GitHub Flavored Markdown Spec

Version 0.28-gfm (2017-08-01)
This formal specification is based on the CommonMark Spec by John MacFarlane and licensed under Creative Commons BY-SA

1Introduction

1.1What is GitHub Flavored Markdown?

GitHub Flavored Markdown, often shortened as GFM, is the dialect of Markdown that is currently supported for user content on GitHub.com and GitHub Enterprise.

This formal specification, based on the CommonMark Spec, defines the syntax and semantics of this dialect.

GFM is a strict superset of CommonMark. All the features which are supported in GitHub user content and that are not specified on the original CommonMark Spec are hence known as extensions, and highlighted as such.

While GFM supports a wide range of inputs, it’s worth noting that GitHub.com and GitHub Enterprise perform additional post-processing and sanitization after GFM is converted to HTML to ensure security and consistency of the website.

1.2What 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. (http://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.

1.3Why 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:

  1. 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.)

  2. 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.)

  3. 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?
    
  4. 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.)

  5. Can list markers be indented? Can ordered list markers be right-aligned?

     8. item 1
     9. item 2
    10. item 2a
    
  6. Is this one list with a thematic break in its second item, or two lists separated by a thematic break?

    * a
    * * * * *
    * b
    
  7. 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
    
  8. 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).
    
  9. What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?

    *foo *bar* baz*
    
  10. 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`
    
  11. Can list items include section headings? (Markdown.pl does not allow this, but does allow blockquotes to include headings.)

    - # Heading
    
  12. Can list items be empty?

    * a
    *
    * b
    
  13. Can link references be defined inside block quotes or list items?

    > Blockquote [foo].
    >
    > [foo]: /url
    
  14. 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.

1.4About 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.

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.

2Preliminaries

2.1Characters 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 newline (U+000A) or carriage return (U+000D), followed by a line ending or by the end of file.

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

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 whitespace character is a space (U+0020), tab (U+0009), newline (U+000A), line tabulation (U+000B), form feed (U+000C), or carriage return (U+000D).

Whitespace is a sequence of one or more whitespace characters.

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

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

A space is U+0020.

A non-whitespace character is any character that is not a whitespace character.

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

A punctuation character is an ASCII punctuation character or anything in the general Unicode categories Pc, Pd, Pe, Pf, Pi, Po, or Ps.

2.2Tabs

Tabs in lines are not expanded to spaces. However, in contexts where whitespace helps 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.)

Example 1
→foo→baz→→bim
<pre><code>foo→baz→→bim
</code></pre>
Example 2
  →foo→baz→→bim
<pre><code>foo→baz→→bim
</code></pre>
Example 3
    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:

Example 4
  - foo

→bar
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
Example 5
- 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.

Example 6
>→→foo
<blockquote>
<pre><code>  foo
</code></pre>
</blockquote>
Example 7
-→→foo
<ul>
<li>
<pre><code>  foo
</code></pre>
</li>
</ul>
Example 8
    foo
→bar
<pre><code>foo
bar
</code></pre>
Example 9
 - foo
   - bar
→ - baz
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>
Example 10
#→Foo
<h1>Foo</h1>
Example 11
*→*→*→
<hr />

2.3Insecure characters

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

3Blocks 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.

3.1Precedence

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:

Example 12
- `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.

3.2Container blocks and leaf blocks

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

4Leaf blocks

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

4.1Thematic breaks

A line consisting of 0-3 spaces of indentation, followed by a sequence of three or more matching -, _, or * characters, each followed optionally by any number of spaces, forms a thematic break.

Example 13
***
---
___
<hr />
<hr />
<hr />

Wrong characters:

Example 14
+++
<p>+++</p>
Example 15
===
<p>===</p>

Not enough characters:

Example 16
--
**
__
<p>--
**
__</p>

One to three spaces indent are allowed:

Example 17
 ***
  ***
   ***
<hr />
<hr />
<hr />

Four spaces is too many:

Example 18
    ***
<pre><code>***
</code></pre>
Example 19
Foo
    ***
<p>Foo
***</p>

More than three characters may be used:

Example 20
_____________________________________
<hr />

Spaces are allowed between the characters:

Example 21
 - - -
<hr />
Example 22
 **  * ** * ** * **
<hr />
Example 23
-     -      -      -
<hr />

Spaces are allowed at the end:

Example 24
- - - -    
<hr />

However, no other characters may occur in the line:

Example 25
_ _ _ _ a

a------

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

It is required that all of the non-whitespace characters be the same. So, this is not a thematic break:

Example 26
 *-*
<p><em>-</em></p>

Thematic breaks do not need blank lines before or after:

Example 27
- foo
***
- bar
<ul>
<li>foo</li>
</ul>
<hr />
<ul>
<li>bar</li>
</ul>

Thematic breaks can interrupt a paragraph:

Example 28
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:

Example 29
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:

Example 30
* 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:

Example 31
- Foo
- * * *
<ul>
<li>Foo</li>
<li>
<hr />
</li>
</ul>

4.2ATX 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 a space or by the end of line. The optional closing sequence of #s must be preceded by a space and may be followed by spaces only. The opening # character may be indented 0-3 spaces. The raw contents of the heading are stripped of leading and trailing spaces before being parsed as inline content. The heading level is equal to the number of # characters in the opening sequence.

Simple headings:

Example 32
# 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:

Example 33
####### foo
<p>####### foo</p>

At least one space 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:

Example 34
#5 bolt

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

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

Example 35
\## foo
<p>## foo</p>

Contents are parsed as inlines:

Example 36
# foo *bar* \*baz\*
<h1>foo <em>bar</em> *baz*</h1>

Leading and trailing blanks are ignored in parsing inline content:

Example 37
#                  foo                     
<h1>foo</h1>

One to three spaces indentation are allowed:

Example 38
 ### foo
  ## foo
   # foo
<h3>foo</h3>
<h2>foo</h2>
<h1>foo</h1>

Four spaces are too much:

Example 39
    # foo
<pre><code># foo
</code></pre>
Example 40
foo
    # bar
<p>foo
# bar</p>

A closing sequence of # characters is optional:

Example 41
## foo ##
  ###   bar    ###
<h2>foo</h2>
<h3>bar</h3>

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

Example 42
# foo ##################################
##### foo ##
<h1>foo</h1>
<h5>foo</h5>

Spaces are allowed after the closing sequence:

Example 43
### foo ###     
<h3>foo</h3>

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

Example 44
### foo ### b
<h3>foo ### b</h3>

The closing sequence must be preceded by a space:

Example 45
# foo#
<h1>foo#</h1>

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

Example 46
### 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:

Example 47
****
## foo
****
<hr />
<h2>foo</h2>
<hr />
Example 48
Foo bar
# baz
Bar foo
<p>Foo bar</p>
<h1>baz</h1>
<p>Bar foo</p>

ATX headings can be empty:

Example 49
## 
#
### ###
<h2></h2>
<h1></h1>
<h3></h3>

4.3Setext headings

A setext heading consists of one or more lines of text, each containing at least one non-whitespace character, with no more than 3 spaces 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, block quote, thematic break, list item, or HTML block.

A setext heading underline is a sequence of = characters or a sequence of - characters, with no more than 3 spaces indentation and any number of trailing spaces. If a line containing a single - can be interpreted as an empty list items, it should be interpreted this way and not as a setext heading underline.

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:

Example 50
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:

Example 51
Foo *bar
baz*
====
<h1>Foo <em>bar
baz</em></h1>

The underlining can be any length:

Example 52
Foo
-------------------------

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

The heading content can be indented up to three spaces, and need not line up with the underlining:

Example 53
   Foo
---

  Foo
-----

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

Four spaces indent is too much:

Example 54
    Foo
    ---

    Foo
---
<pre><code>Foo
---

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

The setext heading underline can be indented up to three spaces, and may have trailing spaces:

Example 55
Foo
   ----      
<h2>Foo</h2>

Four spaces is too much:

Example 56
Foo
    ---
<p>Foo
---</p>

The setext heading underline cannot contain internal spaces:

Example 57
Foo
= =

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

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

Example 58
Foo  
-----
<h2>Foo</h2>

Nor does a backslash at the end:

Example 59
Foo\
----
<h2>Foo\</h2>

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

Example 60
`Foo
----
`

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

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

Example 61
> Foo
---
<blockquote>
<p>Foo</p>
</blockquote>
<hr />
Example 62
> foo
bar
===
<blockquote>
<p>foo
bar
===</p>
</blockquote>
Example 63
- 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:

Example 64
Foo
Bar
---
<h2>Foo
Bar</h2>

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

Example 65
---
Foo
---
Bar
---
Baz
<hr />
<h2>Foo</h2>
<h2>Bar</h2>
<p>Baz</p>

Setext headings cannot be empty:

Example 66

====
<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:

Example 67
---
---
<hr />
<hr />
Example 68
- foo
-----
<ul>
<li>foo</li>
</ul>
<hr />
Example 69
    foo
---
<pre><code>foo
</code></pre>
<hr />
Example 70
> foo
-----
<blockquote>
<p>foo</p>
</blockquote>
<hr />

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

Example 71
\> 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:

  1. paragraph “Foo”, heading “bar”, paragraph “baz”
  2. paragraph “Foo bar”, thematic break, paragraph “baz”
  3. paragraph “Foo bar — baz”
  4. 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:

Example 72
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,

Example 73
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

Example 74
Foo
bar
* * *
baz
<p>Foo
bar</p>
<hr />
<p>baz</p>

Authors who want interpretation 3 can use backslash escapes:

Example 75
Foo
bar
\---
baz
<p>Foo
bar
---
baz</p>

4.4Indented 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 indented four or more spaces. 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.)

Example 76
    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, the list item interpretation takes precedence:

Example 77
  - foo

    bar
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
Example 78
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:

Example 79
    <a/>
    *hi*

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

- one
</code></pre>

Here we have three chunks separated by blank lines:

Example 80
    chunk1

    chunk2
  
 
 
    chunk3
<pre><code>chunk1

chunk2



chunk3
</code></pre>

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

Example 81
    chunk1
      
      chunk2
<pre><code>chunk1
  
  chunk2
</code></pre>

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

Example 82
Foo
    bar
<p>Foo
bar</p>

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

Example 83
    foo
bar
<pre><code>foo
</code></pre>
<p>bar</p>

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

Example 84
# 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 indented more than four spaces:

Example 85
        foo
    bar
<pre><code>    foo
bar
</code></pre>

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

Example 86

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

Trailing spaces are included in the code block’s content:

Example 87
    foo  
<pre><code>foo  
</code></pre>

4.5Fenced 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, indented no more than three spaces.

The line with the opening code fence may optionally contain some text following the code fence; this is trimmed of leading and trailing spaces and called the info string. The info string 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 indented N spaces, 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 less than N spaces, all of the indentation is removed.)

The closing code fence may be indented up to three spaces, and may be followed only by spaces, 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 down side 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:

Example 88
```
<
 >
```
<pre><code>&lt;
 &gt;
</code></pre>

With tildes:

Example 89
~~~
<
 >
~~~
<pre><code>&lt;
 &gt;
</code></pre>

Fewer than three backticks is not enough:

Example 90
``
foo
``
<p><code>foo</code></p>

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

Example 91
```
aaa
~~~
```
<pre><code>aaa
~~~
</code></pre>
Example 92
~~~
aaa
```
~~~
<pre><code>aaa
```
</code></pre>

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

Example 93
````
aaa
```
``````
<pre><code>aaa
```
</code></pre>
Example 94
~~~~
aaa
~~~
~~~~
<pre><code>aaa
~~~
</code></pre>

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

Example 95
```
<pre><code></code></pre>
Example 96
`````

```
aaa
<pre><code>
```
aaa
</code></pre>
Example 97
> ```
> aaa

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

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

Example 98
```

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

A code block can be empty:

Example 99
```
```
<pre><code></code></pre>

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

Example 100
 ```
 aaa
aaa
```
<pre><code>aaa
aaa
</code></pre>
Example 101
  ```
aaa
  aaa
aaa
  ```
<pre><code>aaa
aaa
aaa
</code></pre>
Example 102
   ```
   aaa
    aaa
  aaa
   ```
<pre><code>aaa
 aaa
aaa
</code></pre>

Four spaces indentation produces an indented code block:

Example 103
    ```
    aaa
    ```
<pre><code>```
aaa
```
</code></pre>

Closing fences may be indented by 0-3 spaces, and their indentation need not match that of the opening fence:

Example 104
```
aaa
  ```
<pre><code>aaa
</code></pre>
Example 105
   ```
aaa
  ```
<pre><code>aaa
</code></pre>

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

Example 106
```
aaa
    ```
<pre><code>aaa
    ```
</code></pre>

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

Example 107
``` ```
aaa
<p><code></code>
aaa</p>
Example 108
~~~~~~
aaa
~~~ ~~
<pre><code>aaa
~~~ ~~
</code></pre>

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

Example 109
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:

Example 110
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. Opening and closing spaces will be stripped, and the first word, prefixed with language-, is used as the value for the class attribute of the code element within the enclosing pre element.

Example 111
```ruby
def foo(x)
  return 3
end
```
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
Example 112
~~~~    ruby startline=3 $%@#$
def foo(x)
  return 3
end
~~~~~~~
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
Example 113
````;
````
<pre><code class="language-;"></code></pre>

Info strings for backtick code blocks cannot contain backticks:

Example 114
``` aa ```
foo
<p><code>aa</code>
foo</p>

Closing code fences cannot have info strings:

Example 115
```
``` aaa
```
<pre><code>``` aaa
</code></pre>

4.6HTML 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 spaces optional indentation). It ends with the first subsequent line that meets a matching end condition, or the last line of the document or other container 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.

  1. Start condition: line begins with the string <script, <pre, or <style (case-insensitive), followed by whitespace, the string >, or the end of the line.
    End condition: line contains an end tag </script>, </pre>, or </style> (case-insensitive; it need not match the start tag).

  2. Start condition: line begins with the string <!--.
    End condition: line contains the string -->.

  3. Start condition: line begins with the string <?.
    End condition: line contains the string ?>.

  4. Start condition: line begins with the string <! followed by an uppercase ASCII letter.
    End condition: line contains the character >.

  5. Start condition: line begins with the string <![CDATA[.
    End condition: line contains the string ]]>.

  6. Start condition: line begins 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, meta, nav, noframes, ol, optgroup, option, p, param, section, source, summary, table, tbody, td, tfoot, th, thead, title, tr, track, ul, followed by whitespace, the end of the line, the string >, or the string />.
    End condition: line is followed by a blank line.

  7. Start condition: line begins with a complete open tag or closing tag (with any tag name other than script, style, or pre) followed only by whitespace or 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 a 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:

Example 116
<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 newline — 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:

Example 117
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>

okay.
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>
<p>okay.</p>
Example 118
 <div>
  *hello*
         <foo><a>
 <div>
  *hello*
         <foo><a>

A block can also start with a closing tag:

Example 119
</div>
*foo*
</div>
*foo*

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

Example 120
<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:

Example 121
<div id="foo"
  class="bar">
</div>
<div id="foo"
  class="bar">
</div>
Example 122
<div id="foo" class="bar
  baz">
</div>
<div id="foo" class="bar
  baz">
</div>

An open tag need not be closed:

Example 123
<div>
*foo*

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

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

Example 124
<div id="foo"
*hi*
<div id="foo"
*hi*
Example 125
<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:

Example 126
<div *???-&&&-<---
*foo*
<div *???-&&&-<---
*foo*

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

Example 127
<div><a href="bar">*foo*</a></div>
<div><a href="bar">*foo*</a></div>
Example 128
<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:

Example 129
<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):

Example 130
<a href="foo">
*bar*
</a>
<a href="foo">
*bar*
</a>

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

Example 131
<Warning>
*bar*
</Warning>
<Warning>
*bar*
</Warning>
Example 132
<i class="foo">
*bar*
</i>
<i class="foo">
*bar*
</i>
Example 133
</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:

Example 134
<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:

Example 135
<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.)

Example 136
<del>*foo*</del>
<p><del><em>foo</em></del></p>

HTML tags designed to contain literal content (script, style, pre), 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):

Example 137
<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):

Example 138
<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 style tag (type 1):

Example 139
<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 or list item):

Example 140
<style
  type="text/css">

foo
<style
  type="text/css">

foo
Example 141
> <div>
> foo

bar
<blockquote>
<div>
foo
</blockquote>
<p>bar</p>
Example 142
- <div>
- foo
<ul>
<li>
<div>
</li>
<li>foo</li>
</ul>

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

Example 143
<style>p{color:red;}</style>
*foo*
<style>p{color:red;}</style>
<p><em>foo</em></p>
Example 144
<!-- 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:

Example 145
<script>
foo
</script>1. *bar*
<script>
foo
</script>1. *bar*

A comment (type 2):

Example 146
<!-- Foo

bar
   baz -->
okay
<!-- Foo

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

A processing instruction (type 3):

Example 147
<?php

  echo '>';

?>
okay
<?php

  echo '>';

?>
<p>okay</p>

A declaration (type 4):

Example 148
<!DOCTYPE html>
<!DOCTYPE html>

CDATA (type 5):

Example 149
<![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 indented 1-3 spaces, but not 4:

Example 150
  <!-- foo -->

    <!-- foo -->
  <!-- foo -->
<pre><code>&lt;!-- foo --&gt;
</code></pre>
Example 151
  <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.

Example 152
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:

Example 153
<div>
bar
</div>
*foo*
<div>
bar
</div>
*foo*

HTML blocks of type 7 cannot interrupt a paragraph:

Example 154
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 tabs or spaces.

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

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:

Example 155
<div>

*Emphasized* text.

</div>
<div>
<p><em>Emphasized</em> text.</p>
</div>
Example 156
<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:

Example 157
<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:

Example 158
<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, raw HTML blocks starting with <pre> can contain blank lines.

A link reference definition consists of a link label, indented up to three spaces, followed by a colon (:), optional whitespace (including up to one line ending), a link destination, optional whitespace (including up to one line ending), and an optional link title, which if it is present must be separated from the link destination by whitespace. No further non-whitespace characters may occur on the line.

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.

Example 159
[foo]: /url "title"

[foo]
<p><a href="/url" title="title">foo</a></p>
Example 160
   [foo]: 
      /url  
           'the title'  

[foo]
<p><a href="/url" title="the title">foo</a></p>
Example 161
[Foo*bar\]]:my_(url) 'title (with parens)'

[Foo*bar\]]
<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
Example 162
[Foo bar]:
<my%20url>
'title'

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

The title may extend over multiple lines:

Example 163
[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:

Example 164
[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:

Example 165
[foo]:
/url

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

The link destination may not be omitted:

Example 166
[foo]:

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

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

Example 167
[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:

Example 168
[foo]

[foo]: url
<p><a href="url">foo</a></p>

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

Example 169
[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).

Example 170
[FOO]: /url

[Foo]
<p><a href="/url">Foo</a></p>
Example 171
[ΑΓΩ]: /φου

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

Here is a link reference definition with no corresponding link. It contributes nothing to the document.

Example 172
[foo]: /url

Here is another one:

Example 173
[
foo
]: /url
bar
<p>bar</p>

This is not a link reference definition, because there are non-whitespace characters after the title:

Example 174
[foo]: /url "title" ok
<p>[foo]: /url &quot;title&quot; ok</p>

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

Example 175
[foo]: /url
"title" ok
<p>&quot;title&quot; ok</p>

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

Example 176
    [foo]: /url "title"

[foo]
<pre><code>[foo]: /url &quot;title&quot;
</code></pre>
<p>[foo]</p>

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

Example 177
```
[foo]: /url
```

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

A link reference definition cannot interrupt a paragraph.

Example 178
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.

Example 179
# [Foo]
[foo]: /url
> bar
<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>

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

Example 180
[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:

Example 181
[foo]

> [foo]: /url
<p><a href="/url">foo</a></p>
<blockquote>
</blockquote>

4.8Paragraphs

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 whitespace.

A simple example with two paragraphs:

Example 182
aaa

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

Paragraphs can contain multiple lines, but no blank lines:

Example 183
aaa
bbb

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

Multiple blank lines between paragraph have no effect:

Example 184
aaa


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

Leading spaces are skipped:

Example 185
  aaa
 bbb
<p>aaa
bbb</p>

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

Example 186
aaa
             bbb
                                       ccc
<p>aaa
bbb
ccc</p>

However, the first line may be indented at most three spaces, or an indented code block will be triggered:

Example 187
   aaa
bbb
<p>aaa
bbb</p>
Example 188
    aaa
bbb
<pre><code>aaa
</code></pre>
<p>bbb</p>

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

Example 189
aaa     
bbb     
<p>aaa<br />
bbb</p>

4.9Blank 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.

Example 190
  

aaa
  

# aaa

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

4.10Tables (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 align="center">abc</th>
<th align="right">defghi</th>
</tr>
</thead>
<tbody>
<tr>
<td align="center">bar</td>
<td 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>

5Container 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.)

5.1Block quotes

A block quote marker consists of 0-3 spaces of initial indent, plus (a) the character > together with a following space, or (b) a single character > not followed by a space.

The following rules define block quotes:

  1. 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.

  2. 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 non-whitespace character 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.

  3. 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:

Example 198
> # Foo
> bar
> baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The spaces after the > characters can be omitted:

Example 199
># Foo
>bar
> baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The > characters can be indented 1-3 spaces:

Example 200
   > # Foo
   > bar
 > baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

Four spaces gives us a code block:

Example 201
    > # 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:

Example 202
> # 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:

Example 203
> 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:

Example 204
> 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:

Example 205
> - 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:

Example 206
>     foo
    bar
<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>
Example 207
> ```
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:

Example 208
> 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:

Example 209
>
<blockquote>
</blockquote>
Example 210
>
>  
> 
<blockquote>
</blockquote>

A block quote can have initial or final blank lines:

Example 211
>
> foo
>  
<blockquote>
<p>foo</p>
</blockquote>

A blank line always separates block quotes:

Example 212
> 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:

Example 213
> foo
> bar
<blockquote>
<p>foo
bar</p>
</blockquote>

To get a block quote with two paragraphs, use:

Example 214
> foo
>
> bar
<blockquote>
<p>foo</p>
<p>bar</p>
</blockquote>

Block quotes can interrupt paragraphs:

Example 215
foo
> bar
<p>foo</p>
<blockquote>
<p>bar</p>
</blockquote>

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

Example 216
> 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:

Example 217
> bar
baz
<blockquote>
<p>bar
baz</p>
</blockquote>
Example 218
> bar

baz
<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>
Example 219
> 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:

Example 220
> > > foo
bar
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar</p>
</blockquote>
</blockquote>
</blockquote>
Example 221
>>> 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. So five spaces are needed after the >:

Example 222
>     code

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

5.2List 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:

  1. Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a non-whitespace character and not separated from each other by more than one blank line, and M is a list marker of width W followed by 1 ≤ N ≤ 4 spaces, 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 then that line is not a list item.

For example, let Ls be the lines

Example 223
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:

Example 224
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, and there are three spaces between the list marker and the next non-whitespace character, 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:

Example 225
- one

 two
<ul>
<li>one</li>
</ul>
<p>two</p>
Example 226
- one

  two
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
Example 227
 -    one

     two
<ul>
<li>one</li>
</ul>
<pre><code> two
</code></pre>
Example 228
 -    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 non-whitespace character after the list marker. However, that is not quite right. The spaces 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:

Example 229
   > > 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:

Example 230
>>- one
>>
  >  > two
<blockquote>
<blockquote>
<ul>
<li>one</li>
</ul>
<p>two</p>
</blockquote>
</blockquote>

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

Example 231
-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.

Example 232
- foo


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

A list item may contain any kind of block:

Example 233
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.

Example 234
- 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:

Example 235
123456789. ok
<ol start="123456789">
<li>ok</li>
</ol>
Example 236
1234567890. not ok
<p>1234567890. not ok</p>

A start number may begin with 0s:

Example 237
0. ok
<ol start="0">
<li>ok</li>
</ol>
Example 238
003. ok
<ol start="3">
<li>ok</li>
</ol>

A start number may not be negative:

Example 239
-1. not ok
<p>-1. not ok</p>
  1. Item starting with indented code. If a sequence of lines Ls constitute a sequence of blocks Bs starting with an indented code block and not separated from each other by more than one blank line, and M is a list marker of width W followed by one space, 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 indented four spaces beyond the edge of the region where text will be included in the list item. In the following case that is 6 spaces:

Example 240
- foo

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

And in this case it is 11 spaces:

Example 241
  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 indented one space after the list marker:

Example 242
    indented code

paragraph

    more code
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
Example 243
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 indent is interpreted as space inside the code block:

Example 244
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 non-whitespace character, and (b) cases in which they begin with an indented code block. In a case like the following, where the first block begins with a three-space indent, the rules do not allow us to form a list item by indenting the whole thing and prepending a list marker:

Example 245
   foo

bar
<p>foo</p>
<p>bar</p>
Example 246
-    foo

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

This is not a significant restriction, because when a block begins with 1-3 spaces indent, the indentation can always be removed without a change in interpretation, allowing rule #1 to be applied. So, in the above case:

Example 247
-  foo

   bar
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
  1. 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, not separated from each other by more than one blank line, and M is a list marker of width W, then the result of prepending M 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.

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

Example 248
-
  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:

Example 249
-   
  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:

Example 250
-

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

Here is an empty bullet list item:

Example 251
- foo
-
- bar
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

It does not matter whether there are spaces following the list marker:

Example 252
- foo
-   
- bar
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

Here is an empty ordered list item:

Example 253
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:

Example 254
*
<ul>
<li></li>
</ul>

However, an empty list item cannot interrupt a paragraph:

Example 255
foo
*

foo
1.
<p>foo
*</p>
<p>foo
1.</p>
  1. Indentation. If a sequence of lines Ls constitutes a list item according to rule #1, #2, or #3, then the result of indenting each line of Ls by 1-3 spaces (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:

Example 256
 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:

Example 257
  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:

Example 258
   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:

Example 259
    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>
  1. 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 non-whitespace character 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:

Example 260
  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:

Example 261
  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:

Example 262
> 1. > Blockquote
continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
Example 263
> 1. > Blockquote
> continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
  1. 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. A sublist must be indented the same number of spaces a paragraph would need to be in order to be included in the list item.

So, in this case we need two spaces indent:

Example 264
- 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:

Example 265
- 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:

Example 266
10) foo
    - bar
<ol start="10">
<li>foo
<ul>
<li>bar</li>
</ul>
</li>
</ol>

Three is not enough:

Example 267
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:

Example 268
- - foo
<ul>
<li>
<ul>
<li>foo</li>
</ul>
</li>
</ul>
Example 269
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:

Example 270
- # Foo
- Bar
  ---
  baz
<ul>
<li>
<h1>Foo</h1>
</li>
<li>
<h2>Bar</h2>
baz</li>
</ul>

5.2.1Motivation

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

  1. “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.”

  2. “To make lists look nice, you can wrap items with hanging indents…. But if you don’t want to, you don’t have to.”

  3. “List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab.”

  4. “It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy.”

  5. “To put a blockquote within a list item, the blockquote’s > delimiters need to be indented.”

  6. “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 1–3 spaces indentation of 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

as

<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.

5.3Task 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 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 checked="" disabled="" type="checkbox"> bar</li>
</ul>

Task lists can be arbitrarily nested:

- [x] foo
  - [ ] bar
  - [x] baz
- [ ] bim
<ul>
<li><input checked="" disabled="" type="checkbox"> foo
<ul>
<li><input disabled="" type="checkbox"> bar</li>
<li><input checked="" disabled="" type="checkbox"> baz</li>
</ul>
</li>
<li><input disabled="" type="checkbox"> bim</li>
</ul>

5.4Lists

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:

Example 273
- foo
- bar
+ baz
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<ul>
<li>baz</li>
</ul>
Example 274
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:

Example 275
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 of unwanted lists in paragraphs with hard-wrapped numerals, we allow only lists starting with 1 to interrupt paragraphs. Thus,

Example 276
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

Example 277
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:

Example 278
- foo

- bar


- baz
<ul>
<li>
<p>foo</p>
</li>
<li>
<p>bar</p>
</li>
<li>
<p>baz</p>
</li>
</ul>
Example 279
- 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:

Example 280
- foo
- bar

<!-- -->

- baz
- bim
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<!-- -->
<ul>
<li>baz</li>
<li>bim</li>
</ul>
Example 281
-   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:

Example 282
- a
 - b
  - c
   - d
    - e
   - f
  - g
 - h
- i
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d</li>
<li>e</li>
<li>f</li>
<li>g</li>
<li>h</li>
<li>i</li>
</ul>
Example 283
1. a

  2. b

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

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

Example 284
- 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:

Example 285
* a
*

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

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

Example 286
- a
- b

  c
- d
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
<p>c</p>
</li>
<li>
<p>d</p>
</li>
</ul>
Example 287
- 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:

Example 288
- 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:

Example 289
- 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:

Example 290
* 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:

Example 291
- 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:

Example 292
- a
<ul>
<li>a</li>
</ul>
Example 293
- 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:

Example 294
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:

Example 295
* foo
  * bar

  baz
<ul>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
<p>baz</p>
</li>
</ul>
Example 296
- 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>

6Inlines

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

Example 297
`hi`lo`
<p><code>hi</code>lo`</p>

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

6.1Backslash escapes

Any ASCII punctuation character may be backslash-escaped:

Example 298
\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~
<p>!&quot;#$%&amp;'()*+,-./:;&lt;=&gt;?@[\]^_`{|}~</p>

Backslashes before other characters are treated as literal backslashes:

Example 299
\→\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:

Example 300
\*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"
<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 &quot;not a reference&quot;</p>

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

Example 301
\\*emphasis*
<p>\<em>emphasis</em></p>

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

Example 302
foo\
bar
<p>foo<br />
bar</p>

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

Example 303
`` \[\` ``
<p><code>\[\`</code></p>
Example 304
    \[\]
<pre><code>\[\]
</code></pre>
Example 305
~~~
\[\]
~~~
<pre><code>\[\]
</code></pre>
Example 306
<http://example.com?find=\*>
<p><a href="http://example.com?find=%5C*">http://example.com?find=\*</a></p>
Example 307
<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:

Example 308
[foo](/bar\* "ti\*tle")
<p><a href="/bar*" title="ti*tle">foo</a></p>
Example 309
[foo]

[foo]: /bar\* "ti\*tle"
<p><a href="/bar*" title="ti*tle">foo</a></p>
Example 310
``` foo\+bar
foo
```
<pre><code class="language-foo+bar">foo
</code></pre>

6.2Entity and numeric character references

All valid HTML entity references and numeric character references, except those occuring in code blocks and code spans, are recognized as such and treated as equivalent to the corresponding Unicode characters. 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/multipage/entities.json is used as an authoritative source for the valid entity references and their corresponding code points.

Example 311
&nbsp; &amp; &copy; &AElig; &Dcaron;
&frac34; &HilbertSpace; &DifferentialD;
&ClockwiseContourIntegral; &ngE;
<p>  &amp; © Æ Ď
¾  ⅆ
∲ ≧̸</p>

Decimal numeric character consist of &# + a string of 1–8 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.

Example 312
&#35; &#1234; &#992; &#98765432; &#0;
<p># Ӓ Ϡ  �</p>

Hexadecimal numeric character consist of &# + either X or x + a string of 1-8 hexadecimal digits + ;. They too are parsed as the corresponding Unicode character (this time specified with a hexadecimal numeral instead of decimal).

Example 313
&#X22; &#XD06; &#xcab;
<p>&quot;  ಫ</p>

Here are some nonentities:

Example 314
&nbsp &x; &#; &#x;
&ThisIsNotDefined; &hi?;
<p>&amp;nbsp &amp;x; &amp;#; &amp;#x;
&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:

Example 315
&copy
<p>&amp;copy</p>

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

Example 316
&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:

Example 317
<a href="&ouml;&ouml;.html">
<a href="&ouml;&ouml;.html">
Example 318
[foo](/f&ouml;&ouml; "f&ouml;&ouml;")
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
Example 319
[foo]

[foo]: /f&ouml;&ouml; "f&ouml;&ouml;"
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
Example 320
``` 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:

Example 321
`f&ouml;&ouml;`
<p><code>f&amp;ouml;&amp;ouml;</code></p>
Example 322
    f&ouml;f&ouml;
<pre><code>f&amp;ouml;f&amp;ouml;
</code></pre>

6.3Code 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 the two backtick strings, with leading and trailing spaces and line endings removed, and whitespace collapsed to single spaces.

This is a simple code span:

Example 323
`foo`
<p><code>foo</code></p>

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

Example 324
`` foo ` bar  ``
<p><code>foo ` bar</code></p>

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

Example 325
` `` `
<p><code>``</code></p>

Line endings are treated like spaces:

Example 326
``
foo
``
<p><code>foo</code></p>

Interior spaces and line endings are collapsed into single spaces, just as they would be by a browser:

Example 327
`foo   bar
  baz`
<p><code>foo bar baz</code></p>

Not all Unicode whitespace (for instance, non-breaking space) is collapsed, however:

Example 328
`a  b`
<p><code>a  b</code></p>

Q: Why not just leave the spaces, since browsers will collapse them anyway? A: Because we might be targeting a non-HTML format, and we shouldn’t rely on HTML-specific rendering assumptions.

(Existing implementations differ in their treatment of internal spaces and line endings. Some, including Markdown.pl and showdown, convert an internal line ending into a <br /> tag. But this makes things difficult for those who like to hard-wrap their paragraphs, since a line break in the midst of a code span will cause an unintended line break in the output. Others just leave internal spaces as they are, which is fine if only HTML is being targeted.)

Example 329
`foo `` bar`
<p><code>foo `` bar</code></p>

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

Example 330
`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.

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:

Example 331
*foo`*`
<p>*foo<code>*</code></p>

And this is not parsed as a link:

Example 332
[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:

Example 333
`<a href="`">`
<p><code>&lt;a href=&quot;</code>&quot;&gt;`</p>

But this is an HTML tag:

Example 334
<a href="`">`
<p><a href="`">`</p>

And this is code:

Example 335
`<http://foo.bar.`baz>`
<p><code>&lt;http://foo.bar.</code>baz&gt;`</p>

But this is an autolink:

Example 336
<http://foo.bar.`baz>`
<p><a href="http://foo.bar.%60baz">http://foo.bar.`baz</a>`</p>

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

Example 337
```foo``
<p>```foo``</p>
Example 338
`foo
<p>`foo</p>

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

Example 339
`foo``bar``
<p>`foo<code>bar</code></p>

6.4Emphasis 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 (a) not followed by Unicode whitespace, and (b) not followed by a punctuation character, or preceded by Unicode whitespace or a 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 (a) not preceded by Unicode whitespace, and (b) not preceded by a punctuation character, or followed by Unicode whitespace or a 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.

(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:

  1. A single * character can open emphasis iff (if and only if) it is part of a left-flanking delimiter run.

  2. 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 punctuation.

  3. A single * character can close emphasis iff it is part of a right-flanking delimiter run.

  4. 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 punctuation.

  5. A double ** can open strong emphasis iff it is part of a left-flanking delimiter run.

  6. 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 punctuation.

  7. A double ** can close strong emphasis iff it is part of a right-flanking delimiter run.

  8. 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 punctuation.

  9. 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.

  10. 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.

  11. A literal * character cannot occur at the beginning or end of *-delimited emphasis or **-delimited strong emphasis, unless it is backslash-escaped.

  12. 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:

  1. The number of nestings should be minimized. Thus, for example, an interpretation <strong>...</strong> is always preferred to <em><em>...</em></em>.

  2. An interpretation <em><strong>...</strong></em> is always preferred to <strong><em>...</em></strong>.

  3. 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>.

  4. 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>.

  5. 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:

Example 340
*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:

Example 341
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:

Example 342
a*"foo"*
<p>a*&quot;foo&quot;*</p>

Unicode nonbreaking spaces count as whitespace, too:

Example 343
* a *
<p>* a *</p>

Intraword emphasis with * is permitted:

Example 344
foo*bar*
<p>foo<em>bar</em></p>
Example 345
5*6*78
<p>5<em>6</em>78</p>

Rule 2:

Example 346
_foo bar_
<p><em>foo bar</em></p>

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

Example 347
_ foo bar_
<p>_ foo bar_</p>

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

Example 348
a_"foo"_
<p>a_&quot;foo&quot;_</p>

Emphasis with _ is not allowed inside words:

Example 349
foo_bar_
<p>foo_bar_</p>
Example 350
5_6_78
<p>5_6_78</p>
Example 351
пристаням_стремятся_
<p>пристаням_стремятся_</p>

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

Example 352
aa_"bb"_cc
<p>aa_&quot;bb&quot;_cc</p>

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

Example 353
foo-_(bar)_
<p>foo-<em>(bar)</em></p>

Rule 3:

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

Example 354
_foo*
<p>_foo*</p>

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

Example 355
*foo bar *
<p>*foo bar *</p>

A newline also counts as whitespace:

Example 356
*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:

Example 357
*(*foo)
<p>*(*foo)</p>

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

Example 358
*(*foo*)*
<p><em>(<em>foo</em>)</em></p>

Intraword emphasis with * is allowed:

Example 359
*foo*bar
<p><em>foo</em>bar</p>

Rule 4:

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

Example 360
_foo bar _
<p>_foo bar _</p>

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

Example 361
_(_foo)
<p>_(_foo)</p>

This is emphasis within emphasis:

Example 362
_(_foo_)_
<p><em>(<em>foo</em>)</em></p>

Intraword emphasis is disallowed for _:

Example 363
_foo_bar
<p>_foo_bar</p>
Example 364
_пристаням_стремятся
<p>_пристаням_стремятся</p>
Example 365
_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:

Example 366
_(bar)_.
<p><em>(bar)</em>.</p>

Rule 5:

Example 367
**foo bar**
<p><strong>foo bar</strong></p>

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

Example 368
** 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:

Example 369
a**"foo"**
<p>a**&quot;foo&quot;**</p>

Intraword strong emphasis with ** is permitted:

Example 370
foo**bar**
<p>foo<strong>bar</strong></p>

Rule 6:

Example 371
__foo bar__
<p><strong>foo bar</strong></p>

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

Example 372
__ foo bar__
<p>__ foo bar__</p>

A newline counts as whitespace:

Example 373
__
foo bar__
<p>__
foo bar__</p>

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

Example 374
a__"foo"__
<p>a__&quot;foo&quot;__</p>

Intraword strong emphasis is forbidden with __:

Example 375
foo__bar__
<p>foo__bar__</p>
Example 376
5__6__78
<p>5__6__78</p>
Example 377
пристаням__стремятся__
<p>пристаням__стремятся__</p>
Example 378
__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:

Example 379
foo-__(bar)__
<p>foo-<strong>(bar)</strong></p>

Rule 7:

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

Example 380
**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:

Example 381
**(**foo)
<p>**(**foo)</p>

The point of this restriction is more easily appreciated with these examples:

Example 382
*(**foo**)*
<p><em>(<strong>foo</strong>)</em></p>
Example 383
**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**
<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
<em>Asclepias physocarpa</em>)</strong></p>
Example 384
**foo "*bar*" foo**
<p><strong>foo &quot;<em>bar</em>&quot; foo</strong></p>

Intraword emphasis:

Example 385
**foo**bar
<p><strong>foo</strong>bar</p>

Rule 8:

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

Example 386
__foo bar __
<p>__foo bar __</p>

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

Example 387
__(__foo)
<p>__(__foo)</p>

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

Example 388
_(__foo__)_
<p><em>(<strong>foo</strong>)</em></p>

Intraword strong emphasis is forbidden with __:

Example 389
__foo__bar
<p>__foo__bar</p>
Example 390
__пристаням__стремятся
<p>__пристаням__стремятся</p>
Example 391
__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:

Example 392
__(bar)__.
<p><strong>(bar)</strong>.</p>

Rule 9:

Any nonempty sequence of inline elements can be the contents of an emphasized span.

Example 393
*foo [bar](/url)*
<p><em>foo <a href="/url">bar</a></em></p>
Example 394
*foo
bar*
<p><em>foo
bar</em></p>

In particular, emphasis and strong emphasis can be nested inside emphasis:

Example 395
_foo __bar__ baz_
<p><em>foo <strong>bar</strong> baz</em></p>
Example 396
_foo _bar_ baz_
<p><em>foo <em>bar</em> baz</em></p>
Example 397
__foo_ bar_
<p><em><em>foo</em> bar</em></p>
Example 398
*foo *bar**
<p><em>foo <em>bar</em></em></p>
Example 399
*foo **bar** baz*
<p><em>foo <strong>bar</strong> baz</em></p>
Example 400
*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.

The same condition ensures that the following cases are all strong emphasis nested inside emphasis, even when the interior spaces are omitted:

Example 401
***foo** bar*
<p><em><strong>foo</strong> bar</em></p>
Example 402
*foo **bar***
<p><em>foo <strong>bar</strong></em></p>
Example 403
*foo**bar***
<p><em>foo<strong>bar</strong></em></p>

Indefinite levels of nesting are possible:

Example 404
*foo **bar *baz* bim** bop*
<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>
Example 405
*foo [*bar*](/url)*
<p><em>foo <a href="/url"><em>bar</em></a></em></p>

There can be no empty emphasis or strong emphasis:

Example 406
** is not an empty emphasis
<p>** is not an empty emphasis</p>
Example 407
**** 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.

Example 408
**foo [bar](/url)**
<p><strong>foo <a href="/url">bar</a></strong></p>
Example 409
**foo
bar**
<p><strong>foo
bar</strong></p>

In particular, emphasis and strong emphasis can be nested inside strong emphasis:

Example 410
__foo _bar_ baz__
<p><strong>foo <em>bar</em> baz</strong></p>
Example 411
__foo __bar__ baz__
<p><strong>foo <strong>bar</strong> baz</strong></p>
Example 412
____foo__ bar__
<p><strong><strong>foo</strong> bar</strong></p>
Example 413
**foo **bar****
<p><strong>foo <strong>bar</strong></strong></p>
Example 414
**foo *bar* baz**
<p><strong>foo <em>bar</em> baz</strong></p>
Example 415
**foo*bar*baz**
<p><strong>foo<em>bar</em>baz</strong></p>
Example 416
***foo* bar**
<p><strong><em>foo</em> bar</strong></p>
Example 417
**foo *bar***
<p><strong>foo <em>bar</em></strong></p>

Indefinite levels of nesting are possible:

Example 418
**foo *bar **baz**
bim* bop**
<p><strong>foo <em>bar <strong>baz</strong>
bim</em> bop</strong></p>
Example 419
**foo [*bar*](/url)**
<p><strong>foo <a href="/url"><em>bar</em></a></strong></p>

There can be no empty emphasis or strong emphasis:

Example 420
__ is not an empty emphasis
<p>__ is not an empty emphasis</p>
Example 421
____ is not an empty strong emphasis
<p>____ is not an empty strong emphasis</p>

Rule 11:

Example 422
foo ***
<p>foo ***</p>
Example 423
foo *\**
<p>foo <em>*</em></p>
Example 424
foo *_*
<p>foo <em>_</em></p>
Example 425
foo *****
<p>foo *****</p>
Example 426
foo **\***
<p>foo <strong>*</strong></p>
Example 427
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:

Example 428
**foo*
<p>*<em>foo</em></p>
Example 429
*foo**
<p><em>foo</em>*</p>
Example 430
***foo**
<p>*<strong>foo</strong></p>
Example 431
****foo*
<p>***<em>foo</em></p>
Example 432
**foo***
<p><strong>foo</strong>*</p>
Example 433
*foo****
<p><em>foo</em>***</p>

Rule 12:

Example 434
foo ___
<p>foo ___</p>
Example 435
foo _\__
<p>foo <em>_</em></p>
Example 436
foo _*_
<p>foo <em>*</em></p>
Example 437
foo _____
<p>foo _____</p>
Example 438
foo __\___
<p>foo <strong>_</strong></p>
Example 439
foo __*__
<p>foo <strong>*</strong></p>
Example 440
__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:

Example 441
_foo__
<p><em>foo</em>_</p>
Example 442
___foo__
<p>_<strong>foo</strong></p>
Example 443
____foo_
<p>___<em>foo</em></p>
Example 444
__foo___
<p><strong>foo</strong>_</p>
Example 445
_foo____
<p><em>foo</em>___</p>

Rule 13 implies that if you want emphasis nested directly inside emphasis, you must use different delimiters:

Example 446
**foo**
<p><strong>foo</strong></p>
Example 447
*_foo_*
<p><em><em>foo</em></em></p>
Example 448
__foo__
<p><strong>foo</strong></p>
Example 449
_*foo*_
<p><em><em>foo</em></em></p>

However, strong emphasis within strong emphasis is possible without switching delimiters:

Example 450
****foo****
<p><strong><strong>foo</strong></strong></p>
Example 451
____foo____
<p><strong><strong>foo</strong></strong></p>

Rule 13 can be applied to arbitrarily long sequences of delimiters:

Example 452
******foo******
<p><strong><strong><strong>foo</strong></strong></strong></p>

Rule 14:

Example 453
***foo***
<p><em><strong>foo</strong></em></p>
Example 454
_____foo_____
<p><em><strong><strong>foo</strong></strong></em></p>

Rule 15:

Example 455
*foo _bar* baz_
<p><em>foo _bar</em> baz_</p>
Example 456
*foo __bar *baz bim__ bam*
<p><em>foo <strong>bar *baz bim</strong> bam</em></p>

Rule 16:

Example 457
**foo **bar baz**
<p>**foo <strong>bar baz</strong></p>
Example 458
*foo *bar baz*
<p>*foo <em>bar baz</em></p>

Rule 17:

Example 459
*[bar*](/url)
<p>*<a href="/url">bar*</a></p>
Example 460
_foo [bar_](/url)
<p>_foo <a href="/url">bar_</a></p>
Example 461
*<img src="foo" title="*"/>
<p>*<img src="foo" title="*"/></p>
Example 462
**<a href="**">
<p>**<a href="**"></p>
Example 463
__<a href="__">
<p>__<a href="__"></p>
Example 464
*a `*`*
<p><em>a <code>*</code></em></p>
Example 465
_a `_`_
<p><em>a <code>_</code></em></p>
Example 466
**a<http://foo.bar/?q=**>
<p>**a<a href="http://foo.bar/?q=**">http://foo.bar/?q=**</a></p>
Example 467
__a<http://foo.bar/?q=__>
<p>__a<a href="http://foo.bar/?q=__">http://foo.bar/?q=__</a></p>

6.5Strikethrough (extension)

GFM enables the strikethrough extension, where an additional emphasis type is available.

Strikethrough text is any text wrapped in tildes (~).

~Hi~ Hello, world!
<p><del>Hi</del> Hello, world!</p>

Any number of tildes may be used on either side of the text; they do not need to match, and they cannot be nested.

This ~text~~~~ is ~~~~curious~.
<p>This <del>text</del> is <del>curious</del>.</p>

As with regular emphasis delimiters, a new paragraph will cause the cessation of parsing a strikethrough:

This ~~has a

new paragraph~~.
<p>This ~~has a</p>
<p>new paragraph~~.</p>

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:

A link destination consists of either

A link title consists of either

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 (, optional whitespace, an optional link destination, an optional link title separated from the link destination by whitespace, optional whitespace, and a right parenthesis ). 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:

Example 471
[link](/uri "title")
<p><a href="/uri" title="title">link</a></p>

The title may be omitted:

Example 472
[link](/uri)
<p><a href="/uri">link</a></p>

Both the title and the destination may be omitted:

Example 473
[link]()
<p><a href="">link</a></p>
Example 474
[link](<>)
<p><a href="">link</a></p>

The destination cannot contain spaces or line breaks, even if enclosed in pointy brackets:

Example 475
[link](/my uri)
<p>[link](/my uri)</p>
Example 476
[link](</my uri>)
<p>[link](&lt;/my uri&gt;)</p>
Example 477
[link](foo
bar)
<p>[link](foo
bar)</p>
Example 478
[link](<foo
bar>)
<p>[link](<foo
bar>)</p>

Parentheses inside the link destination may be escaped:

Example 479
[link](\(foo\))
<p><a href="(foo)">link</a></p>

Any number of parentheses are allowed without escaping, as long as they are balanced:

Example 480
[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:

Example 481
[link](foo\(and\(bar\))
<p><a href="foo(and(bar)">link</a></p>
Example 482
[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:

Example 483
[link](foo\)\:)
<p><a href="foo):">link</a></p>

A link can contain fragment identifiers and queries:

Example 484
[link](#fragment)

[link](http://example.com#fragment)

[link](http://example.com?foo=3#frag)
<p><a href="#fragment">link</a></p>
<p><a href="http://example.com#fragment">link</a></p>
<p><a href="http://example.com?foo=3#frag">link</a></p>

Note that a backslash before a non-escapable character is just a backslash:

Example 485
[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.

Example 486
[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:

Example 487
[link]("title")
<p><a href="%22title%22">link</a></p>

Titles may be in single quotes, double quotes, or parentheses:

Example 488
[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:

Example 489
[link](/url "title \"&quot;")
<p><a href="/url" title="title &quot;&quot;">link</a></p>

Titles must be separated from the link using a whitespace. Other Unicode whitespace like non-breaking space doesn’t work.

Example 490
[link](/url "title")
<p><a href="/url%C2%A0%22title%22">link</a></p>

Nested balanced quotes are not allowed without escaping:

Example 491
[link](/url "title "and" title")
<p>[link](/url &quot;title &quot;and&quot; title&quot;)</p>

But it is easy to work around this by using a different quote type:

Example 492
[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.)

Whitespace is allowed around the destination and title:

Example 493
[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:

Example 494
[link] (/uri)
<p>[link] (/uri)</p>

The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:

Example 495
[link [foo [bar]]](/uri)
<p><a href="/uri">link [foo [bar]]</a></p>
Example 496
[link] bar](/uri)
<p>[link] bar](/uri)</p>
Example 497
[link [bar](/uri)
<p>[link <a href="/uri">bar</a></p>
Example 498
[link \[bar](/uri)
<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

Example 499
[link *foo **bar** `#`*](/uri)
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
Example 500
[![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.

Example 501
[foo [bar](/uri)](/uri)
<p>[foo <a href="/uri">bar</a>](/uri)</p>
Example 502
[foo *[bar [baz](/uri)](/uri)*](/uri)
<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>
Example 503
![[[foo](uri1)](uri2)](uri3)
<p><img src="uri3" alt="[foo](uri2)" /></p>

These cases illustrate the precedence of link text grouping over emphasis grouping:

Example 504
*[foo*](/uri)
<p>*<a href="/uri">foo*</a></p>
Example 505
[foo *bar](baz*)
<p><a href="baz*">foo *bar</a></p>

Note that brackets that aren’t part of links do not take precedence:

Example 506
*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:

Example 507
[foo <bar attr="](baz)">
<p>[foo <bar attr="](baz)"></p>
Example 508
[foo`](/uri)`
<p>[foo<code>](/uri)</code></p>
Example 509
[foo<http://example.com/?search=](uri)>
<p>[foo<a href="http://example.com/?search=%5D(uri)">http://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 non-whitespace character. 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 whitespace and collapse consecutive internal whitespace 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 contents of the first 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.

Here is a simple example:

Example 510
[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:

Example 511
[link [foo [bar]]][ref]

[ref]: /uri
<p><a href="/uri">link [foo [bar]]</a></p>
Example 512
[link \[bar][ref]

[ref]: /uri
<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

Example 513
[link *foo **bar** `#`*][ref]

[ref]: /uri
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
Example 514
[![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.

Example 515
[foo [bar](/uri)][ref]

[ref]: /uri
<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>
Example 516
[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:

Example 517
*[foo*][ref]

[ref]: /uri
<p>*<a href="/uri">foo*</a></p>
Example 518
[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:

Example 519
[foo <bar attr="][ref]">

[ref]: /uri
<p>[foo <bar attr="][ref]"></p>
Example 520
[foo`][ref]`

[ref]: /uri
<p>[foo<code>][ref]</code></p>
Example 521
[foo<http://example.com/?search=][ref]>

[ref]: /uri
<p>[foo<a href="http://example.com/?search=%5D%5Bref%5D">http://example.com/?search=][ref]</a></p>

Matching is case-insensitive:

Example 522
[foo][BaR]

[bar]: /url "title"
<p><a href="/url" title="title">foo</a></p>

Unicode case fold is used:

Example 523
[Толпой][Толпой] is a Russian word.

[ТОЛПОЙ]: /url
<p><a href="/url">Толпой</a> is a Russian word.</p>

Consecutive internal whitespace is treated as one space for purposes of determining matching:

Example 524
[Foo
  bar]: /url

[Baz][Foo bar]
<p><a href="/url">Baz</a></p>

No whitespace is allowed between the link text and the link label:

Example 525
[foo] [bar]

[bar]: /url "title"
<p>[foo] <a href="/url" title="title">bar</a></p>
Example 526
[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:

Example 527
[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:

Example 528
[bar][foo\!]

[foo!]: /url
<p>[bar][foo!]</p>

Link labels cannot contain brackets, unless they are backslash-escaped:

Example 529
[foo][ref[]

[ref[]: /uri
<p>[foo][ref[]</p>
<p>[ref[]: /uri</p>
Example 530
[foo][ref[bar]]

[ref[bar]]: /uri
<p>[foo][ref[bar]]</p>
<p>[ref[bar]]: /uri</p>
Example 531
[[[foo]]]

[[[foo]]]: /url
<p>[[[foo]]]</p>
<p>[[[foo]]]: /url</p>
Example 532
[foo][ref\[]

[ref\[]: /uri
<p><a href="/uri">foo</a></p>

Note that in this example ] is not backslash-escaped:

Example 533
[bar\\]: /uri

[bar\\]
<p><a href="/uri">bar\</a></p>

A link label must contain at least one non-whitespace character:

Example 534
[]

[]: /uri
<p>[]</p>
<p>[]: /uri</p>
Example 535
[
 ]

[
 ]: /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 first 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].

Example 536
[foo][]

[foo]: /url "title"
<p><a href="/url" title="title">foo</a></p>
Example 537
[*foo* bar][]

[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>

The link labels are case-insensitive:

Example 538
[Foo][]

[foo]: /url "title"
<p><a href="/url" title="title">Foo</a></p>

As with full reference links, whitespace is not allowed between the two sets of brackets:

Example 539
[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 first 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][].

Example 540
[foo]

[foo]: /url "title"
<p><a href="/url" title="title">foo</a></p>
Example 541
[*foo* bar]

[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
Example 542
[[*foo* bar]]

[*foo* bar]: /url "title"
<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>
Example 543
[[bar [foo]

[foo]: /url
<p>[[bar <a href="/url">foo</a></p>

The link labels are case-insensitive:

Example 544
[Foo]

[foo]: /url "title"
<p><a href="/url" title="title">Foo</a></p>

A space after the link text should be preserved:

Example 545
[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:

Example 546
\[foo]

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

Note that this is a link, because a link label ends with the first following closing bracket:

Example 547
[foo*]: /url

*[foo*]
<p>*<a href="/url">foo*</a></p>

Full and compact references take precedence over shortcut references:

Example 548
[foo][bar]

[foo]: /url1
[bar]: /url2
<p><a href="/url2">foo</a></p>
Example 549
[foo][]

[foo]: /url1
<p><a href="/url1">foo</a></p>

Inline links also take precedence:

Example 550
[foo]()

[foo]: /url1
<p><a href="">foo</a></p>
Example 551
[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:

Example 552
[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:

Example 553
[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):

Example 554
[foo][bar][baz]

[baz]: /url1
[foo]: /url2
<p>[foo]<a href="/url1">bar</a></p>

6.7Images

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.

Example 555
![foo](/url "title")
<p><img src="/url" alt="foo" title="title" /></p>
Example 556
![foo *bar*]

[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 557
![foo ![bar](/url)](/url2)
<p><img src="/url2" alt="foo bar" /></p>
Example 558
![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.

Example 559
![foo *bar*][]

[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 560
![foo *bar*][foobar]

[FOOBAR]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 561
![foo](train.jpg)
<p><img src="train.jpg" alt="foo" /></p>
Example 562
My ![foo bar](/path/to/train.jpg  "title"   )
<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>
Example 563
![foo](<url>)
<p><img src="url" alt="foo" /></p>
Example 564
![](/url)
<p><img src="/url" alt="" /></p>

Reference-style:

Example 565
![foo][bar]

[bar]: /url
<p><img src="/url" alt="foo" /></p>
Example 566
![foo][bar]

[BAR]: /url
<p><img src="/url" alt="foo" /></p>

Collapsed:

Example 567
![foo][]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" /></p>
Example 568
![*foo* bar][]

[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>

The labels are case-insensitive:

Example 569
![Foo][]

[foo]: /url "title"
<p><img src="/url" alt="Foo" title="title" /></p>

As with reference links, whitespace is not allowed between the two sets of brackets:

Example 570
![foo] 
[]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" />
[]</p>

Shortcut:

Example 571
![foo]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" /></p>
Example 572
![*foo* bar]

[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>

Note that link labels cannot contain unescaped brackets:

Example 573
![[foo]]

[[foo]]: /url "title"
<p>![[foo]]</p>
<p>[[foo]]: /url &quot;title&quot;</p>

The link labels are case-insensitive:

Example 574
![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 [:

Example 575
!\[foo]

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

If you want a link after a literal !, backslash-escape the !:

Example 576
\![foo]

[foo]: /url "title"
<p>!<a href="/url" title="title">foo</a></p>

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 not containing <, 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 whitespace and control characters, <, 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:

Example 577
<http://foo.bar.baz>
<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p>
Example 578
<http://foo.bar.baz/test?q=hello&id=22&boolean>
<p><a href="http://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean">http://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean</a></p>
Example 579
<irc://foo.bar:2233/baz>
<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p>

Uppercase is also fine:

Example 580
<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:

Example 581
<a+b+c:d>
<p><a href="a+b+c:d">a+b+c:d</a></p>
Example 582
<made-up-scheme://foo,bar>
<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p>
Example 583
<http://../>
<p><a href="http://../">http://../</a></p>
Example 584
<localhost:5001/foo>
<p><a href="localhost:5001/foo">localhost:5001/foo</a></p>

Spaces are not allowed in autolinks:

Example 585
<http://foo.bar/baz bim>
<p>&lt;http://foo.bar/baz bim&gt;</p>

Backslash-escapes do not work inside autolinks:

Example 586
<http://example.com/\[\>
<p><a href="http://example.com/%5C%5B%5C">http://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:

Example 587
<foo@bar.example.com>
<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>
Example 588
<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:

Example 589
<foo\+@bar.example.com>
<p>&lt;foo+@bar.example.com&gt;</p>

These are not autolinks:

Example 590
<>
<p>&lt;&gt;</p>
Example 591
< http://foo.bar >
<p>&lt; http://foo.bar &gt;</p>
Example 592
<m:abc>
<p>&lt;m:abc&gt;</p>
Example 593
<foo.bar.baz>
<p>&lt;foo.bar.baz&gt;</p>
Example 594
http://example.com
<p>http://example.com</p>
Example 595
foo@bar.example.com
<p>foo@bar.example.com</p>

GFM enables the autolink extension, where autolinks will be recognised in a greater number of conditions.

Autolinks can also be constructed without requiring the use of < and to > to delimit them, although they will be recognized under a smaller set of circumstances. All such recognized autolinks can only come at the beginning of a line, after whitespace, or any of the delimiting characters *, _, ~, and (.

An extended www autolink will be recognized when the text www. is found followed by a valid domain. A valid domain consists of alphanumeric characters, underscores (_), hyphens (-) and periods (.). There must be at least one period, and no underscores may be present in the last two segments of the domain.

The scheme http will be inserted automatically:

www.commonmark.org
<p><a href="http://www.commonmark.org">www.commonmark.org</a></p>

After a valid domain, zero or more non-space non-< characters may follow:

Visit www.commonmark.org/help for more information.
<p>Visit <a href="http://www.commonmark.org/help">www.commonmark.org/help</a> for more information.</p>

We then apply extended autolink path validation as follows:

Trailing punctuation (specifically, ?, !, ., ,, :, *, _, and ~) will not be considered part of the autolink, though they may be included in the interior of the link:

Visit www.commonmark.org.

Visit www.commonmark.org/a.b.
<p>Visit <a href="http://www.commonmark.org">www.commonmark.org</a>.</p>
<p>Visit <a href="http://www.commonmark.org/a.b">www.commonmark.org/a.b</a>.</p>

When an autolink ends in ), we scan the entire autolink for the total number of parentheses. If there is a greater number of closing parentheses than opening ones, we don’t consider the last character part of the autolink, in order to facilitate including an autolink inside a parenthesis:

www.google.com/search?q=Markup+(business)

(www.google.com/search?q=Markup+(business))
<p><a href="http://www.google.com/search?q=Markup+(business)">www.google.com/search?q=Markup+(business)</a></p>
<p>(<a href="http://www.google.com/search?q=Markup+(business)">www.google.com/search?q=Markup+(business)</a>)</p>

This check is only done when the link ends in a closing parentheses ), so if the only parentheses are in the interior of the autolink, no special rules are applied:

www.google.com/search?q=(business))+ok
<p><a href="http://www.google.com/search?q=(business))+ok">www.google.com/search?q=(business))+ok</a></p>

If an autolink ends in a semicolon (;), we check to see if it appears to resemble an entity reference; if the preceding text is & followed by one or more alphanumeric characters. If so, it is excluded from the autolink:

www.google.com/search?q=commonmark&hl=en

www.google.com/search?q=commonmark&hl;
<p><a href="http://www.google.com/search?q=commonmark&amp;hl=en">www.google.com/search?q=commonmark&amp;hl=en</a></p>
<p><a href="http://www.google.com/search?q=commonmark">www.google.com/search?q=commonmark</a>&amp;hl;</p>

< immediately ends an autolink.

www.commonmark.org/he<lp
<p><a href="http://www.commonmark.org/he">www.commonmark.org/he</a>&lt;lp</p>

An extended url autolink will be recognised when one of the schemes http://, https://, or ftp://, followed by a valid domain, then zero or more non-space non-< characters according to extended autolink path validation:

http://commonmark.org

(Visit https://encrypted.google.com/search?q=Markup+(business))

Anonymous FTP is available at ftp://foo.bar.baz.
<p><a href="http://commonmark.org">http://commonmark.org</a></p>
<p>(Visit <a href="https://encrypted.google.com/search?q=Markup+(business)">https://encrypted.google.com/search?q=Markup+(business)</a>)</p>
<p>Anonymous FTP is available at <a href="ftp://foo.bar.baz">ftp://foo.bar.baz</a>.</p>

An extended email autolink will be recognised when an email address is recognised within any text node. Email addresses are recognised according to the following rules:

The scheme mailto: will automatically be added to the generated link:

foo@bar.baz
<p><a href="mailto:foo@bar.baz">foo@bar.baz</a></p>

+ can occur before the @, but not after.

hello@mail+xyz.example isn't valid, but hello+xyz@mail.example is.
<p>hello@mail+xyz.example isn't valid, but <a href="mailto:hello+xyz@mail.example">hello+xyz@mail.example</a> is.</p>

., -, and _ can occur on both sides of the @, but only . may occur at the end of the email address, in which case it will not be considered part of the address:

a.b-c_d@a.b

a.b-c_d@a.b.

a.b-c_d@a.b-

a.b-c_d@a.b_
<p><a href="mailto:a.b-c_d@a.b">a.b-c_d@a.b</a></p>
<p><a href="mailto:a.b-c_d@a.b">a.b-c_d@a.b</a>.</p>
<p>a.b-c_d@a.b-</p>
<p>a.b-c_d@a.b_</p>

6.10Raw 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 whitespace, 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 whitespace, a = character, optional whitespace, 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, ", ', =, <, >, 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 whitespace, an optional / character, and a > character.

A closing tag consists of the string </, a tag name, optional whitespace, and the character >.

An HTML comment consists of <!-- + text + -->, where text does not start with > or ->, does not end with -, and does not contain --. (See the HTML5 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 <!, a name consisting of one or more uppercase ASCII letters, whitespace, a string of 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:

Example 607
<a><bab><c2c>
<p><a><bab><c2c></p>

Empty elements:

Example 608
<a/><b2/>
<p><a/><b2/></p>

Whitespace is allowed:

Example 609
<a  /><b2
data="foo" >
<p><a  /><b2
data="foo" ></p>

With attributes:

Example 610
<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:

Example 611
Foo <responsive-image src="foo.jpg" />
<p>Foo <responsive-image src="foo.jpg" /></p>

Illegal tag names, not parsed as HTML:

Example 612
<33> <__>
<p>&lt;33&gt; &lt;__&gt;</p>

Illegal attribute names:

Example 613
<a h*#ref="hi">
<p>&lt;a h*#ref=&quot;hi&quot;&gt;</p>

Illegal attribute values:

Example 614
<a href="hi'> <a href=hi'>
<p>&lt;a href=&quot;hi'&gt; &lt;a href=hi'&gt;</p>

Illegal whitespace:

Example 615
< a><
foo><bar/ >
<p>&lt; a&gt;&lt;
foo&gt;&lt;bar/ &gt;</p>

Missing whitespace:

Example 616
<a href='bar'title=title>
<p>&lt;a href='bar'title=title&gt;</p>

Closing tags:

Example 617
</a></foo >
<p></a></foo ></p>

Illegal attributes in closing tag:

Example 618
</a href="foo">
<p>&lt;/a href=&quot;foo&quot;&gt;</p>

Comments:

Example 619
foo <!-- this is a
comment - with hyphen -->
<p>foo <!-- this is a
comment - with hyphen --></p>
Example 620
foo <!-- not a comment -- two hyphens -->
<p>foo &lt;!-- not a comment -- two hyphens --&gt;</p>

Not comments:

Example 621
foo <!--> foo -->

foo <!-- foo--->
<p>foo &lt;!--&gt; foo --&gt;</p>
<p>foo &lt;!-- foo---&gt;</p>

Processing instructions:

Example 622
foo <?php echo $a; ?>
<p>foo <?php echo $a; ?></p>

Declarations:

Example 623
foo <!ELEMENT br EMPTY>
<p>foo <!ELEMENT br EMPTY></p>

CDATA sections:

Example 624
foo <![CDATA[>&<]]>
<p>foo <![CDATA[>&<]]></p>

Entity and numeric character references are preserved in HTML attributes:

Example 625
foo <a href="&ouml;">
<p>foo <a href="&ouml;"></p>

Backslash escapes do not work in HTML attributes:

Example 626
foo <a href="\*">
<p>foo <a href="\*"></p>
Example 627
<a href="\"">
<p>&lt;a href=&quot;&quot;&quot;&gt;</p>

6.11Disallowed Raw HTML (extension)

GFM enables the tagfilter extension, where the following HTML tags will be filtered when rendering HTML output:

Filtering is done by replacing the leading < with the entity &lt;. These tags are chosen in particular as they change how HTML is interpreted in a way unique to them (i.e. nested HTML is interpreted differently), and this is usually undesireable in the context of other rendered Markdown content.

All other HTML tags are left untouched.

<strong> <title> <style> <em>

<blockquote>
  <xmp> is disallowed.  <XMP> is also disallowed.
</blockquote>
<p><strong> &lt;title> &lt;style> <em></p>
<blockquote>
  &lt;xmp> is disallowed.  &lt;XMP> is also disallowed.
</blockquote>

6.12Hard line breaks

A line break (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):

Example 629
foo  
baz
<p>foo<br />
baz</p>

For a more visible alternative, a backslash before the line ending may be used instead of two spaces:

Example 630
foo\
baz
<p>foo<br />
baz</p>

More than two spaces can be used:

Example 631
foo       
baz
<p>foo<br />
baz</p>

Leading spaces at the beginning of the next line are ignored:

Example 632
foo  
     bar
<p>foo<br />
bar</p>
Example 633
foo\
     bar
<p>foo<br />
bar</p>

Line breaks can occur inside emphasis, links, and other constructs that allow inline content:

Example 634
*foo  
bar*
<p><em>foo<br />
bar</em></p>
Example 635
*foo\
bar*
<p><em>foo<br />
bar</em></p>

Line breaks do not occur inside code spans

Example 636
`code  
span`
<p><code>code span</code></p>
Example 637
`code\
span`
<p><code>code\ span</code></p>

or HTML tags:

Example 638
<a href="foo  
bar">
<p><a href="foo  
bar"></p>
Example 639
<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:

Example 640
foo\
<p>foo\</p>
Example 641
foo  
<p>foo</p>
Example 642
### foo\
<h3>foo\</h3>
Example 643
### foo  
<h3>foo</h3>

6.13Soft line breaks

A regular line break (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 softbreak 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.)

Example 644
foo
baz
<p>foo
baz</p>

Spaces at the end of the line and beginning of the next line are removed:

Example 645
foo 
 baz
<p>foo
baz</p>

A conforming parser may render a soft line break in HTML either as a line break or as a space.

A renderer may also provide an option to render soft line breaks as hard line breaks.

6.14Textual content

Any characters not given an interpretation by the above rules will be parsed as plain textual content.

Example 646
hello $.;'there
<p>hello $.;'there</p>
Example 647
Foo χρῆν
<p>Foo χρῆν</p>

Internal spaces are preserved verbatim:

Example 648
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:

  1. 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.

  2. 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:

  1. One or more open blocks may be closed.
  2. One or more new blocks may be created as children of the last open block.
  3. 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:

  1. 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.

  2. 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 block.

  3. 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.

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

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

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.

Starting at the top of the delimiter stack, we look backwards through the stack for an opening [ or ![ delimiter.

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 (*, _). Initialize this to stack_bottom.

Then we repeat the following until we run out of potential closers:

After we’re done, we remove all delimiters above stack_bottom from the delimiter stack.