about indentation. But the Markdown version is much easier to read. The nesting of list items is apparent to the eye in the source, not just in the processed document.

Why is a spec needed?

John Gruber's canonical description of Markdown's syntax does not specify the syntax unambiguously. Here are some examples of questions it does not answer:

How much indentation is needed for a sublist? The spec says that continuation paragraphs need to be indented four spaces, but is not fully explicit about sublists. It is natural to think that they, too, must be indented four spaces, but Markdown.pl does not require that. This is hardly a "corner case," and divergences between implementations on this issue often lead to surprises for users in real documents. (See this comment by John Gruber.)
Is a blank line needed before a block quote or heading? Most implementations do not require the blank line. However, this can lead to unexpected results in hard-wrapped text, and also to ambiguities in parsing (note that some implementations put the heading inside the blockquote, while others do not). (John Gruber has also spoken in favor of requiring the blank lines.)
Is a blank line needed before an indented code block? (Markdown.pl requires it, but this is not mentioned in the documentation, and some implementations do not require it.)
```
paragraph
    code?
```
What is the exact rule for determining when list items get wrapped in <p> tags? Can a list be partially "loose" and partially "tight"? What should we do with a list like this?
```
1. one

2. two
3. three
```
Or this?
```
1.  one
    - a

    - b
2.  two
```
(There are some relevant comments by John Gruber here.)
Can list markers be indented? Can ordered list markers be right-aligned?
```
 8. item 1
 9. item 2
10. item 2a
```
Is this one list with a thematic break in its second item, or two lists separated by a thematic break?
```
* a
* * * * *
* b
```
When list markers change from numbers to bullets, do we have two lists or one? (The Markdown syntax description suggests two, but the perl scripts and many other implementations produce one.)
```
1. fee
2. fie
-  foe
-  fum
```
What are the precedence rules for the markers of inline structure? For example, is the following a valid link, or does the code span take precedence ?
```
[a backtick (`)](/url) and [another backtick (`)](/url).
```
What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?
```
*foo *bar* baz*
```
What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed?
```
- `a long code span can contain a hyphen like this
  - and it can screw things up`
```
Can list items include section headings? (Markdown.pl does not allow this, but does allow blockquotes to include headings.)
```
- # Heading
```
Can list items be empty?
```
* a
*
* b
```
Can link references be defined inside block quotes or list items?
```
> Blockquote [foo].
>
> [foo]: /url
```
If there are multiple definitions for the same reference, which takes precedence?
```
[foo]: /url1
[foo]: /url2

[foo][]
```

In the absence of a spec, early implementers consulted Markdown.pl to resolve these ambiguities. But Markdown.pl was quite buggy, and gave manifestly bad results in many cases, so it was not a satisfactory replacement for a spec.

Because there is no unambiguous spec, implementations have diverged considerably. As a result, users are often surprised to find that a document that renders one way on one system (say, a GitHub wiki) renders differently on another (say, converting to docbook using pandoc). To make matters worse, because nothing in Markdown counts as a "syntax error," the divergence often isn't discovered right away.

About this document

This document attempts to specify Markdown syntax unambiguously. It contains many examples with side-by-side Markdown and HTML. These are intended to double as conformance tests. An accompanying script spec_tests.py can be used to run the tests against any Markdown program:

python test/spec_tests.py --spec spec.txt --program PROGRAM

Since this document describes how Markdown is to be parsed into an abstract syntax tree, it would have made sense to use an abstract representation of the syntax tree instead of HTML. But HTML is capable of representing the structural distinctions we need to make, and the choice of HTML for the tests makes it possible to run the tests against an implementation without writing an abstract syntax tree renderer.

Note that not every feature of the HTML samples is mandated by the spec. For example, the spec says what counts as a link destination, but it doesn't mandate that non-ASCII characters in the URL be percent-encoded. To use the automatic tests, implementers will need to provide a renderer that conforms to the expectations of the spec examples (percent-encoding non-ASCII characters in URLs). But a conforming implementation can use a different renderer and may choose not to percent-encode non-ASCII characters in URLs.

This document is generated from a text file, spec.txt, written in Markdown with a small extension for the side-by-side tests. The script tools/makespec.py can be used to convert spec.txt into HTML or CommonMark (which can then be converted into other formats).

In the examples, the → character is used to represent tabs.

Preliminaries

Characters and lines

Any sequence of [characters] is a valid CommonMark document.

A character is a Unicode code point. Although some code points (for example, combining accents) do not correspond to characters in an intuitive sense, all code points count as characters for purposes of this spec.

This spec does not specify an encoding; it thinks of lines as composed of [characters] rather than bytes. A conforming parser may be limited to a certain encoding.

A line is a sequence of zero or more [characters] other than line feed (U+000A) or carriage return (U+000D), followed by a [line ending] or by the end of file.

A line ending is a line feed (U+000A), a carriage return (U+000D) not followed by a line feed, or a carriage return and a following line feed.

A line containing no characters, or a line containing only spaces (U+0020) or tabs (U+0009), is called a blank line.

The following definitions of character classes will be used in this spec:

A Unicode whitespace character is a character in the Unicode Zs general category, or a tab (U+0009), line feed (U+000A), form feed (U+000C), or carriage return (U+000D).

Unicode whitespace is a sequence of one or more [Unicode whitespace characters].

A tab is U+0009.

A space is U+0020.

An ASCII control character is a character between U+0000–1F (both including) or U+007F.

An ASCII punctuation character is !, ", #, $, %, &, ', (, ), *, +, ,, -, ., / (U+0021–2F), :, ;, <, =, >, ?, @ (U+003A–0040), [, \, ], ^, _, ` (U+005B–0060), {, |, }, or ~ (U+007B–007E).

A Unicode punctuation character is a character in the Unicode P (puncuation) or S (symbol) general categories.

Tabs

Tabs in lines are not expanded to [spaces]. However, in contexts where spaces help to define block structure, tabs behave as if they were replaced by spaces with a tab stop of 4 characters.

Thus, for example, a tab can be used instead of four spaces in an indented code block. (Note, however, that internal tabs are passed through as literal tabs, not expanded to spaces.)

→foo→baz→→bim

<pre><code>foo→baz→→bim
</code></pre>

  →foo→baz→→bim

<pre><code>foo→baz→→bim
</code></pre>

    a→a
    ὐ→a

<pre><code>a→a
ὐ→a
</code></pre>

In the following example, a continuation paragraph of a list item is indented with a tab; this has exactly the same effect as indentation with four spaces would:

  - foo

→bar

<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>

- foo

→→bar

<ul>
<li>
<p>foo</p>
<pre><code>  bar
</code></pre>
</li>
</ul>

Normally the > that begins a block quote may be followed optionally by a space, which is not considered part of the content. In the following case > is followed by a tab, which is treated as if it were expanded into three spaces. Since one of these spaces is considered part of the delimiter, foo is considered to be indented six spaces inside the block quote context, so we get an indented code block starting with two spaces.

>→→foo

<blockquote>
<pre><code>  foo
</code></pre>
</blockquote>

-→→foo

<ul>
<li>
<pre><code>  foo
</code></pre>
</li>
</ul>

    foo
→bar

<pre><code>foo
bar
</code></pre>

 - foo
   - bar
→ - baz

<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>

#→Foo

<h1>Foo</h1>

*→*→*→

<hr />

Insecure characters

For security reasons, the Unicode character U+0000 must be replaced with the REPLACEMENT CHARACTER (U+FFFD).

Backslash escapes

Any ASCII punctuation character may be backslash-escaped:

\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~

<p>!"#$%&amp;'()*+,-./:;&lt;=&gt;?@[\]^_`{|}~</p>

Backslashes before other characters are treated as literal backslashes:

\→\A\a\ \3\φ\«

<p>\→\A\a\ \3\φ\«</p>

Escaped characters are treated as regular characters and do not have their usual Markdown meanings:

\*not emphasized*
\<br/> not a tag
\[not a link](/foo)
\`not code`
1\. not a list
\* not a list
\# not a heading
\[foo]: /url "not a reference"
\&ouml; not a character entity

<p>*not emphasized*
&lt;br/&gt; not a tag
[not a link](/foo)
`not code`
1. not a list
* not a list
# not a heading
[foo]: /url "not a reference"
&amp;ouml; not a character entity</p>

If a backslash is itself escaped, the following character is not:

\\*emphasis*

<p>\<em>emphasis</em></p>

A backslash at the end of the line is a [hard line break]:

foo\
bar

<p>foo<br />
bar</p>

Backslash escapes do not work in code blocks, code spans, autolinks, or raw HTML:

`` \[\` ``

<p><code>\[\`</code></p>

    \[\]

<pre><code>\[\]
</code></pre>

~~~
\[\]
~~~

<pre><code>\[\]
</code></pre>

<https://example.com?find=\*>

<p><a href="https://example.com?find=%5C*">https://example.com?find=\*</a></p>

<a href="/bar\/)">

<a href="/bar\/)">

But they work in all other contexts, including URLs and link titles, link references, and [info strings] in [fenced code blocks]:

[foo](/bar\* "ti\*tle")

<p><a href="/bar*" title="ti*tle">foo</a></p>

[foo]

[foo]: /bar\* "ti\*tle"

<p><a href="/bar*" title="ti*tle">foo</a></p>

``` foo\+bar
foo
```

<pre><code class="language-foo+bar">foo
</code></pre>

Entity and numeric character references

Valid HTML entity references and numeric character references can be used in place of the corresponding Unicode character, with the following exceptions:

Entity and character references are not recognized in code blocks and code spans.
Entity and character references cannot stand in place of special characters that define structural elements in CommonMark. For example, although * can be used in place of a literal * character, * cannot replace * in emphasis delimiters, bullet list markers, or thematic breaks.

Conforming CommonMark parsers need not store information about whether a particular character was represented in the source using a Unicode character or an entity reference.

Entity references consist of & + any of the valid HTML5 entity names + ;. The document https://html.spec.whatwg.org/entities.json is used as an authoritative source for the valid entity references and their corresponding code points.

&nbsp; &amp; &copy; &AElig; &Dcaron;
&frac34; &HilbertSpace; &DifferentialD;
&ClockwiseContourIntegral; &ngE;

<p>  &amp; © Æ Ď
¾ ℋ ⅆ
∲ ≧̸</p>

Decimal numeric character references consist of &# + a string of 1--7 arabic digits + ;. A numeric character reference is parsed as the corresponding Unicode character. Invalid Unicode code points will be replaced by the REPLACEMENT CHARACTER (U+FFFD). For security reasons, the code point U+0000 will also be replaced by U+FFFD.

&#35; &#1234; &#992; &#0;

<p># Ӓ Ϡ �</p>

Hexadecimal numeric character references consist of &# + either X or x + a string of 1-6 hexadecimal digits + ;. They too are parsed as the corresponding Unicode character (this time specified with a hexadecimal numeral instead of decimal).

&#X22; &#XD06; &#xcab;

<p>" ആ ಫ</p>

Here are some nonentities:

&nbsp &x; &#; &#x;
&#87654321;
&#abcdef0;
&ThisIsNotDefined; &hi?;

<p>&amp;nbsp &amp;x; &amp;#; &amp;#x;
&amp;#87654321;
&amp;#abcdef0;
&amp;ThisIsNotDefined; &amp;hi?;</p>

Although HTML5 does accept some entity references without a trailing semicolon (such as &copy), these are not recognized here, because it makes the grammar too ambiguous:

&copy

<p>&amp;copy</p>

Strings that are not on the list of HTML5 named entities are not recognized as entity references either:

&MadeUpEntity;

<p>&amp;MadeUpEntity;</p>

Entity and numeric character references are recognized in any context besides code spans or code blocks, including URLs, [link titles], and [fenced code block][] [info strings]:

<a href="&ouml;&ouml;.html">

<a href="&ouml;&ouml;.html">

[foo](/f&ouml;&ouml; "f&ouml;&ouml;")

<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>

[foo]

[foo]: /f&ouml;&ouml; "f&ouml;&ouml;"

<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>

``` f&ouml;&ouml;
foo
```

<pre><code class="language-föö">foo
</code></pre>

Entity and numeric character references are treated as literal text in code spans and code blocks:

`f&ouml;&ouml;`

<p><code>f&amp;ouml;&amp;ouml;</code></p>

    f&ouml;f&ouml;

<pre><code>f&amp;ouml;f&amp;ouml;
</code></pre>

Entity and numeric character references cannot be used in place of symbols indicating structure in CommonMark documents.

&#42;foo&#42;
*foo*

<p>*foo*
<em>foo</em></p>

&#42; foo

* foo

<p>* foo</p>
<ul>
<li>foo</li>
</ul>

foo&#10;&#10;bar

<p>foo

bar</p>

&#9;foo

<p>→foo</p>

[a](url &quot;tit&quot;)

<p>[a](url "tit")</p>

Blocks and inlines

We can think of a document as a sequence of blocks---structural elements like paragraphs, block quotations, lists, headings, rules, and code blocks. Some blocks (like block quotes and list items) contain other blocks; others (like headings and paragraphs) contain inline content---text, links, emphasized text, images, code spans, and so on.

Precedence

Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span:

- `one
- two`

<ul>
<li>`one</li>
<li>two`</li>
</ul>

This means that parsing can proceed in two steps: first, the block structure of the document can be discerned; second, text lines inside paragraphs, headings, and other block constructs can be parsed for inline structure. The second step requires information about link reference definitions that will be available only at the end of the first step. Note that the first step requires processing lines in sequence, but the second can be parallelized, since the inline parsing of one block element does not affect the inline parsing of any other.

Container blocks and leaf blocks

We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot.

Leaf blocks

This section describes the different kinds of leaf block that make up a Markdown document.

Thematic breaks

A line consisting of optionally up to three spaces of indentation, followed by a sequence of three or more matching -, _, or * characters, each followed optionally by any number of spaces or tabs, forms a thematic break.

***
---
___

<hr />
<hr />
<hr />

Wrong characters:

+++

<p>+++</p>

===

<p>===</p>

Not enough characters:

--
**
__

<p>--
**
__</p>

Up to three spaces of indentation are allowed:

 ***
  ***
   ***

<hr />
<hr />
<hr />

Four spaces of indentation is too many:

***

<pre><code>***
</code></pre>

Foo
    ***

<p>Foo
***</p>

More than three characters may be used:

_____________________________________

<hr />

Spaces and tabs are allowed between the characters:

 - - -

<hr />

 **  * ** * ** * **

<hr />

-     -      -      -

<hr />

Spaces and tabs are allowed at the end:

- - - -

<hr />

However, no other characters may occur in the line:

_ _ _ _ a

a------

---a---

<p>_ _ _ _ a</p>
<p>a------</p>
<p>---a---</p>

It is required that all of the characters other than spaces or tabs be the same. So, this is not a thematic break:

*-*

<p><em>-</em></p>

Thematic breaks do not need blank lines before or after:

- foo
***
- bar

<ul>
<li>foo</li>
</ul>
<hr />
<ul>
<li>bar</li>
</ul>

Thematic breaks can interrupt a paragraph:

Foo
***
bar

<p>Foo</p>
<hr />
<p>bar</p>

If a line of dashes that meets the above conditions for being a thematic break could also be interpreted as the underline of a [setext heading], the interpretation as a [setext heading] takes precedence. Thus, for example, this is a setext heading, not a paragraph followed by a thematic break:

Foo
---
bar

<h2>Foo</h2>
<p>bar</p>

When both a thematic break and a list item are possible interpretations of a line, the thematic break takes precedence:

* Foo
* * *
* Bar

<ul>
<li>Foo</li>
</ul>
<hr />
<ul>
<li>Bar</li>
</ul>

If you want a thematic break in a list item, use a different bullet:

- Foo
- * * *

<ul>
<li>Foo</li>
<li>
<hr />
</li>
</ul>

ATX headings

An ATX heading consists of a string of characters, parsed as inline content, between an opening sequence of 1--6 unescaped # characters and an optional closing sequence of any number of unescaped # characters. The opening sequence of # characters must be followed by spaces or tabs, or by the end of line. The optional closing sequence of #s must be preceded by spaces or tabs and may be followed by spaces or tabs only. The opening # character may be preceded by up to three spaces of indentation. The raw contents of the heading are stripped of leading and trailing space or tabs before being parsed as inline content. The heading level is equal to the number of # characters in the opening sequence.

Simple headings:

# foo
## foo
### foo
#### foo
##### foo
###### foo

<h1>foo</h1>
<h2>foo</h2>
<h3>foo</h3>
<h4>foo</h4>
<h5>foo</h5>
<h6>foo</h6>

More than six # characters is not a heading:

####### foo

<p>####### foo</p>

At least one space or tab is required between the # characters and the heading's contents, unless the heading is empty. Note that many implementations currently do not require the space. However, the space was required by the original ATX implementation, and it helps prevent things like the following from being parsed as headings:

#5 bolt

#hashtag

<p>#5 bolt</p>
<p>#hashtag</p>

This is not a heading, because the first # is escaped:

\## foo

<p>## foo</p>

Contents are parsed as inlines:

# foo *bar* \*baz\*

<h1>foo <em>bar</em> *baz*</h1>

Leading and trailing spaces or tabs are ignored in parsing inline content:

#                  foo

<h1>foo</h1>

Up to three spaces of indentation are allowed:

 ### foo
  ## foo
   # foo

<h3>foo</h3>
<h2>foo</h2>
<h1>foo</h1>

Four spaces of indentation is too many:

    # foo

<pre><code># foo
</code></pre>

foo
    # bar

<p>foo
# bar</p>

A closing sequence of # characters is optional:

## foo ##
  ###   bar    ###

<h2>foo</h2>
<h3>bar</h3>

It need not be the same length as the opening sequence:

# foo ##################################
##### foo ##

<h1>foo</h1>
<h5>foo</h5>

Spaces or tabs are allowed after the closing sequence:

### foo ###

<h3>foo</h3>

A sequence of # characters with anything but spaces or tabs following it is not a closing sequence, but counts as part of the contents of the heading:

### foo ### b

<h3>foo ### b</h3>

The closing sequence must be preceded by a space or tab:

# foo#

<h1>foo#</h1>

Backslash-escaped # characters do not count as part of the closing sequence:

### foo \###
## foo #\##
# foo \#

<h3>foo ###</h3>
<h2>foo ###</h2>
<h1>foo #</h1>

ATX headings need not be separated from surrounding content by blank lines, and they can interrupt paragraphs:

****
## foo
****

<hr />
<h2>foo</h2>
<hr />

Foo bar
# baz
Bar foo

<p>Foo bar</p>
<h1>baz</h1>
<p>Bar foo</p>

ATX headings can be empty:

## 
#
### ###

<h2></h2>
<h1></h1>
<h3></h3>

Setext headings

A setext heading consists of one or more lines of text, not interrupted by a blank line, of which the first line does not have more than 3 spaces of indentation, followed by a [setext heading underline]. The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a [code fence], [ATX heading][ATX headings], [block quote][block quotes], [thematic break][thematic breaks], [list item][list items], or [HTML block][HTML blocks].

A setext heading underline is a sequence of = characters or a sequence of - characters, with no more than 3 spaces of indentation and any number of trailing spaces or tabs.

The heading is a level 1 heading if = characters are used in the [setext heading underline], and a level 2 heading if - characters are used. The contents of the heading are the result of parsing the preceding lines of text as CommonMark inline content.

In general, a setext heading need not be preceded or followed by a blank line. However, it cannot interrupt a paragraph, so when a setext heading comes after a paragraph, a blank line is needed between them.

Simple examples:

Foo *bar*
=========

Foo *bar*
---------

<h1>Foo <em>bar</em></h1>
<h2>Foo <em>bar</em></h2>

The content of the header may span more than one line:

Foo *bar
baz*
====

<h1>Foo <em>bar
baz</em></h1>

The contents are the result of parsing the headings's raw content as inlines. The heading's raw content is formed by concatenating the lines and removing initial and final spaces or tabs.

  Foo *bar
baz*→
====

<h1>Foo <em>bar
baz</em></h1>

The underlining can be any length:

Foo
-------------------------

Foo
=

<h2>Foo</h2>
<h1>Foo</h1>

The heading content can be preceded by up to three spaces of indentation, and need not line up with the underlining:

   Foo
---

  Foo
-----

  Foo
  ===

<h2>Foo</h2>
<h2>Foo</h2>
<h1>Foo</h1>

Four spaces of indentation is too many:

    Foo
    ---

    Foo
---

<pre><code>Foo
---

Foo
</code></pre>
<hr />

The setext heading underline can be preceded by up to three spaces of indentation, and may have trailing spaces or tabs:

Foo
   ----

<h2>Foo</h2>

Four spaces of indentation is too many:

Foo
    ---

<p>Foo
---</p>

The setext heading underline cannot contain internal spaces or tabs:

Foo
= =

Foo
--- -

<p>Foo
= =</p>
<p>Foo</p>
<hr />

Trailing spaces or tabs in the content line do not cause a hard line break:

Foo  
-----

<h2>Foo</h2>

Nor does a backslash at the end:

Foo\
----

<h2>Foo\</h2>

Since indicators of block structure take precedence over indicators of inline structure, the following are setext headings:

`Foo
----
`

<a title="a lot
---
of dashes"/>

<h2>`Foo</h2>
<p>`</p>
<h2>&lt;a title="a lot</h2>
<p>of dashes"/&gt;</p>

The setext heading underline cannot be a [lazy continuation line] in a list item or block quote:

> Foo
---

<blockquote>
<p>Foo</p>
</blockquote>
<hr />

> foo
bar
===

<blockquote>
<p>foo
bar
===</p>
</blockquote>

- Foo
---

<ul>
<li>Foo</li>
</ul>
<hr />

A blank line is needed between a paragraph and a following setext heading, since otherwise the paragraph becomes part of the heading's content:

Foo
Bar
---

<h2>Foo
Bar</h2>

But in general a blank line is not required before or after setext headings:

---
Foo
---
Bar
---
Baz

<hr />
<h2>Foo</h2>
<h2>Bar</h2>
<p>Baz</p>

Setext headings cannot be empty:


====

<p>====</p>

Setext heading text lines must not be interpretable as block constructs other than paragraphs. So, the line of dashes in these examples gets interpreted as a thematic break:

---
---

<hr />
<hr />

- foo
-----

<ul>
<li>foo</li>
</ul>
<hr />

    foo
---

<pre><code>foo
</code></pre>
<hr />

> foo
-----

<blockquote>
<p>foo</p>
</blockquote>
<hr />

If you want a heading with > foo as its literal text, you can use backslash escapes:

\> foo
------

<h2>&gt; foo</h2>

Compatibility note: Most existing Markdown implementations do not allow the text of setext headings to span multiple lines. But there is no consensus about how to interpret

Foo
bar
---
baz

One can find four different interpretations:

paragraph "Foo", heading "bar", paragraph "baz"
paragraph "Foo bar", thematic break, paragraph "baz"
paragraph "Foo bar --- baz"
heading "Foo bar", paragraph "baz"

We find interpretation 4 most natural, and interpretation 4 increases the expressive power of CommonMark, by allowing multiline headings. Authors who want interpretation 1 can put a blank line after the first paragraph:

Foo

bar
---
baz

<p>Foo</p>
<h2>bar</h2>
<p>baz</p>

Authors who want interpretation 2 can put blank lines around the thematic break,

Foo
bar

---

baz

<p>Foo
bar</p>
<hr />
<p>baz</p>

or use a thematic break that cannot count as a [setext heading underline], such as

Foo
bar
* * *
baz

<p>Foo
bar</p>
<hr />
<p>baz</p>

Authors who want interpretation 3 can use backslash escapes:

Foo
bar
\---
baz

<p>Foo
bar
---
baz</p>

Indented code blocks

An indented code block is composed of one or more [indented chunks] separated by blank lines. An indented chunk is a sequence of non-blank lines, each preceded by four or more spaces of indentation. The contents of the code block are the literal contents of the lines, including trailing [line endings], minus four spaces of indentation. An indented code block has no [info string].

An indented code block cannot interrupt a paragraph, so there must be a blank line between a paragraph and a following indented code block. (A blank line is not needed, however, between a code block and a following paragraph.)

    a simple
      indented code block

<pre><code>a simple
  indented code block
</code></pre>

If there is any ambiguity between an interpretation of indentation as a code block and as indicating that material belongs to a [list item][list items], the list item interpretation takes precedence:

  - foo

    bar

<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>

1.  foo

    - bar

<ol>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
</li>
</ol>

The contents of a code block are literal text, and do not get parsed as Markdown:

    <a/>
    *hi*

    - one

<pre><code>&lt;a/&gt;
*hi*

- one
</code></pre>

Here we have three chunks separated by blank lines:

    chunk1

    chunk2
  
 
 
    chunk3

<pre><code>chunk1

chunk2



chunk3
</code></pre>

Any initial spaces or tabs beyond four spaces of indentation will be included in the content, even in interior blank lines:

    chunk1
      
      chunk2

<pre><code>chunk1
  
  chunk2
</code></pre>

An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.)

Foo
    bar

<p>Foo
bar</p>

However, any non-blank line with fewer than four spaces of indentation ends the code block immediately. So a paragraph may occur immediately after indented code:

    foo
bar

<pre><code>foo
</code></pre>
<p>bar</p>

And indented code can occur immediately before and after other kinds of blocks:

# Heading
    foo
Heading
------
    foo
----

<h1>Heading</h1>
<pre><code>foo
</code></pre>
<h2>Heading</h2>
<pre><code>foo
</code></pre>
<hr />

The first line can be preceded by more than four spaces of indentation:

        foo
    bar

<pre><code>    foo
bar
</code></pre>

Blank lines preceding or following an indented code block are not included in it:

foo

<pre><code>foo
</code></pre>

Trailing spaces or tabs are included in the code block's content:

foo

<pre><code>foo  
</code></pre>

Fenced code blocks

A code fence is a sequence of at least three consecutive backtick characters (`) or tildes (~). (Tildes and backticks cannot be mixed.) A fenced code block begins with a code fence, preceded by up to three spaces of indentation.

The line with the opening code fence may optionally contain some text following the code fence; this is trimmed of leading and trailing spaces or tabs and called the info string. If the [info string] comes after a backtick fence, it may not contain any backtick characters. (The reason for this restriction is that otherwise some inline code would be incorrectly interpreted as the beginning of a fenced code block.)

The content of the code block consists of all subsequent lines, until a closing [code fence] of the same type as the code block began with (backticks or tildes), and with at least as many backticks or tildes as the opening code fence. If the leading code fence is preceded by N spaces of indentation, then up to N spaces of indentation are removed from each line of the content (if present). (If a content line is not indented, it is preserved unchanged. If it is indented N spaces or less, all of the indentation is removed.)

The closing code fence may be preceded by up to three spaces of indentation, and may be followed only by spaces or tabs, which are ignored. If the end of the containing block (or document) is reached and no closing code fence has been found, the code block contains all of the lines after the opening code fence until the end of the containing block (or document). (An alternative spec would require backtracking in the event that a closing code fence is not found. But this makes parsing much less efficient, and there seems to be no real downside to the behavior described here.)

A fenced code block may interrupt a paragraph, and does not require a blank line either before or after.

The content of a code fence is treated as literal text, not parsed as inlines. The first word of the [info string] is typically used to specify the language of the code sample, and rendered in the class attribute of the code tag. However, this spec does not mandate any particular treatment of the [info string].

Here is a simple example with backticks:

```
<
 >
```

<pre><code>&lt;
 &gt;
</code></pre>

With tildes:

~~~
<
 >
~~~

<pre><code>&lt;
 &gt;
</code></pre>

Fewer than three backticks is not enough:

``
foo
``

<p><code>foo</code></p>

The closing code fence must use the same character as the opening fence:

```
aaa
~~~
```

<pre><code>aaa
~~~
</code></pre>

~~~
aaa
```
~~~

<pre><code>aaa
```
</code></pre>

The closing code fence must be at least as long as the opening fence:

````
aaa
```
``````

<pre><code>aaa
```
</code></pre>

~~~~
aaa
~~~
~~~~

<pre><code>aaa
~~~
</code></pre>

Unclosed code blocks are closed by the end of the document (or the enclosing [block quote][block quotes] or [list item][list items]):

```

<pre><code></code></pre>

`````

```
aaa

<pre><code>
```
aaa
</code></pre>

> ```
> aaa

bbb

<blockquote>
<pre><code>aaa
</code></pre>
</blockquote>
<p>bbb</p>

A code block can have all empty lines as its content:

```

  
```

<pre><code>
  
</code></pre>

A code block can be empty:

```
```

<pre><code></code></pre>

Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:

 ```
 aaa
aaa
```

<pre><code>aaa
aaa
</code></pre>

  ```
aaa
  aaa
aaa
  ```

<pre><code>aaa
aaa
aaa
</code></pre>

   ```
   aaa
    aaa
  aaa
   ```

<pre><code>aaa
 aaa
aaa
</code></pre>

Four spaces of indentation is too many:

    ```
    aaa
    ```

<pre><code>```
aaa
```
</code></pre>

Closing fences may be preceded by up to three spaces of indentation, and their indentation need not match that of the opening fence:

```
aaa
  ```

<pre><code>aaa
</code></pre>

   ```
aaa
  ```

<pre><code>aaa
</code></pre>

This is not a closing fence, because it is indented 4 spaces:

```
aaa
    ```

<pre><code>aaa
    ```
</code></pre>

Code fences (opening and closing) cannot contain internal spaces or tabs:

``` ```
aaa

<p><code> </code>
aaa</p>

~~~~~~
aaa
~~~ ~~

<pre><code>aaa
~~~ ~~
</code></pre>

Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between:

foo
```
bar
```
baz

<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>

Other blocks can also occur before and after fenced code blocks without an intervening blank line:

foo
---
~~~
bar
~~~
# baz

<h2>foo</h2>
<pre><code>bar
</code></pre>
<h1>baz</h1>

An [info string] can be provided after the opening code fence. Although this spec doesn't mandate any particular treatment of the info string, the first word is typically used to specify the language of the code block. In HTML output, the language is normally indicated by adding a class to the code element consisting of language- followed by the language name.

```ruby
def foo(x)
  return 3
end
```

<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>

~~~~    ruby startline=3 $%@#$
def foo(x)
  return 3
end
~~~~~~~

<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>

````;
````

<pre><code class="language-;"></code></pre>

[Info strings] for backtick code blocks cannot contain backticks:

``` aa ```
foo

<p><code>aa</code>
foo</p>

[Info strings] for tilde code blocks can contain backticks and tildes:

~~~ aa ``` ~~~
foo
~~~

<pre><code class="language-aa">foo
</code></pre>

Closing code fences cannot have [info strings]:

```
``` aaa
```

<pre><code>``` aaa
</code></pre>

HTML blocks

An HTML block is a group of lines that is treated as raw HTML (and will not be escaped in HTML output).

There are seven kinds of [HTML block], which can be defined by their start and end conditions. The block begins with a line that meets a start condition (after up to three optional spaces of indentation). It ends with the first subsequent line that meets a matching end condition, or the last line of the document, or the last line of the container block containing the current HTML block, if no line is encountered that meets the [end condition]. If the first line meets both the [start condition] and the [end condition], the block will contain just that line.

Start condition: line begins with the string <pre, <script, <style, or <textarea (case-insensitive), followed by a space, a tab, the string >, or the end of the line.
End condition: line contains an end tag </pre>, </script>, </style>, or </textarea> (case-insensitive; it need not match the start tag).
Start condition: line begins with the string .
Start condition: line begins with the string <?.
End condition: line contains the string ?>.
Start condition: line begins with the string <! followed by an ASCII letter.
End condition: line contains the character >.
Start condition: line begins with the string <![CDATA[.
End condition: line contains the string ]]>.
Start condition: line begins with the string < or </ followed by one of the strings (case-insensitive) address, article, aside, base, basefont, blockquote, body, caption, center, col, colgroup, dd, details, dialog, dir, div, dl, dt, fieldset, figcaption, figure, footer, form, frame, frameset, h1, h2, h3, h4, h5, h6, head, header, hr, html, iframe, legend, li, link, main, menu, menuitem, nav, noframes, ol, optgroup, option, p, param, search, section, summary, table, tbody, td, tfoot, th, thead, title, tr, track, ul, followed by a space, a tab, the end of the line, the string >, or the string />.
End condition: line is followed by a [blank line].
Start condition: line begins with a complete [open tag] (with any [tag name] other than pre, script, style, or textarea) or a complete [closing tag], followed by zero or more spaces and tabs, followed by the end of the line.
End condition: line is followed by a [blank line].

HTML blocks continue until they are closed by their appropriate [end condition], or the last line of the document or other container block. This means any HTML within an HTML block that might otherwise be recognised as a start condition will be ignored by the parser and passed through as-is, without changing the parser's state.

For instance, <pre> within an HTML block started by <table> will not affect the parser state; as the HTML block was started in by start condition 6, it will end at any blank line. This can be surprising:

<table><tr><td>
<pre>
**Hello**,

_world_.
</pre>
</td></tr></table>

<table><tr><td>
<pre>
**Hello**,
<p><em>world</em>.
</pre></p>
</td></tr></table>

In this case, the HTML block is terminated by the blank line — the **Hello** text remains verbatim — and regular parsing resumes, with a paragraph, emphasised world and inline and block HTML following.

All types of [HTML blocks] except type 7 may interrupt a paragraph. Blocks of type 7 may not interrupt a paragraph. (This restriction is intended to prevent unwanted interpretation of long tags inside a wrapped paragraph as starting HTML blocks.)

Some simple examples follow. Here are some basic HTML blocks of type 6:

<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>

okay.

<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>
<p>okay.</p>

 <div>
  *hello*
         <foo><a>

 <div>
  *hello*
         <foo><a>

A block can also start with a closing tag:

</div>
*foo*

</div>
*foo*

Here we have two HTML blocks with a Markdown paragraph between them:

<DIV CLASS="foo">

*Markdown*

</DIV>

<DIV CLASS="foo">
<p><em>Markdown</em></p>
</DIV>

The tag on the first line can be partial, as long as it is split where there would be whitespace:

<div id="foo"
  class="bar">
</div>

<div id="foo"
  class="bar">
</div>

<div id="foo" class="bar
  baz">
</div>

<div id="foo" class="bar
  baz">
</div>

An open tag need not be closed:

<div>
*foo*

*bar*

<div>
*foo*
<p><em>bar</em></p>

A partial tag need not even be completed (garbage in, garbage out):

<div id="foo"
*hi*

<div id="foo"
*hi*

<div class
foo

<div class
foo

The initial tag doesn't even need to be a valid tag, as long as it starts like one:

<div *???-&&&-<---
*foo*

<div *???-&&&-<---
*foo*

In type 6 blocks, the initial tag need not be on a line by itself:

<div><a href="bar">*foo*</a></div>

<div><a href="bar">*foo*</a></div>

<table><tr><td>
foo
</td></tr></table>

<table><tr><td>
foo
</td></tr></table>

Everything until the next blank line or end of document gets included in the HTML block. So, in the following example, what looks like a Markdown code block is actually part of the HTML block, which continues until a blank line or the end of the document is reached:

<div></div>
``` c
int x = 33;
```

<div></div>
``` c
int x = 33;
```

To start an [HTML block] with a tag that is not in the list of block-level tags in (6), you must put the tag by itself on the first line (and it must be complete):

<a href="foo">
*bar*
</a>

<a href="foo">
*bar*
</a>

In type 7 blocks, the [tag name] can be anything:

<Warning>
*bar*
</Warning>

<Warning>
*bar*
</Warning>

<i class="foo">
*bar*
</i>

<i class="foo">
*bar*
</i>

</ins>
*bar*

</ins>
*bar*

These rules are designed to allow us to work with tags that can function as either block-level or inline-level tags. The <del> tag is a nice example. We can surround content with <del> tags in three different ways. In this case, we get a raw HTML block, because the <del> tag is on a line by itself:

<del>
*foo*
</del>

<del>
*foo*
</del>

In this case, we get a raw HTML block that just includes the <del> tag (because it ends with the following blank line). So the contents get interpreted as CommonMark:

<del>

*foo*

</del>

<del>
<p><em>foo</em></p>
</del>

Finally, in this case, the <del> tags are interpreted as [raw HTML] inside the CommonMark paragraph. (Because the tag is not on a line by itself, we get inline HTML rather than an [HTML block].)

<del>*foo*</del>

<p><del><em>foo</em></del></p>

HTML tags designed to contain literal content (pre, script, style, textarea), comments, processing instructions, and declarations are treated somewhat differently. Instead of ending at the first blank line, these blocks end at the first line containing a corresponding end tag. As a result, these blocks can contain blank lines:

A pre tag (type 1):

<pre language="haskell"><code>
import Text.HTML.TagSoup

main :: IO ()
main = print $ parseTags tags
</code></pre>
okay

<pre language="haskell"><code>
import Text.HTML.TagSoup

main :: IO ()
main = print $ parseTags tags
</code></pre>
<p>okay</p>

A script tag (type 1):

<script type="text/javascript">
// JavaScript example

document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
okay

<script type="text/javascript">
// JavaScript example

document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
<p>okay</p>

A textarea tag (type 1):

<textarea>

*foo*

_bar_

</textarea>

<textarea>

*foo*

_bar_

</textarea>

A style tag (type 1):

<style
  type="text/css">
h1 {color:red;}

p {color:blue;}
</style>
okay

<style
  type="text/css">
h1 {color:red;}

p {color:blue;}
</style>
<p>okay</p>

If there is no matching end tag, the block will end at the end of the document (or the enclosing [block quote][block quotes] or [list item][list items]):

<style
  type="text/css">

foo

<style
  type="text/css">

foo

> <div>
> foo

bar

<blockquote>
<div>
foo
</blockquote>
<p>bar</p>

- <div>
- foo

<ul>
<li>
<div>
</li>
<li>foo</li>
</ul>

The end tag can occur on the same line as the start tag:

<style>p{color:red;}</style>
*foo*

<style>p{color:red;}</style>
<p><em>foo</em></p>

<!-- foo -->*bar*
*baz*

<!-- foo -->*bar*
<p><em>baz</em></p>

Note that anything on the last line after the end tag will be included in the [HTML block]:

<script>
foo
</script>1. *bar*

<script>
foo
</script>1. *bar*

A comment (type 2):

<!-- Foo

bar
   baz -->
okay

<!-- Foo

bar
   baz -->
<p>okay</p>

A processing instruction (type 3):

<?php

  echo '>';

?>
okay

<?php

  echo '>';

?>
<p>okay</p>

A declaration (type 4):

<!DOCTYPE html>

<!DOCTYPE html>

CDATA (type 5):

<![CDATA[
function matchwo(a,b)
{
  if (a < b && a < 0) then {
    return 1;

  } else {

    return 0;
  }
}
]]>
okay

<![CDATA[
function matchwo(a,b)
{
  if (a < b && a < 0) then {
    return 1;

  } else {

    return 0;
  }
}
]]>
<p>okay</p>

The opening tag can be preceded by up to three spaces of indentation, but not four:

  <!-- foo -->

    <!-- foo -->

  <!-- foo -->
<pre><code>&lt;!-- foo --&gt;
</code></pre>

  <div>

    <div>

  <div>
<pre><code>&lt;div&gt;
</code></pre>

An HTML block of types 1--6 can interrupt a paragraph, and need not be preceded by a blank line.

Foo
<div>
bar
</div>

<p>Foo</p>
<div>
bar
</div>

However, a following blank line is needed, except at the end of a document, and except for blocks of types 1--5, [above][HTML block]:

<div>
bar
</div>
*foo*

<div>
bar
</div>
*foo*

HTML blocks of type 7 cannot interrupt a paragraph:

Foo
<a href="bar">
baz

<p>Foo
<a href="bar">
baz</p>

This rule differs from John Gruber's original Markdown syntax specification, which says:

The only restrictions are that block-level HTML elements — e.g. <div>, <table>, <pre>, <p>, etc. — must be separated from surrounding content by blank lines, and the start and end tags of the block should not be indented with spaces or tabs.

In some ways Gruber's rule is more restrictive than the one given here:

It requires that an HTML block be preceded by a blank line.
It does not allow the start tag to be indented.
It requires a matching end tag, which it also does not allow to be indented.

Most Markdown implementations (including some of Gruber's own) do not respect all of these restrictions.

There is one respect, however, in which Gruber's rule is more liberal than the one given here, since it allows blank lines to occur inside an HTML block. There are two reasons for disallowing them here. First, it removes the need to parse balanced tags, which is expensive and can require backtracking from the end of the document if no matching end tag is found. Second, it provides a very simple and flexible way of including Markdown content inside HTML tags: simply separate the Markdown from the HTML using blank lines:

Compare:

<div>

*Emphasized* text.

</div>

<div>
<p><em>Emphasized</em> text.</p>
</div>

<div>
*Emphasized* text.
</div>

<div>
*Emphasized* text.
</div>

Some Markdown implementations have adopted a convention of interpreting content inside tags as text if the open tag has the attribute markdown=1. The rule given above seems a simpler and more elegant way of achieving the same expressive power, which is also much simpler to parse.

The main potential drawback is that one can no longer paste HTML blocks into Markdown documents with 100% reliability. However, in most cases this will work fine, because the blank lines in HTML are usually followed by HTML block tags. For example:

<table>

<tr>

<td>
Hi
</td>

</tr>

</table>

<table>
<tr>
<td>
Hi
</td>
</tr>
</table>

There are problems, however, if the inner tags are indented and separated by spaces, as then they will be interpreted as an indented code block:

<table>

  <tr>

    <td>
      Hi
    </td>

  </tr>

</table>

<table>
  <tr>
<pre><code>&lt;td&gt;
  Hi
&lt;/td&gt;
</code></pre>
  </tr>
</table>

Fortunately, blank lines are usually not necessary and can be deleted. The exception is inside <pre> tags, but as described [above][HTML blocks], raw HTML blocks starting with <pre> can contain blank lines.

Link reference definitions

A link reference definition consists of a [link label], optionally preceded by up to three spaces of indentation, followed by a colon (:), optional spaces or tabs (including up to one [line ending]), a [link destination], optional spaces or tabs (including up to one [line ending]), and an optional [link title], which if it is present must be separated from the [link destination] by spaces or tabs. No further character may occur.

A [link reference definition] does not correspond to a structural element of a document. Instead, it defines a label which can be used in [reference links] and reference-style [images] elsewhere in the document. [Link reference definitions] can come either before or after the links that use them.

[foo]: /url "title"

[foo]

<p><a href="/url" title="title">foo</a></p>

   [foo]: 
      /url  
           'the title'  

[foo]

<p><a href="/url" title="the title">foo</a></p>

[Foo*bar\]]:my_(url) 'title (with parens)'

[Foo*bar\]]

<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>

[Foo bar]:
<my url>
'title'

[Foo bar]

<p><a href="my%20url" title="title">Foo bar</a></p>

The title may extend over multiple lines:

[foo]: /url '
title
line1
line2
'

[foo]

<p><a href="/url" title="
title
line1
line2
">foo</a></p>

However, it may not contain a [blank line]:

[foo]: /url 'title

with blank line'

[foo]

<p>[foo]: /url 'title</p>
<p>with blank line'</p>
<p>[foo]</p>

The title may be omitted:

[foo]:
/url

[foo]

<p><a href="/url">foo</a></p>

The link destination may not be omitted:

[foo]:

[foo]

<p>[foo]:</p>
<p>[foo]</p>

However, an empty link destination may be specified using angle brackets:

[foo]: <>

[foo]

<p><a href="">foo</a></p>

The title must be separated from the link destination by spaces or tabs:

[foo]: <bar>(baz)

[foo]

<p>[foo]: <bar>(baz)</p>
<p>[foo]</p>

Both title and destination can contain backslash escapes and literal backslashes:

[foo]: /url\bar\*baz "foo\"bar\baz"

[foo]

<p><a href="/url%5Cbar*baz" title="foo&quot;bar\baz">foo</a></p>

A link can come before its corresponding definition:

[foo]

[foo]: url

<p><a href="url">foo</a></p>

If there are several matching definitions, the first one takes precedence:

[foo]

[foo]: first
[foo]: second

<p><a href="first">foo</a></p>

As noted in the section on [Links], matching of labels is case-insensitive (see [matches]).

[FOO]: /url

[Foo]

<p><a href="/url">Foo</a></p>

[ΑΓΩ]: /φου

[αγω]

<p><a href="/%CF%86%CE%BF%CF%85">αγω</a></p>

Whether something is a [link reference definition] is independent of whether the link reference it defines is used in the document. Thus, for example, the following document contains just a link reference definition, and no visible content:

[foo]: /url

Here is another one:

[
foo
]: /url
bar

<p>bar</p>

This is not a link reference definition, because there are characters other than spaces or tabs after the title:

[foo]: /url "title" ok

<p>[foo]: /url "title" ok</p>

This is a link reference definition, but it has no title:

[foo]: /url
"title" ok

<p>"title" ok</p>

This is not a link reference definition, because it is indented four spaces:

    [foo]: /url "title"

[foo]

<pre><code>[foo]: /url "title"
</code></pre>
<p>[foo]</p>

This is not a link reference definition, because it occurs inside a code block:

```
[foo]: /url
```

[foo]

<pre><code>[foo]: /url
</code></pre>
<p>[foo]</p>

A [link reference definition] cannot interrupt a paragraph.

Foo
[bar]: /baz

[bar]

<p>Foo
[bar]: /baz</p>
<p>[bar]</p>

However, it can directly follow other block elements, such as headings and thematic breaks, and it need not be followed by a blank line.

# [Foo]
[foo]: /url
> bar

<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>

[foo]: /url
bar
===
[foo]

<h1>bar</h1>
<p><a href="/url">foo</a></p>

[foo]: /url
===
[foo]

<p>===
<a href="/url">foo</a></p>

Several [link reference definitions] can occur one after another, without intervening blank lines.

[foo]: /foo-url "foo"
[bar]: /bar-url
  "bar"
[baz]: /baz-url

[foo],
[bar],
[baz]

<p><a href="/foo-url" title="foo">foo</a>,
<a href="/bar-url" title="bar">bar</a>,
<a href="/baz-url">baz</a></p>

[Link reference definitions] can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined:

[foo]

> [foo]: /url

<p><a href="/url">foo</a></p>
<blockquote>
</blockquote>

Paragraphs

A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph's raw content as inlines. The paragraph's raw content is formed by concatenating the lines and removing initial and final spaces or tabs.

A simple example with two paragraphs:

aaa

bbb

<p>aaa</p>
<p>bbb</p>

Paragraphs can contain multiple lines, but no blank lines:

aaa
bbb

ccc
ddd

<p>aaa
bbb</p>
<p>ccc
ddd</p>

Multiple blank lines between paragraphs have no effect:

aaa


bbb

<p>aaa</p>
<p>bbb</p>

Leading spaces or tabs are skipped:

  aaa
 bbb

<p>aaa
bbb</p>

Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs.

aaa
             bbb
                                       ccc

<p>aaa
bbb
ccc</p>

However, the first line may be preceded by up to three spaces of indentation. Four spaces of indentation is too many:

   aaa
bbb

<p>aaa
bbb</p>

    aaa
bbb

<pre><code>aaa
</code></pre>
<p>bbb</p>

Final spaces or tabs are stripped before inline parsing, so a paragraph that ends with two or more spaces will not end with a [hard line break]:

aaa     
bbb

<p>aaa<br />
bbb</p>

Blank lines

[Blank lines] between block-level elements are ignored, except for the role they play in determining whether a [list] is [tight] or [loose].

Blank lines at the beginning and end of the document are also ignored.

  

aaa
  

# aaa

<p>aaa</p>
<h1>aaa</h1>

Container blocks

A container block is a block that has other blocks as its contents. There are two basic kinds of container blocks: [block quotes] and [list items]. [Lists] are meta-containers for [list items].

We define the syntax for container blocks recursively. The general form of the definition is:

If X is a sequence of blocks, then the result of transforming X in such-and-such a way is a container of type Y with these blocks as its content.

So, we explain what counts as a block quote or list item by explaining how these can be generated from their contents. This should suffice to define the syntax, although it does not give a recipe for parsing these constructions. (A recipe is provided below in the section entitled A parsing strategy.)

Block quotes

A block quote marker, optionally preceded by up to three spaces of indentation, consists of (a) the character > together with a following space of indentation, or (b) a single character > not followed by a space of indentation.

The following rules define [block quotes]:

Basic case. If a string of lines Ls constitute a sequence of blocks Bs, then the result of prepending a [block quote marker] to the beginning of each line in Ls is a block quote containing Bs.
Laziness. If a string of lines Ls constitute a block quote with contents Bs, then the result of deleting the initial [block quote marker] from one or more lines in which the next character other than a space or tab after the [block quote marker] is [paragraph continuation text] is a block quote with Bs as its content. Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph.
Consecutiveness. A document cannot contain two [block quotes] in a row unless there is a [blank line] between them.

Nothing else counts as a block quote.

Here is a simple example:

> # Foo
> bar
> baz

<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The space or tab after the > characters can be omitted:

># Foo
>bar
> baz

<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The > characters can be preceded by up to three spaces of indentation:

   > # Foo
   > bar
 > baz

<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

Four spaces of indentation is too many:

    > # Foo
    > bar
    > baz

<pre><code>&gt; # Foo
&gt; bar
&gt; baz
</code></pre>

The Laziness clause allows us to omit the > before [paragraph continuation text]:

> # Foo
> bar
baz

<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

A block quote can contain some lazy and some non-lazy continuation lines:

> bar
baz
> foo

<blockquote>
<p>bar
baz
foo</p>
</blockquote>

Laziness only applies to lines that would have been continuations of paragraphs had they been prepended with [block quote markers]. For example, the > cannot be omitted in the second line of

> foo
> ---

without changing the meaning:

> foo
---

<blockquote>
<p>foo</p>
</blockquote>
<hr />

Similarly, if we omit the > in the second line of

> - foo
> - bar

then the block quote ends after the first line:

> - foo
- bar

<blockquote>
<ul>
<li>foo</li>
</ul>
</blockquote>
<ul>
<li>bar</li>
</ul>

For the same reason, we can't omit the > in front of subsequent lines of an indented or fenced code block:

>     foo
    bar

<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>

> ```
foo
```

<blockquote>
<pre><code></code></pre>
</blockquote>
<p>foo</p>
<pre><code></code></pre>

Note that in the following case, we have a [lazy continuation line]:

> foo
    - bar

<blockquote>
<p>foo
- bar</p>
</blockquote>

To see why, note that in

> foo
>     - bar

the - bar is indented too far to start a list, and can't be an indented code block because indented code blocks cannot interrupt paragraphs, so it is [paragraph continuation text].

A block quote can be empty:

<blockquote>
</blockquote>

>
>  
>

<blockquote>
</blockquote>

A block quote can have initial or final blank lines:

>
> foo
>

<blockquote>
<p>foo</p>
</blockquote>

A blank line always separates block quotes:

> foo

> bar

<blockquote>
<p>foo</p>
</blockquote>
<blockquote>
<p>bar</p>
</blockquote>

(Most current Markdown implementations, including John Gruber's original Markdown.pl, will parse this example as a single block quote with two paragraphs. But it seems better to allow the author to decide whether two block quotes or one are wanted.)

Consecutiveness means that if we put these block quotes together, we get a single block quote:

> foo
> bar

<blockquote>
<p>foo
bar</p>
</blockquote>

To get a block quote with two paragraphs, use:

> foo
>
> bar

<blockquote>
<p>foo</p>
<p>bar</p>
</blockquote>

Block quotes can interrupt paragraphs:

foo
> bar

<p>foo</p>
<blockquote>
<p>bar</p>
</blockquote>

In general, blank lines are not needed before or after block quotes:

> aaa
***
> bbb

<blockquote>
<p>aaa</p>
</blockquote>
<hr />
<blockquote>
<p>bbb</p>
</blockquote>

However, because of laziness, a blank line is needed between a block quote and a following paragraph:

> bar
baz

<blockquote>
<p>bar
baz</p>
</blockquote>

> bar

baz

<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>

> bar
>
baz

<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>

It is a consequence of the Laziness rule that any number of initial >s may be omitted on a continuation line of a nested block quote:

> > > foo
bar

<blockquote>
<blockquote>
<blockquote>
<p>foo
bar</p>
</blockquote>
</blockquote>
</blockquote>

>>> foo
> bar
>>baz

<blockquote>
<blockquote>
<blockquote>
<p>foo
bar
baz</p>
</blockquote>
</blockquote>
</blockquote>

When including an indented code block in a block quote, remember that the [block quote marker] includes both the > and a following space of indentation. So five spaces are needed after the >:

>     code

>    not code

<blockquote>
<pre><code>code
</code></pre>
</blockquote>
<blockquote>
<p>not code</p>
</blockquote>

List items

A list marker is a [bullet list marker] or an [ordered list marker].

A bullet list marker is a -, +, or * character.

An ordered list marker is a sequence of 1--9 arabic digits (0-9), followed by either a . character or a ) character. (The reason for the length limit is that with 10 digits we start seeing integer overflows in some browsers.)

The following rules define [list items]:

Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a character other than a space or tab, and M is a list marker of width W followed by 1 ≤ N ≤ 4 spaces of indentation, then the result of prepending M and the following spaces to the first line of Ls, and indenting subsequent lines of Ls by W + N spaces, is a list item with Bs as its contents. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.

Exceptions:
1. When the first list item in a [list] interrupts a paragraph---that is, when it starts on a line that would otherwise count as [paragraph continuation text]---then (a) the lines Ls must not begin with a blank line, and (b) if the list item is ordered, the start number must be 1.
2. If any line is a [thematic break][thematic breaks] then that line is not a list item.

For example, let Ls be the lines

A paragraph
with two lines.

    indented code

> A block quote.

<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>

And let M be the marker 1., and N = 2. Then rule #1 says that the following is an ordered list item with start number 1, and the same contents as Ls:

1.  A paragraph
    with two lines.

        indented code

    > A block quote.

<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

The most important thing to notice is that the position of the text after the list marker determines how much indentation is needed in subsequent blocks in the list item. If the list marker takes up two spaces of indentation, and there are three spaces between the list marker and the next character other than a space or tab, then blocks must be indented five spaces in order to fall under the list item.

Here are some examples showing how far content must be indented to be put under the list item:

- one

 two

<ul>
<li>one</li>
</ul>
<p>two</p>

- one

  two

<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>

 -    one

     two

<ul>
<li>one</li>
</ul>
<pre><code> two
</code></pre>

 -    one

      two

<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>

It is tempting to think of this in terms of columns: the continuation blocks must be indented at least to the column of the first character other than a space or tab after the list marker. However, that is not quite right. The spaces of indentation after the list marker determine how much relative indentation is needed. Which column this indentation reaches will depend on how the list item is embedded in other constructions, as shown by this example:

   > > 1.  one
>>
>>     two

<blockquote>
<blockquote>
<ol>
<li>
<p>one</p>
<p>two</p>
</li>
</ol>
</blockquote>
</blockquote>

Here two occurs in the same column as the list marker 1., but is actually contained in the list item, because there is sufficient indentation after the last containing blockquote marker.

The converse is also possible. In the following example, the word two occurs far to the right of the initial text of the list item, one, but it is not considered part of the list item, because it is not indented far enough past the blockquote marker:

>>- one
>>
  >  > two

<blockquote>
<blockquote>
<ul>
<li>one</li>
</ul>
<p>two</p>
</blockquote>
</blockquote>

Note that at least one space or tab is needed between the list marker and any following content, so these are not list items:

-one

2.two

<p>-one</p>
<p>2.two</p>

A list item may contain blocks that are separated by more than one blank line.

- foo


  bar

<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>

A list item may contain any kind of block:

1.  foo

    ```
    bar
    ```

    baz

    > bam

<ol>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>
<blockquote>
<p>bam</p>
</blockquote>
</li>
</ol>

A list item that contains an indented code block will preserve empty lines within the code block verbatim.

- Foo

      bar


      baz

<ul>
<li>
<p>Foo</p>
<pre><code>bar


baz
</code></pre>
</li>
</ul>

Note that ordered list start numbers must be nine digits or less:

123456789. ok

<ol start="123456789">
<li>ok</li>
</ol>

1234567890. not ok

<p>1234567890. not ok</p>

A start number may begin with 0s:

0. ok

<ol start="0">
<li>ok</li>
</ol>

003. ok

<ol start="3">
<li>ok</li>
</ol>

A start number may not be negative:

-1. not ok

<p>-1. not ok</p>

Item starting with indented code. If a sequence of lines Ls constitute a sequence of blocks Bs starting with an indented code block, and M is a list marker of width W followed by one space of indentation, then the result of prepending M and the following space to the first line of Ls, and indenting subsequent lines of Ls by W + 1 spaces, is a list item with Bs as its contents. If a line is empty, then it need not be indented. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.

An indented code block will have to be preceded by four spaces of indentation beyond the edge of the region where text will be included in the list item. In the following case that is 6 spaces:

- foo

      bar

<ul>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ul>

And in this case it is 11 spaces:

  10.  foo

           bar

<ol start="10">
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ol>

If the first block in the list item is an indented code block, then by rule #2, the contents must be preceded by one space of indentation after the list marker:

    indented code

paragraph

    more code

<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>

1.     indented code

   paragraph

       more code

<ol>
<li>
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>

Note that an additional space of indentation is interpreted as space inside the code block:

1.      indented code

   paragraph

       more code

<ol>
<li>
<pre><code> indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>

Note that rules #1 and #2 only apply to two cases: (a) cases in which the lines to be included in a list item begin with a character other than a space or tab, and (b) cases in which they begin with an indented code block. In a case like the following, where the first block begins with three spaces of indentation, the rules do not allow us to form a list item by indenting the whole thing and prepending a list marker:

   foo

bar

<p>foo</p>
<p>bar</p>

-    foo

  bar

<ul>
<li>foo</li>
</ul>
<p>bar</p>

This is not a significant restriction, because when a block is preceded by up to three spaces of indentation, the indentation can always be removed without a change in interpretation, allowing rule #1 to be applied. So, in the above case:

-  foo

   bar

<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>

Item starting with a blank line. If a sequence of lines Ls starting with a single [blank line] constitute a (possibly empty) sequence of blocks Bs, and M is a list marker of width W, then the result of prepending M to the first line of Ls, and preceding subsequent lines of Ls by W + 1 spaces of indentation, is a list item with Bs as its contents. If a line is empty, then it need not be indented. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.

Here are some list items that start with a blank line but are not empty:

-
  foo
-
  ```
  bar
  ```
-
      baz

<ul>
<li>foo</li>
<li>
<pre><code>bar
</code></pre>
</li>
<li>
<pre><code>baz
</code></pre>
</li>
</ul>

When the list item starts with a blank line, the number of spaces following the list marker doesn't change the required indentation:

-   
  foo

<ul>
<li>foo</li>
</ul>

A list item can begin with at most one blank line. In the following example, foo is not part of the list item:

-

  foo

<ul>
<li></li>
</ul>
<p>foo</p>

Here is an empty bullet list item:

- foo
-
- bar

<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

It does not matter whether there are spaces or tabs following the [list marker]:

- foo
-   
- bar

<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

Here is an empty ordered list item:

1. foo
2.
3. bar

<ol>
<li>foo</li>
<li></li>
<li>bar</li>
</ol>

A list may start or end with an empty list item:

<ul>
<li></li>
</ul>

However, an empty list item cannot interrupt a paragraph:

foo
*

foo
1.

<p>foo
*</p>
<p>foo
1.</p>

Indentation. If a sequence of lines Ls constitutes a list item according to rule #1, #2, or #3, then the result of preceding each line of Ls by up to three spaces of indentation (the same for each line) also constitutes a list item with the same contents and attributes. If a line is empty, then it need not be indented.

Indented one space:

 1.  A paragraph
     with two lines.

         indented code

     > A block quote.

<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indented two spaces:

  1.  A paragraph
      with two lines.

          indented code

      > A block quote.

<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indented three spaces:

   1.  A paragraph
       with two lines.

           indented code

       > A block quote.

<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Four spaces indent gives a code block:

    1.  A paragraph
        with two lines.

            indented code

        > A block quote.

<pre><code>1.  A paragraph
    with two lines.

        indented code

    &gt; A block quote.
</code></pre>

Laziness. If a string of lines Ls constitute a list item with contents Bs, then the result of deleting some or all of the indentation from one or more lines in which the next character other than a space or tab after the indentation is [paragraph continuation text] is a list item with the same contents and attributes. The unindented lines are called lazy continuation lines.

Here is an example with [lazy continuation lines]:

  1.  A paragraph
with two lines.

          indented code

      > A block quote.

<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indentation can be partially deleted:

  1.  A paragraph
    with two lines.

<ol>
<li>A paragraph
with two lines.</li>
</ol>

These examples show how laziness can work in nested structures:

> 1. > Blockquote
continued here.

<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>

> 1. > Blockquote
> continued here.

<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>

That's all. Nothing that is not counted as a list item by rules #1--5 counts as a list item.

The rules for sublists follow from the general rules [above][List items]. A sublist must be indented the same number of spaces of indentation a paragraph would need to be in order to be included in the list item.

So, in this case we need two spaces indent:

- foo
  - bar
    - baz
      - boo

<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz
<ul>
<li>boo</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>

One is not enough:

- foo
 - bar
  - baz
   - boo

<ul>
<li>foo</li>
<li>bar</li>
<li>baz</li>
<li>boo</li>
</ul>

Here we need four, because the list marker is wider:

10) foo
    - bar

<ol start="10">
<li>foo
<ul>
<li>bar</li>
</ul>
</li>
</ol>

Three is not enough:

10) foo
   - bar

<ol start="10">
<li>foo</li>
</ol>
<ul>
<li>bar</li>
</ul>

A list may be the first block in a list item:

- - foo

<ul>
<li>
<ul>
<li>foo</li>
</ul>
</li>
</ul>

1. - 2. foo

<ol>
<li>
<ul>
<li>
<ol start="2">
<li>foo</li>
</ol>
</li>
</ul>
</li>
</ol>

A list item can contain a heading:

- # Foo
- Bar
  ---
  baz

<ul>
<li>
<h1>Foo</h1>
</li>
<li>
<h2>Bar</h2>
baz</li>
</ul>

Motivation

John Gruber's Markdown spec says the following about list items:

"List markers typically start at the left margin, but may be indented by up to three spaces. List markers must be followed by one or more spaces or a tab."
"To make lists look nice, you can wrap items with hanging indents.... But if you don't want to, you don't have to."
"List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab."
"It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy."
"To put a blockquote within a list item, the blockquote's > delimiters need to be indented."
"To put a code block within a list item, the code block needs to be indented twice — 8 spaces or two tabs."

These rules specify that a paragraph under a list item must be indented four spaces (presumably, from the left margin, rather than the start of the list marker, but this is not said), and that code under a list item must be indented eight spaces instead of the usual four. They also say that a block quote must be indented, but not by how much; however, the example given has four spaces indentation. Although nothing is said about other kinds of block-level content, it is certainly reasonable to infer that all block elements under a list item, including other lists, must be indented four spaces. This principle has been called the four-space rule.

The four-space rule is clear and principled, and if the reference implementation Markdown.pl had followed it, it probably would have become the standard. However, Markdown.pl allowed paragraphs and sublists to start with only two spaces indentation, at least on the outer level. Worse, its behavior was inconsistent: a sublist of an outer-level list needed two spaces indentation, but a sublist of this sublist needed three spaces. It is not surprising, then, that different implementations of Markdown have developed very different rules for determining what comes under a list item. (Pandoc and python-Markdown, for example, stuck with Gruber's syntax description and the four-space rule, while discount, redcarpet, marked, PHP Markdown, and others followed Markdown.pl's behavior more closely.)

Unfortunately, given the divergences between implementations, there is no way to give a spec for list items that will be guaranteed not to break any existing documents. However, the spec given here should correctly handle lists formatted with either the four-space rule or the more forgiving Markdown.pl behavior, provided they are laid out in a way that is natural for a human to read.

The strategy here is to let the width and indentation of the list marker determine the indentation necessary for blocks to fall under the list item, rather than having a fixed and arbitrary number. The writer can think of the body of the list item as a unit which gets indented to the right enough to fit the list marker (and any indentation on the list marker). (The laziness rule, #5, then allows continuation lines to be unindented if needed.)

This rule is superior, we claim, to any rule requiring a fixed level of indentation from the margin. The four-space rule is clear but unnatural. It is quite unintuitive that

- foo

  bar

  - baz

should be parsed as two lists with an intervening paragraph,

<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>

as the four-space rule demands, rather than a single list,

<ul>
<li>
<p>foo</p>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
</li>
</ul>

The choice of four spaces is arbitrary. It can be learned, but it is not likely to be guessed, and it trips up beginners regularly.

Would it help to adopt a two-space rule? The problem is that such a rule, together with the rule allowing up to three spaces of indentation for the initial list marker, allows text that is indented less than the original list marker to be included in the list item. For example, Markdown.pl parses

   - one

  two

as a single list item, with two a continuation paragraph:

<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>

and similarly

>   - one
>
>  two

<blockquote>
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
</blockquote>

This is extremely unintuitive.

Rather than requiring a fixed indent from the margin, we could require a fixed indent (say, two spaces, or even one space) from the list marker (which may itself be indented). This proposal would remove the last anomaly discussed. Unlike the spec presented above, it would count the following as a list item with a subparagraph, even though the paragraph bar is not indented as far as the first paragraph foo:

 10. foo

   bar

Arguably this text does read like a list item with bar as a subparagraph, which may count in favor of the proposal. However, on this proposal indented code would have to be indented six spaces after the list marker. And this would break a lot of existing Markdown, which has the pattern:

1.  foo

        indented code

where the code is indented eight spaces. The spec above, by contrast, will parse this text as expected, since the code block's indentation is measured from the beginning of foo.

The one case that needs special treatment is a list item that starts with indented code. How much indentation is required in that case, since we don't have a "first paragraph" to measure from? Rule #2 simply stipulates that in such cases, we require one space indentation from the list marker (and then the normal four spaces for the indented code). This will match the four-space rule in cases where the list marker plus its initial indentation takes four spaces (a common case), but diverge in other cases.

Lists

A list is a sequence of one or more list items [of the same type]. The list items may be separated by any number of blank lines.

Two list items are of the same type if they begin with a [list marker] of the same type. Two list markers are of the same type if (a) they are bullet list markers using the same character (-, +, or *) or (b) they are ordered list numbers with the same delimiter (either . or )).

A list is an ordered list if its constituent list items begin with [ordered list markers], and a bullet list if its constituent list items begin with [bullet list markers].

The start number of an [ordered list] is determined by the list number of its initial list item. The numbers of subsequent list items are disregarded.

A list is loose if any of its constituent list items are separated by blank lines, or if any of its constituent list items directly contain two block-level elements with a blank line between them. Otherwise a list is tight. (The difference in HTML output is that paragraphs in a loose list are wrapped in <p> tags, while paragraphs in a tight list are not.)

Changing the bullet or ordered list delimiter starts a new list:

- foo
- bar
+ baz

<ul>
<li>foo</li>
<li>bar</li>
</ul>
<ul>
<li>baz</li>
</ul>

1. foo
2. bar
3) baz

<ol>
<li>foo</li>
<li>bar</li>
</ol>
<ol start="3">
<li>baz</li>
</ol>

In CommonMark, a list can interrupt a paragraph. That is, no blank line is needed to separate a paragraph from a following list:

Foo
- bar
- baz

<p>Foo</p>
<ul>
<li>bar</li>
<li>baz</li>
</ul>

Markdown.pl does not allow this, through fear of triggering a list via a numeral in a hard-wrapped line:

The number of windows in my house is
14.  The number of doors is 6.

Oddly, though, Markdown.pl does allow a blockquote to interrupt a paragraph, even though the same considerations might apply.

In CommonMark, we do allow lists to interrupt paragraphs, for two reasons. First, it is natural and not uncommon for people to start lists without blank lines:

I need to buy
- new shoes
- a coat
- a plane ticket

Second, we are attracted to a

principle of uniformity: if a chunk of text has a certain meaning, it will continue to have the same meaning when put into a container block (such as a list item or blockquote).

(Indeed, the spec for [list items] and [block quotes] presupposes this principle.) This principle implies that if

  * I need to buy
    - new shoes
    - a coat
    - a plane ticket

is a list item containing a paragraph followed by a nested sublist, as all Markdown implementations agree it is (though the paragraph may be rendered without <p> tags, since the list is "tight"), then

I need to buy
- new shoes
- a coat
- a plane ticket

by itself should be a paragraph followed by a nested sublist.

Since it is well established Markdown practice to allow lists to interrupt paragraphs inside list items, the [principle of uniformity] requires us to allow this outside list items as well. (reStructuredText takes a different approach, requiring blank lines before lists even inside other list items.)

In order to solve the problem of unwanted lists in paragraphs with hard-wrapped numerals, we allow only lists starting with 1 to interrupt paragraphs. Thus,

The number of windows in my house is
14.  The number of doors is 6.

<p>The number of windows in my house is
14.  The number of doors is 6.</p>

We may still get an unintended result in cases like

The number of windows in my house is
1.  The number of doors is 6.

<p>The number of windows in my house is</p>
<ol>
<li>The number of doors is 6.</li>
</ol>

but this rule should prevent most spurious list captures.

There can be any number of blank lines between items:

- foo

- bar


- baz

<ul>
<li>
<p>foo</p>
</li>
<li>
<p>bar</p>
</li>
<li>
<p>baz</p>
</li>
</ul>

- foo
  - bar
    - baz


      bim

<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>
<p>baz</p>
<p>bim</p>
</li>
</ul>
</li>
</ul>
</li>
</ul>

To separate consecutive lists of the same type, or to separate a list from an indented code block that would otherwise be parsed as a subparagraph of the final list item, you can insert a blank HTML comment:

- foo
- bar

<!-- -->

- baz
- bim

<ul>
<li>foo</li>
<li>bar</li>
</ul>
<!-- -->
<ul>
<li>baz</li>
<li>bim</li>
</ul>

-   foo

    notcode

-   foo

<!-- -->

    code

<ul>
<li>
<p>foo</p>
<p>notcode</p>
</li>
<li>
<p>foo</p>
</li>
</ul>
<!-- -->
<pre><code>code
</code></pre>

List items need not be indented to the same level. The following list items will be treated as items at the same list level, since none is indented enough to belong to the previous list item:

- a
 - b
  - c
   - d
  - e
 - f
- g

<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d</li>
<li>e</li>
<li>f</li>
<li>g</li>
</ul>

1. a

  2. b

   3. c

<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ol>

Note, however, that list items may not be preceded by more than three spaces of indentation. Here - e is treated as a paragraph continuation line, because it is indented more than three spaces:

- a
 - b
  - c
   - d
    - e

<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d
- e</li>
</ul>

And here, 3. c is treated as in indented code block, because it is indented four spaces and preceded by a blank line.

1. a

  2. b

    3. c

<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
</ol>
<pre><code>3. c
</code></pre>

This is a loose list, because there is a blank line between two of the list items:

- a
- b

- c

<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ul>

So is this, with a empty second item:

* a
*

* c

<ul>
<li>
<p>a</p>
</li>
<li></li>
<li>
<p>c</p>
</li>
</ul>

These are loose lists, even though there are no blank lines between the items, because one of the items directly contains two block-level elements with a blank line between them:

- a
- b

  c
- d

<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
<p>c</p>
</li>
<li>
<p>d</p>
</li>
</ul>

- a
- b

  [ref]: /url
- d

<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>d</p>
</li>
</ul>

This is a tight list, because the blank lines are in a code block:

- a
- ```
  b


  ```
- c

<ul>
<li>a</li>
<li>
<pre><code>b


</code></pre>
</li>
<li>c</li>
</ul>

This is a tight list, because the blank line is between two paragraphs of a sublist. So the sublist is loose while the outer list is tight:

- a
  - b

    c
- d

<ul>
<li>a
<ul>
<li>
<p>b</p>
<p>c</p>
</li>
</ul>
</li>
<li>d</li>
</ul>

This is a tight list, because the blank line is inside the block quote:

* a
  > b
  >
* c

<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
</li>
<li>c</li>
</ul>

This list is tight, because the consecutive block elements are not separated by blank lines:

- a
  > b
  ```
  c
  ```
- d

<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
<pre><code>c
</code></pre>
</li>
<li>d</li>
</ul>

A single-paragraph list is tight:

- a

<ul>
<li>a</li>
</ul>

- a
  - b

<ul>
<li>a
<ul>
<li>b</li>
</ul>
</li>
</ul>

This list is loose, because of the blank line between the two block elements in the list item:

1. ```
   foo
   ```

   bar

<ol>
<li>
<pre><code>foo
</code></pre>
<p>bar</p>
</li>
</ol>

Here the outer list is loose, the inner list tight:

* foo
  * bar

  baz

<ul>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
<p>baz</p>
</li>
</ul>

- a
  - b
  - c

- d
  - e
  - f

<ul>
<li>
<p>a</p>
<ul>
<li>b</li>
<li>c</li>
</ul>
</li>
<li>
<p>d</p>
<ul>
<li>e</li>
<li>f</li>
</ul>
</li>
</ul>

Inlines

Inlines are parsed sequentially from the beginning of the character stream to the end (left to right, in left-to-right languages). Thus, for example, in

`hi`lo`

<p><code>hi</code>lo`</p>

hi is parsed as code, leaving the backtick at the end as a literal backtick.

Code spans

A backtick string is a string of one or more backtick characters (`) that is neither preceded nor followed by a backtick.

A code span begins with a backtick string and ends with a backtick string of equal length. The contents of the code span are the characters between these two backtick strings, normalized in the following ways:

First, [line endings] are converted to [spaces].
If the resulting string both begins and ends with a [space] character, but does not consist entirely of [space] characters, a single [space] character is removed from the front and back. This allows you to include code that begins or ends with backtick characters, which must be separated by whitespace from the opening or closing backtick strings.

This is a simple code span:

`foo`

<p><code>foo</code></p>

Here two backticks are used, because the code contains a backtick. This example also illustrates stripping of a single leading and trailing space:

`` foo ` bar ``

<p><code>foo ` bar</code></p>

This example shows the motivation for stripping leading and trailing spaces:

` `` `

<p><code>``</code></p>

Note that only one space is stripped:

`  ``  `

<p><code> `` </code></p>

The stripping only happens if the space is on both sides of the string:

` a`

<p><code> a</code></p>

Only [spaces], and not [unicode whitespace] in general, are stripped in this way:

` b `

<p><code> b </code></p>

No stripping occurs if the code span contains only spaces:

` `
`  `

<p><code> </code>
<code>  </code></p>

[Line endings] are treated like spaces:

``
foo
bar  
baz
``

<p><code>foo bar   baz</code></p>

``
foo 
``

<p><code>foo </code></p>

Interior spaces are not collapsed:

`foo   bar 
baz`

<p><code>foo   bar  baz</code></p>

Note that browsers will typically collapse consecutive spaces when rendering <code> elements, so it is recommended that the following CSS be used:

code{white-space: pre-wrap;}

Note that backslash escapes do not work in code spans. All backslashes are treated literally:

`foo\`bar`

<p><code>foo\</code>bar`</p>

Backslash escapes are never needed, because one can always choose a string of n backtick characters as delimiters, where the code does not contain any strings of exactly n backtick characters.

``foo`bar``

<p><code>foo`bar</code></p>

` foo `` bar `

<p><code>foo `` bar</code></p>

Code span backticks have higher precedence than any other inline constructs except HTML tags and autolinks. Thus, for example, this is not parsed as emphasized text, since the second * is part of a code span:

*foo`*`

<p>*foo<code>*</code></p>

And this is not parsed as a link:

[not a `link](/foo`)

<p>[not a <code>link](/foo</code>)</p>

Code spans, HTML tags, and autolinks have the same precedence. Thus, this is code:

`<a href="`">`

<p><code>&lt;a href="</code>"&gt;`</p>

But this is an HTML tag:

<a href="`">`

<p><a href="`">`</p>

And this is code:

`<https://foo.bar.`baz>`

<p><code>&lt;https://foo.bar.</code>baz&gt;`</p>

But this is an autolink:

<https://foo.bar.`baz>`

<p><a href="https://foo.bar.%60baz">https://foo.bar.`baz</a>`</p>

When a backtick string is not closed by a matching backtick string, we just have literal backticks:

```foo``

<p>```foo``</p>

`foo

<p>`foo</p>

The following case also illustrates the need for opening and closing backtick strings to be equal in length:

`foo``bar``

<p>`foo<code>bar</code></p>

Emphasis and strong emphasis

John Gruber's original Markdown syntax description says:

Markdown treats asterisks (*) and underscores (_) as indicators of emphasis. Text wrapped with one * or _ will be wrapped with an HTML <em> tag; double *'s or _'s will be wrapped with an HTML <strong> tag.

This is enough for most users, but these rules leave much undecided, especially when it comes to nested emphasis. The original Markdown.pl test suite makes it clear that triple *** and ___ delimiters can be used for strong emphasis, and most implementations have also allowed the following patterns:

***strong emph***
***strong** in emph*
***emph* in strong**
**in strong *emph***
*in emph **strong***

The following patterns are less widely supported, but the intent is clear and they are useful (especially in contexts like bibliography entries):

*emph *with emph* in it*
**strong **with strong** in it**

Many implementations have also restricted intraword emphasis to the * forms, to avoid unwanted emphasis in words containing internal underscores. (It is best practice to put these in code spans, but users often do not.)

internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz

The rules given below capture all of these patterns, while allowing for efficient parsing strategies that do not backtrack.

First, some definitions. A delimiter run is either a sequence of one or more * characters that is not preceded or followed by a non-backslash-escaped * character, or a sequence of one or more _ characters that is not preceded or followed by a non-backslash-escaped _ character.

A left-flanking delimiter run is a [delimiter run] that is (1) not followed by [Unicode whitespace], and either (2a) not followed by a [Unicode punctuation character], or (2b) followed by a [Unicode punctuation character] and preceded by [Unicode whitespace] or a [Unicode punctuation character]. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.

A right-flanking delimiter run is a [delimiter run] that is (1) not preceded by [Unicode whitespace], and either (2a) not preceded by a [Unicode punctuation character], or (2b) preceded by a [Unicode punctuation character] and followed by [Unicode whitespace] or a [Unicode punctuation character]. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.

Here are some examples of delimiter runs.

left-flanking but not right-flanking:
```
***abc
  _abc
**"abc"
 _"abc"
```
right-flanking but not left-flanking:
```
 abc***
 abc_
"abc"**
"abc"_
```
Both left and right-flanking:
```
 abc***def
"abc"_"def"
```
Neither left nor right-flanking:
```
abc *** def
a _ b
```

(The idea of distinguishing left-flanking and right-flanking delimiter runs based on the character before and the character after comes from Roopesh Chander's vfmd. vfmd uses the terminology "emphasis indicator string" instead of "delimiter run," and its rules for distinguishing left- and right-flanking runs are a bit more complex than the ones given here.)

The following rules define emphasis and strong emphasis:

A single * character can open emphasis iff (if and only if) it is part of a [left-flanking delimiter run].
A single _ character [can open emphasis] iff it is part of a [left-flanking delimiter run] and either (a) not part of a [right-flanking delimiter run] or (b) part of a [right-flanking delimiter run] preceded by a [Unicode punctuation character].
A single * character can close emphasis iff it is part of a [right-flanking delimiter run].
A single _ character [can close emphasis] iff it is part of a [right-flanking delimiter run] and either (a) not part of a [left-flanking delimiter run] or (b) part of a [left-flanking delimiter run] followed by a [Unicode punctuation character].
A double ** can open strong emphasis iff it is part of a [left-flanking delimiter run].
A double __ [can open strong emphasis] iff it is part of a [left-flanking delimiter run] and either (a) not part of a [right-flanking delimiter run] or (b) part of a [right-flanking delimiter run] preceded by a [Unicode punctuation character].
A double ** can close strong emphasis iff it is part of a [right-flanking delimiter run].
A double __ [can close strong emphasis] iff it is part of a [right-flanking delimiter run] and either (a) not part of a [left-flanking delimiter run] or (b) part of a [left-flanking delimiter run] followed by a [Unicode punctuation character].
Emphasis begins with a delimiter that [can open emphasis] and ends with a delimiter that [can close emphasis], and that uses the same character (_ or *) as the opening delimiter. The opening and closing delimiters must belong to separate [delimiter runs]. If one of the delimiters can both open and close emphasis, then the sum of the lengths of the delimiter runs containing the opening and closing delimiters must not be a multiple of 3 unless both lengths are multiples of 3.
Strong emphasis begins with a delimiter that [can open strong emphasis] and ends with a delimiter that [can close strong emphasis], and that uses the same character (_ or *) as the opening delimiter. The opening and closing delimiters must belong to separate [delimiter runs]. If one of the delimiters can both open and close strong emphasis, then the sum of the lengths of the delimiter runs containing the opening and closing delimiters must not be a multiple of 3 unless both lengths are multiples of 3.
A literal * character cannot occur at the beginning or end of *-delimited emphasis or **-delimited strong emphasis, unless it is backslash-escaped.
A literal _ character cannot occur at the beginning or end of _-delimited emphasis or __-delimited strong emphasis, unless it is backslash-escaped.

Where rules 1--12 above are compatible with multiple parsings, the following principles resolve ambiguity:

The number of nestings should be minimized. Thus, for example, an interpretation <strong>...</strong> is always preferred to <em><em>...</em></em>.
An interpretation <em><strong>...</strong></em> is always preferred to <strong><em>...</em></strong>.
When two potential emphasis or strong emphasis spans overlap, so that the second begins before the first ends and ends after the first ends, the first takes precedence. Thus, for example, *foo _bar* baz_ is parsed as <em>foo _bar</em> baz_ rather than *foo <em>bar* baz</em>.
When there are two potential emphasis or strong emphasis spans with the same closing delimiter, the shorter one (the one that opens later) takes precedence. Thus, for example, **foo **bar baz** is parsed as **foo <strong>bar baz</strong> rather than <strong>foo **bar baz</strong>.
Inline code spans, links, images, and HTML tags group more tightly than emphasis. So, when there is a choice between an interpretation that contains one of these elements and one that does not, the former always wins. Thus, for example, *[foo*](bar) is parsed as *<a href="bar">foo*</a> rather than as <em>[foo</em>](bar).

These rules can be illustrated through a series of examples.

Rule 1:

*foo bar*

<p><em>foo bar</em></p>

This is not emphasis, because the opening * is followed by whitespace, and hence not part of a [left-flanking delimiter run]:

a * foo bar*

<p>a * foo bar*</p>

This is not emphasis, because the opening * is preceded by an alphanumeric and followed by punctuation, and hence not part of a [left-flanking delimiter run]:

a*"foo"*

<p>a*"foo"*</p>

Unicode nonbreaking spaces count as whitespace, too:

* a *

<p>* a *</p>

Unicode symbols count as punctuation, too:

*$*alpha.

*£*bravo.

*€*charlie.

<p>*$*alpha.</p>
<p>*£*bravo.</p>
<p>*€*charlie.</p>

Intraword emphasis with * is permitted:

foo*bar*

<p>foo<em>bar</em></p>

5*6*78

<p>5<em>6</em>78</p>

Rule 2:

_foo bar_

<p><em>foo bar</em></p>

This is not emphasis, because the opening _ is followed by whitespace:

_ foo bar_

<p>_ foo bar_</p>

This is not emphasis, because the opening _ is preceded by an alphanumeric and followed by punctuation:

a_"foo"_

<p>a_"foo"_</p>

Emphasis with _ is not allowed inside words:

foo_bar_

<p>foo_bar_</p>

5_6_78

<p>5_6_78</p>

пристаням_стремятся_

<p>пристаням_стремятся_</p>

Here _ does not generate emphasis, because the first delimiter run is right-flanking and the second left-flanking:

aa_"bb"_cc

<p>aa_"bb"_cc</p>

This is emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:

foo-_(bar)_

<p>foo-<em>(bar)</em></p>

Rule 3:

This is not emphasis, because the closing delimiter does not match the opening delimiter:

_foo*

<p>_foo*</p>

This is not emphasis, because the closing * is preceded by whitespace:

*foo bar *

<p>*foo bar *</p>

A line ending also counts as whitespace:

*foo bar
*

<p>*foo bar
*</p>

This is not emphasis, because the second * is preceded by punctuation and followed by an alphanumeric (hence it is not part of a [right-flanking delimiter run]:

*(*foo)

<p>*(*foo)</p>

The point of this restriction is more easily appreciated with this example:

*(*foo*)*

<p><em>(<em>foo</em>)</em></p>

Intraword emphasis with * is allowed:

*foo*bar

<p><em>foo</em>bar</p>

Rule 4:

This is not emphasis, because the closing _ is preceded by whitespace:

_foo bar _

<p>_foo bar _</p>

This is not emphasis, because the second _ is preceded by punctuation and followed by an alphanumeric:

_(_foo)

<p>_(_foo)</p>

This is emphasis within emphasis:

_(_foo_)_

<p><em>(<em>foo</em>)</em></p>

Intraword emphasis is disallowed for _:

_foo_bar

<p>_foo_bar</p>

_пристаням_стремятся

<p>_пристаням_стремятся</p>

_foo_bar_baz_

<p><em>foo_bar_baz</em></p>

This is emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:

_(bar)_.

<p><em>(bar)</em>.</p>

Rule 5:

**foo bar**

<p><strong>foo bar</strong></p>

This is not strong emphasis, because the opening delimiter is followed by whitespace:

** foo bar**

<p>** foo bar**</p>

This is not strong emphasis, because the opening ** is preceded by an alphanumeric and followed by punctuation, and hence not part of a [left-flanking delimiter run]:

a**"foo"**

<p>a**"foo"**</p>

Intraword strong emphasis with ** is permitted:

foo**bar**

<p>foo<strong>bar</strong></p>

Rule 6:

__foo bar__

<p><strong>foo bar</strong></p>

This is not strong emphasis, because the opening delimiter is followed by whitespace:

__ foo bar__

<p>__ foo bar__</p>

A line ending counts as whitespace:

__
foo bar__

<p>__
foo bar__</p>

This is not strong emphasis, because the opening __ is preceded by an alphanumeric and followed by punctuation:

a__"foo"__

<p>a__"foo"__</p>

Intraword strong emphasis is forbidden with __:

foo__bar__

<p>foo__bar__</p>

5__6__78

<p>5__6__78</p>

пристаням__стремятся__

<p>пристаням__стремятся__</p>

__foo, __bar__, baz__

<p><strong>foo, <strong>bar</strong>, baz</strong></p>

This is strong emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:

foo-__(bar)__

<p>foo-<strong>(bar)</strong></p>

Rule 7:

This is not strong emphasis, because the closing delimiter is preceded by whitespace:

**foo bar **

<p>**foo bar **</p>

(Nor can it be interpreted as an emphasized *foo bar *, because of Rule 11.)

This is not strong emphasis, because the second ** is preceded by punctuation and followed by an alphanumeric:

**(**foo)

<p>**(**foo)</p>

The point of this restriction is more easily appreciated with these examples:

*(**foo**)*

<p><em>(<strong>foo</strong>)</em></p>

**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**

<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
<em>Asclepias physocarpa</em>)</strong></p>

**foo "*bar*" foo**

<p><strong>foo "<em>bar</em>" foo</strong></p>

Intraword emphasis:

**foo**bar

<p><strong>foo</strong>bar</p>

Rule 8:

This is not strong emphasis, because the closing delimiter is preceded by whitespace:

__foo bar __

<p>__foo bar __</p>

This is not strong emphasis, because the second __ is preceded by punctuation and followed by an alphanumeric:

__(__foo)

<p>__(__foo)</p>

The point of this restriction is more easily appreciated with this example:

_(__foo__)_

<p><em>(<strong>foo</strong>)</em></p>

Intraword strong emphasis is forbidden with __:

__foo__bar

<p>__foo__bar</p>

__пристаням__стремятся

<p>__пристаням__стремятся</p>

__foo__bar__baz__

<p><strong>foo__bar__baz</strong></p>

This is strong emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:

__(bar)__.

<p><strong>(bar)</strong>.</p>

Rule 9:

Any nonempty sequence of inline elements can be the contents of an emphasized span.

*foo [bar](/url)*

<p><em>foo <a href="/url">bar</a></em></p>

*foo
bar*

<p><em>foo
bar</em></p>

In particular, emphasis and strong emphasis can be nested inside emphasis:

_foo __bar__ baz_

<p><em>foo <strong>bar</strong> baz</em></p>

_foo _bar_ baz_

<p><em>foo <em>bar</em> baz</em></p>

__foo_ bar_

<p><em><em>foo</em> bar</em></p>

*foo *bar**

<p><em>foo <em>bar</em></em></p>

*foo **bar** baz*

<p><em>foo <strong>bar</strong> baz</em></p>

*foo**bar**baz*

<p><em>foo<strong>bar</strong>baz</em></p>

Note that in the preceding case, the interpretation

<p><em>foo</em><em>bar<em></em>baz</em></p>

is precluded by the condition that a delimiter that can both open and close (like the * after foo) cannot form emphasis if the sum of the lengths of the delimiter runs containing the opening and closing delimiters is a multiple of 3 unless both lengths are multiples of 3.

For the same reason, we don't get two consecutive emphasis sections in this example:

*foo**bar*

<p><em>foo**bar</em></p>

The same condition ensures that the following cases are all strong emphasis nested inside emphasis, even when the interior whitespace is omitted:

***foo** bar*

<p><em><strong>foo</strong> bar</em></p>

*foo **bar***

<p><em>foo <strong>bar</strong></em></p>

*foo**bar***

<p><em>foo<strong>bar</strong></em></p>

When the lengths of the interior closing and opening delimiter runs are both multiples of 3, though, they can match to create emphasis:

foo***bar***baz

<p>foo<em><strong>bar</strong></em>baz</p>

foo******bar*********baz

<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>

Indefinite levels of nesting are possible:

*foo **bar *baz* bim** bop*

<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>

*foo [*bar*](/url)*

<p><em>foo <a href="/url"><em>bar</em></a></em></p>

There can be no empty emphasis or strong emphasis:

** is not an empty emphasis

<p>** is not an empty emphasis</p>

**** is not an empty strong emphasis

<p>**** is not an empty strong emphasis</p>

Rule 10:

Any nonempty sequence of inline elements can be the contents of an strongly emphasized span.

**foo [bar](/url)**

<p><strong>foo <a href="/url">bar</a></strong></p>

**foo
bar**

<p><strong>foo
bar</strong></p>

In particular, emphasis and strong emphasis can be nested inside strong emphasis:

__foo _bar_ baz__

<p><strong>foo <em>bar</em> baz</strong></p>

__foo __bar__ baz__

<p><strong>foo <strong>bar</strong> baz</strong></p>

____foo__ bar__

<p><strong><strong>foo</strong> bar</strong></p>

**foo **bar****

<p><strong>foo <strong>bar</strong></strong></p>

**foo *bar* baz**

<p><strong>foo <em>bar</em> baz</strong></p>

**foo*bar*baz**

<p><strong>foo<em>bar</em>baz</strong></p>

***foo* bar**

<p><strong><em>foo</em> bar</strong></p>

**foo *bar***

<p><strong>foo <em>bar</em></strong></p>

Indefinite levels of nesting are possible:

**foo *bar **baz**
bim* bop**

<p><strong>foo <em>bar <strong>baz</strong>
bim</em> bop</strong></p>

**foo [*bar*](/url)**

<p><strong>foo <a href="/url"><em>bar</em></a></strong></p>

There can be no empty emphasis or strong emphasis:

__ is not an empty emphasis

<p>__ is not an empty emphasis</p>

____ is not an empty strong emphasis

<p>____ is not an empty strong emphasis</p>

Rule 11:

foo ***

<p>foo ***</p>

foo *\**

<p>foo <em>*</em></p>

foo *_*

<p>foo <em>_</em></p>

foo *****

<p>foo *****</p>

foo **\***

<p>foo <strong>*</strong></p>

foo **_**

<p>foo <strong>_</strong></p>

Note that when delimiters do not match evenly, Rule 11 determines that the excess literal * characters will appear outside of the emphasis, rather than inside it:

**foo*

<p>*<em>foo</em></p>

*foo**

<p><em>foo</em>*</p>

***foo**

<p>*<strong>foo</strong></p>

****foo*

<p>***<em>foo</em></p>

**foo***

<p><strong>foo</strong>*</p>

*foo****

<p><em>foo</em>***</p>

Rule 12:

foo ___

<p>foo ___</p>

foo _\__

<p>foo <em>_</em></p>

foo _*_

<p>foo <em>*</em></p>

foo _____

<p>foo _____</p>

foo __\___

<p>foo <strong>_</strong></p>

foo __*__

<p>foo <strong>*</strong></p>

__foo_

<p>_<em>foo</em></p>

Note that when delimiters do not match evenly, Rule 12 determines that the excess literal _ characters will appear outside of the emphasis, rather than inside it:

_foo__

<p><em>foo</em>_</p>

___foo__

<p>_<strong>foo</strong></p>

____foo_

<p>___<em>foo</em></p>

__foo___

<p><strong>foo</strong>_</p>

_foo____

<p><em>foo</em>___</p>

Rule 13 implies that if you want emphasis nested directly inside emphasis, you must use different delimiters:

**foo**

<p><strong>foo</strong></p>

*_foo_*

<p><em><em>foo</em></em></p>

__foo__

<p><strong>foo</strong></p>

_*foo*_

<p><em><em>foo</em></em></p>

However, strong emphasis within strong emphasis is possible without switching delimiters:

****foo****

<p><strong><strong>foo</strong></strong></p>

____foo____

<p><strong><strong>foo</strong></strong></p>

Rule 13 can be applied to arbitrarily long sequences of delimiters:

******foo******

<p><strong><strong><strong>foo</strong></strong></strong></p>

Rule 14:

***foo***

<p><em><strong>foo</strong></em></p>

_____foo_____

<p><em><strong><strong>foo</strong></strong></em></p>

Rule 15:

*foo _bar* baz_

<p><em>foo _bar</em> baz_</p>

*foo __bar *baz bim__ bam*

<p><em>foo <strong>bar *baz bim</strong> bam</em></p>

Rule 16:

**foo **bar baz**

<p>**foo <strong>bar baz</strong></p>

*foo *bar baz*

<p>*foo <em>bar baz</em></p>

Rule 17:

*[bar*](/url)

<p>*<a href="/url">bar*</a></p>

_foo [bar_](/url)

<p>_foo <a href="/url">bar_</a></p>

*<img src="foo" title="*"/>

<p>*<img src="foo" title="*"/></p>

**<a href="**">

<p>**<a href="**"></p>

__<a href="__">

<p>__<a href="__"></p>

*a `*`*

<p><em>a <code>*</code></em></p>

_a `_`_

<p><em>a <code>_</code></em></p>

**a<https://foo.bar/?q=**>

<p>**a<a href="https://foo.bar/?q=**">https://foo.bar/?q=**</a></p>

__a<https://foo.bar/?q=__>

<p>__a<a href="https://foo.bar/?q=__">https://foo.bar/?q=__</a></p>

Links

A link contains [link text] (the visible text), a [link destination] (the URI that is the link destination), and optionally a [link title]. There are two basic kinds of links in Markdown. In [inline links] the destination and title are given immediately after the link text. In [reference links] the destination and title are defined elsewhere in the document.

A link text consists of a sequence of zero or more inline elements enclosed by square brackets ([ and ]). The following rules apply:

Links may not contain other links, at any level of nesting. If multiple otherwise valid link definitions appear nested inside each other, the inner-most definition is used.
Brackets are allowed in the [link text] only if (a) they are backslash-escaped or (b) they appear as a matched pair of brackets, with an open bracket [, a sequence of zero or more inlines, and a close bracket ].
Backtick [code spans], [autolinks], and raw [HTML tags] bind more tightly than the brackets in link text. Thus, for example, [foo`]` could not be a link text, since the second ] is part of a code span.
The brackets in link text bind more tightly than markers for [emphasis and strong emphasis]. Thus, for example, *[foo*](url) is a link.

A link destination consists of either

a sequence of zero or more characters between an opening < and a closing > that contains no line endings or unescaped < or > characters, or
a nonempty sequence of characters that does not start with <, does not include [ASCII control characters][ASCII control character] or [space] character, and includes parentheses only if (a) they are backslash-escaped or (b) they are part of a balanced pair of unescaped parentheses. (Implementations may impose limits on parentheses nesting to avoid performance issues, but at least three levels of nesting should be supported.)

A link title consists of either

a sequence of zero or more characters between straight double-quote characters ("), including a " character only if it is backslash-escaped, or
a sequence of zero or more characters between straight single-quote characters ('), including a ' character only if it is backslash-escaped, or
a sequence of zero or more characters between matching parentheses ((...)), including a ( or ) character only if it is backslash-escaped.

Although [link titles] may span multiple lines, they may not contain a [blank line].

An inline link consists of a [link text] followed immediately by a left parenthesis (, an optional [link destination], an optional [link title], and a right parenthesis ). These four components may be separated by spaces, tabs, and up to one line ending. If both [link destination] and [link title] are present, they must be separated by spaces, tabs, and up to one line ending.

The link's text consists of the inlines contained in the [link text] (excluding the enclosing square brackets). The link's URI consists of the link destination, excluding enclosing <...> if present, with backslash-escapes in effect as described above. The link's title consists of the link title, excluding its enclosing delimiters, with backslash-escapes in effect as described above.

Here is a simple inline link:

[link](/uri "title")

<p><a href="/uri" title="title">link</a></p>

The title, the link text and even the destination may be omitted:

[link](/uri)

<p><a href="/uri">link</a></p>

[](./target.md)

<p><a href="./target.md"></a></p>

[link]()

<p><a href="">link</a></p>

[link](<>)

<p><a href="">link</a></p>

[]()

<p><a href=""></a></p>

The destination can only contain spaces if it is enclosed in pointy brackets:

[link](/my uri)

<p>[link](/my uri)</p>

[link](</my uri>)

<p><a href="/my%20uri">link</a></p>

The destination cannot contain line endings, even if enclosed in pointy brackets:

[link](foo
bar)

<p>[link](foo
bar)</p>

[link](<foo
bar>)

<p>[link](<foo
bar>)</p>

The destination can contain ) if it is enclosed in pointy brackets:

[a](<b)c>)

<p><a href="b)c">a</a></p>

Pointy brackets that enclose links must be unescaped:

[link](<foo\>)

<p>[link](&lt;foo&gt;)</p>

These are not links, because the opening pointy bracket is not matched properly:

[a](<b)c
[a](<b)c>
[a](<b>c)

<p>[a](&lt;b)c
[a](&lt;b)c&gt;
[a](<b>c)</p>

Parentheses inside the link destination may be escaped:

[link](\(foo\))

<p><a href="(foo)">link</a></p>

Any number of parentheses are allowed without escaping, as long as they are balanced:

[link](foo(and(bar)))

<p><a href="foo(and(bar))">link</a></p>

However, if you have unbalanced parentheses, you need to escape or use the <...> form:

[link](foo(and(bar))

<p>[link](foo(and(bar))</p>

[link](foo\(and\(bar\))

<p><a href="foo(and(bar)">link</a></p>

[link](<foo(and(bar)>)

<p><a href="foo(and(bar)">link</a></p>

Parentheses and other symbols can also be escaped, as usual in Markdown:

[link](foo\)\:)

<p><a href="foo):">link</a></p>

A link can contain fragment identifiers and queries:

[link](#fragment)

[link](https://example.com#fragment)

[link](https://example.com?foo=3#frag)

<p><a href="#fragment">link</a></p>
<p><a href="https://example.com#fragment">link</a></p>
<p><a href="https://example.com?foo=3#frag">link</a></p>

Note that a backslash before a non-escapable character is just a backslash:

[link](foo\bar)

<p><a href="foo%5Cbar">link</a></p>

URL-escaping should be left alone inside the destination, as all URL-escaped characters are also valid URL characters. Entity and numerical character references in the destination will be parsed into the corresponding Unicode code points, as usual. These may be optionally URL-escaped when written as HTML, but this spec does not enforce any particular policy for rendering URLs in HTML or other formats. Renderers may make different decisions about how to escape or normalize URLs in the output.

[link](foo%20b&auml;)

<p><a href="foo%20b%C3%A4">link</a></p>

Note that, because titles can often be parsed as destinations, if you try to omit the destination and keep the title, you'll get unexpected results:

[link]("title")

<p><a href="%22title%22">link</a></p>

Titles may be in single quotes, double quotes, or parentheses:

[link](/url "title")
[link](/url 'title')
[link](/url (title))

<p><a href="/url" title="title">link</a>
<a href="/url" title="title">link</a>
<a href="/url" title="title">link</a></p>

Backslash escapes and entity and numeric character references may be used in titles:

[link](/url "title \"&quot;")

<p><a href="/url" title="title &quot;&quot;">link</a></p>

Titles must be separated from the link using spaces, tabs, and up to one line ending. Other [Unicode whitespace] like non-breaking space doesn't work.

[link](/url "title")

<p><a href="/url%C2%A0%22title%22">link</a></p>

Nested balanced quotes are not allowed without escaping:

[link](/url "title "and" title")

<p>[link](/url "title "and" title")</p>

But it is easy to work around this by using a different quote type:

[link](/url 'title "and" title')

<p><a href="/url" title="title &quot;and&quot; title">link</a></p>

(Note: Markdown.pl did allow double quotes inside a double-quoted title, and its test suite included a test demonstrating this. But it is hard to see a good rationale for the extra complexity this brings, since there are already many ways---backslash escaping, entity and numeric character references, or using a different quote type for the enclosing title---to write titles containing double quotes. Markdown.pl's handling of titles has a number of other strange features. For example, it allows single-quoted titles in inline links, but not reference links. And, in reference links but not inline links, it allows a title to begin with " and end with ). Markdown.pl 1.0.1 even allows titles with no closing quotation mark, though 1.0.2b8 does not. It seems preferable to adopt a simple, rational rule that works the same way in inline links and link reference definitions.)

Spaces, tabs, and up to one line ending is allowed around the destination and title:

[link](   /uri
  "title"  )

<p><a href="/uri" title="title">link</a></p>

But it is not allowed between the link text and the following parenthesis:

[link] (/uri)

<p>[link] (/uri)</p>

The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:

[link [foo [bar]]](/uri)

<p><a href="/uri">link [foo [bar]]</a></p>

[link] bar](/uri)

<p>[link] bar](/uri)</p>

[link [bar](/uri)

<p>[link <a href="/uri">bar</a></p>

[link \[bar](/uri)

<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

[link *foo **bar** `#`*](/uri)

<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>

[![moon](moon.jpg)](/uri)

<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>

However, links may not contain other links, at any level of nesting.

[foo [bar](/uri)](/uri)

<p>[foo <a href="/uri">bar</a>](/uri)</p>

[foo *[bar [baz](/uri)](/uri)*](/uri)

<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>

![[[foo](uri1)](uri2)](uri3)

<p><img src="uri3" alt="[foo](uri2)" /></p>

These cases illustrate the precedence of link text grouping over emphasis grouping:

*[foo*](/uri)

<p>*<a href="/uri">foo*</a></p>

[foo *bar](baz*)

<p><a href="baz*">foo *bar</a></p>

Note that brackets that aren't part of links do not take precedence:

*foo [bar* baz]

<p><em>foo [bar</em> baz]</p>

These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:

[foo <bar attr="](baz)">

<p>[foo <bar attr="](baz)"></p>

[foo`](/uri)`

<p>[foo<code>](/uri)</code></p>

[foo<https://example.com/?search=](uri)>

<p>[foo<a href="https://example.com/?search=%5D(uri)">https://example.com/?search=](uri)</a></p>

There are three kinds of reference links: full, collapsed, and shortcut.

A full reference link consists of a [link text] immediately followed by a [link label] that [matches] a [link reference definition] elsewhere in the document.

A link label begins with a left bracket ([) and ends with the first right bracket (]) that is not backslash-escaped. Between these brackets there must be at least one character that is not a space, tab, or line ending. Unescaped square bracket characters are not allowed inside the opening and closing square brackets of [link labels]. A link label can have at most 999 characters inside the square brackets.

One label matches another just in case their normalized forms are equal. To normalize a label, strip off the opening and closing brackets, perform the Unicode case fold, strip leading and trailing spaces, tabs, and line endings, and collapse consecutive internal spaces, tabs, and line endings to a single space. If there are multiple matching reference link definitions, the one that comes first in the document is used. (It is desirable in such cases to emit a warning.)

The link's URI and title are provided by the matching [link reference definition].

Here is a simple example:

[foo][bar]

[bar]: /url "title"

<p><a href="/url" title="title">foo</a></p>

The rules for the [link text] are the same as with [inline links]. Thus:

The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:

[link [foo [bar]]][ref]

[ref]: /uri

<p><a href="/uri">link [foo [bar]]</a></p>

[link \[bar][ref]

[ref]: /uri

<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

[link *foo **bar** `#`*][ref]

[ref]: /uri

<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>

[![moon](moon.jpg)][ref]

[ref]: /uri

<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>

However, links may not contain other links, at any level of nesting.

[foo [bar](/uri)][ref]

[ref]: /uri

<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>

[foo *bar [baz][ref]*][ref]

[ref]: /uri

<p>[foo <em>bar <a href="/uri">baz</a></em>]<a href="/uri">ref</a></p>

(In the examples above, we have two [shortcut reference links] instead of one [full reference link].)

The following cases illustrate the precedence of link text grouping over emphasis grouping:

*[foo*][ref]

[ref]: /uri

<p>*<a href="/uri">foo*</a></p>

[foo *bar][ref]*

[ref]: /uri

<p><a href="/uri">foo *bar</a>*</p>

These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:

[foo <bar attr="][ref]">

[ref]: /uri

<p>[foo <bar attr="][ref]"></p>

[foo`][ref]`

[ref]: /uri

<p>[foo<code>][ref]</code></p>

[foo<https://example.com/?search=][ref]>

[ref]: /uri

<p>[foo<a href="https://example.com/?search=%5D%5Bref%5D">https://example.com/?search=][ref]</a></p>

Matching is case-insensitive:

[foo][BaR]

[bar]: /url "title"

<p><a href="/url" title="title">foo</a></p>

Unicode case fold is used:

[ẞ]

[SS]: /url

<p><a href="/url">ẞ</a></p>

Consecutive internal spaces, tabs, and line endings are treated as one space for purposes of determining matching:

[Foo
  bar]: /url

[Baz][Foo bar]

<p><a href="/url">Baz</a></p>

No spaces, tabs, or line endings are allowed between the [link text] and the [link label]:

[foo] [bar]

[bar]: /url "title"

<p>[foo] <a href="/url" title="title">bar</a></p>

[foo]
[bar]

[bar]: /url "title"

<p>[foo]
<a href="/url" title="title">bar</a></p>

This is a departure from John Gruber's original Markdown syntax description, which explicitly allows whitespace between the link text and the link label. It brings reference links in line with [inline links], which (according to both original Markdown and this spec) cannot have whitespace after the link text. More importantly, it prevents inadvertent capture of consecutive [shortcut reference links]. If whitespace is allowed between the link text and the link label, then in the following we will have a single reference link, not two shortcut reference links, as intended:

[foo]
[bar]

[foo]: /url1
[bar]: /url2

(Note that [shortcut reference links] were introduced by Gruber himself in a beta version of Markdown.pl, but never included in the official syntax description. Without shortcut reference links, it is harmless to allow space between the link text and link label; but once shortcut references are introduced, it is too dangerous to allow this, as it frequently leads to unintended results.)

When there are multiple matching [link reference definitions], the first is used:

[foo]: /url1

[foo]: /url2

[bar][foo]

<p><a href="/url1">bar</a></p>

Note that matching is performed on normalized strings, not parsed inline content. So the following does not match, even though the labels define equivalent inline content:

[bar][foo\!]

[foo!]: /url

<p>[bar][foo!]</p>

[Link labels] cannot contain brackets, unless they are backslash-escaped:

[foo][ref[]

[ref[]: /uri

<p>[foo][ref[]</p>
<p>[ref[]: /uri</p>

[foo][ref[bar]]

[ref[bar]]: /uri

<p>[foo][ref[bar]]</p>
<p>[ref[bar]]: /uri</p>

[[[foo]]]

[[[foo]]]: /url

<p>[[[foo]]]</p>
<p>[[[foo]]]: /url</p>

[foo][ref\[]

[ref\[]: /uri

<p><a href="/uri">foo</a></p>

Note that in this example ] is not backslash-escaped:

[bar\\]: /uri

[bar\\]

<p><a href="/uri">bar\</a></p>

A [link label] must contain at least one character that is not a space, tab, or line ending:

[]

[]: /uri

<p>[]</p>
<p>[]: /uri</p>

[
 ]

[
 ]: /uri

<p>[
]</p>
<p>[
]: /uri</p>

A collapsed reference link consists of a [link label] that [matches] a [link reference definition] elsewhere in the document, followed by the string []. The contents of the link label are parsed as inlines, which are used as the link's text. The link's URI and title are provided by the matching reference link definition. Thus, [foo][] is equivalent to [foo][foo].

[foo][]

[foo]: /url "title"

<p><a href="/url" title="title">foo</a></p>

[*foo* bar][]

[*foo* bar]: /url "title"

<p><a href="/url" title="title"><em>foo</em> bar</a></p>

The link labels are case-insensitive:

[Foo][]

[foo]: /url "title"

<p><a href="/url" title="title">Foo</a></p>

As with full reference links, spaces, tabs, or line endings are not allowed between the two sets of brackets:

[foo] 
[]

[foo]: /url "title"

<p><a href="/url" title="title">foo</a>
[]</p>

A shortcut reference link consists of a [link label] that [matches] a [link reference definition] elsewhere in the document and is not followed by [] or a link label. The contents of the link label are parsed as inlines, which are used as the link's text. The link's URI and title are provided by the matching link reference definition. Thus, [foo] is equivalent to [foo][].

[foo]

[foo]: /url "title"

<p><a href="/url" title="title">foo</a></p>

[*foo* bar]

[*foo* bar]: /url "title"

<p><a href="/url" title="title"><em>foo</em> bar</a></p>

[[*foo* bar]]

[*foo* bar]: /url "title"

<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>

[[bar [foo]

[foo]: /url

<p>[[bar <a href="/url">foo</a></p>

The link labels are case-insensitive:

[Foo]

[foo]: /url "title"

<p><a href="/url" title="title">Foo</a></p>

A space after the link text should be preserved:

[foo] bar

[foo]: /url

<p><a href="/url">foo</a> bar</p>

If you just want bracketed text, you can backslash-escape the opening bracket to avoid links:

\[foo]

[foo]: /url "title"

<p>[foo]</p>

Note that this is a link, because a link label ends with the first following closing bracket:

[foo*]: /url

*[foo*]

<p>*<a href="/url">foo*</a></p>

Full and collapsed references take precedence over shortcut references:

[foo][bar]

[foo]: /url1
[bar]: /url2

<p><a href="/url2">foo</a></p>

[foo][]

[foo]: /url1

<p><a href="/url1">foo</a></p>

Inline links also take precedence:

[foo]()

[foo]: /url1

<p><a href="">foo</a></p>

[foo](not a link)

[foo]: /url1

<p><a href="/url1">foo</a>(not a link)</p>

In the following case [bar][baz] is parsed as a reference, [foo] as normal text:

[foo][bar][baz]

[baz]: /url

<p>[foo]<a href="/url">bar</a></p>

Here, though, [foo][bar] is parsed as a reference, since [bar] is defined:

[foo][bar][baz]

[baz]: /url1
[bar]: /url2

<p><a href="/url2">foo</a><a href="/url1">baz</a></p>

Here [foo] is not parsed as a shortcut reference, because it is followed by a link label (even though [bar] is not defined):

[foo][bar][baz]

[baz]: /url1
[foo]: /url2

<p>[foo]<a href="/url1">bar</a></p>

Images

Syntax for images is like the syntax for links, with one difference. Instead of [link text], we have an image description. The rules for this are the same as for [link text], except that (a) an image description starts with ![ rather than [, and (b) an image description may contain links. An image description has inline elements as its contents. When an image is rendered to HTML, this is standardly used as the image's alt attribute.

![foo](/url "title")

<p><img src="/url" alt="foo" title="title" /></p>

![foo *bar*]

[foo *bar*]: train.jpg "train & tracks"

<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>

![foo ![bar](/url)](/url2)

<p><img src="/url2" alt="foo bar" /></p>

![foo [bar](/url)](/url2)

<p><img src="/url2" alt="foo bar" /></p>

Though this spec is concerned with parsing, not rendering, it is recommended that in rendering to HTML, only the plain string content of the [image description] be used. Note that in the above example, the alt attribute's value is foo bar, not foo [bar](/url) or foo <a href="/url">bar</a>. Only the plain string content is rendered, without formatting.

![foo *bar*][]

[foo *bar*]: train.jpg "train & tracks"

<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>

![foo *bar*][foobar]

[FOOBAR]: train.jpg "train & tracks"

<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>

![foo](train.jpg)

<p><img src="train.jpg" alt="foo" /></p>

My ![foo bar](/path/to/train.jpg  "title"   )

<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>

![foo](<url>)

<p><img src="url" alt="foo" /></p>

![](/url)

<p><img src="/url" alt="" /></p>

Reference-style:

![foo][bar]

[bar]: /url

<p><img src="/url" alt="foo" /></p>

![foo][bar]

[BAR]: /url

<p><img src="/url" alt="foo" /></p>

Collapsed:

![foo][]

[foo]: /url "title"

<p><img src="/url" alt="foo" title="title" /></p>

![*foo* bar][]

[*foo* bar]: /url "title"

<p><img src="/url" alt="foo bar" title="title" /></p>

The labels are case-insensitive:

![Foo][]

[foo]: /url "title"

<p><img src="/url" alt="Foo" title="title" /></p>

As with reference links, spaces, tabs, and line endings, are not allowed between the two sets of brackets:

![foo] 
[]

[foo]: /url "title"

<p><img src="/url" alt="foo" title="title" />
[]</p>

Shortcut:

![foo]

[foo]: /url "title"

<p><img src="/url" alt="foo" title="title" /></p>

![*foo* bar]

[*foo* bar]: /url "title"

<p><img src="/url" alt="foo bar" title="title" /></p>

Note that link labels cannot contain unescaped brackets:

![[foo]]

[[foo]]: /url "title"

<p>![[foo]]</p>
<p>[[foo]]: /url "title"</p>

The link labels are case-insensitive:

![Foo]

[foo]: /url "title"

<p><img src="/url" alt="Foo" title="title" /></p>

If you just want a literal ! followed by bracketed text, you can backslash-escape the opening [:

!\[foo]

[foo]: /url "title"

<p>![foo]</p>

If you want a link after a literal !, backslash-escape the !:

\![foo]

[foo]: /url "title"

<p>!<a href="/url" title="title">foo</a></p>

Autolinks

Autolinks are absolute URIs and email addresses inside < and >. They are parsed as links, with the URL or email address as the link label.

A URI autolink consists of <, followed by an [absolute URI] followed by >. It is parsed as a link to the URI, with the URI as the link's label.

An absolute URI, for these purposes, consists of a [scheme] followed by a colon (:) followed by zero or more characters other than [ASCII control characters][ASCII control character], [space], <, and >. If the URI includes these characters, they must be percent-encoded (e.g. %20 for a space).

For purposes of this spec, a scheme is any sequence of 2--32 characters beginning with an ASCII letter and followed by any combination of ASCII letters, digits, or the symbols plus ("+"), period ("."), or hyphen ("-").

Here are some valid autolinks:

<http://foo.bar.baz>

<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p>

<https://foo.bar.baz/test?q=hello&id=22&boolean>

<p><a href="https://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean">https://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean</a></p>

<irc://foo.bar:2233/baz>

<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p>

Uppercase is also fine:

<MAILTO:FOO@BAR.BAZ>

<p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>

Note that many strings that count as [absolute URIs] for purposes of this spec are not valid URIs, because their schemes are not registered or because of other problems with their syntax:

<a+b+c:d>

<p><a href="a+b+c:d">a+b+c:d</a></p>

<made-up-scheme://foo,bar>

<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p>

<https://../>

<p><a href="https://../">https://../</a></p>

<localhost:5001/foo>

<p><a href="localhost:5001/foo">localhost:5001/foo</a></p>

Spaces are not allowed in autolinks:

<https://foo.bar/baz bim>

<p>&lt;https://foo.bar/baz bim&gt;</p>

Backslash-escapes do not work inside autolinks:

<https://example.com/\[\>

<p><a href="https://example.com/%5C%5B%5C">https://example.com/\[\</a></p>

An email autolink consists of <, followed by an [email address], followed by >. The link's label is the email address, and the URL is mailto: followed by the email address.

An email address, for these purposes, is anything that matches the non-normative regex from the HTML5 spec:

/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?
(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/

Examples of email autolinks:

<foo@bar.example.com>

<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>

<foo+special@Bar.baz-bar0.com>

<p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p>

Backslash-escapes do not work inside email autolinks:

<foo\+@bar.example.com>

<p>&lt;foo+@bar.example.com&gt;</p>

These are not autolinks:

<>

<p>&lt;&gt;</p>

< https://foo.bar >

<p>&lt; https://foo.bar &gt;</p>

<m:abc>

<p>&lt;m:abc&gt;</p>

<foo.bar.baz>

<p>&lt;foo.bar.baz&gt;</p>

https://example.com

<p>https://example.com</p>

foo@bar.example.com

<p>foo@bar.example.com</p>

Raw HTML

Text between < and > that looks like an HTML tag is parsed as a raw HTML tag and will be rendered in HTML without escaping. Tag and attribute names are not limited to current HTML tags, so custom tags (and even, say, DocBook tags) may be used.

Here is the grammar for tags:

A tag name consists of an ASCII letter followed by zero or more ASCII letters, digits, or hyphens (-).

An attribute consists of spaces, tabs, and up to one line ending, an [attribute name], and an optional [attribute value specification].

An attribute name consists of an ASCII letter, _, or :, followed by zero or more ASCII letters, digits, _, ., :, or -. (Note: This is the XML specification restricted to ASCII. HTML5 is laxer.)

An attribute value specification consists of optional spaces, tabs, and up to one line ending, a = character, optional spaces, tabs, and up to one line ending, and an [attribute value].

An attribute value consists of an [unquoted attribute value], a [single-quoted attribute value], or a [double-quoted attribute value].

An unquoted attribute value is a nonempty string of characters not including spaces, tabs, line endings, ", ', =, <, >, or `.

A single-quoted attribute value consists of ', zero or more characters not including ', and a final '.

A double-quoted attribute value consists of ", zero or more characters not including ", and a final ".

An open tag consists of a < character, a [tag name], zero or more [attributes], optional spaces, tabs, and up to one line ending, an optional / character, and a > character.

A closing tag consists of the string </, a [tag name], optional spaces, tabs, and up to one line ending, and the character >.

An HTML comment consists of , or , and --> (see the HTML spec).

A processing instruction consists of the string <?, a string of characters not including the string ?>, and the string ?>.

A declaration consists of the string <!, an ASCII letter, zero or more characters not including the character >, and the character >.

A CDATA section consists of the string <![CDATA[, a string of characters not including the string ]]>, and the string ]]>.

An HTML tag consists of an [open tag], a [closing tag], an [HTML comment], a [processing instruction], a [declaration], or a [CDATA section].

Here are some simple open tags:

<a><bab><c2c>

<p><a><bab><c2c></p>

Empty elements:

<a/><b2/>

<p><a/><b2/></p>

Whitespace is allowed:

<a  /><b2
data="foo" >

<p><a  /><b2
data="foo" ></p>

With attributes:

<a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 />

<p><a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 /></p>

Custom tag names can be used:

Foo <responsive-image src="foo.jpg" />

<p>Foo <responsive-image src="foo.jpg" /></p>

Illegal tag names, not parsed as HTML:

<33> <__>

<p>&lt;33&gt; &lt;__&gt;</p>

Illegal attribute names:

<a h*#ref="hi">

<p>&lt;a h*#ref="hi"&gt;</p>

Illegal attribute values:

<a href="hi'> <a href=hi'>

<p>&lt;a href="hi'&gt; &lt;a href=hi'&gt;</p>

Illegal whitespace:

< a><
foo><bar/ >
<foo bar=baz
bim!bop />

<p>&lt; a&gt;&lt;
foo&gt;&lt;bar/ &gt;
&lt;foo bar=baz
bim!bop /&gt;</p>

Missing whitespace:

<a href='bar'title=title>

<p>&lt;a href='bar'title=title&gt;</p>

Closing tags:

</a></foo >

<p></a></foo ></p>

Illegal attributes in closing tag:

</a href="foo">

<p>&lt;/a href="foo"&gt;</p>

Comments:

foo <!-- this is a --
comment - with hyphens -->

<p>foo <!-- this is a --
comment - with hyphens --></p>

foo <!--> foo -->

foo <!---> foo -->

<p>foo <!--> foo --&gt;</p>
<p>foo <!---> foo --&gt;</p>

Processing instructions:

foo <?php echo $a; ?>

<p>foo <?php echo $a; ?></p>

Declarations:

foo <!ELEMENT br EMPTY>

<p>foo <!ELEMENT br EMPTY></p>

CDATA sections:

foo <![CDATA[>&<]]>

<p>foo <![CDATA[>&<]]></p>

Entity and numeric character references are preserved in HTML attributes:

foo <a href="&ouml;">

<p>foo <a href="&ouml;"></p>

Backslash escapes do not work in HTML attributes:

foo <a href="\*">

<p>foo <a href="\*"></p>

<a href="\"">

<p>&lt;a href="""&gt;</p>

Hard line breaks

A line ending (not in a code span or HTML tag) that is preceded by two or more spaces and does not occur at the end of a block is parsed as a hard line break (rendered in HTML as a <br /> tag):

foo  
baz

<p>foo<br />
baz</p>

For a more visible alternative, a backslash before the [line ending] may be used instead of two or more spaces:

foo\
baz

<p>foo<br />
baz</p>

More than two spaces can be used:

foo       
baz

<p>foo<br />
baz</p>

Leading spaces at the beginning of the next line are ignored:

foo  
     bar

<p>foo<br />
bar</p>

foo\
     bar

<p>foo<br />
bar</p>

Hard line breaks can occur inside emphasis, links, and other constructs that allow inline content:

*foo  
bar*

<p><em>foo<br />
bar</em></p>

*foo\
bar*

<p><em>foo<br />
bar</em></p>

Hard line breaks do not occur inside code spans

`code  
span`

<p><code>code   span</code></p>

`code\
span`

<p><code>code\ span</code></p>

or HTML tags:

<a href="foo  
bar">

<p><a href="foo  
bar"></p>

<a href="foo\
bar">

<p><a href="foo\
bar"></p>

Hard line breaks are for separating inline content within a block. Neither syntax for hard line breaks works at the end of a paragraph or other block element:

foo\

<p>foo\</p>

foo

<p>foo</p>

### foo\

<h3>foo\</h3>

### foo

<h3>foo</h3>

Soft line breaks

A regular line ending (not in a code span or HTML tag) that is not preceded by two or more spaces or a backslash is parsed as a softbreak. (A soft line break may be rendered in HTML either as a [line ending] or as a space. The result will be the same in browsers. In the examples here, a [line ending] will be used.)

foo
baz

<p>foo
baz</p>

Spaces at the end of the line and beginning of the next line are removed:

foo 
 baz

<p>foo
baz</p>

A conforming parser may render a soft line break in HTML either as a line ending or as a space.

A renderer may also provide an option to render soft line breaks as hard line breaks.

Textual content

Any characters not given an interpretation by the above rules will be parsed as plain textual content.

hello $.;'there

<p>hello $.;'there</p>

Foo χρῆν

<p>Foo χρῆν</p>

Internal spaces are preserved verbatim:

Multiple     spaces

<p>Multiple     spaces</p>

Appendix: A parsing strategy

In this appendix we describe some features of the parsing strategy used in the CommonMark reference implementations.

Overview

Parsing has two phases:

In the first phase, lines of input are consumed and the block structure of the document---its division into paragraphs, block quotes, list items, and so on---is constructed. Text is assigned to these blocks but not parsed. Link reference definitions are parsed and a map of links is constructed.
In the second phase, the raw text contents of paragraphs and headings are parsed into sequences of Markdown inline elements (strings, code spans, links, emphasis, and so on), using the map of link references constructed in phase 1.

At each point in processing, the document is represented as a tree of blocks. The root of the tree is a document block. The document may have any number of other blocks as children. These children may, in turn, have other blocks as children. The last child of a block is normally considered open, meaning that subsequent lines of input can alter its contents. (Blocks that are not open are closed.) Here, for example, is a possible document tree, with the open blocks marked by arrows:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"

Phase 1: block structure

Each line that is processed has an effect on this tree. The line is analyzed and, depending on its contents, the document may be altered in one or more of the following ways:

One or more open blocks may be closed.
One or more new blocks may be created as children of the last open block.
Text may be added to the last (deepest) open block remaining on the tree.

Once a line has been incorporated into the tree in this way, it can be discarded, so input can be read in a stream.

For each line, we follow this procedure:

First we iterate through the open blocks, starting with the root document, and descending through last children down to the last open block. Each block imposes a condition that the line must satisfy if the block is to remain open. For example, a block quote requires a > character. A paragraph requires a non-blank line. In this phase we may match all or just some of the open blocks. But we cannot close unmatched blocks yet, because we may have a [lazy continuation line].
Next, after consuming the continuation markers for existing blocks, we look for new block starts (e.g. > for a block quote). If we encounter a new block start, we close any blocks unmatched in step 1 before creating the new block as a child of the last matched container block.
Finally, we look at the remainder of the line (after block markers like >, list markers, and indentation have been consumed). This is text that can be incorporated into the last open block (a paragraph, code block, heading, or raw HTML).

Setext headings are formed when we see a line of a paragraph that is a [setext heading underline].

Reference link definitions are detected when a paragraph is closed; the accumulated text lines are parsed to see if they begin with one or more reference link definitions. Any remainder becomes a normal paragraph.

We can see how this works by considering how the tree above is generated by four lines of Markdown:

> Lorem ipsum dolor
sit amet.
> - Qui *quodsi iracundia*
> - aliquando id

At the outset, our document model is just

-> document

The first line of our text,

> Lorem ipsum dolor

causes a block_quote block to be created as a child of our open document block, and a paragraph block as a child of the block_quote. Then the text is added to the last open block, the paragraph:

-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor"

The next line,

sit amet.

is a "lazy continuation" of the open paragraph, so it gets added to the paragraph's text:

-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor\nsit amet."

The third line,

> - Qui *quodsi iracundia*

causes the paragraph block to be closed, and a new list block opened as a child of the block_quote. A list_item is also added as a child of the list, and a paragraph as a child of the list_item. The text is then added to the new paragraph:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
      -> list_item
        -> paragraph
             "Qui *quodsi iracundia*"

The fourth line,

> - aliquando id

causes the list_item (and its child the paragraph) to be closed, and a new list_item opened up as child of the list. A paragraph is added as a child of the new list_item, to contain the text. We thus obtain the final tree:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"

Phase 2: inline structure

Once all of the input has been parsed, all open blocks are closed.

We then "walk the tree," visiting every node, and parse raw string contents of paragraphs and headings as inlines. At this point we have seen all the link reference definitions, so we can resolve reference links as we go.

document
  block_quote
    paragraph
      str "Lorem ipsum dolor"
      softbreak
      str "sit amet."
    list (type=bullet tight=true bullet_char=-)
      list_item
        paragraph
          str "Qui "
          emph
            str "quodsi iracundia"
      list_item
        paragraph
          str "aliquando id"

Notice how the [line ending] in the first paragraph has been parsed as a softbreak, and the asterisks in the first list item have become an emph.

An algorithm for parsing nested emphasis and links

By far the trickiest part of inline parsing is handling emphasis, strong emphasis, links, and images. This is done using the following algorithm.

When we're parsing inlines and we hit either

a run of * or _ characters, or
a [ or ![

we insert a text node with these symbols as its literal content, and we add a pointer to this text node to the delimiter stack.

The [delimiter stack] is a doubly linked list. Each element contains a pointer to a text node, plus information about

the type of delimiter ([, ![, *, _)
the number of delimiters,
whether the delimiter is "active" (all are active to start), and
whether the delimiter is a potential opener, a potential closer, or both (which depends on what sort of characters precede and follow the delimiters).

When we hit a ] character, we call the look for link or image procedure (see below).

When we hit the end of the input, we call the process emphasis procedure (see below), with stack_bottom = NULL.

look for link or image

Starting at the top of the delimiter stack, we look backwards through the stack for an opening [ or ![ delimiter.

If we don't find one, we return a literal text node ].
If we do find one, but it's not active, we remove the inactive delimiter from the stack, and return a literal text node ].
If we find one and it's active, then we parse ahead to see if we have an inline link/image, reference link/image, collapsed reference link/image, or shortcut reference link/image.
- If we don't, then we remove the opening delimiter from the delimiter stack and return a literal text node ].
- If we do, then
  - We return a link or image node whose children are the inlines after the text node pointed to by the opening delimiter.
  - We run process emphasis on these inlines, with the [ opener as stack_bottom.
  - We remove the opening delimiter.
  - If we have a link (and not an image), we also set all [ delimiters before the opening delimiter to inactive. (This will prevent us from getting links within links.)

process emphasis

Parameter stack_bottom sets a lower bound to how far we descend in the [delimiter stack]. If it is NULL, we can go all the way to the bottom. Otherwise, we stop before visiting stack_bottom.

Let current_position point to the element on the [delimiter stack] just above stack_bottom (or the first element if stack_bottom is NULL).

We keep track of the openers_bottom for each delimiter type (*, _), indexed to the length of the closing delimiter run (modulo 3) and to whether the closing delimiter can also be an opener. Initialize this to stack_bottom.

Then we repeat the following until we run out of potential closers:

Move current_position forward in the delimiter stack (if needed) until we find the first potential closer with delimiter * or _. (This will be the potential closer closest to the beginning of the input -- the first one in parse order.)
Now, look back in the stack (staying above stack_bottom and the openers_bottom for this delimiter type) for the first matching potential opener ("matching" means same delimiter).
If one is found:
- Figure out whether we have emphasis or strong emphasis: if both closer and opener spans have length >= 2, we have strong, otherwise regular.
- Insert an emph or strong emph node accordingly, after the text node corresponding to the opener.
- Remove any delimiters between the opener and closer from the delimiter stack.
- Remove 1 (for regular emph) or 2 (for strong emph) delimiters from the opening and closing text nodes. If they become empty as a result, remove them and remove the corresponding element of the delimiter stack. If the closing node is removed, reset current_position to the next element in the stack.
If none is found:
- Set openers_bottom to the element before current_position. (We know that there are no openers for this kind of closer up to and including this point, so this puts a lower bound on future searches.)
- If the closer at current_position is not a potential opener, remove it from the delimiter stack (since we know it can't be a closer either).
- Advance current_position to the next element in the stack.

After we're done, we remove all delimiters above stack_bottom from the delimiter stack.

Smart punctuation

Open quotes are matched with closed quotes. The same method is used for matching openers and closers as is used in emphasis parsing:

"Hello," said the spider.
"'Shelob' is my name."

<p>“Hello,” said the spider.
“‘Shelob’ is my name.”</p>

'A', 'B', and 'C' are letters.

<p>‘A’, ‘B’, and ‘C’ are letters.</p>

'Oak,' 'elm,' and 'beech' are names of trees.
So is 'pine.'

<p>‘Oak,’ ‘elm,’ and ‘beech’ are names of trees.
So is ‘pine.’</p>

'He said, "I want to go."'

<p>‘He said, “I want to go.”’</p>

A single quote that isn't an open quote matched with a close quote will be treated as an apostrophe:

Were you alive in the 70's?

<p>Were you alive in the 70’s?</p>

Here is some quoted '`code`' and a "[quoted link](url)".

<p>Here is some quoted ‘<code>code</code>’ and a “<a href="url">quoted link</a>”.</p>

Here the first ' is treated as an apostrophe, not an open quote, because the final single quote is matched by the single quote before jolly:

'tis the season to be 'jolly'

<p>’tis the season to be ‘jolly’</p>

Multiple apostrophes should not be marked as open/closing quotes.

'We'll use Jane's boat and John's truck,' Jenna said.

<p>‘We’ll use Jane’s boat and John’s truck,’ Jenna said.</p>

An unmatched double quote will be interpreted as a left double quote, to facilitate this style:

"A paragraph with no closing quote.

"Second paragraph by same speaker, in fiction."

<p>“A paragraph with no closing quote.</p>
<p>“Second paragraph by same speaker, in fiction.”</p>

A quote following a ] or ) character cannot be an open quote:

[a]'s b'

<p>[a]’s b’</p>

Quotes that are escaped come out as literal straight quotes:

\"This is not smart.\"
This isn\'t either.
5\'8\"

<p>"This is not smart."
This isn't either.
5'8"</p>

Two hyphens form an en-dash, three an em-dash.

Some dashes:  em---em
en--en
em --- em
en -- en
2--3

<p>Some dashes:  em—em
en–en
em — em
en – en
2–3</p>

A sequence of more than three hyphens is parsed as a sequence of em and/or en dashes, with no hyphens. If possible, a homogeneous sequence of dashes is used (so, 10 hyphens = 5 en dashes, and 9 hyphens = 3 em dashes). When a heterogeneous sequence must be used, the em dashes come first, followed by the en dashes, and as few en dashes as possible are used (so, 7 hyphens = 2 em dashes an 1 en dash).

one-
two--
three---
four----
five-----
six------
seven-------
eight--------
nine---------
thirteen-------------.

<p>one-
two–
three—
four––
five—–
six——
seven—––
eight––––
nine———
thirteen———––.</p>

Hyphens can be escaped:

Escaped hyphens: \-- \-\-\-.

<p>Escaped hyphens: -- ---.</p>

Three periods form an ellipsis:

Ellipses...and...and....

<p>Ellipses…and…and….</p>

Periods can be escaped if ellipsis-formation is not wanted:

No ellipses\.\.\.

<p>No ellipses...</p>

// This is a slightly modified (replaced example strikethrough by example) of the // table section from GitHub's spec.txt

Strikethrough (extension)

GFM enables the strikethrough extension, where an additional emphasis type is available.

Strikethrough text is any text wrapped in a matching pair of one or two tildes (~).

~~Hi~~ Hello, ~there~ world!

<p><del>Hi</del> Hello, <del>there</del> world!</p>

As with regular emphasis delimiters, a new paragraph will cause strikethrough parsing to cease:

This ~~has a

new paragraph~~.

<p>This ~~has a</p>
<p>new paragraph~~.</p>

Three or more tildes do not create a strikethrough:

This will ~~~not~~~ strike.

<p>This will ~~~not~~~ strike.</p>

This is an extension of gfm_strikethrough.txt. Some of these tests are also pulled from commonmark-hs.

Two tildes

Basic strikethrough is between two tildes:

~~This is *stricken out*~~

<p><del>This is <em>stricken out</em></del></p>

Backslash escapes:

~~This is \~\~stricken~~

<p><del>This is ~~stricken</del></p>

Intraword strikeout:

This~~is~~stricken

<p>This<del>is</del>stricken</p>

~~This~~is~~stricken~~

<p><del>This</del>is<del>stricken</del></p>

Punctuation is ignored for purposes of determining flankingness on two tildes:

Here I strike out an exclamation point~~!~~.

<p>Here I strike out an exclamation point<del>!</del>.</p>

One tilde

One tilde—and this is where we differ from commonmark-hs—is allowed in certain situations:

~This is stricken out~

<p><del>This is stricken out</del></p>

Backslash escapes:

~This is \~stricken~

<p><del>This is ~stricken</del></p>

Intraword strikeout requires two tildes:

This~is~nothing

<p>This~is~nothing</p>

~This~is~nothing~

<p><del>This~is~nothing</del></p>

Punctuation is used for purposes of determining flankingness:

Here I fail to strike out an exclamation point~!~.

<p>Here I fail to strike out an exclamation point~!~.</p>

Tilde runs can't mix.

Here I fail to strike out a tilde ~~~.

<p>Here I fail to strike out a tilde ~~~.</p>

Here I fail to match up ~~tildes~.

<p>Here I fail to match up ~~tildes~.</p>

Here I fail to match up ~tildes~~.

<p>Here I fail to match up ~tildes~~.</p>

Double tildes are allowed to contain single tildes, and the other way around:

~~This ~is stricken.~~

<p><del>This ~is stricken.</del></p>

~This ~~is stricken.~

<p><del>This ~~is stricken.</del></p>

The first one wins.

~This ~~is stricken~ but this is not~~

<p><del>This ~~is stricken</del> but this is not~~</p>

// This is a slightly modified (replaced example table by example, // different rendering of code inside table) copy of the // table section from GFM's spec.txt

Tables (extension)

GFM enables the table extension, where an additional leaf block type is available.

A table is an arrangement of data with rows and columns, consisting of a single header row, a [delimiter row] separating the header from the data, and zero or more data rows.

Each row consists of cells containing arbitrary text, in which [inlines] are parsed, separated by pipes (|). A leading and trailing pipe is also recommended for clarity of reading, and if there's otherwise parsing ambiguity. Spaces between pipes and cell content are trimmed. Block-level elements cannot be inserted in a table.

The delimiter row consists of cells whose only content are hyphens (-), and optionally, a leading or trailing colon (:), or both, to indicate left, right, or center alignment respectively.

| foo | bar |
| --- | --- |
| baz | bim |

<table>
<thead>
<tr>
<th>foo</th>
<th>bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>baz</td>
<td>bim</td>
</tr>
</tbody>
</table>

Cells in one column don't need to match length, though it's easier to read if they are. Likewise, use of leading and trailing pipes may be inconsistent:

| abc | defghi |
:-: | -----------:
bar | baz

<table>
<thead>
<tr>
<th style="text-align: center">abc</th>
<th style="text-align: right">defghi</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align: center">bar</td>
<td style="text-align: right">baz</td>
</tr>
</tbody>
</table>

Include a pipe in a cell's content by escaping it, including inside other inline spans:

| f\|oo  |
| ------ |
| b `\|` az |
| b **\|** im |

<table>
<thead>
<tr>
<th>f|oo</th>
</tr>
</thead>
<tbody>
<tr>
<td>b <code>|</code> az</td>
</tr>
<tr>
<td>b <strong>|</strong> im</td>
</tr>
</tbody>
</table>

The table is broken at the first empty line, or beginning of another block-level structure:

| abc | def |
| --- | --- |
| bar | baz |
> bar

<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>
<blockquote>
<p>bar</p>
</blockquote>

| abc | def |
| --- | --- |
| bar | baz |
bar

bar

<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
<tr>
<td>bar</td>
<td></td>
</tr>
</tbody>
</table>
<p>bar</p>

The header row must match the [delimiter row] in the number of cells. If not, a table will not be recognized:

| abc | def |
| --- |
| bar |

<p>| abc | def |
| --- |
| bar |</p>

The remainder of the table's rows may vary in the number of cells. If there are a number of cells fewer than the number of cells in the header row, empty cells are inserted. If there are greater, the excess is ignored:

| abc | def |
| --- | --- |
| bar |
| bar | baz | boo |

<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td></td>
</tr>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>

If there are no rows in the body, no <tbody> is generated in HTML output:

| abc | def |
| --- | --- |

<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody></tbody>
</table>

Tables can interrupt paragraphs:

Hello World
| abc | def |
| --- | --- |
| bar | baz |

<p>Hello World</p>
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>

Run this with cargo test --features gen-tests suite::table.

Adapted from original by replacing td elements by th elements inside thead and disabling the third test.

False match

Test header
-----------

<h2>Test header</h2>

True match

Test|Table
----|-----

<table><thead><tr><th>Test</th><th>Table</th></tr></thead><tbody></tbody>
</table>

Actual rows in it

Test|Table
----|-----
Test row
Test|2

Test ending
.
<table><thead><tr><th>Test</th><th>Table</th></tr></thead><tbody>
<tr><td>Test row</td></tr>
<tr><td>Test</td><td>2</td></tr>
</tbody></table>
<p>Test ending</p>

Test with quote

> Test  | Table
> ------|------
> Row 1 | Every
> Row 2 | Day
>
> Paragraph

<blockquote>
<table><thead><tr><th>Test</th><th>Table</th></tr></thead><tbody>
<tr><td>Row 1</td><td>Every</td></tr>
<tr><td>Row 2</td><td>Day</td></tr>
</tbody></table>
<p>Paragraph</p>
</blockquote>

Test with list

 1. First entry
 2. Second entry

    Col 1|Col 2
    -|-
    Row 1|Part 2
    Row 2|Part 2

<ol>
<li>
<p>First entry</p>
</li>
<li>
<p>Second entry</p>
<table><thead><tr><th>Col 1</th><th>Col 2</th></tr></thead><tbody>
<tr><td>Row 1</td><td>Part 2</td></tr>
<tr><td>Row 2</td><td>Part 2</td></tr>
</tbody></table>
</li>
</ol>

Test with border

|Col 1|Col 2|
|-----|-----|
|R1C1 |R1C2 |
|R2C1 |R2C2 |

<table><thead><tr><th>Col 1</th><th>Col 2</th></tr></thead><tbody>
<tr><td>R1C1</td><td>R1C2</td></tr>
<tr><td>R2C1</td><td>R2C2</td></tr>
</tbody></table>

Test with empty cells

Empty cells should work.

| Col 1 | Col 2 |
|-------|-------|
|       |       |
|       |       |

<table><thead><tr><th>Col 1</th><th>Col 2</th></tr></thead><tbody>
<tr><td></td><td></td></tr>
<tr><td></td><td></td></tr>
</tbody></table>

... and properly mix with filled cells.

| Col 1 | Col 2 |
|-------|-------|
|   x   |       |
|       |    x  |

<table><thead><tr><th>Col 1</th><th>Col 2</th></tr></thead><tbody>
<tr><td>x</td><td></td></tr>
<tr><td></td><td>x</td></tr>
</tbody></table>

Table with UTF-8

Basic example.

|Col 1|Col 2|
|-----|-----|
|✓    |✓    |
|✓    |✓    |

<table><thead><tr><th>Col 1</th><th>Col 2</th></tr></thead><tbody>
<tr><td>✓</td><td>✓</td></tr>
<tr><td>✓</td><td>✓</td></tr>
</tbody></table>

More advanced example.

|  Target                       | std |rustc|cargo| notes                      |
|-------------------------------|-----|-----|-----|----------------------------|
| `x86_64-unknown-linux-musl`   |  ✓  |     |     | 64-bit Linux with MUSL     |
| `arm-linux-androideabi`       |  ✓  |     |     | ARM Android                |
| `arm-unknown-linux-gnueabi`   |  ✓  |  ✓  |     | ARM Linux (2.6.18+)        |
| `arm-unknown-linux-gnueabihf` |  ✓  |  ✓  |     | ARM Linux (2.6.18+)        |
| `aarch64-unknown-linux-gnu`   |  ✓  |     |     | ARM64 Linux (2.6.18+)      |
| `mips-unknown-linux-gnu`      |  ✓  |     |     | MIPS Linux (2.6.18+)       |
| `mipsel-unknown-linux-gnu`    |  ✓  |     |     | MIPS (LE) Linux (2.6.18+)  |

<table><thead><tr><th>Target</th><th>std</th><th>rustc</th><th>cargo</th><th>notes</th></tr></thead><tbody>
<tr><td><code>x86_64-unknown-linux-musl</code></td><td>✓</td><td></td><td></td><td>64-bit Linux with MUSL</td></tr>
<tr><td><code>arm-linux-androideabi</code></td><td>✓</td><td></td><td></td><td>ARM Android</td></tr>
<tr><td><code>arm-unknown-linux-gnueabi</code></td><td>✓</td><td>✓</td><td></td><td>ARM Linux (2.6.18+)</td></tr>
<tr><td><code>arm-unknown-linux-gnueabihf</code></td><td>✓</td><td>✓</td><td></td><td>ARM Linux (2.6.18+)</td></tr>
<tr><td><code>aarch64-unknown-linux-gnu</code></td><td>✓</td><td></td><td></td><td>ARM64 Linux (2.6.18+)</td></tr>
<tr><td><code>mips-unknown-linux-gnu</code></td><td>✓</td><td></td><td></td><td>MIPS Linux (2.6.18+)</td></tr>
<tr><td><code>mipsel-unknown-linux-gnu</code></td><td>✓</td><td></td><td></td><td>MIPS (LE) Linux (2.6.18+)</td></tr>
</tbody></table>

Hiragana-containing pseudo-table.

|-|-|
|ぃ|い|

<p>|-|-|
|ぃ|い|</p>

Hiragana-containing actual table.

|ぁ|ぃ|
|-|-|
|ぃ|ぃ|

<table><thead><tr><th>ぁ</th><th>ぃ</th></tr></thead><tbody>
<tr><td>ぃ</td><td>ぃ</td></tr>
</tbody></table>

Test russian symbols.

|Колонка 1|Колонка 2|
|---------|---------|
|Ячейка 1 |Ячейка 2 |

<table><thead><tr><th>Колонка 1</th><th>Колонка 2</th></tr></thead><tbody>
<tr><td>Ячейка 1</td><td>Ячейка 2</td></tr>
</tbody></table>

Test cases based on https://github.com/github/cmark-gfm/issues/180 and https://github.com/raphlinus/pulldown-cmark/issues/540.

Pulldown-cmark gives "heavy" tables (with a leading |) higher priority than "light" tables for paragraph interruption. This is compatible with older versions of GitHub, but 2023-06-21 GitHub treats them the same.

A quick run through https://babelmark.github.io shows, of the implementations that support pipe tables at all¹:

Implementation ->	a	b	c	d	e	f	g	h	i	j
pulldown-cmark	✓	x	x	x	x	✓	x	x	✓	x
github/cmark-gfm	✓	x	✓	x	✓	✓	✓	✓	✓	x
pycmarkgfm	✓	x	✓	x	✓	✓	✓	✓	✓	x
DFM²	✓	✓	✓	✓	✓	✓	✓	-	-	x
league/commonmark GFM	✓	✓	✓	✓	✓	✓	✓	x	✓	x
s9e/TextFormatter	✓	✓	✓	✓	✓	✓	✓	x	x	x
markdown-it[^3]³	✓	x	✓	x	✓	✓	✓	x	-	-
php-markdown-extra	✓	x	✓	x	✓	✓	✓	x	x	x
markdig	x	x	x	x	x	x	x	x	x	x
MD4C	x	x	x	x	x	x	x	x	x	x
parsedown	x	x	x	x	x	x	x	x	x	x
maruku⁴	x	x	x	x	x	x	x	x	x	-
cebe	x	x	x	x	x	x	x	x	x	x
pandoc	x	x	x	x	x	x	x	x	x	x
multimarkdown	x	x	x	x	x	x	x	x	x	x

While pulldown-cmark could probably afford to align itself with GFM, tables interrupting paragraphs is not a portable feature, and anyone trying to write widely-compatible markdown syntax can't rely on it.

Implementations that "support pipe tables at all" were identified by including "table a" without a preceding paragraph and seeing if it produced an HTML table for that.

On "table h" and "table i", DFM sees a table with no cells, followed by a paragraph with b in it.

⁴

On "table j", maruku and markdown-it see a table with no cells, followed by a paragraph with b in it.

On "table i", markdown-it sees a table with no cells, followed by a paragraph with b in it.

table a
|  a  |  b  |
| --- | --- |
|  c  |  d  |


table b
    |  a  |  b  |
    | --- | --- |
    |  c  |  d  |


table c
 a  |  b
--- | ---
 c  |  d


table d
    a | b
    --|--
    c | d


table e
a | b
--|--
c | d

table f
  |  a  |  b  |
  | --- | --- |
  |  c  |  d  |


table g
   a  |  b
  --- | ---
   c  |  d

table h
a
|-|
b

table i
| a
|-
b

table j
| a
-
b

<p>table a</p>
<table><thead><tr><th>a</th><th>b</th></tr></thead><tbody>
<tr><td>c</td><td>d</td></tr>
</tbody></table>
<p>table b
|  a  |  b  |
| --- | --- |
|  c  |  d  |</p>
<p>table c
a  |  b
--- | ---
c  |  d</p>
<p>table d
a | b
--|--
c | d</p>
<p>table e
a | b
--|--
c | d</p>
<p>table f</p>
<table><thead><tr><th>a</th><th>b</th></tr></thead><tbody>
<tr><td>c</td><td>d</td></tr>
</tbody></table>
<p>table g
a  |  b
--- | ---
c  |  d</p>
<p>table h
a
|-|
b</p>
<p>table i</p>
<table><thead><tr><th>a</th></tr></thead><tbody>
<tr><td>b</td></tr>
</tbody></table>
<h2>table j
| a</h2>
<p>b</p>

Test case based on https://github.com/github/cmark-gfm/issues/333. Normally, we'd follow GFM's lead, but this one seems like a bug, and Pandoc, the other big, authoritative Extended Markdown parser, treats it as a table.

Like the example above showing interruption, this ambiguity is tested:

Implementation ->	parsed as
pulldown-cmark	table
github/cmark-gfm	list
pycmarkgfm	list
markdown-it	table
php-markdown-extra	table
DFM	table
league/commonmark GFM	list
s9e/TextFormatter	table
markdig	table
MD4C	list
parsedown	table
maruku	table
cebe	table
pandoc	table
multimarkdown	table

a | b
- | -
1 | 2

<table><thead><tr><th>a</th><th>b</th></tr></thead><tbody>
<tr><td>1</td><td>2</td></tr>
</tbody></table>

Hard line breaks are not allowed at the end of table header lines. It's about a 50:50 split on babelmark, but this change makes pulldown-cmark consistent with GFM and Pandoc.

Implementation ->	parsed as
pulldown-cmark	paragraph
github/cmark-gfm	paragraph
pycmarkgfm	paragraph
markdown-it	table
php-markdown-extra	table
DFM	table
league/commonmark GFM	paragraph
s9e/TextFormatter	table
markdig	paragraph
MD4C	paragraph
parsedown	table
maruku	table
cebe	paragraph
pandoc	paragraph
multimarkdown	table

a | b\
- | -
1 | 2

<p>a | b\</p>
<ul>
<li>| -
1 | 2</li>
</ul>

As a block structure, table parsing has a higher priority than backslashes. This is consistent with how other paragraph-interrupting structures work, and is what GitHub does.

a\
| b | c |
|---|---|
| d | e |

<p>a\</p>
<table><thead><tr><th>b</th><th>c</th></tr></thead><tbody>
<tr><td>d</td><td>e</td></tr>
</tbody></table>

As a special case, pipes in inline code in tables are escaped with backslashes.

The parsing rule for CommonMark is that block structures are parsed before inline structures are. Normally, this means backslashes aren't allowed to have any effect on block structures at all. Tables do consider backslashes, but not the same way inline syntax does: they have higher precedence than anything else, as-if they were parsed in a completely separate pass.

This rule should be identical to GitHub's. See https://gist.github.com/notriddle/c027512ee849f12098fec3a3256c89d3 for what they do. This is totally different from Pandoc's commonmark_x, for example, and from many other Markdown parsers, because of some weird corner cases it creates.

| Description | Test case |
|-------------|-----------|
| Single      | `\`       |
| Double      | `\\`      |
| Basic test  | `\|`      |
| Basic test 2| `\|\|\`   |
| Basic test 3| `x\|y\|z\`|
| Not pipe    | `\.`      |
| Combo       | `\.\|\`   |
| Extra       | `\\\.`    |
| Wait, what? | `\\|`     |
| Wait, what? | `\\\|`    |
| Wait, what? | `\\\\|`   |
| Wait, what? | `\\\\\|`  |
| Wait, what? |          \|
| Wait, what? |         \\|
| Wait, what? |        \\\|
| Wait, what?x|          \|x
| Wait, what?x|         \\|x
| Wait, what?x|        \\\|x
| Direct trail|         \.|x

<table><thead><tr><th>Description</th><th>Test case</th></tr></thead><tbody>
<tr><td>Single</td><td><code>\</code></td></tr>
<tr><td>Double</td><td><code>\\</code></td></tr>
<tr><td>Basic test</td><td><code>|</code></td></tr>
<tr><td>Basic test 2</td><td><code>||\</code></td></tr>
<tr><td>Basic test 3</td><td><code>x|y|z\</code></td></tr>
<tr><td>Not pipe</td><td><code>\.</code></td></tr>
<tr><td>Combo</td><td><code>\.|\</code></td></tr>
<tr><td>Extra</td><td><code>\\\.</code></td></tr>
<tr><td>Wait, what?</td><td><code>\|</code></td></tr>
<tr><td>Wait, what?</td><td><code>\\|</code></td></tr>
<tr><td>Wait, what?</td><td><code>\\\|</code></td></tr>
<tr><td>Wait, what?</td><td><code>\\\\|</code></td></tr>
<tr><td>Wait, what?</td><td>|</td></tr>
<tr><td>Wait, what?</td><td>|</td></tr>
<tr><td>Wait, what?</td><td>\|</td></tr>
<tr><td>Wait, what?x</td><td>|x</td></tr>
<tr><td>Wait, what?x</td><td>|x</td></tr>
<tr><td>Wait, what?x</td><td>\|x</td></tr>
<tr><td>Direct trail</td><td>.</td></tr>
</tbody></table>

Example with code blocks in the table's header.

| Single | `\|` |
|--|--|
| Single | `\|` |


| Double | `\\|` |
|--|--|
| Double | `\\|` |


| Double Twice | `\\|\\|` |
|--|--|
| Double Twice | `\\|\\|` |


| Triple | `\\\|` |
|--|--|
| Triple | `\\\|` |

<table><thead><tr><th>Single</th><th><code>|</code></th></tr></thead><tbody>
<tr><td>Single</td><td><code>|</code></td></tr>
</tbody></table>
<table><thead><tr><th>Double</th><th><code>\|</code></th></tr></thead><tbody>
<tr><td>Double</td><td><code>\|</code></td></tr>
</tbody></table>
<table><thead><tr><th>Double Twice</th><th><code>\|\|</code></th></tr></thead><tbody>
<tr><td>Double Twice</td><td><code>\|\|</code></td></tr>
</tbody></table>
<table><thead><tr><th>Triple</th><th><code>\\|</code></th></tr></thead><tbody>
<tr><td>Triple</td><td><code>\\|</code></td></tr>
</tbody></table>

Table rows must have at least one cell. A single pipe, on its own, doesn't count.

The behavior on babelmark is pretty diverse, but this behavior is chosen to align with GFM.

✓ means it treats the zero cell lines with only a pipe as not being valid rows, but otherwise parses as a table.

x means it treats the zero cell lines with only a pipe as a valid row.

? means it does something else.

Implementation ->	first	second	third
pulldown-cmark	✓	✓	✓
github/cmark-gfm	✓	✓	✓
pycmarkgfm	✓	✓	✓
pandoc gfm³	✓	✓	✓
parsedown	x	x	✓
php-markdown-extra	x	x	✓
markdown-it	x	x	✓⁴
DFM	x	x	x
league/commonmark GFM	x	x	x
s9e/TextFormatter	x	x	x
MD4C	x	x	x
cebe	x	x	x
pandoc	x	x	x
multimarkdown	?¹	?¹	?¹
markdig	?¹	?¹	?¹
maruku	?²	?²	?²

babelmark test cases

Multimarkdown and Markdig don't recognize these at tables at all, even though I know they support pipe tables.

Maruku swallows much of the text entirely in these test cases.

⁴

Markdown-it changed some time between the version on their playground https://markdown-it.github.io/#md3=%7B%22source%22%3A%22%7C%20Table%20%7C%20Header%20%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%20Table%20%7C%20Body%20%20%20%7C%5Cn%7C%5Cn%7C%20Not%20%20%20%7C%20Enough%20%7C%5Cn%5Cn%7C%20Table%20%7C%20Header%20%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%5Cn%5Cn%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%20Table%20%7C%20Body%20%20%20%7C%5Cn%22%2C%22defaults%22%3A%7B%22html%22%3Afalse%2C%22xhtmlOut%22%3Afalse%2C%22breaks%22%3Afalse%2C%22langPrefix%22%3A%22language-%22%2C%22linkify%22%3Atrue%2C%22typographer%22%3Atrue%2C%22_highlight%22%3Atrue%2C%22_strict%22%3Afalse%2C%22_view%22%3A%22html%22%7D%7D and the version Babelmark 3 uses.

Not on babelmark 3. I ran it on https://pandoc.org/try/?params=%7B%22text%22%3A%22%7C+Table+%7C+Header+%7C%5Cn%7C-------%7C--------%7C%5Cn%7C+Table+%7C+Body+++%7C%5Cn%7C%5Cn%7C+Not+++%7C+Enough+%7C%5Cn%5Cn%7C+Table+%7C+Header+%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%5Cn%5Cn%7C%5Cn%7C-------%7C--------%7C%5Cn%7C+Table+%7C+Body+++%7C%5Cn%22%2C%22to%22%3A%22html5%22%2C%22from%22%3A%22gfm%22%2C%22standalone%22%3Afalse%2C%22embed-resources%22%3Afalse%2C%22table-of-contents%22%3Afalse%2C%22number-sections%22%3Afalse%2C%22citeproc%22%3Afalse%2C%22html-math-method%22%3A%22plain%22%2C%22wrap%22%3A%22auto%22%2C%22highlight-style%22%3Anull%2C%22files%22%3A%7B%7D%2C%22template%22%3Anull%7D

| Table | Header |
|-------|--------|
| Table | Body   |
|
| Not   | Enough |


| Table | Header |
|-------|--------|
| Table | Body   |
|→
| Not   | Enough |

<table><thead><tr><th>Table</th><th>Header</th></tr></thead><tbody>
<tr><td>Table</td><td>Body</td></tr>
</tbody></table>
<p>|
| Not   | Enough |</p>
<table><thead><tr><th>Table</th><th>Header</th></tr></thead><tbody>
<tr><td>Table</td><td>Body</td></tr>
</tbody></table>
<p>|
| Not   | Enough |</p>

| Table | Header |
|-------|--------|
|

<table><thead><tr><th>Table</th><th>Header</th></tr></thead><tbody>
</tbody></table>
<p>|</p>

|
|-------|--------|
| Table | Body   |

<p>|
|-------|--------|
| Table | Body   |</p>

Double escaping in link URLs

https://markdown-it.github.io/#md3=%7B%22source%22%3A%22%7C%20Single%20%7C%20%5Btest%5D%28first%5C%5C%7Csecond%29%20%7C%5Cn%7C--%7C--%7C%5Cn%5Cn%7C%20Double%20%7C%20%5Btest%5D%28first%5C%5C%5C%5C%7Csecond%29%20%7C%5Cn%7C--%7C--%7C%5Cn%5Cn%7C%20Triple%20%7C%20%5Btest%5D%28first%5C%5C%5C%5C%5C%5C%7Csecond%29%20%7C%5Cn%7C--%7C--%7C%5Cn%22%2C%22defaults%22%3A%7B%22html%22%3Afalse%2C%22xhtmlOut%22%3Afalse%2C%22breaks%22%3Afalse%2C%22langPrefix%22%3A%22language-%22%2C%22linkify%22%3Atrue%2C%22typographer%22%3Atrue%2C%22_highlight%22%3Atrue%2C%22_strict%22%3Afalse%2C%22_view%22%3A%22src%22%7D%7D

https://gist.github.com/notriddle/a789116ccec90d261242f3e5fd3bf0ee

| Single | [test](first\|second) |
|--|--|

| Double | [test](first\\|second) |
|--|--|

| Triple | [test](first\\\|second) |
|--|--|

<table><thead><tr><th>Single</th><th><a href="first%7Csecond">test</a></th></tr></thead><tbody>
</tbody></table>
<table><thead><tr><th>Double</th><th><a href="first%7Csecond">test</a></th></tr></thead><tbody>
</tbody></table>
<table><thead><tr><th>Triple</th><th><a href="first%5C%7Csecond">test</a></th></tr></thead><tbody>
</tbody></table>

Double escaping in link references

https://gist.github.com/notriddle/02d6dfe2559a1a05fa9308e0bf4a9c39

https://markdown-it.github.io/#md3=%7B%22source%22%3A%22%7C%20Single%20%7C%20%5Bfirst%5C%5C%7Csecond%5D%20%7C%5Cn%7C--%7C--%7C%5Cn%5Cn%7C%20Double%20%7C%20%5Bfirst%5C%5C%5C%5C%7Csecond%5D%20%7C%5Cn%7C--%7C--%7C%5Cn%5Cn%7C%20Triple%20%7C%20%5Bfirst%5C%5C%5C%5C%5C%5C%7Csecond%5D%20%7C%5Cn%7C--%7C--%7C%5Cn%5Cn%5Bfirst%5C%5C%7Csecond%5D%3A%20https%3A%2F%2Frust-lang.org%5Cn%5Cn%5Bfirst%5C%5C%5C%5C%7Csecond%5D%3A%20https%3A%2F%2Fdocs.rs%22%2C%22defaults%22%3A%7B%22html%22%3Afalse%2C%22xhtmlOut%22%3Afalse%2C%22breaks%22%3Afalse%2C%22langPrefix%22%3A%22language-%22%2C%22linkify%22%3Atrue%2C%22typographer%22%3Atrue%2C%22_highlight%22%3Atrue%2C%22_strict%22%3Afalse%2C%22_view%22%3A%22html%22%7D%7D

| Single | [first\|second] |
|--|--|

| Double | [first\\|second] |
|--|--|

| Triple | [first\\\|second] |
|--|--|

[first\|second]: https://rust-lang.org

[first\\|second]: https://docs.rs

<table><thead><tr><th>Single</th><th>[first|second]</th></tr></thead><tbody>
</tbody></table>
<table><thead><tr><th>Double</th><th><a href="https://rust-lang.org">first|second</a></th></tr></thead><tbody>
</tbody></table>
<table><thead><tr><th>Triple</th><th><a href="https://docs.rs">first\|second</a></th></tr></thead><tbody>
</tbody></table>

Double escaping in table interruption

Q: Knock knock.
A: Who's there.
Q: Interrupting cow.
A: Interrupting —?
| `Moo\\|ooo` |
|-------------|
| `ooo\\|ooo` |

<p>Q: Knock knock.
A: Who's there.
Q: Interrupting cow.
A: Interrupting —?</p>
<table><thead><tr><th><code>Moo\|ooo</code></th></tr></thead><tbody>
<tr><td><code>ooo\|ooo</code></td></tr>
</tbody></table>

Double escaping in image alt text

| ![Moo\\|Moo](image.png) |
|-------------|
| ![Moo\\\|Moo](image.png) |

<table><thead><tr><th><img src="image.png" alt="Moo|Moo" /></th></tr></thead><tbody>
<tr><td><img src="image.png" alt="Moo\|Moo" /></td></tr>
</tbody></table>

Double escaping in link title

| [Moo](https://example.org "Example\\|Link") |
|---------------------------------------------|
| [Moo](https://example.org "Example\\\|Link") |

<table><thead><tr><th><a href="https://example.org" title="Example|Link">Moo</a></th></tr></thead><tbody>
<tr><td><a href="https://example.org" title="Example\|Link">Moo</a></td></tr>
</tbody></table>

Blank list items and lists that don't start at one can interrupt tables.

moo | moo
----|----
moo | moo
*

<table><thead><tr><th>moo</th><th>moo</th></tr></thead><tbody>
<tr><td>moo</td><td>moo</td></tr>
</tbody></table>
<ul>
<li></li>
</ul>

moo | moo
----|----
moo | moo
2.

<table><thead><tr><th>moo</th><th>moo</th></tr></thead><tbody>
<tr><td>moo</td><td>moo</td></tr>
</tbody></table>
<ol start="2">
<li></li>
</ol>

// This is a slightly modified (replaced example disabled by example) of the // table section from GitHub's spec.txt

Task list items (extension)

GFM enables the tasklist extension, where an additional processing step is performed on [list items].

A task list item is a [list item][list items] where the first block in it is a paragraph which begins with a [task list item marker] and at least one whitespace character before any other content.

A task list item marker consists of an optional number of spaces, a left bracket ([), either a whitespace character or the letter x in either lowercase or uppercase, and then a right bracket (]).

When rendered, the [task list item marker] is replaced with a semantic checkbox element; in an HTML output, this would be an <input type="checkbox"> element.

If the character between the brackets is a whitespace character, the checkbox is unchecked. Otherwise, the checkbox is checked.

This spec does not define how the checkbox elements are interacted with: in practice, implementors are free to render the checkboxes as disabled or inmutable elements, or they may dynamically handle dynamic interactions (i.e. checking, unchecking) in the final rendered document.

- [ ] foo
- [x] bar

<ul>
<li><input disabled="" type="checkbox"/>
foo</li>
<li><input disabled="" type="checkbox" checked=""/>
bar</li>
</ul>

Task lists can be arbitrarily nested:

- [x] foo
  - [ ] bar
  - [x] baz
- [ ] bim

<ul>
<li><input disabled="" type="checkbox" checked=""/>
foo
<ul>
<li><input disabled="" type="checkbox"/>
bar</li>
<li><input disabled="" type="checkbox" checked=""/>
baz</li>
</ul>
</li>
<li><input disabled="" type="checkbox"/>
bim</li>
</ul>

Run this with cargo test --features gen-tests suite::footnotes.

Parts of this test case are based on https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702. Other parts were based on trial-and-error with GitHub's issue comment preview.

This spec describes nearly identical parsing to GFM, but not identical rendering. The HTML outputting module may be extended with features such as backreferences, but these are not built into the parser, and are unrelated to the syntax in any case. Alternative footnote rendering styles can be found in /examples/footnote-rewrite.rs.

Lorem ipsum.[^a] [^missing]

[^a]: Cool.

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup> [^missing]</p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Cool.</p>
</div>

Footnotes can be used inside blockquotes:

> This is the song that never ends.\
> Yes it goes on and on my friends.[^lambchops]
>
> [^lambchops]: <https://www.youtube.com/watch?v=0U2zJOryHKQ>

<blockquote>
<p>This is the song that never ends.<br />
Yes it goes on and on my friends.<sup class="footnote-reference"><a href="#lambchops">1</a></sup></p>
<div class="footnote-definition" id="lambchops"><sup class="footnote-definition-label">1</sup>
<p><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">https://www.youtube.com/watch?v=0U2zJOryHKQ</a></p>
</div>
</blockquote>

Footnotes can be complex block structures, but their contents must be indented first!

Without four spaces of indentation, no nesting:

Songs that simply loop are a popular way to annoy people. [^examples]

[^examples]:
 * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ)
 * [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls)
 * [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ)

<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples">1</a></sup></p>
<div class="footnote-definition" id="examples"><sup class="footnote-definition-label">1</sup></div>
<ul>
<li><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a></li>
<li><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a></li>
<li><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a></li>
</ul>

Yes, nesting:

Songs that simply loop are a popular way to annoy people. [^examples]

[^examples]:
    * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ)
    * [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls)
    * [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ)

<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples">1</a></sup></p>
<div class="footnote-definition" id="examples"><sup class="footnote-definition-label">1</sup>
<ul>
<li><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a></li>
<li><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a></li>
<li><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a></li>
</ul>
</div>

How this works with indented code blocks, blockquotes:

[^not-code] [^code] [^quote] [^not-quote] [^indented-quote]

[^not-code]:         not code

[^code]:
        code

[^quote]: > quote

[^not-quote]:
 > external quote

[^indented-quote]:
    > indented quote

<p><sup class="footnote-reference"><a href="#not-code">1</a></sup> <sup class="footnote-reference"><a href="#code">2</a></sup> <sup class="footnote-reference"><a href="#quote">3</a></sup> <sup class="footnote-reference"><a href="#not-quote">4</a></sup> <sup class="footnote-reference"><a href="#indented-quote">5</a></sup></p>
<div class="footnote-definition" id="not-code"><sup class="footnote-definition-label">1</sup>
<p>not code</p>
</div>
<div class="footnote-definition" id="code"><sup class="footnote-definition-label">2</sup>
<pre><code>code
</code></pre>
</div>
<div class="footnote-definition" id="quote"><sup class="footnote-definition-label">3</sup>
<blockquote><p>quote</p></blockquote>
</div>
<div class="footnote-definition" id="not-quote"><sup class="footnote-definition-label">4</sup></div>
<blockquote><p>external quote</p></blockquote><div class="footnote-definition" id="indented-quote"><sup class="footnote-definition-label">5</sup>
<blockquote><p>indented quote</p></blockquote>
</div>

Line breaks in paragraphs, however, are okay. Note that GitHub comments have hard line breaks without escapes, while Gists don't. To see compatible rendering with the examples in this file, try the code in a GitHub Gist file with a .md extension.

[^ab] [^cd]

[^ab]: a
b

[^cd]: c\
d

<p><sup class="footnote-reference"><a href="#ab">1</a></sup> <sup class="footnote-reference"><a href="#cd">2</a></sup></p>
<div class="footnote-definition" id="ab"><sup class="footnote-definition-label">1</sup>
<p>a
b</p>
</div>
<div class="footnote-definition" id="cd"><sup class="footnote-definition-label">2</sup>
<p>c<br>
d</p>
</div>

Footnotes can even have multiple paragraphs. You need to indent, though.

[^lorem]: If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.

I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.

[^ipsum]: If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.

    I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.

<div class="footnote-definition" id="lorem"><sup class="footnote-definition-label">1</sup>
<p>If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.</p>
</div>
<p>I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.</p>
<div class="footnote-definition" id="ipsum"><sup class="footnote-definition-label">2</sup>
<p>If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.</p>
<p>I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.</p>
</div>

A footnote will end on a single line break if the following paragraph isn't indented. Note that this behavior changed in version 0.1.0, to become more like hoedown. See issue #21.

[^ipsum]: How much wood would a woodchuck chuck.

If a woodchuck could chuck wood.


# Forms of entertainment that aren't childish

<div class="footnote-definition" id="ipsum"><sup class="footnote-definition-label">1</sup>
<p>How much wood would a woodchuck chuck.</p>
</div>
<p>If a woodchuck could chuck wood.</p>
<h1>Forms of entertainment that aren't childish</h1>

If the following footnote is indented, however, then it'll be included, regardless of how many blank lines separate it from the footnote definition. See https://gist.github.com/notriddle/39ffd52799604bc2cf923fe21a1ea0a9.

Footnotes [^one] [^many].

[^one]:





    first paragraph inside footnote

[^many]: first paragraph inside footnote





    second paragraph still inside footnote

<p>Footnotes <sup class="footnote-reference"><a href="#one">1</a></sup> <sup class="footnote-reference"><a href="#many">2</a></sup>.</p>
<div class="footnote-definition" id="one"><sup class="footnote-definition-label">1</sup>
<p>first paragraph inside footnote</p>
</div>
<div class="footnote-definition" id="many"><sup class="footnote-definition-label">2</sup>
<p>first paragraph inside footnote</p>
<p>second paragraph still inside footnote</p>
</div>

A footnote will also break if it's inside another container.

> He's also really stupid. [^why]
>
> [^why]: Because your mamma!

As such, we can guarantee that the non-childish forms of entertainment are probably more entertaining to adults, since, having had a whole childhood doing the childish ones, the non-childish ones are merely the ones that haven't gotten boring yet.

<blockquote>
<p>He's also really stupid. <sup class="footnote-reference"><a href="#why">1</a></sup></p>
<div class="footnote-definition" id="why"><sup class="footnote-definition-label">1</sup>
<p>Because your mamma!</p>
</div>
</blockquote>
<p>As such, we can guarantee that the non-childish forms of entertainment are probably more entertaining to adults, since, having had a whole childhood doing the childish ones, the non-childish ones are merely the ones that haven't gotten boring yet.</p>

As a special exception, footnotes cannot be nested directly inside each other. If they are, they act like they're declared beside each other.

Nested footnotes are considered poor style. [^a] [^xkcd] [^indent1] [^indent2]

[^a]: This does not mean that footnotes cannot reference each other. [^b]

[^b]: This means that a footnote definition cannot be directly inside another footnote definition.
> This means that a footnote cannot be directly inside another footnote's body. [^e]
>
> [^e]: They can, however, be inside anything else.

[^xkcd]: [The other kind of nested footnote is, however, considered poor style.](https://xkcd.com/1208/)

[^indent1]: indent1

    [^indent2]: indent2

<p>Nested footnotes are considered poor style. <sup class="footnote-reference"><a href="#a">1</a></sup> <sup class="footnote-reference"><a href="#xkcd">2</a></sup> <sup class="footnote-reference"><a href="#indent1">3</a></sup> <sup class="footnote-reference"><a href="#indent2">4</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>This does not mean that footnotes cannot reference each other. <sup class="footnote-reference"><a href="#b">5</a></sup></p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">5</sup>
<p>This means that a footnote definition cannot be directly inside another footnote definition.</p>
</div>
<blockquote>
<p>This means that a footnote cannot be directly inside another footnote's body. <sup class="footnote-reference"><a href="#e">6</a></sup></p>
<div class="footnote-definition" id="e"><sup class="footnote-definition-label">6</sup>
<p>They can, however, be inside anything else.</p>
</div>
</blockquote>
<div class="footnote-definition" id="xkcd"><sup class="footnote-definition-label">2</sup>
<p><a href="https://xkcd.com/1208/">The other kind of nested footnote is, however, considered poor style.</a></p>
</div>
<div class="footnote-definition" id="indent1"><sup class="footnote-definition-label">3</sup>
<p>indent1</p>
</div>
<div class="footnote-definition" id="indent2"><sup class="footnote-definition-label">4</sup>
<p>indent2</p>
</div>

[^foo] [^bar]

[^foo]: [^bar]: 1

<p><sup class="footnote-reference"><a href="#foo">1</a></sup> <sup class="footnote-reference"><a href="#bar">2</a></sup></p>
<div class="footnote-definition" id="foo"><sup class="footnote-definition-label">1</sup></div>
<div class="footnote-definition" id="bar"><sup class="footnote-definition-label">2</sup>
<p>1</p>
</div>

They do not need one line between each other.

[^Doh] Ray Me Fa So La Te Do! [^1]

[^Doh]: I know. Wrong Doe. And it won't render right.
[^1]: Common for people practicing music.

<p><sup class="footnote-reference"><a href="#Doh">1</a></sup> Ray Me Fa So La Te Do! <sup class="footnote-reference"><a href="#1">2</a></sup></p>
<div class="footnote-definition" id="Doh"><sup class="footnote-definition-label">1</sup>
<p>I know. Wrong Doe. And it won't render right.</p>
</div>
<div class="footnote-definition" id="1"><sup class="footnote-definition-label">2</sup>
<p>Common for people practicing music.</p>
</div>

ISSUE 413

Lorem ipsum.[^a]

An unordered list before the footnotes:
* Ipsum
* Lorem

[^a]: Cool.

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup></p>
<p>An unordered list before the footnotes:</p>
<ul>
<li>Ipsum</li>
<li>Lorem</li>
</ul>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Cool.</p>
</div>

Test case for more footnotes with list. Note that, while the DOM produced here is similar to GitHub's DOM (except GitHub reorders all footnotes to the end), they have a weird flexbox setup that causes lists nested inside footnotes to look really weird.

https://gist.github.com/notriddle/07360ab169c650b72ab8921089b12456

Songs that simply loop are a popular way to annoy people. [^examples]

[^examples]: * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ)
* [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls)
* [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ)


Songs that simply loop are a popular way to annoy people. [^examples2]

[^examples2]: * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ) 2
    * [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls) 2
    - [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ) 2


Songs that simply loop are a popular way to annoy people. [^examples3]

[^examples3]: * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ) 3

    * [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls) 3

    * [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ) 3

<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples">1</a></sup></p>
<div class="footnote-definition" id="examples"><sup class="footnote-definition-label">1</sup>
<ul>
<li><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a></li>
</ul>
</div>
<ul>
<li><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a></li>
<li><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a></li>
</ul>
<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples2">2</a></sup></p>
<div class="footnote-definition" id="examples2"><sup class="footnote-definition-label">2</sup>
<ul>
<li><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a> 2</li>
<li><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a> 2</li>
</ul>
<ul>
<li><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a> 2</li>
</ul>
</div>
<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples3">3</a></sup></p>
<div class="footnote-definition" id="examples3"><sup class="footnote-definition-label">3</sup>
<ul>
<li><p><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a> 3</p>
</li><li><p><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a> 3</p>
</li><li><p><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a> 3</p></li>
</ul>
</div>

Test case for the relationship between link references and footnotes. https://gist.github.com/notriddle/5625a3c1cb70b3067a01d7465f9d10f1

GitHub writes My [cmark-gfm]<sup class="footnote-reference"><a href="#examples">1</a></sup>. in the first paragraph. This seems like a bug, and it's easier to fix it than to remain compatible with it https://github.com/pulldown-cmark/pulldown-cmark/pull/929.

My [cmark-gfm][^c].

My [cmark-gfm][cmark-gfm][^c].

My [cmark-gfm][][^c].

My [cmark-gfm] [^c].

My [cmark-gfm[^c]].

[cmark-gfm]: https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702

[^c]: cmark-gfm is under the MIT license, so incorporating parts of its
    test suite into pulldown-cmark should be fine.


My [otherlink[^c]].

[otherlink[^c]]: https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702

<p>My <a href="https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702">cmark-gfm</a><sup class="footnote-reference"><a href="#c">1</a></sup>.</p>
<p>My <a href="https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702">cmark-gfm</a><sup class="footnote-reference"><a href="#c">1</a></sup>.</p>
<p>My <a href="https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702">cmark-gfm</a><sup class="footnote-reference"><a href="#c">1</a></sup>.</p>
<p>My <a href="https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702">cmark-gfm</a> <sup class="footnote-reference"><a href="#c">1</a></sup>.</p>
<p>My [cmark-gfm<sup class="footnote-reference"><a href="#c">1</a></sup>].</p>
<div class="footnote-definition" id="c"><sup class="footnote-definition-label">1</sup>
<p>cmark-gfm is under the MIT license, so incorporating parts of its
test suite into pulldown-cmark should be fine.</p>
</div>
<p>My [otherlink<sup class="footnote-reference"><a href="#c">1</a></sup>].</p>
<p>[otherlink<sup class="footnote-reference"><a href="#c">1</a></sup>]: https://github.com/github/cmark-gfm/blob/1e230827a584ebc9938c3eadc5059c55ef3c9abf/test/extensions.txt#L702</p>

If a footnote definition is followed by text indented four spaces or one tab, that text will be treated as part of the footnote instead of being an indented code block. To preserve the current behavior, writing code that will be interpreted the same way under both editions, separate the code block from the footnote using an un-indented HTML comment:

[^1]: footnote definition text

<!-- -->

    // indented code block
    fn main() {
        println!("hello world!");
    }

<div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup>
<p>footnote definition text</p>
</div>
<!-- -->
<pre><code>// indented code block
fn main() {
    println!("hello world!");
}
</code></pre>

Footnote definitions no longer need to be separated by blank lines. To preserve the current behavior, if you intentionally want to write a footnote reference followed by a colon at the start of a line, use a backslash escape:

[^1]: footnote definition text
[^1]\: this is a reference, rather than a definition

<div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup>
<p>footnote definition text
<sup class="footnote-reference"><a href="#1">1</a></sup>: this is a reference, rather than a definition</p>
</div>

If a footnote definition is immediately followed by a list, block quote, or table, it needs to be indented by four spaces to be considered part of the footnote. To preserve the current behavior, writing code that will be interpreted the same way in either edition, you'll need to fall back to HTML syntax, since there's no easy way to write code that the new syntax will accept as part of the footnote without the old syntax considering it a code block.

When migrating to the new Edition, a table within a footnote can be written like this (under the old Edition, the table is treated as source code):

[^1]:

    | column1 | column2 |
    |---------|---------|
    | row1a   | row1b   |
    | row2a   | row2b   |

<div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup><table>
<thead>
<tr><th>column1</th><th>column2</th></tr>
</thead>
<tbody>
<tr><td>row1a</td><td>row1b</td></tr>
<tr><td>row2a</td><td>row2b</td></tr>
</tbody>
</table>
</div>

While Markdown Guide claims that you can't put footnote definitions inside lists, block quotes, or tables. When I try it in GitHub, footnote definitions are parsed when nested within lists and blockquotes, but not tables. A footnote definition will interrupt a table if it's at the start of a line, or just not be parsed if the table has a pipe at the start of the line.

* First
  [^1]: test
* Second [^1] test


> first
> [^2]: test
> Second [^2] test


   First   | Second
-----------|----------
first      | second
[^3]: test | test [^3]


|    First   | Second    |
|------------|-----------|
| first      | second    |
| [^4]: test | test [^4] |

> [^5]: * test [^5]

<ul>
<li>First
<div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup>
<p>test</p>
</div>
</li>
<li>Second <sup class="footnote-reference"><a href="#1">1</a></sup> test</li>
</ul>
<blockquote>
<p>first</p>
<div class="footnote-definition" id="2"><sup class="footnote-definition-label">2</sup>
<p>test
Second <sup class="footnote-reference"><a href="#2">2</a></sup> test</p>
</div>
</blockquote>
<table><thead><tr><th>First</th><th>Second</th></tr></thead><tbody>
<tr><td>first</td><td>second</td></tr>
</tbody></table>
<div class="footnote-definition" id="3"><sup class="footnote-definition-label">3</sup>
<p>test | test <sup class="footnote-reference"><a href="#3">3</a></sup></p>
</div>
<table><thead><tr><th>First</th><th>Second</th></tr></thead><tbody>
<tr><td>first</td><td>second</td></tr>
<tr><td>[^4]: test</td><td>test [^4]</td></tr>
</tbody></table>
<blockquote>
<div class="footnote-definition" id="5"><sup class="footnote-definition-label">4</sup>
<ul>
<li>test <sup class="footnote-reference"><a href="#5">4</a></sup></li>
</ul>
</div>
</blockquote>

Footnote labels cannot be empty.

Test [^] link

[^]: https://rust-lang.org

<p>Test <a href="https://rust-lang.org">^</a> link</p>

Footnote labels cannot contain line breaks, even where link labels can.

[^foo\
bar]: not a footnote definition

[baz\
quux]: https://rust-lang.org

[first
second]: https://rust-lang.org

[^third
fourth]: not a footnote definition

[baz\
quux]
[^foo\
bar]
[first
second]
[^third
fourth]

<p>[^foo<br>
bar]: not a footnote definition</p>
<p>[^third
fourth]: not a footnote definition</p>
<p><a href="https://rust-lang.org">baz<br>
quux</a>
[^foo<br>
bar]
<a href="https://rust-lang.org">first
second</a>
[^third
fourth]</p>

The above rules imply a few cases where something starts with ^, but actually parses as a link definition.

[^foo
]: https://rust-lang.org

[^foo
]

<p><a href="https://rust-lang.org">^foo
</a></p>

Footnote definitions can be indented up to three spaces. At four spaces, it becomes a code block.

footnote [^baz]
footnote [^quux]

    [^quux]: x

   [^baz]: x

<p>footnote <sup class="footnote-reference"><a href="#baz">1</a></sup>
footnote [^quux]</p>
<pre><code>[^quux]: x
</code></pre>
<div class="footnote-definition" id="baz"><sup class="footnote-definition-label">1</sup>
<p>x</p>
</div>

Multiple footnotes should be able without needing whitespace in between.

Lorem ipsum.[^a][^b]

[^a]: Foo
[^b]: Bar

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup><sup class="footnote-reference"><a href="#b">2</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Foo</p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">2</sup>
<p>Bar</p>
</div>

This does work already:

Lorem ipsum.[^a] [^b]

[^a]: Foo
[^b]: Bar

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup> <sup class="footnote-reference"><a href="#b">2</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Foo</p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">2</sup>
<p>Bar</p>
</div>

Run this with cargo test --features gen-tests suite::old_footnotes.

This is how footnotes are basically used.

Lorem ipsum.[^a]

[^a]: Cool.

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Cool.</p>
</div>

Footnotes can be used inside blockquotes:

> This is the song that never ends.\
> Yes it goes on and on my friends.[^lambchops]
>
> [^lambchops]: <https://www.youtube.com/watch?v=0U2zJOryHKQ>

<blockquote>
<p>This is the song that never ends.<br />
Yes it goes on and on my friends.<sup class="footnote-reference"><a href="#lambchops">1</a></sup></p>
<div class="footnote-definition" id="lambchops"><sup class="footnote-definition-label">1</sup>
<p><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">https://www.youtube.com/watch?v=0U2zJOryHKQ</a></p>
</div>
</blockquote>

Footnotes can be complex block structures:

Songs that simply loop are a popular way to annoy people. [^examples]

[^examples]:
 * [The song that never ends](https://www.youtube.com/watch?v=0U2zJOryHKQ)
 * [I know a song that gets on everybody's nerves](https://www.youtube.com/watch?v=TehWI09qxls)
 * [Ninety-nine bottles of beer on the wall](https://www.youtube.com/watch?v=qVjCag8XoHQ)

<p>Songs that simply loop are a popular way to annoy people. <sup class="footnote-reference"><a href="#examples">1</a></sup></p>
<div class="footnote-definition" id="examples"><sup class="footnote-definition-label">1</sup>
<ul>
<li><a href="https://www.youtube.com/watch?v=0U2zJOryHKQ">The song that never ends</a></li>
<li><a href="https://www.youtube.com/watch?v=TehWI09qxls">I know a song that gets on everybody's nerves</a></li>
<li><a href="https://www.youtube.com/watch?v=qVjCag8XoHQ">Ninety-nine bottles of beer on the wall</a></li>
</ul>
</div>

Footnotes can even have multiple paragraphs. They also don't need to be referenced to show up.

[^lorem]: If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.

I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.

<div class="footnote-definition" id="lorem"><sup class="footnote-definition-label">1</sup>
<p>If heaven ever wishes to grant me a boon, it will be a total effacing of the results of a mere chance which fixed my eye on a certain stray piece of shelf-paper. It was nothing on which I would naturally have stumbled in the course of my daily round, for it was an old number of an Australian journal, the Sydney Bulletin for April 18, 1925. It had escaped even the cutting bureau which had at the time of its issuance been avidly collecting material for my uncle's research.</p>
</div>
<p>I had largely given over my inquiries into what Professor Angell called the "Cthulhu Cult", and was visiting a learned friend in Paterson, New Jersey; the curator of a local museum and a mineralogist of note. Examining one day the reserve specimens roughly set on the storage shelves in a rear room of the museum, my eye was caught by an odd picture in one of the old papers spread beneath the stones. It was the Sydney Bulletin I have mentioned, for my friend had wide affiliations in all conceivable foreign parts; and the picture was a half-tone cut of a hideous stone image almost identical with that which Legrasse had found in the swamp.</p>

A footnote will end on a single line break. Note that this behavior changed in version 0.1.0, to become more like hoedown. See issue #21.

[^ipsum]: How much wood would a woodchuck chuck.

If a woodchuck could chuck wood.


# Forms of entertainment that aren't childish

<div class="footnote-definition" id="ipsum"><sup class="footnote-definition-label">1</sup>
<p>How much wood would a woodchuck chuck.</p>
</div>
<p>If a woodchuck could chuck wood.</p>
<h1>Forms of entertainment that aren't childish</h1>

A footnote will also break if it's inside another container.

> He's also really stupid. [^why]
>
> [^why]: Because your mamma!

As such, we can guarantee that the non-childish forms of entertainment are probably more entertaining to adults, since, having had a whole childhood doing the childish ones, the non-childish ones are merely the ones that haven't gotten boring yet.

<blockquote>
<p>He's also really stupid. <sup class="footnote-reference"><a href="#why">1</a></sup></p>
<div class="footnote-definition" id="why"><sup class="footnote-definition-label">1</sup>
<p>Because your mamma!</p>
</div>
</blockquote>
<p>As such, we can guarantee that the non-childish forms of entertainment are probably more entertaining to adults, since, having had a whole childhood doing the childish ones, the non-childish ones are merely the ones that haven't gotten boring yet.</p>

As a special exception, footnotes cannot be nested directly inside each other.

Nested footnotes are considered poor style. [^a] [^xkcd]

[^a]: This does not mean that footnotes cannot reference each other. [^b]

[^b]: This means that a footnote definition cannot be directly inside another footnote definition.
> This means that a footnote cannot be directly inside another footnote's body. [^e]
>
> [^e]: They can, however, be inside anything else.

[^xkcd]: [The other kind of nested footnote is, however, considered poor style.](https://xkcd.com/1208/)

<p>Nested footnotes are considered poor style. <sup class="footnote-reference"><a href="#a">1</a></sup> <sup class="footnote-reference"><a href="#xkcd">2</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>This does not mean that footnotes cannot reference each other. <sup class="footnote-reference"><a href="#b">3</a></sup></p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">3</sup>
<p>This means that a footnote definition cannot be directly inside another footnote definition.</p>
<blockquote>
<p>This means that a footnote cannot be directly inside another footnote's body. <sup class="footnote-reference"><a href="#e">4</a></sup></p>
<div class="footnote-definition" id="e"><sup class="footnote-definition-label">4</sup>
<p>They can, however, be inside anything else.</p>
</div>
</blockquote>
</div>
<div class="footnote-definition" id="xkcd"><sup class="footnote-definition-label">2</sup>
<p><a href="https://xkcd.com/1208/">The other kind of nested footnote is, however, considered poor style.</a></p>
</div>

They do need one line between each other.

[^Doh] Ray Me Fa So La Te Do! [^1]

[^Doh]: I know. Wrong Doe. And it will render right.
[^1]: Common for people practicing music.

<p><sup class="footnote-reference"><a href="#Doh">1</a></sup> Ray Me Fa So La Te Do! <sup class="footnote-reference"><a href="#1">2</a></sup></p>
<div class="footnote-definition" id="Doh"><sup class="footnote-definition-label">1</sup><p>I know. Wrong Doe. And it will render right.</p></div>
<div class="footnote-definition" id="1"><sup class="footnote-definition-label">2</sup><p>Common for people practicing music.</p></div>

Second check for footnotes:

[Reference to footnotes A[^1], B[^2] and C[^3].

[^1]: Footnote A.
[^2]: Footnote B.
[^3]: Footnote C.

<p>[Reference to footnotes A<sup class="footnote-reference"><a href="#1">1</a></sup>, B<sup class="footnote-reference"><a href="#2">2</a></sup> and C<sup class="footnote-reference"><a href="#3">3</a></sup>.</p><div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup><p>Footnote A.</p></div><div class="footnote-definition" id="2"><sup class="footnote-definition-label">2</sup><p>Footnote B.</p></div><div class="footnote-definition" id="3"><sup class="footnote-definition-label">3</sup><p>Footnote C.</p></div>

Multiple footnotes should be able without needing whitespace in between.

Lorem ipsum.[^a][^b]

[^a]: Foo
[^b]: Bar

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup><sup class="footnote-reference"><a href="#b">2</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Foo</p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">2</sup>
<p>Bar</p>
</div>

This does work already:

Lorem ipsum.[^a] [^b]

[^a]: Foo
[^b]: Bar

<p>Lorem ipsum.<sup class="footnote-reference"><a href="#a">1</a></sup> <sup class="footnote-reference"><a href="#b">2</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Foo</p>
</div>
<div class="footnote-definition" id="b"><sup class="footnote-definition-label">2</sup>
<p>Bar</p>
</div>

Run this with cargo test --features gen-tests suite::math.

`$`-delimited LaTeX Math in pulldown-cmark

Mathematical expressions extension. Syntax based on https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/math.md.

Inline mode mathematical expressions:

This sentence uses `$` delimiters to show math inline: $\sqrt{3x-1}+(1+x)^2$
$\sum_{k=1}^n a_k b_k$: Mathematical expression at head of line

`\` may follow just after the first `$`: $\{1, 2, 3\}$

<p>This sentence uses <code>$</code> delimiters to show math inline: <span class="math math-inline">\sqrt{3x-1}+(1+x)^2</span>
<span class="math math-inline">\sum_{k=1}^n a_k b_k</span>: Mathematical expression at head of line</p>
<p><code>\</code> may follow just after the first <code>$</code>: <span class="math math-inline">\{1, 2, 3\}</span></p>

Display mode mathematical expressions:

**The Cauchy-Schwarz Inequality**

$$\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)$$

<p><strong>The Cauchy-Schwarz Inequality</strong></p>
<p><span class="math math-display">\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)</span></p>

Inline math expressions cannot be empty, but display mode expressions can.

Oops empty $$ expression.

$$$$

<p>Oops empty $$ expression.</p>
<p><span class="math math-display"></span></p>

This is a greedy, left-to-right parser.

$x$$$$$$$y$$

$x$$$$$$y$$

$$x$$$$$$y$$

<p><span class="math math-inline">x</span><span class="math math-display"></span><span class="math math-display">y</span></p>
<p><span class="math math-inline">x</span><span class="math math-display"></span><span class="math math-inline">y</span>$</p>
<p><span class="math math-display">x</span><span class="math math-display"></span>y$$</p>

Math expressions pass their content through as-is, ignoring any other inline Markdown constructs:

$a<b>c</b>$

$${a*b*c} _c_ d$$

$not `code`$

$![not an](/image)$

$<https://not.a.link/>$

$&alpha;$

<p><span class="math math-inline">a&lt;b&gt;c&lt;/b&gt;</span></p>
<p><span class="math math-display">{a*b*c} _c_ d</span></p>
<p><span class="math math-inline">not `code`</span></p>
<p><span class="math math-inline">![not an](/image)</span></p>
<p><span class="math math-inline">&lt;https://not.a.link/&gt;</span></p>
<p><span class="math math-inline">&amp;alpha;</span></p>

Sole $ characters without a matching pair in the same block element are handled as normal text.

Hello $world.

Dollar at end of line$

<p>Hello $world.</p>
<p>Dollar at end of line$</p>

Mathematical expressions can continue across multiple lines:

$5x + 2 =
17$

$$\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right)
\left( \sum_{k=1}^n b_k^2 \right)$$

<p><span class="math math-inline">5x + 2 =
17</span></p>
<p><span class="math math-display">\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right)
\left( \sum_{k=1}^n b_k^2 \right)</span></p>

Markdown hard breaks are also not recognized inside math expressions:

$not a\
hard break  
either$

<p><span class="math math-inline">not a\
hard break  
either</span></p>

$ character can be escaped with backslash in mathematical expressions:

$\$$

$$y = \$ x$$

<p><span class="math math-inline">\$</span></p>
<p><span class="math math-display">y = \$ x</span></p>

Inline mode math expressions cannot contain unescaped $ characters. Neither can display math.

$x $ x$

$$ $ $$

<p>$x $ x$</p>
<p>$$ $ $$</p>

alpha$$beta$gamma$$delta

<p>alpha$<span class="math math-inline">beta</span>gamma$$delta</p>

Inline math expressions cannot start or end with whitespace, including newlines:

these are not math texts: $ y=x$, $y=x $, $
y=x$ and $y=x
$

>The start of a line counts as whitespace $2 +
>$

While displays can start with whitespace, {${
they should not allow inlines to do that $$2 +
$*$

<p>these are not math texts: $ y=x$, $y=x $, $
y=x$ and $y=x
$</p>
<blockquote>
<p>The start of a line counts as whitespace $2 +
$</p>
</blockquote>
<p>While displays can start with whitespace, {${
they should not allow inlines to do that $$2 +
<span class="math math-inline">*</span></p>

Inline math expressions do not need to be surrounded with whitespace:

these are math texts: foo$y=x$bar and $y=x$bar and foo$y=x$ bar

<p>these are math texts: foo<span class="math math-inline">y=x</span>bar and <span class="math math-inline">y=x</span>bar and foo<span class="math math-inline">y=x</span> bar</p>

Inline math expressions can be surrounded by punctuation:

math texts: $x=y$! and $x=y$? and $x=y$: and $x=y$. and $x=y$"

also math texts: !$x=y$! and ?$x=y$? and :$x=y$: and .$x=y$. and "$x=y$"

braces: ($x=y$) [$x=y$] {$x=y$}

<p>math texts: <span class="math math-inline">x=y</span>! and <span class="math math-inline">x=y</span>? and <span class="math math-inline">x=y</span>: and <span class="math math-inline">x=y</span>. and <span class="math math-inline">x=y</span>"</p>
<p>also math texts: !<span class="math math-inline">x=y</span>! and ?<span class="math math-inline">x=y</span>? and :<span class="math math-inline">x=y</span>: and .<span class="math math-inline">x=y</span>. and "<span class="math math-inline">x=y</span>"</p>
<p>braces: (<span class="math math-inline">x=y</span>) [<span class="math math-inline">x=y</span>] {<span class="math math-inline">x=y</span>}</p>

Math expression as only item on a line:

$x=y$

<p><span class="math math-inline">x=y</span></p>

Math expressions can be immediately followed by other math expressions:

$a$$b$

$a$$$b$$

$$a$$$b$

$$a$$$$b$$

<p><span class="math math-inline">a</span><span class="math math-inline">b</span></p>
<p><span class="math math-inline">a</span><span class="math math-display">b</span></p>
<p><span class="math math-display">a</span><span class="math math-inline">b</span></p>
<p><span class="math math-display">a</span><span class="math math-display">b</span></p>

Both inline and display mode math expressions are inline elements with the same precedence as code spans. The leftmost valid element takes priority:

$Inline `first$ then` code

`Code $first` then$ inline

$$ Display `first $$ then` code

`Code $$ first` then $$ display

<p><span class="math math-inline">Inline `first</span> then` code</p>
<p><code>Code $first</code> then$ inline</p>
<p><span class="math math-display"> Display `first </span> then` code</p>
<p><code>Code $$ first</code> then $$ display</p>

Indicators of block structure take precedence over math expressions:

$x + y - z$

$x + y
- z$

$$ x + y
> z $$

<p><span class="math math-inline">x + y - z</span></p>
<p>$x + y</p>
<ul>
<li>z$</li>
</ul>
<p>$$ x + y</p>
<blockquote>
<p>z $$</p>
</blockquote>

This also means that math expressions cannot contain empty lines, since they start a new paragraph:

$not

math$

$$
not

math
$$

<p>$not</p>
<p>math$</p>
<p>$$
not</p>
<p>math
$$</p>

It also implies that math notation has lower parsing power than block elements.

- $not
    - *
  math$

<ul>
<li>$not
<ul>
<li>
<ul>
<li>
</li>
</ul>
</li>
</ul>
math$</li>
</ul>

Note that math can contain embedded math. In scanning for a closing delimiter, we skip material in balanced curly braces:

This is display math:
$$
\text{Hello $x^2$}
$$
And this is inline math:
$\text{Hello $x$ there!}$

<p>This is display math:
<span class="math math-display">
\text{Hello $x^2$}
</span>
And this is inline math:
<span class="math math-inline">\text{Hello $x$ there!}</span></p>

Math expressions must be nested within balanced curly braces. Backslash-escaped braces do not count.

This is not valid math: $}{$

Neither is this: { $}{$ }

This is: $\}\{$

This is: $\}$

Math environment contains 2+2: $}$2+2$

Math environment contains y: $x {$ $ } $y$

<p>This is not valid math: $}{$</p>
<p>Neither is this: { $}{$ }</p>
<p>This is: <span class="math math-inline">\}\{</span></p>
<p>This is: <span class="math math-inline">\}</span></p>
<p>Math environment contains 2+2: $}<span class="math math-inline">2+2</span></p>
<p>Math environment contains y: $x {$ $ } <span class="math math-inline">y</span></p>

Math expressions must contain properly nested braces.

This is not display math. It is inline math:

$$\text{first $$ second}$

$$$\text{first $$ second}$

This is display math:

$$\text{first $$ second}$$

$$$\text{first $$ second}$$

This is also display math, but (counterintuitively) it's allowed to be empty
and expected to be as short as possible:

$$$$\text{first $$ second}$$

<p>This is not display math. It is inline math:</p>
<p>$<span class="math math-inline">\text{first $$ second}</span></p>
<p>$$<span class="math math-inline">\text{first $$ second}</span></p>
<p>This is display math:</p>
<p><span class="math math-display">\text{first $$ second}</span></p>
<p>$<span class="math math-display">\text{first $$ second}</span></p>
<p>This is also display math, but (counterintuitively) it's allowed to be empty
and expected to be as short as possible:</p>
<p><span class="math math-display"></span>\text{first $$ second}$$</p>

Dollar signs must also be backslash-escaped if they occur within math:

$\text{\$}$

$$x$x$$

${$^$$

$}$$$$

$}$] $$

<p><span class="math math-inline">\text{\$}</span></p>
<p>$<span class="math math-inline">x</span>x$$</p>
<p>${<span class="math math-inline">^</span>$</p>
<p>$}<span class="math math-display"></span></p>
<p>$}$] $$</p>

Edge case tests comparison with GitHub

Test cases https://raw.githubusercontent.com/nschloe/github-math-bugs/db938ff690ab7c534d8195fe4a1a5163c20b1134/README.md

Inline math wrapped in quotes

$x$ $`y`$

<p><span class="math math-inline">x</span> <span class="math math-inline">`y`</span></p>

Inline and display math in the same list

- $a$

  ```math
  a
  ```

  $$
  a
  $$

- ```math
  b
  ```

  $$
  b
  $$

<ul>
<li>
<p><span class="math math-inline">a</span></p>
<pre><code class="language-math">a
</code></pre>
<p><span class="math math-display">
a
</span></p>
</li>
<li>
<pre><code class="language-math">b
</code></pre>
<p><span class="math math-display">
b
</span></p>
</li>
</ul>

Images and math in the same list

- ![node logo](https://nodejs.org/static/images/logo.svg)
- $x$

<ul>
<li><img src="https://nodejs.org/static/images/logo.svg" alt="node logo" /></li>
<li><span class="math math-inline">x</span></li>
</ul>

Inline and display math in <details>

<details>

$A = 5$

$$
A = 5
$$

</details>

<details>
<p><span class="math math-inline">A = 5</span></p>
<p><span class="math math-display">
A = 5
</span></p>
</details>

< without surrounding whitespace

$a<b$

$$a<b$$

<p><span class="math math-inline">a&lt;b</span></p>
<p><span class="math math-display">a&lt;b</span></p>

Math in footnotes

[^a]

[^a]: Lorem $a$

<p><sup class="footnote-reference"><a href="#a">1</a></sup></p>
<div class="footnote-definition" id="a"><sup class="footnote-definition-label">1</sup>
<p>Lorem <span class="math math-inline">a</span></p>
</div>

Math in links

[$a$](x)

<p>
<a href="x"><span class="math math-inline">a</span></a>
</p>

Math preceded by an alphabetical character

a$x$

-$x$

1$x$

<p>a<span class="math math-inline">x</span></p>
<p>-<span class="math math-inline">x</span></p>
<p>1<span class="math math-inline">x</span></p>

Inline math at the end of italic text

_$a$ equals $b$_

_$a$ equals $b$_

**$a$ equals $b$**

<p><em><span class="math math-inline">a</span> equals <span class="math math-inline">b</span></em></p>
<p><em><span class="math math-inline">a</span> equals <span class="math math-inline">b</span></em></p>
<p><strong><span class="math math-inline">a</span> equals <span class="math math-inline">b</span></strong></p>

Dollar in \text

$$
a
$$

- $$
  \text{$b$}
  $$

<p><span class="math math-display">
a
</span>
</p><ul>
<li><span class="math math-display">
\text{$b$}
</span></li>
</ul>

Backslashes in $-math

$\{a\,b\}$

<p><span class="math math-inline">\{a\,b\}</span></p>

Math vs. HTML mix-up

$a <b > c$

$[(a+b)c](d+e)$

${a}_b c_{d}$

<p><span class="math math-inline">a &lt;b &gt; c</span></p>
<p><span class="math math-inline">[(a+b)c](d+e)</span></p>
<p><span class="math math-inline">{a}_b c_{d}</span></p>

Dollar-math with spaces

When $a \ne 0$, there are two solutions to $(ax^2 + bx + c = 0)$ and they are
$$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$

<p>When <span class="math math-inline">a \ne 0</span>, there are two solutions to <span class="math math-inline">(ax^2 + bx + c = 0)</span> and they are
<span class="math math-display"> x = {-b \pm \sqrt{b^2-4ac} \over 2a} </span></p>

Spacing around dollar sign in math mode

$x = \$$

<p><span class="math math-inline">x = \$</span></p>

Math in italic text

_Equation $\Omega(69)$ in italic text_

<p><em>Equation <span class="math math-inline">\Omega(69)</span> in italic text</em></p>

Inline math can't be preceded by brackets, quotation marks etc.

$\pi$
'$\pi$
"$\pi$
($\pi$
[$\pi$
{$\pi$
/$\pi$

<p>
<span class="math math-inline">\pi</span>
'<span class="math math-inline">\pi</span>
"<span class="math math-inline">\pi</span>
(<span class="math math-inline">\pi</span>
[<span class="math math-inline">\pi</span>
{<span class="math math-inline">\pi</span>
/<span class="math math-inline">\pi</span>
</p>

Relationship with tables

As a block element, tables parsing is stronger than math.

| first $|$ second |
|--------|---------|
| a ${   | }$ b    |

<table><thead>
<tr><th>first $</th><th>$ second</th></tr>
</thead><tbody>
<tr><td>a ${</td><td>}$ b</td></tr>
</tbody></table>

As a special case, pipes in math environments in tables are escaped with backslashes. Though backslash-escaped characters in math environments are normally passed through verbatim to the LaTeX engine, escaped pipes in tables are an exception like they are in code spans.

The behavior of the table parser should be as-if it found the bounds of the table cell in a separate pass that only looked for the strings | and \|, treating pipes as boundaries and removing the escaping backslash before passing the string to the inline parser.

| first $\|$ second |
|-------------------|
| a ${   \| }$ b    |

<table><thead>
<tr><th>first <span class="math math-inline">|</span> second</th></tr>
</thead><tbody>
<tr><td>a <span class="math math-inline">{   | }</span> b</td></tr>
</tbody></table>

| Description | Test case |
|-------------|-----------|
| Single      | $\$       |
| Double      | $\\$      |
| Basic test  | $\|$      |
| Basic test 2| $\|\|\$   |
| Basic test 3| $x\|y\|z\$|
| Not pipe    | $\.$      |
| Combo       | $\.\|$    |
| Combo 2     | $\.\|\$   |
| Extra       | $\\\.$    |
| Wait, what? | $\\|$     |
| Wait, what? | $\\\|$    |
| Wait, what? | $\\\\|$   |
| Wait, what? | $\\\\\|$  |

<table><thead><tr><th>Description</th><th>Test case</th></tr></thead><tbody>
<tr><td>Single</td><td>$$</td></tr>
<tr><td>Double</td><td><span class="math math-inline">\\</span></td></tr>
<tr><td>Basic test</td><td><span class="math math-inline">|</span></td></tr>
<tr><td>Basic test 2</td><td>$||$</td></tr>
<tr><td>Basic test 3</td><td>$x|y|z$</td></tr>
<tr><td>Not pipe</td><td><span class="math math-inline">\.</span></td></tr>
<tr><td>Combo</td><td><span class="math math-inline">\.|</span></td></tr>
<tr><td>Combo 2</td><td>$.|$</td></tr>
<tr><td>Extra</td><td><span class="math math-inline">\\\.</span></td></tr>
<tr><td>Wait, what?</td><td><span class="math math-inline">\|</span></td></tr>
<tr><td>Wait, what?</td><td><span class="math math-inline">\\|</span></td></tr>
<tr><td>Wait, what?</td><td><span class="math math-inline">\\\|</span></td></tr>
<tr><td>Wait, what?</td><td><span class="math math-inline">\\\\|</span></td></tr>
</tbody></table>

Implementation limits

Implementations may impose limits on brace nesting to avoid performance issues, but at least three levels of nesting should be supported.

Pulldown-cmark imposes the following limits:

At 25 levels of nesting, it switches from tracking nested pairs to simply counting the number of braces. This means the below example will spurriously recognize a math environment with the correct number of braces, but not nested correctly.

This is not an inline math environment: $}{$
But, because it's nested too deeply, this is parsed as an inline math environment:
{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
improperly $}{$ nested
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
But this still isn't, because the braces are still counted: $}{$

<p>This is not an inline math environment: $}{$
But, because it's nested too deeply, this is parsed as an inline math environment:
{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
improperly <span class="math math-inline">}{</span> nested
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
But this still isn't, because the braces are still counted: $}{$</p>

This is also deeply nested, but, unlike the first example,
they don't have an equal number of close braces and open braces,
so aren't detected as math.
{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
improperly $}$ nested ${$ example
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
This, however, is detected ${}$

${{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
another improperly nested example
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}$

<p>This is also deeply nested, but, unlike the first example,
they don't have an equal number of close braces and open braces,
so aren't detected as math.
{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
improperly $}$ nested ${$ example
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
This, however, is detected <span class="math math-inline">{}</span></p>
<p><span class="math math-inline">{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{
another improperly nested example
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}</span></p>

At 255 distinct brace-delimited groups, the counter rolls over. This means the below example will spurriously recognize an incorrectly-nested inline math environment.

${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  20 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  40 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  60 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  80 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 100 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 120 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 140 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 160 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 180 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 200 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 220 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 240 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{$ 255 brace pairs and one unclosed brace

<p><span class="math math-inline">{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  20 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  40 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  60 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}  80 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 100 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 120 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 140 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 160 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 180 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 200 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 220 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{} 240 brace pairs
{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{</span> 255 brace pairs and one unclosed brace</p>

Thanks to rule 1, though, deeply-nested structures won't chew through all of the ID space. This means that the below example, even though it nests 255 levels deep, parses correctly anyway.

${{{{{{{{{{{{{{{{{{{{ 20 open braces
{{{{{{{{{{{{{{{{{{{{  40 open braces
{{{{{{{{{{{{{{{{{{{{  60 open braces
{{{{{{{{{{{{{{{{{{{{  80 open braces
{{{{{{{{{{{{{{{{{{{{ 100 open braces
{{{{{{{{{{{{{{{{{{{{ 110 open braces
{{{{{{{{{{{{{{{{{{{{ 120 open braces
{{{{{{{{{{{{{{{{{{{{ 140 open braces
{{{{{{{{{{{{{{{{{{{{ 160 open braces
{{{{{{{{{{{{{{{{{{{{ 180 open braces
{{{{{{{{{{{{{{{{{{{{ 200 open braces
{{{{{{{{{{{{{{{{{{{{ 220 open braces
{{{{{{{{{{{{{{{{{{{{ 240 open braces
{{{{{{{{{{{{{{{ 255 open braces
}}}}}}}}}}}}}}}}}}}}  20 close braces
}}}}}}}}}}}}}}}}}}}}  40 close braces
}}}}}}}}}}}}}}}}}}}}  60 close braces
}}}}}}}}}}}}}}}}}}}}  80 close braces
}}}}}}}}}}}}}}}}}}}} 100 close braces
}}}}}}}}}}}}}}}}}}}} 120 close braces
}}}}}}}}}}}}}}}}}}}} 140 close braces
}}}}}}}}}}}}}}}}}}}} 160 close braces
}}}}}}}}}}}}}}}}}}}} 180 close braces
}}}}}}}}}}}}}}}}}}}} 200 close braces
}}}}}}}}}}}}}}}}}}}} 220 close braces
}}}}}}}}}}}}}}}}}}}} 240 close braces
}}}}}}}}}}}}}}}{$ 255 close braces and one open brace

<p>${{{{{{{{{{{{{{{{{{{{ 20 open braces
{{{{{{{{{{{{{{{{{{{{  40 open braces
{{{{{{{{{{{{{{{{{{{{  60 open braces
{{{{{{{{{{{{{{{{{{{{  80 open braces
{{{{{{{{{{{{{{{{{{{{ 100 open braces
{{{{{{{{{{{{{{{{{{{{ 110 open braces
{{{{{{{{{{{{{{{{{{{{ 120 open braces
{{{{{{{{{{{{{{{{{{{{ 140 open braces
{{{{{{{{{{{{{{{{{{{{ 160 open braces
{{{{{{{{{{{{{{{{{{{{ 180 open braces
{{{{{{{{{{{{{{{{{{{{ 200 open braces
{{{{{{{{{{{{{{{{{{{{ 220 open braces
{{{{{{{{{{{{{{{{{{{{ 240 open braces
{{{{{{{{{{{{{{{ 255 open braces
}}}}}}}}}}}}}}}}}}}}  20 close braces
}}}}}}}}}}}}}}}}}}}}  40 close braces
}}}}}}}}}}}}}}}}}}}}  60 close braces
}}}}}}}}}}}}}}}}}}}}  80 close braces
}}}}}}}}}}}}}}}}}}}} 100 close braces
}}}}}}}}}}}}}}}}}}}} 120 close braces
}}}}}}}}}}}}}}}}}}}} 140 close braces
}}}}}}}}}}}}}}}}}}}} 160 close braces
}}}}}}}}}}}}}}}}}}}} 180 close braces
}}}}}}}}}}}}}}}}}}}} 200 close braces
}}}}}}}}}}}}}}}}}}}} 220 close braces
}}}}}}}}}}}}}}}}}}}} 240 close braces
}}}}}}}}}}}}}}}{$ 255 close braces and one open brace</p>

Run this with cargo test --features gen-tests suite::heading_attrs.

Examples and edge cases for attribute blocks for headings.

Basic usage

Attribute blocks are attributes wrapped by curly braces ({ and }) at the end of heading content.

Strictly speaking, attribute blocks satisfies the conditions below:

Placed at the end of the heading, with optional following spaces.
Starts with { and ends with }.
Does not contain any other { and } except for opening and ending of the block.
Does not contain any <s (less-than operators), >s (greater-than operators), \s (backslashes), or newline characters.

Attributes are separated by ASCII non-newline whitespaces. An ID attribute is specified as #id-fragment-here, and a class attribute is specified as .class-here. Custom attributes has not a prefix and can optionally have a value (myattr, myattr=myvalue).

Basic usage with setext headings:

with the ID {#myh1}
===================
with a class {.myclass}
------------
with a custom attribute {myattr=myvalue}
========================================
multiple! {.myclass1 myattr #myh3 otherattr=value .myclass2}
--

<h1 id="myh1">with the ID</h1>
<h2 class="myclass">with a class</h2>
<h1 myattr="myvalue">with a custom attribute</h1>
<h2 id="myh3" class="myclass1 myclass2" myattr="" otherattr="value">multiple!</h2>

Basic usage with ATX headings:

# with the ID {#myh1}
## with a class {.myclass}
#### with a custom attribute {myattr=myvalue}
### multiple! {.myclass1 myattr #myh3 otherattr=value .myclass2}

<h1 id="myh1">with the ID</h1>
<h2 class="myclass">with a class</h2>
<h4 myattr="myvalue">with a custom attribute</h4>
<h3 id="myh3" class="myclass1 myclass2" myattr="" otherattr="value">multiple!</h3>

If an ATX heading has closing #s, the attribute block should be placed after them.

# H1 # {#id1}
## H2 ## with ## multiple ## hashes ## {#id2}
### with trailing hash # ### {#id3}

#### non-attribute-block {#id4} ####

<h1 id="id1">H1</h1>
<h2 id="id2">H2 ## with ## multiple ## hashes</h2>
<h3 id="id3">with trailing hash #</h3>
<h4>non-attribute-block {#id4}</h4>

Trailing ASCII spaces are allowed.

# spaces {#myid1}    
## tabs {#myid2}

<h1 id="myid1">spaces</h1>
<h2 id="myid2">tabs</h2>

ATX heading cannot have multiple lines.

# H1 \
nextline

<h1>H1 \</h1>
<p>nextline</p>

Of course, attribute blocks cannot be another line in ATX headings.

# H1 \
{#myid}

## H2 \
nextline {.class}

### H3 [link
](https://example.com/) {#myid3}

<h1>H1 \</h1>
<p>{#myid}</p>
<h2>H2 \</h2>
<p>nextline {.class}</p>
<h3>H3 [link</h3>
<p>](https://example.com/) {#myid3}</p>

Setext header can have multiple lines.

H1
cont
{#myid}
==

<h1 id="myid">H1
cont
</h1>

However, an attribute block itself cannot have newline characters.

H1
{
  .class1
  .class2
}
==

<h1>H1
{
.class1
.class2
}</h1>

Leading spaces

It is recommended to separate heading content and attribute blocks by spaces. Some implementations supports this style, and some others not.

FYI: Attribute blocks without leading spaces are supported by cebe (markdown-extra) 1.2.0, markdig (advanced) 0.26.0.0, maruku 0.7.3.beta1, and pandoc 2.14.2. On the other hand, it is not supported by kramdown 1.2.0 and php-markdown-extra 1.9.0, while they supports attribute blocks with leading spaces.

This style is supported since the majority of other implementations seems to support this, but be careful about incompatibility. Put spaces before attribute blocks for more compatibility.

# without space, not recommended{#id1}
## recommended style with spaces {#id2}

<h1 id="id1">without space, not recommended</h1>
<h2 id="id2">recommended style with spaces</h2>

Spaces inside braces

Braces can be optionally separated by spaces, and contiguous spaces are handled in the same way as a single whitespace.

# H1 { #id1 }
## H2 {.myclass      #id2 }
### H3 {     .myclass}

<h1 id="id1">H1</h1>
<h2 id="id2" class="myclass">H2</h2>
<h3 class="myclass">H3</h3>

Separators

Separators are mandatory. There are no automagical separation.

# H1 {#id1.class1.class2 .class3}
## H2 {.class1#id2.class2}

<h1 id="id1.class1.class2" class="class3">H1</h1>
<h2 class="class1#id2.class2">H2</h2>

Unclosed braces

If a left curly brace is present but is not closed, it is not parsed as an attribute blocks.

# H1 { #id1
## H2 {#id2

<h1>H1 { #id1</h1>
<h2>H2 {#id2</h2>

Not-opened braces are also treated as a usual text content.

# H1 #id1 }
## H2 #id2}

<h1>H1 #id1 }</h1>
<h2>H2 #id2}</h2>

Non-suffix block

Attribute blocks should be at the end of the heading.

# H1 { #id1 } foo
## H2 {#id2} <!-- hello -->

<h1>H1 { #id1 } foo</h1>
<h2>H2 {#id2} <!-- hello --></h2>

Inlines

Inlines can be used in headings.

# *H1* { #id1 }
## **H2** {#id2}
### _H3_ {#id3}
#### ~~H4~~ {#id4}
##### [text](uri) {#id5}

<h1 id="id1"><em>H1</em></h1>
<h2 id="id2"><strong>H2</strong></h2>
<h3 id="id3"><em>H3</em></h3>
<h4 id="id4"><del>H4</del></h4>
<h5 id="id5"><a href="uri">text</a></h5>

Ordering and deduplication of attributes

Attributes are handle in quite simple way.

ID

When a heading have multiple IDs, the last one wins.

# H1 {#first #second #last}

<h1 id="last">H1</h1>

Classes

There are no reordering of classes.

# H1 {.z .a .zz}

<h1 class="z a zz">H1</h1>

There are no deduplication of classes.

# H1 {.a .a .a}

<h1 class="a a a">H1</h1>

Combined

When both an ID and classes are specified, ID attribute is always emitted first.

# H1 {.myclass #myid}
## H2 {.z #m .a}

<h1 id="myid" class="myclass">H1</h1>
<h2 id="m" class="z a">H2</h2>

Custom attributes

Custom attributes can optionally have a value. If they don't have a = symbol, their value is None and they are written in HTML with an empty value. When nothing follows a = symbol, their value is an empty string and they are written in HTML with an empty value too. When something follows a = symbol, that characters are their value.

# H1 {foo}
## H2 {#myid unknown this#is.ignored attr=value .myclass}

<h1 foo="">H1</h1>
<h2 id="myid" class="myclass" unknown="" this#is.ignored="" attr="value">H2</h2>

# Header # {myattr=value other_attr}

<h1 myattr="value" other_attr="">Header</h1>

#### Header {#id myattr= .class1 other_attr=false}

<h4 id="id" class="class1" myattr="" other_attr="false">Header</h4>

Forbidden characters

Some characters cannot appear in attribute blocks. Forbidden characters are:

left curly brace ({),
right curly brace (}),
less-than operator (<),
greater-than operator (>),
backslash (\), and
newline.

Left curly brace (note that {unknown} and {.bar} are treated as attribute blocks):

# H1 {.foo{unknown}
## H2 {.foo{.bar}

<h1 unknown="">H1 {.foo</h1>
<h2 class="bar">H2 {.foo</h2>

Right curly brace:

# H1 {.foo}bar}

<h1>H1 {.foo}bar}</h1>

less-than operator and greater-than operator:

# H1 {<i>foo</i>}

<h1>H1 {<i>foo</i>}</h1>

Backslash:

# H1 {.foo\}

<h1>H1 {.foo}</h1>

Newline:

H1 {.foo
.bar}
==

<h1>H1 {.foo
.bar}</h1>

Cancelling parsing of attribute blocks

If you want to avoid the last {...} from being treated as an attribute block, you can do it in some ways.

Put a dummy empty attribute block {} at the end of line:

H1 {} {}
=====

## H2 {} {}

<h1>H1 {}</h1>
<h2>H2 {}</h2>

Or, put closing hashes for ATX heading:

## H2 {} ##

<h2>H2 {}</h2>

Or, include forbidden characters (a backslash and newline in this example) between the braces.

# H1 {\}
## this is also ok \{\}

newline can be used for setext heading {
}
--

<h1>H1 {}</h1>
<h2>this is also ok {}</h2>
<h2>newline can be used for setext heading {
}</h2>

Note that escaping only opening braces does not prevent the braces from starting attribute blocks. Forbidden characters between braces are important.

# H1 \{.foo}
## H2 \\{.bar}
### stray backslash at the end is preserved \

<h1 class="foo">H1 \</h1>
<h2 class="bar">H2 \</h2>
<h3>stray backslash at the end is preserved \</h3>

You may notice the backslash in # H1 \{.foo} is preserved. The parser strips the {.foo} attribute block before processing backslash escapes, so the heading content H1 \ is extracted and the stray backslash at the end of the heading is preserved as usual.

Same applies to setext headings with stray trailing backslashes.

H1 \{.foo}
==
H2 \\{.bar}
--

stray backslash at the end is preserved \
--

<h1 class="foo">H1 \</h1>
<h2 class="bar">H2 \</h2>
<h2>stray backslash at the end is preserved \</h2>

Disabled inlines

Inlines are disabled and special characters such as _ and * will be treated as a plain character inside attribute blocks.

# H1 {#`code`}
## H2 {#foo__bar__baz}
### H3 {#foo**bar**baz}

<h1 id="`code`">H1</h1>
<h2 id="foo__bar__baz">H2</h2>
<h3 id="foo**bar**baz">H3</h3>

H1 {#`code`}
==

H2-1 {#foo__bar__baz}
----

H2-2 {#foo**bar**baz}
--

<h1 id="`code`">H1</h1>
<h2 id="foo__bar__baz">H2-1</h2>
<h2 id="foo**bar**baz">H2-2</h2>

Parsing of attribute blocks takes precedence, even when unclosed inlines are present before the attribute blocks.

# H1 __{#my__id1}
## H2 **{#my**id2}
### H3 `{.code` }
#### H4 ~~{.strike~~ }
.
<h1 id="my__id1">H1 __</h1>
<h2 id="my**id2">H2 **</h2>
<h3 class="code`">H3 `</h3>
<h4 class=".strike~~">H4 ~~</h4>

# H1__ {#my__id1}
## H2** {#my**id2}
### H3` {.code` }
#### H4~~ {.strike~~ }
.
<h1 id="my__id1">H1__ </h1>
<h2 id="my**id2">H2** </h2>
<h3 class="code`">H3` </h3>
<h4 class=".strike~~">H4~~ </h4>

# H1__ {.foo__bar**baz}
qux**
.
<h1 class="foo__bar**baz">H1__</h1>
<p>qux**</p>

This behavior helps source writers and generators add attribute blocks to headings without necessity to know and parse the heading content.

Escapes

Only double quotes (") and ampersands (&) are escaped into HTML character entity references (" and & respectively), and no more extra escape and sanitizing are applied. Writers are responsible to write valid (and safe) IDs and classes.

Note that ' won't be escaped since pulldown-cmark always uses double quotes to wrap attributes and it is safe to put single quotes inside them.

# H1 {.foo#bar}
## H2 {#foo.bar}
### H3 {.a"b'c&d}

<h1 class="foo#bar">H1</h1>
<h2 id="foo.bar">H2</h2>
<h3 class="a&quot;b&#39;c&amp;d">H3</h3>

Edge cases

Markdown processor cannot abort on syntax error, so some output is required for any input, even if it is obviously wrong or looks very weird. However, attribute blocks is extended syntax with neither de jure nor de facto standard, so uncommon cases will lead to possibly unexpected output that is incompatible with other implementations.

Users should not rely on the current behavior on these weird edge case inputs.

Empty IDs and classes

If IDs and classes are specified with empty name, they are simply ignored.

# H1 {#}
## H2 {.}

<h1>H1</h1>
<h2>H2</h2>

# H1 {#foo #}
# H1 {.foo . . .bar}

<h1 id="foo">H1</h1>
<h1 class="foo bar">H1</h1>

Empty headers

# {}
## {}
### {\}
#### {} {}

#{}

<h1></h1>
<h2></h2>
<h3>{}</h3>
<h4>{}</h4>
<p>#{}</p>

{}
==

\{}
--

\
--

{\}
==

{}{}
--

<h1></h1>
<h2>\</h2>
<h2>\</h2>
<h1>{}</h1>
<h2>{}</h2>

Whitespaces

Trailing ASCII whitespaces

Trailing non-U+0020 spaces in the heading content are preserved, even when an attribute block follows.

# horizontal tab	
# horizontal tab	{#ht}
## form feed
## form feed{#ff}
### vertical tab
### vertical tab{#vt}

<h1>horizontal tab	</h1>
<h1 id="ht">horizontal tab	</h1>
<h2>form feed</h2>
<h2 id="ff">form feed</h2>
<h3>vertical tab</h3>
<h3 id="vt">vertical tab</h3>

Attributes separators

Attributes are separated by ASCII non-newline whitespaces, as described previously. More precisely, the three characters below can be used as a separator:

U+0020 SPACE,
U+0009 HORIZONTAL TAB, and
U+000C FORM FEED.

Note that U+000B VERTICAL TAB is not considered as a non-newline whitespace.

# horizontal tab (U+000A) {#ht	.myclass}
## form feed (U+000C) {#ff.myclass}

# vertical tab (U+000B) {#vt.myclass}

<h1 id="ht" class="myclass">horizontal tab (U+000A)</h1>
<h2 id="ff" class="myclass">form feed (U+000C)</h2>
<h1 id="vt.myclass">vertical tab (U+000B)</h1>

Non-ASCII whitespaces don't work as separators.

# EN SPACE (U+2002) {#en-space .myclass}
## IDEOGRAPHIC SPACE (U+3000) {#ideographic-space　.myclass}

<h1 id="en-space .myclass">EN SPACE (U+2002)</h1>
<h2 id="ideographic-space　.myclass">IDEOGRAPHIC SPACE (U+3000)</h2>

Run this with cargo test --features gen-tests suite::metadata_blocks.

Examples and edge cases for metadata blocks.

YAML-style and pluses-style metadata blocks are supported.

YAML-style metadata blocks:

Start by a --- line.
End by a --- or a ... line.

Pluses-style metadata blocks:

Start by a +++ line.
End by a +++ line.

In both cases, only trailing spaces can be in the delimiting lines.

All metadata blocks must be ended, EOF is not a valid ending of the block.

Basic usage:

---
title: example
another_field: 0
---

EOF is not a valid ending of the block:

---
title: example
another_field: 0

<hr>
<p>title: example
another_field: 0</p>

A metadata block cannot be empty:

---
---

<hr>
<hr>

A blank line cannot be after the start of the metadata block:

---

title: example
another_field: 0
---

<hr>
<h2>title: example
another_field: 0</h2>

A metadata block cannot interrumpt a paragraph:

My paragraph here.
---
title: example
another_field: 0
---

<h2>My paragraph here.</h2>
<h2>title: example
another_field: 0</h2>

But it can be after a blank line:

My paragraph here.

---
title: example
another_field: 0
---

<p>My paragraph here.</p>

After the start sequence nothing but spaces can be in the same line:

---    
title: example
another_field: 0
---

--- -
title: example
another_field: 0
---

<hr>
<h2>title: example
another_field: 0</h2>

The same applies for the end sequence:

---
title: example
another_field: 0
---        

---
title: example
another_field: 0
---a

<hr>
<p>title: example
another_field: 0
---a</p>

The end sequence for YAML-style blocks can be three dots:

---
title: example
another_field: 0
...

Finally, pluses-style blocks also work following the same rules except the closing sequence must be +++:

+++
title: example
another_field: 0
+++

Metadata blocks cannot be indented:

    ---
    Things
    ---

<pre><code>---
Things
---
</code></pre>

Metadata blocks may contain lists:

---
- Item 1
- Item 2
---

pulldown-cmark guide

Custom heading