CommonMark Spec by John MacFarlane is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Markdown is a plain text format for writing structured documents,
based on conventions for indicating formatting in email
and usenet posts. It was developed by John Gruber (with
help from Aaron Swartz) and released in 2004 in the form of a
syntax description
and a Perl script (Markdown.pl) for converting Markdown to
HTML. In the next decade, dozens of implementations were
developed in many languages. Some extended the original
Markdown syntax with conventions for footnotes, tables, and
other document elements. Some allowed Markdown documents to be
rendered in formats other than HTML. Websites like Reddit,
StackOverflow, and GitHub had millions of people using Markdown.
And Markdown started to be used beyond the web, to author books,
articles, slide shows, letters, and lecture notes.
What distinguishes Markdown from many other lightweight markup syntaxes, which are often easier to write, is its readability. As Gruber writes:
The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions. (https://daringfireball.net/projects/markdown/)
The point can be illustrated by comparing a sample of AsciiDoc with an equivalent sample of Markdown. Here is a sample of AsciiDoc from the AsciiDoc manual:
1. List item one.
+
List item one continued with a second paragraph followed by an
Indented block.
+
.................
$ ls *.sh
$ mv *.sh ~/tmp
.................
+
List item continued with a third paragraph.
2. List item two continued with an open block.
+
--
This paragraph is part of the preceding list item.
a. This list is nested and does not require explicit item
continuation.
+
This paragraph is part of the preceding list item.
b. List item b.
This paragraph belongs to item two of the outer list.
--
And here is the equivalent in Markdown:
1. List item one.
List item one continued with a second paragraph followed by an
Indented block.
$ ls *.sh
$ mv *.sh ~/tmp
List item continued with a third paragraph.
2. List item two continued with an open block.
This paragraph is part of the preceding list item.
1. This list is nested and does not require explicit item continuation.
This paragraph is part of the preceding list item.
2. List item b.
This paragraph belongs to item two of the outer list.
The AsciiDoc version is, arguably, easier to write. You don’t need to worry about indentation. But the Markdown version is much easier to read. The nesting of list items is apparent to the eye in the source, not just in the processed document.
John Gruber’s canonical description of Markdown’s syntax does not specify the syntax unambiguously. Here are some examples of questions it does not answer:
How much indentation is needed for a sublist? The spec says that
continuation paragraphs need to be indented four spaces, but is
not fully explicit about sublists. It is natural to think that
they, too, must be indented four spaces, but Markdown.pl does
not require that. This is hardly a “corner case,” and divergences
between implementations on this issue often lead to surprises for
users in real documents. (See this comment by John
Gruber.)
Is a blank line needed before a block quote or heading? Most implementations do not require the blank line. However, this can lead to unexpected results in hard-wrapped text, and also to ambiguities in parsing (note that some implementations put the heading inside the blockquote, while others do not). (John Gruber has also spoken in favor of requiring the blank lines.)
Is a blank line needed before an indented code block?
(Markdown.pl requires it, but this is not mentioned in the
documentation, and some implementations do not require it.)
paragraph
code?
What is the exact rule for determining when list items get
wrapped in <p> tags? Can a list be partially “loose” and partially
“tight”? What should we do with a list like this?
1. one
2. two
3. three
Or this?
1. one
- a
- b
2. two
(There are some relevant comments by John Gruber here.)
Can list markers be indented? Can ordered list markers be right-aligned?
8. item 1
9. item 2
10. item 2a
Is this one list with a thematic break in its second item, or two lists separated by a thematic break?
* a
* * * * *
* b
When list markers change from numbers to bullets, do we have two lists or one? (The Markdown syntax description suggests two, but the perl scripts and many other implementations produce one.)
1. fee
2. fie
- foe
- fum
What are the precedence rules for the markers of inline structure? For example, is the following a valid link, or does the code span take precedence ?
[a backtick (`)](/url) and [another backtick (`)](/url).
What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?
*foo *bar* baz*
What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed?
- `a long code span can contain a hyphen like this
- and it can screw things up`
Can list items include section headings? (Markdown.pl does not
allow this, but does allow blockquotes to include headings.)
- # Heading
Can list items be empty?
* a
*
* b
Can link references be defined inside block quotes or list items?
> Blockquote [foo].
>
> [foo]: /url
If there are multiple definitions for the same reference, which takes precedence?
[foo]: /url1
[foo]: /url2
[foo][]
In the absence of a spec, early implementers consulted Markdown.pl
to resolve these ambiguities. But Markdown.pl was quite buggy, and
gave manifestly bad results in many cases, so it was not a
satisfactory replacement for a spec.
Because there is no unambiguous spec, implementations have diverged considerably. As a result, users are often surprised to find that a document that renders one way on one system (say, a GitHub wiki) renders differently on another (say, converting to docbook using pandoc). To make matters worse, because nothing in Markdown counts as a “syntax error,” the divergence often isn’t discovered right away.
This document attempts to specify Markdown syntax unambiguously.
It contains many examples with side-by-side Markdown and
HTML. These are intended to double as conformance tests. An
accompanying script spec_tests.py can be used to run the tests
against any Markdown program:
python test/spec_tests.py --spec spec.txt --program PROGRAM
Since this document describes how Markdown is to be parsed into an abstract syntax tree, it would have made sense to use an abstract representation of the syntax tree instead of HTML. But HTML is capable of representing the structural distinctions we need to make, and the choice of HTML for the tests makes it possible to run the tests against an implementation without writing an abstract syntax tree renderer.
Note that not every feature of the HTML samples is mandated by the spec. For example, the spec says what counts as a link destination, but it doesn’t mandate that non-ASCII characters in the URL be percent-encoded. To use the automatic tests, implementers will need to provide a renderer that conforms to the expectations of the spec examples (percent-encoding non-ASCII characters in URLs). But a conforming implementation can use a different renderer and may choose not to percent-encode non-ASCII characters in URLs.
This document is generated from a text file, spec.txt, written
in Markdown with a small extension for the side-by-side tests.
The script tools/makespec.py can be used to convert spec.txt into
HTML or CommonMark (which can then be converted into other formats).
In the examples, the → character is used to represent tabs.
Any sequence of characters is a valid CommonMark document.
A character is a Unicode code point. Although some code points (for example, combining accents) do not correspond to characters in an intuitive sense, all code points count as characters for purposes of this spec.
This spec does not specify an encoding; it thinks of lines as composed of characters rather than bytes. A conforming parser may be limited to a certain encoding.
A line is a sequence of zero or more characters
other than line feed (U+000A) or carriage return (U+000D),
followed by a line ending or by the end of file.
A line ending is a line feed (U+000A), a carriage return
(U+000D) not followed by a line feed, or a carriage return and a
following line feed.
A line containing no characters, or a line containing only spaces
(U+0020) or tabs (U+0009), is called a blank line.
The following definitions of character classes will be used in this spec:
A Unicode whitespace character is a character in the Unicode Zs general
category, or a tab (U+0009), line feed (U+000A), form feed (U+000C), or
carriage return (U+000D).
Unicode whitespace is a sequence of one or more Unicode whitespace characters.
A tab is U+0009.
A space is U+0020.
An ASCII control character is a character between U+0000–1F (both
including) or U+007F.
An ASCII punctuation character
is !, ", #, $, %, &, ', (, ),
*, +, ,, -, ., / (U+0021–2F),
:, ;, <, =, >, ?, @ (U+003A–0040),
[, \, ], ^, _, ` (U+005B–0060),
{, |, }, or ~ (U+007B–007E).
A Unicode punctuation character is a character in the Unicode P
(puncuation) or S (symbol) general categories.
Tabs in lines are not expanded to spaces. However, in contexts where spaces help to define block structure, tabs behave as if they were replaced by spaces with a tab stop of 4 characters.
Thus, for example, a tab can be used instead of four spaces in an indented code block. (Note, however, that internal tabs are passed through as literal tabs, not expanded to spaces.)
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:
Normally the > that begins a block quote may be followed
optionally by a space, which is not considered part of the
content. In the following case > is followed by a tab,
which is treated as if it were expanded into three spaces.
Since one of these spaces is considered part of the
delimiter, foo is considered to be indented six spaces
inside the block quote context, so we get an indented
code block starting with two spaces.
- foo
- bar
→ - baz
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>
For security reasons, the Unicode character U+0000 must be replaced
with the REPLACEMENT CHARACTER (U+FFFD).
Any ASCII punctuation character may be backslash-escaped:
\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~
<p>!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~</p>
Backslashes before other characters are treated as literal backslashes:
Escaped characters are treated as regular characters and do not have their usual Markdown meanings:
\*not emphasized*
\<br/> not a tag
\[not a link](/foo)
\`not code`
1\. not a list
\* not a list
\# not a heading
\[foo]: /url "not a reference"
\ö not a character entity
<p>*not emphasized*
<br/> not a tag
[not a link](/foo)
`not code`
1. not a list
* not a list
# not a heading
[foo]: /url "not a reference"
&ouml; not a character entity</p>
If a backslash is itself escaped, the following character is not:
A backslash at the end of the line is a hard line break:
Backslash escapes do not work in code blocks, code spans, autolinks, or raw HTML:
<https://example.com?find=\*>
<p><a href="https://example.com?find=%5C*">https://example.com?find=\*</a></p>
But they work in all other contexts, including URLs and link titles, link references, and info strings in fenced code blocks:
Valid HTML entity references and numeric character references can be used in place of the corresponding Unicode character, with the following exceptions:
Entity and character references are not recognized in code blocks and code spans.
Entity and character references cannot stand in place of
special characters that define structural elements in
CommonMark. For example, although * can be used
in place of a literal * character, * cannot replace
* in emphasis delimiters, bullet list markers, or thematic
breaks.
Conforming CommonMark parsers need not store information about whether a particular character was represented in the source using a Unicode character or an entity reference.
Entity references consist of & + any of the valid
HTML5 entity names + ;. The
document https://html.spec.whatwg.org/entities.json
is used as an authoritative source for the valid entity
references and their corresponding code points.
& © Æ Ď
¾ ℋ ⅆ
∲ ≧̸
<p> & © Æ Ď
¾ ℋ ⅆ
∲ ≧̸</p>
Decimal numeric character
references
consist of &# + a string of 1–7 arabic digits + ;. A
numeric character reference is parsed as the corresponding
Unicode character. Invalid Unicode code points will be replaced by
the REPLACEMENT CHARACTER (U+FFFD). For security reasons,
the code point U+0000 will also be replaced by U+FFFD.
Hexadecimal numeric character
references consist of &# +
either X or x + a string of 1-6 hexadecimal digits + ;.
They too are parsed as the corresponding Unicode character (this
time specified with a hexadecimal numeral instead of decimal).
Here are some nonentities:
  &x; &#; &#x;
�
&#abcdef0;
&ThisIsNotDefined; &hi?;
<p>&nbsp &x; &#; &#x;
&#87654321;
&#abcdef0;
&ThisIsNotDefined; &hi?;</p>
Although HTML5 does accept some entity references
without a trailing semicolon (such as ©), these are not
recognized here, because it makes the grammar too ambiguous:
Strings that are not on the list of HTML5 named entities are not recognized as entity references either:
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:
[foo](/föö "föö")
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
[foo]
[foo]: /föö "föö"
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
Entity and numeric character references are treated as literal text in code spans and code blocks:
Entity and numeric character references cannot be used in place of symbols indicating structure in CommonMark documents.
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.
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:
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.
We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot.
This section describes the different kinds of leaf block that make up a Markdown document.
A line consisting of optionally up to three spaces of indentation, followed by a
sequence of three or more matching -, _, or * characters, each followed
optionally by any number of spaces or tabs, forms a
thematic break.
Wrong characters:
Not enough characters:
Up to three spaces of indentation are allowed:
Four spaces of indentation is too many:
More than three characters may be used:
Spaces and tabs are allowed between the characters:
Spaces and tabs are allowed at the end:
However, no other characters may occur in the line:
It is required that all of the characters other than spaces or tabs be the same. So, this is not a thematic break:
Thematic breaks do not need blank lines before or after:
Thematic breaks can interrupt a paragraph:
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:
When both a thematic break and a list item are possible interpretations of a line, the thematic break takes precedence:
If you want a thematic break in a list item, use a different bullet:
An ATX heading
consists of a string of characters, parsed as inline content, between an
opening sequence of 1–6 unescaped # characters and an optional
closing sequence of any number of unescaped # characters.
The opening sequence of # characters must be followed by spaces or tabs, or
by the end of line. The optional closing sequence of #s must be preceded by
spaces or tabs and may be followed by spaces or tabs only. The opening
# character may be preceded by up to three spaces of indentation. The raw
contents of the heading are stripped of leading and trailing space or tabs
before being parsed as inline content. The heading level is equal to the number
of # characters in the opening sequence.
Simple headings:
# foo
## foo
### foo
#### foo
##### foo
###### foo
<h1>foo</h1>
<h2>foo</h2>
<h3>foo</h3>
<h4>foo</h4>
<h5>foo</h5>
<h6>foo</h6>
More than six # characters is not a heading:
At least one space or tab is required between the # characters and the
heading’s contents, unless the heading is empty. Note that many
implementations currently do not require the space. However, the
space was required by the
original ATX implementation,
and it helps prevent things like the following from being parsed as
headings:
This is not a heading, because the first # is escaped:
Contents are parsed as inlines:
Leading and trailing spaces or tabs are ignored in parsing inline content:
Up to three spaces of indentation are allowed:
Four spaces of indentation is too many:
A closing sequence of # characters is optional:
It need not be the same length as the opening sequence:
Spaces or tabs are allowed after the closing sequence:
A sequence of # characters with anything but spaces or tabs following it
is not a closing sequence, but counts as part of the contents of the
heading:
The closing sequence must be preceded by a space or tab:
Backslash-escaped # characters do not count as part
of the closing sequence:
ATX headings need not be separated from surrounding content by blank lines, and they can interrupt paragraphs:
ATX headings can be empty:
A setext heading consists of one or more lines of text, not interrupted by a blank line, of which the first line does not have more than 3 spaces of indentation, followed by a setext heading underline. The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a code fence, ATX heading, 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 of indentation and any number of trailing spaces or tabs.
The heading is a level 1 heading if = characters are used in
the setext heading underline, and a level 2 heading if -
characters are used. The contents of the heading are the result
of parsing the preceding lines of text as CommonMark inline
content.
In general, a setext heading need not be preceded or followed by a blank line. However, it cannot interrupt a paragraph, so when a setext heading comes after a paragraph, a blank line is needed between them.
Simple examples:
Foo *bar*
=========
Foo *bar*
---------
<h1>Foo <em>bar</em></h1>
<h2>Foo <em>bar</em></h2>
The content of the header may span more than one line:
The contents are the result of parsing the headings’s raw content as inlines. The heading’s raw content is formed by concatenating the lines and removing initial and final spaces or tabs.
The underlining can be any length:
The heading content can be preceded by up to three spaces of indentation, and need not line up with the underlining:
Four spaces of indentation is too many:
The setext heading underline can be preceded by up to three spaces of indentation, and may have trailing spaces or tabs:
Four spaces of indentation is too many:
The setext heading underline cannot contain internal spaces or tabs:
Trailing spaces or tabs in the content line do not cause a hard line break:
Nor does a backslash at the end:
Since indicators of block structure take precedence over indicators of inline structure, the following are setext headings:
`Foo
----
`
<a title="a lot
---
of dashes"/>
<h2>`Foo</h2>
<p>`</p>
<h2><a title="a lot</h2>
<p>of dashes"/></p>
The setext heading underline cannot be a lazy continuation line in a list item or block quote:
A blank line is needed between a paragraph and a following setext heading, since otherwise the paragraph becomes part of the heading’s content:
But in general a blank line is not required before or after setext headings:
Setext headings cannot be empty:
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:
If you want a heading with > foo as its literal text, you can
use backslash escapes:
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:
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:
Authors who want interpretation 2 can put blank lines around the thematic break,
or use a thematic break that cannot count as a setext heading underline, such as
Authors who want interpretation 3 can use backslash escapes:
An indented code block is composed of one or more indented chunks separated by blank lines. An indented chunk is a sequence of non-blank lines, each preceded by four or more spaces of indentation. The contents of the code block are the literal contents of the lines, including trailing line endings, minus four spaces of indentation. An indented code block has no info string.
An indented code block cannot interrupt a paragraph, so there must be a blank line between a paragraph and a following indented code block. (A blank line is not needed, however, between a code block and a following paragraph.)
a simple
indented code block
<pre><code>a simple
indented code block
</code></pre>
If there is any ambiguity between an interpretation of indentation as a code block and as indicating that material belongs to a list item, the list item interpretation takes precedence:
The contents of a code block are literal text, and do not get parsed as Markdown:
Here we have three chunks separated by blank lines:
Any initial spaces or tabs beyond four spaces of indentation will be included in the content, even in interior blank lines:
An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.)
However, any non-blank line with fewer than four spaces of indentation ends the code block immediately. So a paragraph may occur immediately after indented code:
And indented code can occur immediately before and after other kinds of blocks:
# Heading
foo
Heading
------
foo
----
<h1>Heading</h1>
<pre><code>foo
</code></pre>
<h2>Heading</h2>
<pre><code>foo
</code></pre>
<hr />
The first line can be preceded by more than four spaces of indentation:
Blank lines preceding or following an indented code block are not included in it:
Trailing spaces or tabs are included in the code block’s content:
A code fence is a sequence
of at least three consecutive backtick characters (`) or
tildes (~). (Tildes and backticks cannot be mixed.)
A fenced code block
begins with a code fence, preceded by up to three spaces of indentation.
The line with the opening code fence may optionally contain some text following the code fence; this is trimmed of leading and trailing spaces or tabs and called the info string. If the info string comes after a backtick fence, it may not contain any backtick characters. (The reason for this restriction is that otherwise some inline code would be incorrectly interpreted as the beginning of a fenced code block.)
The content of the code block consists of all subsequent lines, until a closing code fence of the same type as the code block began with (backticks or tildes), and with at least as many backticks or tildes as the opening code fence. If the leading code fence is preceded by N spaces of indentation, then up to N spaces of indentation are removed from each line of the content (if present). (If a content line is not indented, it is preserved unchanged. If it is indented N spaces or less, all of the indentation is removed.)
The closing code fence may be preceded by up to three spaces of indentation, and may be followed only by spaces or tabs, which are ignored. If the end of the containing block (or document) is reached and no closing code fence has been found, the code block contains all of the lines after the opening code fence until the end of the containing block (or document). (An alternative spec would require backtracking in the event that a closing code fence is not found. But this makes parsing much less efficient, and there seems to be no real downside to the behavior described here.)
A fenced code block may interrupt a paragraph, and does not require a blank line either before or after.
The content of a code fence is treated as literal text, not parsed
as inlines. The first word of the info string is typically used to
specify the language of the code sample, and rendered in the class
attribute of the code tag. However, this spec does not mandate any
particular treatment of the info string.
Here is a simple example with backticks:
With tildes:
Fewer than three backticks is not enough:
The closing code fence must use the same character as the opening fence:
The closing code fence must be at least as long as the opening fence:
Unclosed code blocks are closed by the end of the document (or the enclosing block quote or list item):
> ```
> aaa
bbb
<blockquote>
<pre><code>aaa
</code></pre>
</blockquote>
<p>bbb</p>
A code block can have all empty lines as its content:
A code block can be empty:
Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:
Four spaces of indentation is too many:
Closing fences may be preceded by up to three spaces of indentation, and their indentation need not match that of the opening fence:
This is not a closing fence, because it is indented 4 spaces:
Code fences (opening and closing) cannot contain internal spaces or tabs:
Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between:
Other blocks can also occur before and after fenced code blocks without an intervening blank line:
An info string can be provided after the opening code fence.
Although this spec doesn’t mandate any particular treatment of
the info string, the first word is typically used to specify
the language of the code block. In HTML output, the language is
normally indicated by adding a class to the code element consisting
of language- followed by the language name.
```ruby
def foo(x)
return 3
end
```
<pre><code class="language-ruby">def foo(x)
return 3
end
</code></pre>
~~~~ ruby startline=3 $%@#$
def foo(x)
return 3
end
~~~~~~~
<pre><code class="language-ruby">def foo(x)
return 3
end
</code></pre>
Info strings for backtick code blocks cannot contain backticks:
Info strings for tilde code blocks can contain backticks and tildes:
Closing code fences cannot have info strings:
An HTML block is a group of lines that is treated as raw HTML (and will not be escaped in HTML output).
There are seven kinds of HTML block, which can be defined by their start and end conditions. The block begins with a line that meets a start condition (after up to three optional spaces of indentation). It ends with the first subsequent line that meets a matching end condition, or the last line of the document, or the last line of the container block containing the current HTML block, if no line is encountered that meets the end condition. If the first line meets both the start condition and the end condition, the block will contain just that line.
Start condition: line begins with the string <pre,
<script, <style, or <textarea (case-insensitive), followed by a space,
a tab, the string >, or the end of the line.
End condition: line contains an end tag
</pre>, </script>, </style>, or </textarea> (case-insensitive; it
need not match the start tag).
Start condition: line begins with the string <!--.
End condition: line contains the string -->.
Start condition: line begins with the string <?.
End condition: line contains the string ?>.
Start condition: line begins with the string <!
followed by an ASCII letter.
End condition: line contains the character >.
Start condition: line begins with the string
<![CDATA[.
End condition: line contains the string ]]>.
Start condition: line begins with the string < or </
followed by one of the strings (case-insensitive) address,
article, aside, base, basefont, blockquote, body,
caption, center, col, colgroup, dd, details, dialog,
dir, div, dl, dt, fieldset, figcaption, figure,
footer, form, frame, frameset,
h1, h2, h3, h4, h5, h6, head, header, hr,
html, iframe, legend, li, link, main, menu, menuitem,
nav, noframes, ol, optgroup, option, p, param,
search, section, summary, table, tbody, td,
tfoot, th, thead, title, tr, track, ul, followed
by a space, a tab, the end of the line, the string >, or
the string />.
End condition: line is followed by a blank line.
Start condition: line begins with a complete open tag
(with any tag name other than pre, script,
style, or textarea) or a complete closing tag,
followed by zero or more spaces and tabs, followed by the end of the line.
End condition: line is followed by a blank line.
HTML blocks continue until they are closed by their appropriate end condition, or the last line of the document or other container block. This means any HTML within an HTML block that might otherwise be recognised as a start condition will be ignored by the parser and passed through as-is, without changing the parser’s state.
For instance, <pre> within an HTML block started by <table> will not affect
the parser state; as the HTML block was started in by start condition 6, it
will end at any blank line. This can be surprising:
<table><tr><td>
<pre>
**Hello**,
_world_.
</pre>
</td></tr></table>
<table><tr><td>
<pre>
**Hello**,
<p><em>world</em>.
</pre></p>
</td></tr></table>
In this case, the HTML block is terminated by the blank line — the **Hello**
text remains verbatim — and regular parsing resumes, with a paragraph,
emphasised world and inline and block HTML following.
All types of HTML blocks except type 7 may interrupt a paragraph. Blocks of type 7 may not interrupt a paragraph. (This restriction is intended to prevent unwanted interpretation of long tags inside a wrapped paragraph as starting HTML blocks.)
Some simple examples follow. Here are some basic HTML blocks of type 6:
<table>
<tr>
<td>
hi
</td>
</tr>
</table>
okay.
<table>
<tr>
<td>
hi
</td>
</tr>
</table>
<p>okay.</p>
A block can also start with a closing tag:
Here we have two HTML blocks with a Markdown paragraph between them:
<DIV CLASS="foo">
*Markdown*
</DIV>
<DIV CLASS="foo">
<p><em>Markdown</em></p>
</DIV>
The tag on the first line can be partial, as long as it is split where there would be whitespace:
An open tag need not be closed:
A partial tag need not even be completed (garbage in, garbage out):
The initial tag doesn’t even need to be a valid tag, as long as it starts like one:
In type 6 blocks, the initial tag need not be on a line by itself:
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:
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):
In type 7 blocks, the tag name can be anything:
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:
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:
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.)
HTML tags designed to contain literal content
(pre, script, style, textarea), comments, processing instructions,
and declarations are treated somewhat differently.
Instead of ending at the first blank line, these blocks
end at the first line containing a corresponding end tag.
As a result, these blocks can contain blank lines:
A pre tag (type 1):
<pre language="haskell"><code>
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
</code></pre>
okay
<pre language="haskell"><code>
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
</code></pre>
<p>okay</p>
A script tag (type 1):
<script type="text/javascript">
// JavaScript example
document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
okay
<script type="text/javascript">
// JavaScript example
document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
<p>okay</p>
A textarea tag (type 1):
A style tag (type 1):
<style
type="text/css">
h1 {color:red;}
p {color:blue;}
</style>
okay
<style
type="text/css">
h1 {color:red;}
p {color:blue;}
</style>
<p>okay</p>
If there is no matching end tag, the block will end at the end of the document (or the enclosing block quote or list item):
The end tag can occur on the same line as the start tag:
<style>p{color:red;}</style>
*foo*
<style>p{color:red;}</style>
<p><em>foo</em></p>
Note that anything on the last line after the end tag will be included in the HTML block:
A comment (type 2):
A processing instruction (type 3):
A declaration (type 4):
CDATA (type 5):
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then {
return 1;
} else {
return 0;
}
}
]]>
okay
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then {
return 1;
} else {
return 0;
}
}
]]>
<p>okay</p>
The opening tag can be preceded by up to three spaces of indentation, but not four:
<!-- foo -->
<!-- foo -->
<!-- foo -->
<pre><code><!-- foo -->
</code></pre>
An HTML block of types 1–6 can interrupt a paragraph, and need not be preceded by a blank line.
However, a following blank line is needed, except at the end of a document, and except for blocks of types 1–5, above:
HTML blocks of type 7 cannot interrupt a paragraph:
This rule differs from John Gruber’s original Markdown syntax specification, which says:
The only restrictions are that block-level HTML elements — e.g.
<div>,<table>,<pre>,<p>, etc. — must be separated from surrounding content by blank lines, and the start and end tags of the block should not be indented with spaces or tabs.
In some ways Gruber’s rule is more restrictive than the one given here:
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:
Some Markdown implementations have adopted a convention of
interpreting content inside tags as text if the open tag has
the attribute markdown=1. The rule given above seems a simpler and
more elegant way of achieving the same expressive power, which is also
much simpler to parse.
The main potential drawback is that one can no longer paste HTML blocks into Markdown documents with 100% reliability. However, in most cases this will work fine, because the blank lines in HTML are usually followed by HTML block tags. For example:
<table>
<tr>
<td>
Hi
</td>
</tr>
</table>
<table>
<tr>
<td>
Hi
</td>
</tr>
</table>
There are problems, however, if the inner tags are indented and separated by spaces, as then they will be interpreted as an indented code block:
<table>
<tr>
<td>
Hi
</td>
</tr>
</table>
<table>
<tr>
<pre><code><td>
Hi
</td>
</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, optionally preceded by up to three spaces of
indentation, followed
by a colon (:), optional spaces or tabs (including up to one
line ending), a link destination,
optional spaces or tabs (including up to one
line ending), and an optional link
title, which if it is present must be separated
from the link destination by spaces or tabs.
No further character may occur.
A link reference definition does not correspond to a structural element of a document. Instead, it defines a label which can be used in reference links and reference-style images elsewhere in the document. Link reference definitions can come either before or after the links that use them.
[Foo*bar\]]:my_(url) 'title (with parens)'
[Foo*bar\]]
<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
[Foo bar]:
<my url>
'title'
[Foo bar]
<p><a href="my%20url" title="title">Foo bar</a></p>
The title may extend over multiple lines:
[foo]: /url '
title
line1
line2
'
[foo]
<p><a href="/url" title="
title
line1
line2
">foo</a></p>
However, it may not contain a blank line:
[foo]: /url 'title
with blank line'
[foo]
<p>[foo]: /url 'title</p>
<p>with blank line'</p>
<p>[foo]</p>
The title may be omitted:
The link destination may not be omitted:
However, an empty link destination may be specified using angle brackets:
The title must be separated from the link destination by spaces or tabs:
Both title and destination can contain backslash escapes and literal backslashes:
[foo]: /url\bar\*baz "foo\"bar\baz"
[foo]
<p><a href="/url%5Cbar*baz" title="foo"bar\baz">foo</a></p>
A link can come before its corresponding definition:
If there are several matching definitions, the first one takes precedence:
As noted in the section on Links, matching of labels is case-insensitive (see matches).
Whether something is a link reference definition is independent of whether the link reference it defines is used in the document. Thus, for example, the following document contains just a link reference definition, and no visible content:
Here is another one:
This is not a link reference definition, because there are characters other than spaces or tabs after the title:
This is a link reference definition, but it has no title:
This is not a link reference definition, because it is indented four spaces:
[foo]: /url "title"
[foo]
<pre><code>[foo]: /url "title"
</code></pre>
<p>[foo]</p>
This is not a link reference definition, because it occurs inside a code block:
A link reference definition cannot interrupt a paragraph.
However, it can directly follow other block elements, such as headings and thematic breaks, and it need not be followed by a blank line.
# [Foo]
[foo]: /url
> bar
<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>
Several link reference definitions can occur one after another, without intervening blank lines.
[foo]: /foo-url "foo"
[bar]: /bar-url
"bar"
[baz]: /baz-url
[foo],
[bar],
[baz]
<p><a href="/foo-url" title="foo">foo</a>,
<a href="/bar-url" title="bar">bar</a>,
<a href="/baz-url">baz</a></p>
Link reference definitions can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined:
A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph’s raw content as inlines. The paragraph’s raw content is formed by concatenating the lines and removing initial and final spaces or tabs.
A simple example with two paragraphs:
Paragraphs can contain multiple lines, but no blank lines:
Multiple blank lines between paragraphs have no effect:
Leading spaces or tabs are skipped:
Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs.
However, the first line may be preceded by up to three spaces of indentation. Four spaces of indentation is too many:
Final spaces or tabs are stripped before inline parsing, so a paragraph that ends with two or more spaces will not end with a hard line break:
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.
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.)
A block quote marker,
optionally preceded by up to three spaces of indentation,
consists of (a) the character > together with a following space of
indentation, or (b) a single character > not followed by a space of
indentation.
The following rules define block quotes:
Basic case. If a string of lines Ls constitute a sequence of blocks Bs, then the result of prepending a block quote marker to the beginning of each line in Ls is a block quote containing Bs.
Laziness. If a string of lines Ls constitute a block quote with contents Bs, then the result of deleting the initial block quote marker from one or more lines in which the next character other than a space or tab after the block quote marker is paragraph continuation text is a block quote with Bs as its content. Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph.
Consecutiveness. A document cannot contain two block quotes in a row unless there is a blank line between them.
Nothing else counts as a block quote.
Here is a simple example:
The space or tab after the > characters can be omitted:
The > characters can be preceded by up to three spaces of indentation:
Four spaces of indentation is too many:
The Laziness clause allows us to omit the > before
paragraph continuation text:
A block quote can contain some lazy and some non-lazy continuation lines:
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:
Similarly, if we omit the > in the second line of
> - foo
> - bar
then the block quote ends after the first line:
> - foo
- bar
<blockquote>
<ul>
<li>foo</li>
</ul>
</blockquote>
<ul>
<li>bar</li>
</ul>
For the same reason, we can’t omit the > in front of
subsequent lines of an indented or fenced code block:
> foo
bar
<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>
> ```
foo
```
<blockquote>
<pre><code></code></pre>
</blockquote>
<p>foo</p>
<pre><code></code></pre>
Note that in the following case, we have a lazy continuation line:
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:
A block quote can have initial or final blank lines:
A blank line always separates block quotes:
> foo
> bar
<blockquote>
<p>foo</p>
</blockquote>
<blockquote>
<p>bar</p>
</blockquote>
(Most current Markdown implementations, including John Gruber’s
original Markdown.pl, will parse this example as a single block quote
with two paragraphs. But it seems better to allow the author to decide
whether two block quotes or one are wanted.)
Consecutiveness means that if we put these block quotes together, we get a single block quote:
To get a block quote with two paragraphs, use:
Block quotes can interrupt paragraphs:
In general, blank lines are not needed before or after block quotes:
> aaa
***
> bbb
<blockquote>
<p>aaa</p>
</blockquote>
<hr />
<blockquote>
<p>bbb</p>
</blockquote>
However, because of laziness, a blank line is needed between a block quote and a following paragraph:
It is a consequence of the Laziness rule that any number
of initial >s may be omitted on a continuation line of a
nested block quote:
> > > foo
bar
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar</p>
</blockquote>
</blockquote>
</blockquote>
>>> foo
> bar
>>baz
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar
baz</p>
</blockquote>
</blockquote>
</blockquote>
When including an indented code block in a block quote,
remember that the block quote marker includes
both the > and a following space of indentation. So five spaces are needed
after the >:
> code
> not code
<blockquote>
<pre><code>code
</code></pre>
</blockquote>
<blockquote>
<p>not code</p>
</blockquote>
A list marker is a bullet list marker or an ordered list marker.
A bullet list marker
is a -, +, or * character.
An ordered list marker
is a sequence of 1–9 arabic digits (0-9), followed by either a
. character or a ) character. (The reason for the length
limit is that with 10 digits we start seeing integer overflows
in some browsers.)
The following rules define list items:
Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a character other than a space or tab, and M is a list marker of width W followed by 1 ≤ N ≤ 4 spaces of indentation, then the result of prepending M and the following spaces to the first line of Ls, and indenting subsequent lines of Ls by W + N spaces, is a list item with Bs as its contents. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.
Exceptions:
For example, let Ls be the lines
A paragraph
with two lines.
indented code
> A block quote.
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
And let M be the marker 1., and N = 2. Then rule #1 says
that the following is an ordered list item with start number 1,
and the same contents as Ls:
1. A paragraph
with two lines.
indented code
> A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
The most important thing to notice is that the position of the text after the list marker determines how much indentation is needed in subsequent blocks in the list item. If the list marker takes up two spaces of indentation, and there are three spaces between the list marker and the next character other than a space or tab, then blocks must be indented five spaces in order to fall under the list item.
Here are some examples showing how far content must be indented to be put under the list item:
It is tempting to think of this in terms of columns: the continuation blocks must be indented at least to the column of the first character other than a space or tab after the list marker. However, that is not quite right. The spaces of indentation after the list marker determine how much relative indentation is needed. Which column this indentation reaches will depend on how the list item is embedded in other constructions, as shown by this example:
> > 1. one
>>
>> two
<blockquote>
<blockquote>
<ol>
<li>
<p>one</p>
<p>two</p>
</li>
</ol>
</blockquote>
</blockquote>
Here two occurs in the same column as the list marker 1.,
but is actually contained in the list item, because there is
sufficient indentation after the last containing blockquote marker.
The converse is also possible. In the following example, the word two
occurs far to the right of the initial text of the list item, one, but
it is not considered part of the list item, because it is not indented
far enough past the blockquote marker:
>>- one
>>
> > two
<blockquote>
<blockquote>
<ul>
<li>one</li>
</ul>
<p>two</p>
</blockquote>
</blockquote>
Note that at least one space or tab is needed between the list marker and any following content, so these are not list items:
A list item may contain blocks that are separated by more than one blank line.
A list item may contain any kind of block:
1. foo
```
bar
```
baz
> bam
<ol>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>
<blockquote>
<p>bam</p>
</blockquote>
</li>
</ol>
A list item that contains an indented code block will preserve empty lines within the code block verbatim.
Note that ordered list start numbers must be nine digits or less:
A start number may begin with 0s:
A start number may not be negative:
An indented code block will have to be preceded by four spaces of indentation beyond the edge of the region where text will be included in the list item. In the following case that is 6 spaces:
And in this case it is 11 spaces:
10. foo
bar
<ol start="10">
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ol>
If the first block in the list item is an indented code block, then by rule #2, the contents must be preceded by one space of indentation after the list marker:
indented code
paragraph
more code
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
1. indented code
paragraph
more code
<ol>
<li>
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>
Note that an additional space of indentation is interpreted as space inside the code block:
1. indented code
paragraph
more code
<ol>
<li>
<pre><code> indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>
Note that rules #1 and #2 only apply to two cases: (a) cases in which the lines to be included in a list item begin with a character other than a space or tab, and (b) cases in which they begin with an indented code block. In a case like the following, where the first block begins with three spaces of indentation, the rules do not allow us to form a list item by indenting the whole thing and prepending a list marker:
This is not a significant restriction, because when a block is preceded by up to three spaces of indentation, the indentation can always be removed without a change in interpretation, allowing rule #1 to be applied. So, in the above case:
Here are some list items that start with a blank line but are not empty:
-
foo
-
```
bar
```
-
baz
<ul>
<li>foo</li>
<li>
<pre><code>bar
</code></pre>
</li>
<li>
<pre><code>baz
</code></pre>
</li>
</ul>
When the list item starts with a blank line, the number of spaces following the list marker doesn’t change the required indentation:
A list item can begin with at most one blank line.
In the following example, foo is not part of the list
item:
Here is an empty bullet list item:
It does not matter whether there are spaces or tabs following the list marker:
Here is an empty ordered list item:
A list may start or end with an empty list item:
However, an empty list item cannot interrupt a paragraph:
Indented one space:
1. A paragraph
with two lines.
indented code
> A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
Indented two spaces:
1. A paragraph
with two lines.
indented code
> A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
Indented three spaces:
1. A paragraph
with two lines.
indented code
> A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
Four spaces indent gives a code block:
1. A paragraph
with two lines.
indented code
> A block quote.
<pre><code>1. A paragraph
with two lines.
indented code
> A block quote.
</code></pre>
Here is an example with lazy continuation lines:
1. A paragraph
with two lines.
indented code
> A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
Indentation can be partially deleted:
These examples show how laziness can work in nested structures:
> 1. > Blockquote
continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
> 1. > Blockquote
> continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
The rules for sublists follow from the general rules above. A sublist must be indented the same number of spaces of indentation a paragraph would need to be in order to be included in the list item.
So, in this case we need two spaces indent:
- foo
- bar
- baz
- boo
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz
<ul>
<li>boo</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
One is not enough:
- foo
- bar
- baz
- boo
<ul>
<li>foo</li>
<li>bar</li>
<li>baz</li>
<li>boo</li>
</ul>
Here we need four, because the list marker is wider:
Three is not enough:
A list may be the first block in a list item:
1. - 2. foo
<ol>
<li>
<ul>
<li>
<ol start="2">
<li>foo</li>
</ol>
</li>
</ul>
</li>
</ol>
A list item can contain a heading:
- # Foo
- Bar
---
baz
<ul>
<li>
<h1>Foo</h1>
</li>
<li>
<h2>Bar</h2>
baz</li>
</ul>
John Gruber’s Markdown spec says the following about list items:
“List markers typically start at the left margin, but may be indented by up to three spaces. List markers must be followed by one or more spaces or a tab.”
“To make lists look nice, you can wrap items with hanging indents…. But if you don’t want to, you don’t have to.”
“List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab.”
“It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy.”
“To put a blockquote within a list item, the blockquote’s >
delimiters need to be indented.”
“To put a code block within a list item, the code block needs to be indented twice — 8 spaces or two tabs.”
These rules specify that a paragraph under a list item must be indented four spaces (presumably, from the left margin, rather than the start of the list marker, but this is not said), and that code under a list item must be indented eight spaces instead of the usual four. They also say that a block quote must be indented, but not by how much; however, the example given has four spaces indentation. Although nothing is said about other kinds of block-level content, it is certainly reasonable to infer that all block elements under a list item, including other lists, must be indented four spaces. This principle has been called the four-space rule.
The four-space rule is clear and principled, and if the reference
implementation Markdown.pl had followed it, it probably would have
become the standard. However, Markdown.pl allowed paragraphs and
sublists to start with only two spaces indentation, at least on the
outer level. Worse, its behavior was inconsistent: a sublist of an
outer-level list needed two spaces indentation, but a sublist of this
sublist needed three spaces. It is not surprising, then, that different
implementations of Markdown have developed very different rules for
determining what comes under a list item. (Pandoc and python-Markdown,
for example, stuck with Gruber’s syntax description and the four-space
rule, while discount, redcarpet, marked, PHP Markdown, and others
followed Markdown.pl’s behavior more closely.)
Unfortunately, given the divergences between implementations, there
is no way to give a spec for list items that will be guaranteed not
to break any existing documents. However, the spec given here should
correctly handle lists formatted with either the four-space rule or
the more forgiving Markdown.pl behavior, provided they are laid out
in a way that is natural for a human to read.
The strategy here is to let the width and indentation of the list marker determine the indentation necessary for blocks to fall under the list item, rather than having a fixed and arbitrary number. The writer can think of the body of the list item as a unit which gets indented to the right enough to fit the list marker (and any indentation on the list marker). (The laziness rule, #5, then allows continuation lines to be unindented if needed.)
This rule is superior, we claim, to any rule requiring a fixed level of indentation from the margin. The four-space rule is clear but unnatural. It is quite unintuitive that
- foo
bar
- baz
should be parsed as two lists with an intervening paragraph,
<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
as the four-space rule demands, rather than a single list,
<ul>
<li>
<p>foo</p>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
</li>
</ul>
The choice of four spaces is arbitrary. It can be learned, but it is not likely to be guessed, and it trips up beginners regularly.
Would it help to adopt a two-space rule? The problem is that such
a rule, together with the rule allowing up to three spaces of indentation for
the initial list marker, allows text that is indented less than the
original list marker to be included in the list item. For example,
Markdown.pl parses
- one
two
as a single list item, with two a continuation paragraph:
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
and similarly
> - one
>
> two
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.
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:
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:
Markdown.pl does not allow this, through fear of triggering a list
via a numeral in a hard-wrapped line:
The number of windows in my house is
14. The number of doors is 6.
Oddly, though, Markdown.pl does allow a blockquote to
interrupt a paragraph, even though the same considerations might
apply.
In CommonMark, we do allow lists to interrupt paragraphs, for two reasons. First, it is natural and not uncommon for people to start lists without blank lines:
I need to buy
- new shoes
- a coat
- a plane ticket
Second, we are attracted to a
principle of uniformity: if a chunk of text has a certain meaning, it will continue to have the same meaning when put into a container block (such as a list item or blockquote).
(Indeed, the spec for list items and block quotes presupposes this principle.) This principle implies that if
* I need to buy
- new shoes
- a coat
- a plane ticket
is a list item containing a paragraph followed by a nested sublist,
as all Markdown implementations agree it is (though the paragraph
may be rendered without <p> tags, since the list is “tight”),
then
I need to buy
- new shoes
- a coat
- a plane ticket
by itself should be a paragraph followed by a nested sublist.
Since it is well established Markdown practice to allow lists to interrupt paragraphs inside list items, the principle of uniformity requires us to allow this outside list items as well. (reStructuredText takes a different approach, requiring blank lines before lists even inside other list items.)
In order to solve the problem of unwanted lists in paragraphs with
hard-wrapped numerals, we allow only lists starting with 1 to
interrupt paragraphs. Thus,
The number of windows in my house is
14. The number of doors is 6.
<p>The number of windows in my house is
14. The number of doors is 6.</p>
We may still get an unintended result in cases like
The number of windows in my house is
1. The number of doors is 6.
<p>The number of windows in my house is</p>
<ol>
<li>The number of doors is 6.</li>
</ol>
but this rule should prevent most spurious list captures.
There can be any number of blank lines between items:
- foo
- bar
- baz
<ul>
<li>
<p>foo</p>
</li>
<li>
<p>bar</p>
</li>
<li>
<p>baz</p>
</li>
</ul>
- foo
- bar
- baz
bim
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>
<p>baz</p>
<p>bim</p>
</li>
</ul>
</li>
</ul>
</li>
</ul>
To separate consecutive lists of the same type, or to separate a list from an indented code block that would otherwise be parsed as a subparagraph of the final list item, you can insert a blank HTML comment:
- foo
- bar
<!-- -->
- baz
- bim
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<!-- -->
<ul>
<li>baz</li>
<li>bim</li>
</ul>
- foo
notcode
- foo
<!-- -->
code
<ul>
<li>
<p>foo</p>
<p>notcode</p>
</li>
<li>
<p>foo</p>
</li>
</ul>
<!-- -->
<pre><code>code
</code></pre>
List items need not be indented to the same level. The following list items will be treated as items at the same list level, since none is indented enough to belong to the previous list item:
- a
- b
- c
- d
- e
- f
- g
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d</li>
<li>e</li>
<li>f</li>
<li>g</li>
</ul>
1. a
2. b
3. c
<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ol>
Note, however, that list items may not be preceded by more than
three spaces of indentation. Here - e is treated as a paragraph continuation
line, because it is indented more than three spaces:
And here, 3. c is treated as in indented code block,
because it is indented four spaces and preceded by a
blank line.
1. a
2. b
3. c
<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
</ol>
<pre><code>3. c
</code></pre>
This is a loose list, because there is a blank line between two of the list items:
- a
- b
- c
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ul>
So is this, with a empty second item:
These are loose lists, even though there are no blank lines between the items, because one of the items directly contains two block-level elements with a blank line between them:
- a
- b
c
- d
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
<p>c</p>
</li>
<li>
<p>d</p>
</li>
</ul>
- a
- b
[ref]: /url
- d
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>d</p>
</li>
</ul>
This is a tight list, because the blank lines are in a code block:
- a
- ```
b
```
- c
<ul>
<li>a</li>
<li>
<pre><code>b
</code></pre>
</li>
<li>c</li>
</ul>
This is a tight list, because the blank line is between two paragraphs of a sublist. So the sublist is loose while the outer list is tight:
- a
- b
c
- d
<ul>
<li>a
<ul>
<li>
<p>b</p>
<p>c</p>
</li>
</ul>
</li>
<li>d</li>
</ul>
This is a tight list, because the blank line is inside the block quote:
* a
> b
>
* c
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
</li>
<li>c</li>
</ul>
This list is tight, because the consecutive block elements are not separated by blank lines:
- a
> b
```
c
```
- d
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
<pre><code>c
</code></pre>
</li>
<li>d</li>
</ul>
A single-paragraph list is tight:
This list is loose, because of the blank line between the two block elements in the list item:
Here the outer list is loose, the inner list tight:
* foo
* bar
baz
<ul>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
<p>baz</p>
</li>
</ul>
- a
- b
- c
- d
- e
- f
<ul>
<li>
<p>a</p>
<ul>
<li>b</li>
<li>c</li>
</ul>
</li>
<li>
<p>d</p>
<ul>
<li>e</li>
<li>f</li>
</ul>
</li>
</ul>
Inlines are parsed sequentially from the beginning of the character stream to the end (left to right, in left-to-right languages). Thus, for example, in
hi is parsed as code, leaving the backtick at the end as a literal
backtick.
A backtick string
is a string of one or more backtick characters (`) that is neither
preceded nor followed by a backtick.
A code span begins with a backtick string and ends with a backtick string of equal length. The contents of the code span are the characters between these two backtick strings, normalized in the following ways:
This is a simple code span:
Here two backticks are used, because the code contains a backtick. This example also illustrates stripping of a single leading and trailing space:
This example shows the motivation for stripping leading and trailing spaces:
Note that only one space is stripped:
The stripping only happens if the space is on both sides of the string:
Only spaces, and not unicode whitespace in general, are stripped in this way:
No stripping occurs if the code span contains only spaces:
Line endings are treated like spaces:
Interior spaces are not collapsed:
Note that browsers will typically collapse consecutive spaces
when rendering <code> elements, so it is recommended that
the following CSS be used:
code{white-space: pre-wrap;}
Note that backslash escapes do not work in code spans. All backslashes are treated literally:
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:
And this is not parsed as a link:
Code spans, HTML tags, and autolinks have the same precedence. Thus, this is code:
But this is an HTML tag:
And this is code:
But this is an autolink:
<https://foo.bar.`baz>`
<p><a href="https://foo.bar.%60baz">https://foo.bar.`baz</a>`</p>
When a backtick string is not closed by a matching backtick string, we just have literal backticks:
The following case also illustrates the need for opening and closing backtick strings to be equal in length:
John Gruber’s original Markdown syntax description says:
Markdown treats asterisks (
*) and underscores (_) as indicators of emphasis. Text wrapped with one*or_will be wrapped with an HTML<em>tag; double*’s or_’s will be wrapped with an HTML<strong>tag.
This is enough for most users, but these rules leave much undecided,
especially when it comes to nested emphasis. The original
Markdown.pl test suite makes it clear that triple *** and
___ delimiters can be used for strong emphasis, and most
implementations have also allowed the following patterns:
***strong emph***
***strong** in emph*
***emph* in strong**
**in strong *emph***
*in emph **strong***
The following patterns are less widely supported, but the intent is clear and they are useful (especially in contexts like bibliography entries):
*emph *with emph* in it*
**strong **with strong** in it**
Many implementations have also restricted intraword emphasis to
the * forms, to avoid unwanted emphasis in words containing
internal underscores. (It is best practice to put these in code
spans, but users often do not.)
internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz
The rules given below capture all of these patterns, while allowing for efficient parsing strategies that do not backtrack.
First, some definitions. A delimiter run is either
a sequence of one or more * characters that is not preceded or
followed by a non-backslash-escaped * character, or a sequence
of one or more _ characters that is not preceded or followed by
a non-backslash-escaped _ character.
A left-flanking delimiter run is a delimiter run that is (1) not followed by Unicode whitespace, and either (2a) not followed by a Unicode punctuation character, or (2b) followed by a Unicode punctuation character and preceded by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.
A right-flanking delimiter run is a delimiter run that is (1) not preceded by Unicode whitespace, and either (2a) not preceded by a Unicode punctuation character, or (2b) preceded by a Unicode punctuation character and followed by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.
Here are some examples of delimiter runs.
left-flanking but not right-flanking:
***abc
_abc
**"abc"
_"abc"
right-flanking but not left-flanking:
abc***
abc_
"abc"**
"abc"_
Both left and right-flanking:
abc***def
"abc"_"def"
Neither left nor right-flanking:
abc *** def
a _ b
(The idea of distinguishing left-flanking and right-flanking delimiter runs based on the character before and the character after comes from Roopesh Chander’s vfmd. vfmd uses the terminology “emphasis indicator string” instead of “delimiter run,” and its rules for distinguishing left- and right-flanking runs are a bit more complex than the ones given here.)
The following rules define emphasis and strong emphasis:
A single * character can open emphasis
iff (if and only if) it is part of a left-flanking delimiter run.
A single _ character can open emphasis iff
it is part of a left-flanking delimiter run
and either (a) not part of a right-flanking delimiter run
or (b) part of a right-flanking delimiter run
preceded by a Unicode punctuation character.
A single * character can close emphasis
iff it is part of a right-flanking delimiter run.
A single _ character can close emphasis iff
it is part of a right-flanking delimiter run
and either (a) not part of a left-flanking delimiter run
or (b) part of a left-flanking delimiter run
followed by a Unicode punctuation character.
A double ** can open strong emphasis
iff it is part of a left-flanking delimiter run.
A double __ can open strong emphasis iff
it is part of a left-flanking delimiter run
and either (a) not part of a right-flanking delimiter run
or (b) part of a right-flanking delimiter run
preceded by a Unicode punctuation character.
A double ** can close strong emphasis
iff it is part of a right-flanking delimiter run.
A double __ can close strong emphasis iff
it is part of a right-flanking delimiter run
and either (a) not part of a left-flanking delimiter run
or (b) part of a left-flanking delimiter run
followed by a Unicode punctuation character.
Emphasis begins with a delimiter that can open emphasis and ends
with a delimiter that can close emphasis, and that uses the same
character (_ or *) as the opening delimiter. The
opening and closing delimiters must belong to separate
delimiter runs. If one of the delimiters can both
open and close emphasis, then the sum of the lengths of the
delimiter runs containing the opening and closing delimiters
must not be a multiple of 3 unless both lengths are
multiples of 3.
Strong emphasis begins with a delimiter that
can open strong emphasis and ends with a delimiter that
can close strong emphasis, and that uses the same character
(_ or *) as the opening delimiter. The
opening and closing delimiters must belong to separate
delimiter runs. If one of the delimiters can both open
and close strong emphasis, then the sum of the lengths of
the delimiter runs containing the opening and closing
delimiters must not be a multiple of 3 unless both lengths
are multiples of 3.
A literal * character cannot occur at the beginning or end of
*-delimited emphasis or **-delimited strong emphasis, unless it
is backslash-escaped.
A literal _ character cannot occur at the beginning or end of
_-delimited emphasis or __-delimited strong emphasis, unless it
is backslash-escaped.
Where rules 1–12 above are compatible with multiple parsings, the following principles resolve ambiguity:
The number of nestings should be minimized. Thus, for example,
an interpretation <strong>...</strong> is always preferred to
<em><em>...</em></em>.
An interpretation <em><strong>...</strong></em> is always
preferred to <strong><em>...</em></strong>.
When two potential emphasis or strong emphasis spans overlap,
so that the second begins before the first ends and ends after
the first ends, the first takes precedence. Thus, for example,
*foo _bar* baz_ is parsed as <em>foo _bar</em> baz_ rather
than *foo <em>bar* baz</em>.
When there are two potential emphasis or strong emphasis spans
with the same closing delimiter, the shorter one (the one that
opens later) takes precedence. Thus, for example,
**foo **bar baz** is parsed as **foo <strong>bar baz</strong>
rather than <strong>foo **bar baz</strong>.
Inline code spans, links, images, and HTML tags group more tightly
than emphasis. So, when there is a choice between an interpretation
that contains one of these elements and one that does not, the
former always wins. Thus, for example, *[foo*](bar) is
parsed as *<a href="bar">foo*</a> rather than as
<em>[foo</em>](bar).
These rules can be illustrated through a series of examples.
Rule 1:
This is not emphasis, because the opening * is followed by
whitespace, and hence not part of a left-flanking delimiter run:
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:
Unicode nonbreaking spaces count as whitespace, too:
Unicode symbols count as punctuation, too:
*$*alpha.
*£*bravo.
*€*charlie.
<p>*$*alpha.</p>
<p>*£*bravo.</p>
<p>*€*charlie.</p>
Intraword emphasis with * is permitted:
Rule 2:
This is not emphasis, because the opening _ is followed by
whitespace:
This is not emphasis, because the opening _ is preceded
by an alphanumeric and followed by punctuation:
Emphasis with _ is not allowed inside words:
Here _ does not generate emphasis, because the first delimiter run
is right-flanking and the second left-flanking:
This is emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:
Rule 3:
This is not emphasis, because the closing delimiter does not match the opening delimiter:
This is not emphasis, because the closing * is preceded by
whitespace:
A line ending also counts as whitespace:
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:
The point of this restriction is more easily appreciated with this example:
Intraword emphasis with * is allowed:
Rule 4:
This is not emphasis, because the closing _ is preceded by
whitespace:
This is not emphasis, because the second _ is
preceded by punctuation and followed by an alphanumeric:
This is emphasis within emphasis:
Intraword emphasis is disallowed for _:
This is emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:
Rule 5:
This is not strong emphasis, because the opening delimiter is followed by whitespace:
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:
Intraword strong emphasis with ** is permitted:
Rule 6:
This is not strong emphasis, because the opening delimiter is followed by whitespace:
A line ending counts as whitespace:
This is not strong emphasis, because the opening __ is preceded
by an alphanumeric and followed by punctuation:
Intraword strong emphasis is forbidden with __:
This is strong emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:
Rule 7:
This is not strong emphasis, because the closing delimiter is preceded by whitespace:
(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:
The point of this restriction is more easily appreciated with these examples:
**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**
<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
<em>Asclepias physocarpa</em>)</strong></p>
Intraword emphasis:
Rule 8:
This is not strong emphasis, because the closing delimiter is preceded by whitespace:
This is not strong emphasis, because the second __ is
preceded by punctuation and followed by an alphanumeric:
The point of this restriction is more easily appreciated with this example:
Intraword strong emphasis is forbidden with __:
This is strong emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:
Rule 9:
Any nonempty sequence of inline elements can be the contents of an emphasized span.
In particular, emphasis and strong emphasis can be nested inside emphasis:
Note that in the preceding case, the interpretation
<p><em>foo</em><em>bar<em></em>baz</em></p>
is precluded by the condition that a delimiter that
can both open and close (like the * after foo)
cannot form emphasis if the sum of the lengths of
the delimiter runs containing the opening and
closing delimiters is a multiple of 3 unless
both lengths are multiples of 3.
For the same reason, we don’t get two consecutive emphasis sections in this example:
The same condition ensures that the following cases are all strong emphasis nested inside emphasis, even when the interior whitespace is omitted:
When the lengths of the interior closing and opening delimiter runs are both multiples of 3, though, they can match to create emphasis:
foo******bar*********baz
<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>
Indefinite levels of nesting are possible:
*foo **bar *baz* bim** bop*
<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>
There can be no empty emphasis or strong emphasis:
Rule 10:
Any nonempty sequence of inline elements can be the contents of an strongly emphasized span.
In particular, emphasis and strong emphasis can be nested inside strong emphasis:
Indefinite levels of nesting are possible:
**foo *bar **baz**
bim* bop**
<p><strong>foo <em>bar <strong>baz</strong>
bim</em> bop</strong></p>
There can be no empty emphasis or strong emphasis:
Rule 11:
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:
Rule 12:
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:
Rule 13 implies that if you want emphasis nested directly inside emphasis, you must use different delimiters:
However, strong emphasis within strong emphasis is possible without switching delimiters:
Rule 13 can be applied to arbitrarily long sequences of delimiters:
Rule 14:
Rule 15:
Rule 16:
Rule 17:
**a<https://foo.bar/?q=**>
<p>**a<a href="https://foo.bar/?q=**">https://foo.bar/?q=**</a></p>
__a<https://foo.bar/?q=__>
<p>__a<a href="https://foo.bar/?q=__">https://foo.bar/?q=__</a></p>
A link contains link text (the visible text), a link destination (the URI that is the link destination), and optionally a link title. There are two basic kinds of links in Markdown. In inline links the destination and title are given immediately after the link text. In reference links the destination and title are defined elsewhere in the document.
A link text consists of a sequence of zero or more
inline elements enclosed by square brackets ([ and ]). The
following rules apply:
Links may not contain other links, at any level of nesting. If multiple otherwise valid link definitions appear nested inside each other, the inner-most definition is used.
Brackets are allowed in the link text only if (a) they
are backslash-escaped or (b) they appear as a matched pair of brackets,
with an open bracket [, a sequence of zero or more inlines, and
a close bracket ].
Backtick code spans, autolinks, and raw HTML tags bind more tightly
than the brackets in link text. Thus, for example,
[foo`]` could not be a link text, since the second ]
is part of a code span.
The brackets in link text bind more tightly than markers for
emphasis and strong emphasis. Thus, for example, *[foo*](url) is a link.
A link destination consists of either
a sequence of zero or more characters between an opening < and a
closing > that contains no line endings or unescaped
< or > characters, or
a nonempty sequence of characters that does not start with <,
does not include ASCII control characters
or space character, and includes parentheses only if (a) they are
backslash-escaped or (b) they are part of a balanced pair of
unescaped parentheses.
(Implementations may impose limits on parentheses nesting to
avoid performance issues, but at least three levels of nesting
should be supported.)
A link title consists of either
a sequence of zero or more characters between straight double-quote
characters ("), including a " character only if it is
backslash-escaped, or
a sequence of zero or more characters between straight single-quote
characters ('), including a ' character only if it is
backslash-escaped, or
a sequence of zero or more characters between matching parentheses
((...)), including a ( or ) character only if it is
backslash-escaped.
Although link titles may span multiple lines, they may not contain a blank line.
An inline link consists of a link text followed immediately
by a left parenthesis (, an optional link destination, an optional
link title, and a right parenthesis ).
These four components may be separated by spaces, tabs, and up to one line
ending.
If both link destination and link title are present, they must be
separated by spaces, tabs, and up to one line ending.
The link’s text consists of the inlines contained
in the link text (excluding the enclosing square brackets).
The link’s URI consists of the link destination, excluding enclosing
<...> if present, with backslash-escapes in effect as described
above. The link’s title consists of the link title, excluding its
enclosing delimiters, with backslash-escapes in effect as described
above.
Here is a simple inline link:
The title, the link text and even the destination may be omitted:
The destination can only contain spaces if it is enclosed in pointy brackets:
The destination cannot contain line endings, even if enclosed in pointy brackets:
The destination can contain ) if it is enclosed
in pointy brackets:
Pointy brackets that enclose links must be unescaped:
These are not links, because the opening pointy bracket is not matched properly:
Parentheses inside the link destination may be escaped:
Any number of parentheses are allowed without escaping, as long as they are balanced:
However, if you have unbalanced parentheses, you need to escape or use the
<...> form:
Parentheses and other symbols can also be escaped, as usual in Markdown:
A link can contain fragment identifiers and queries:
[link](#fragment)
[link](https://example.com#fragment)
[link](https://example.com?foo=3#frag)
<p><a href="#fragment">link</a></p>
<p><a href="https://example.com#fragment">link</a></p>
<p><a href="https://example.com?foo=3#frag">link</a></p>
Note that a backslash before a non-escapable character is just a backslash:
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.
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:
Titles may be in single quotes, double quotes, or parentheses:
[link](/url "title")
[link](/url 'title')
[link](/url (title))
<p><a href="/url" title="title">link</a>
<a href="/url" title="title">link</a>
<a href="/url" title="title">link</a></p>
Backslash escapes and entity and numeric character references may be used in titles:
[link](/url "title \""")
<p><a href="/url" title="title """>link</a></p>
Titles must be separated from the link using spaces, tabs, and up to one line ending. Other Unicode whitespace like non-breaking space doesn’t work.
Nested balanced quotes are not allowed without escaping:
[link](/url "title "and" title")
<p>[link](/url "title "and" title")</p>
But it is easy to work around this by using a different quote type:
[link](/url 'title "and" title')
<p><a href="/url" title="title "and" title">link</a></p>
(Note: Markdown.pl did allow double quotes inside a double-quoted
title, and its test suite included a test demonstrating this.
But it is hard to see a good rationale for the extra complexity this
brings, since there are already many ways—backslash escaping,
entity and numeric character references, or using a different
quote type for the enclosing title—to write titles containing
double quotes. Markdown.pl’s handling of titles has a number
of other strange features. For example, it allows single-quoted
titles in inline links, but not reference links. And, in
reference links but not inline links, it allows a title to begin
with " and end with ). Markdown.pl 1.0.1 even allows
titles with no closing quotation mark, though 1.0.2b8 does not.
It seems preferable to adopt a simple, rational rule that works
the same way in inline links and link reference definitions.)
Spaces, tabs, and up to one line ending is allowed around the destination and title:
But it is not allowed between the link text and the following parenthesis:
The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:
The link text may contain inline content:
[link *foo **bar** `#`*](/uri)
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
[](/uri)
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>
However, links may not contain other links, at any level of nesting.
[foo *[bar [baz](/uri)](/uri)*](/uri)
<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>
These cases illustrate the precedence of link text grouping over emphasis grouping:
Note that brackets that aren’t part of links do not take precedence:
These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:
[foo<https://example.com/?search=](uri)>
<p>[foo<a href="https://example.com/?search=%5D(uri)">https://example.com/?search=](uri)</a></p>
There are three kinds of reference links: full, collapsed, and shortcut.
A full reference link consists of a link text immediately followed by a link label that matches a link reference definition elsewhere in the document.
A link label begins with a left bracket ([) and ends
with the first right bracket (]) that is not backslash-escaped.
Between these brackets there must be at least one character that is not a space,
tab, or line ending.
Unescaped square bracket characters are not allowed inside the
opening and closing square brackets of link labels. A link
label can have at most 999 characters inside the square
brackets.
One label matches another just in case their normalized forms are equal. To normalize a label, strip off the opening and closing brackets, perform the Unicode case fold, strip leading and trailing spaces, tabs, and line endings, and collapse consecutive internal spaces, tabs, and line endings to a single space. If there are multiple matching reference link definitions, the one that comes first in the document is used. (It is desirable in such cases to emit a warning.)
The link’s URI and title are provided by the matching link reference definition.
Here is a simple example:
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:
The link text may contain inline content:
[link *foo **bar** `#`*][ref]
[ref]: /uri
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
[][ref]
[ref]: /uri
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>
However, links may not contain other links, at any level of nesting.
[foo [bar](/uri)][ref]
[ref]: /uri
<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>
[foo *bar [baz][ref]*][ref]
[ref]: /uri
<p>[foo <em>bar <a href="/uri">baz</a></em>]<a href="/uri">ref</a></p>
(In the examples above, we have two shortcut reference links instead of one full reference link.)
The following cases illustrate the precedence of link text grouping over emphasis grouping:
These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:
[foo<https://example.com/?search=][ref]>
[ref]: /uri
<p>[foo<a href="https://example.com/?search=%5D%5Bref%5D">https://example.com/?search=][ref]</a></p>
Matching is case-insensitive:
Unicode case fold is used:
Consecutive internal spaces, tabs, and line endings are treated as one space for purposes of determining matching:
No spaces, tabs, or line endings are allowed between the link text and the link label:
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:
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:
Link labels cannot contain brackets, unless they are backslash-escaped:
Note that in this example ] is not backslash-escaped:
A link label must contain at least one character that is not a space, tab, or line ending:
A collapsed reference link
consists of a link label that matches a
link reference definition elsewhere in the
document, followed by the string [].
The contents of the link label are parsed as inlines,
which are used as the link’s text. The link’s URI and title are
provided by the matching reference link definition. Thus,
[foo][] is equivalent to [foo][foo].
[*foo* bar][]
[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
The link labels are case-insensitive:
As with full reference links, spaces, tabs, or line endings are not allowed between the two sets of brackets:
A shortcut reference link
consists of a link label that matches a
link reference definition elsewhere in the
document and is not followed by [] or a link label.
The contents of the link label are parsed as inlines,
which are used as the link’s text. The link’s URI and title
are provided by the matching link reference definition.
Thus, [foo] is equivalent to [foo][].
[*foo* bar]
[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
[[*foo* bar]]
[*foo* bar]: /url "title"
<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>
The link labels are case-insensitive:
A space after the link text should be preserved:
If you just want bracketed text, you can backslash-escape the opening bracket to avoid links:
Note that this is a link, because a link label ends with the first following closing bracket:
Full and collapsed references take precedence over shortcut references:
Inline links also take precedence:
In the following case [bar][baz] is parsed as a reference,
[foo] as normal text:
Here, though, [foo][bar] is parsed as a reference, since
[bar] is defined:
[foo][bar][baz]
[baz]: /url1
[bar]: /url2
<p><a href="/url2">foo</a><a href="/url1">baz</a></p>
Here [foo] is not parsed as a shortcut reference, because it
is followed by a link label (even though [bar] is not defined):
Syntax for images is like the syntax for links, with one
difference. Instead of link text, we have an
image description. The rules for this are the
same as for link text, except that (a) an
image description starts with ![ rather than [, and
(b) an image description may contain links.
An image description has inline elements
as its contents. When an image is rendered to HTML,
this is standardly used as the image’s alt attribute.
![foo *bar*]
[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
Though this spec is concerned with parsing, not rendering, it is
recommended that in rendering to HTML, only the plain string content
of the image description be used. Note that in
the above example, the alt attribute’s value is foo bar, not foo [bar](/url) or foo <a href="/url">bar</a>. Only the plain string
content is rendered, without formatting.
![foo *bar*][]
[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
![foo *bar*][foobar]
[FOOBAR]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
My 
<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>
Reference-style:
Collapsed:
![*foo* bar][]
[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>
The labels are case-insensitive:
As with reference links, spaces, tabs, and line endings, are not allowed between the two sets of brackets:
![foo]
[]
[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" />
[]</p>
Shortcut:
![*foo* bar]
[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>
Note that link labels cannot contain unescaped brackets:
![[foo]]
[[foo]]: /url "title"
<p>![[foo]]</p>
<p>[[foo]]: /url "title"</p>
The link labels are case-insensitive:
If you just want a literal ! followed by bracketed text, you can
backslash-escape the opening [:
If you want a link after a literal !, backslash-escape the
!:
Autolinks are absolute URIs and email addresses inside
< and >. They are parsed as links, with the URL or email address
as the link label.
A URI autolink consists of <, followed by an
absolute URI followed by >. It is parsed as
a link to the URI, with the URI as the link’s label.
An absolute URI,
for these purposes, consists of a scheme followed by a colon (:)
followed by zero or more characters other than ASCII control
characters, space, <, and >.
If the URI includes these characters, they must be percent-encoded
(e.g. %20 for a space).
For purposes of this spec, a scheme is any sequence of 2–32 characters beginning with an ASCII letter and followed by any combination of ASCII letters, digits, or the symbols plus (“+”), period (“.”), or hyphen (“-”).
Here are some valid autolinks:
<https://foo.bar.baz/test?q=hello&id=22&boolean>
<p><a href="https://foo.bar.baz/test?q=hello&id=22&boolean">https://foo.bar.baz/test?q=hello&id=22&boolean</a></p>
<irc://foo.bar:2233/baz>
<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p>
Uppercase is also fine:
<MAILTO:[email protected]>
<p><a href="MAILTO:[email protected]">MAILTO:[email protected]</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:
<made-up-scheme://foo,bar>
<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p>
Spaces are not allowed in autolinks:
Backslash-escapes do not work inside autolinks:
<https://example.com/\[\>
<p><a href="https://example.com/%5C%5B%5C">https://example.com/\[\</a></p>
An email autolink
consists of <, followed by an email address,
followed by >. The link’s label is the email address,
and the URL is mailto: followed by the email address.
An email address, for these purposes, is anything that matches the non-normative regex from the HTML5 spec:
/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?
(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/
Examples of email autolinks:
<p><a href="mailto:[email protected]">[email protected]</a></p>
<p><a href="mailto:[email protected]">[email protected]</a></p>
Backslash-escapes do not work inside email autolinks:
These are not autolinks:
Text between < and > that looks like an HTML tag is parsed as a
raw HTML tag and will be rendered in HTML without escaping.
Tag and attribute names are not limited to current HTML tags,
so custom tags (and even, say, DocBook tags) may be used.
Here is the grammar for tags:
A tag name consists of an ASCII letter
followed by zero or more ASCII letters, digits, or
hyphens (-).
An attribute consists of spaces, tabs, and up to one line ending, an attribute name, and an optional attribute value specification.
An attribute name
consists of an ASCII letter, _, or :, followed by zero or more ASCII
letters, digits, _, ., :, or -. (Note: This is the XML
specification restricted to ASCII. HTML5 is laxer.)
An attribute value specification
consists of optional spaces, tabs, and up to one line ending,
a = character, optional spaces, tabs, and up to one line ending,
and an attribute value.
An attribute value consists of an unquoted attribute value, a single-quoted attribute value, or a double-quoted attribute value.
An unquoted attribute value
is a nonempty string of characters not
including spaces, tabs, line endings, ", ', =, <, >, or `.
A single-quoted attribute value
consists of ', zero or more
characters not including ', and a final '.
A double-quoted attribute value
consists of ", zero or more
characters not including ", and a final ".
An open tag consists of a < character, a tag name,
zero or more attributes, optional spaces, tabs, and up to one line ending,
an optional / character, and a > character.
A closing tag consists of the string </, a
tag name, optional spaces, tabs, and up to one line ending, and the character
>.
An HTML comment consists of <!-->, <!--->, or <!--, a string of
characters not including the string -->, and --> (see the
HTML spec).
A processing instruction
consists of the string <?, a string
of characters not including the string ?>, and the string
?>.
A declaration consists of the string <!, an ASCII letter, zero or more
characters not including the character >, and the character >.
A CDATA section consists of
the string <![CDATA[, a string of characters not including the string
]]>, and the string ]]>.
An HTML tag consists of an open tag, a closing tag, an HTML comment, a processing instruction, a declaration, or a CDATA section.
Here are some simple open tags:
Empty elements:
Whitespace is allowed:
With attributes:
<a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 />
<p><a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 /></p>
Custom tag names can be used:
Foo <responsive-image src="foo.jpg" />
<p>Foo <responsive-image src="foo.jpg" /></p>
Illegal tag names, not parsed as HTML:
Illegal attribute names:
Illegal attribute values:
Illegal whitespace:
< a><
foo><bar/ >
<foo bar=baz
bim!bop />
<p>< a><
foo><bar/ >
<foo bar=baz
bim!bop /></p>
Missing whitespace:
Closing tags:
Illegal attributes in closing tag:
Comments:
foo <!-- this is a --
comment - with hyphens -->
<p>foo <!-- this is a --
comment - with hyphens --></p>
foo <!--> foo -->
foo <!---> foo -->
<p>foo <!--> foo --></p>
<p>foo <!---> foo --></p>
Processing instructions:
Declarations:
CDATA sections:
Entity and numeric character references are preserved in HTML attributes:
Backslash escapes do not work in HTML attributes:
A line ending (not in a code span or HTML tag) that is preceded
by two or more spaces and does not occur at the end of a block
is parsed as a hard line break (rendered
in HTML as a <br /> tag):
For a more visible alternative, a backslash before the line ending may be used instead of two or more spaces:
More than two spaces can be used:
Leading spaces at the beginning of the next line are ignored:
Hard line breaks can occur inside emphasis, links, and other constructs that allow inline content:
Hard line breaks do not occur inside code spans
or HTML tags:
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:
A regular line ending (not in a code span or HTML tag) that is not preceded by two or more spaces or a backslash is parsed as a softbreak. (A soft line break may be rendered in HTML either as a line ending or as a space. The result will be the same in browsers. In the examples here, a line ending will be used.)
Spaces at the end of the line and beginning of the next line are removed:
A conforming parser may render a soft line break in HTML either as a line ending or as a space.
A renderer may also provide an option to render soft line breaks as hard line breaks.
Any characters not given an interpretation by the above rules will be parsed as plain textual content.
Internal spaces are preserved verbatim:
In this appendix we describe some features of the parsing strategy used in the CommonMark reference implementations.
Parsing has two phases:
In the first phase, lines of input are consumed and the block structure of the document—its division into paragraphs, block quotes, list items, and so on—is constructed. Text is assigned to these blocks but not parsed. Link reference definitions are parsed and a map of links is constructed.
In the second phase, the raw text contents of paragraphs and headings are parsed into sequences of Markdown inline elements (strings, code spans, links, emphasis, and so on), using the map of link references constructed in phase 1.
At each point in processing, the document is represented as a tree of
blocks. The root of the tree is a document block. The document
may have any number of other blocks as children. These children
may, in turn, have other blocks as children. The last child of a block
is normally considered open, meaning that subsequent lines of input
can alter its contents. (Blocks that are not open are closed.)
Here, for example, is a possible document tree, with the open blocks
marked by arrows:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui *quodsi iracundia*"
-> list_item
-> paragraph
"aliquando id"
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:
Once a line has been incorporated into the tree in this way, it can be discarded, so input can be read in a stream.
For each line, we follow this procedure:
First we iterate through the open blocks, starting with the
root document, and descending through last children down to the last
open block. Each block imposes a condition that the line must satisfy
if the block is to remain open. For example, a block quote requires a
> character. A paragraph requires a non-blank line.
In this phase we may match all or just some of the open
blocks. But we cannot close unmatched blocks yet, because we may have a
lazy continuation line.
Next, after consuming the continuation markers for existing
blocks, we look for new block starts (e.g. > for a block quote).
If we encounter a new block start, we close any blocks unmatched
in step 1 before creating the new block as a child of the last
matched container block.
Finally, we look at the remainder of the line (after block
markers like >, list markers, and indentation have been consumed).
This is text that can be incorporated into the last open
block (a paragraph, code block, heading, or raw HTML).
Setext headings are formed when we see a line of a paragraph that is a setext heading underline.
Reference link definitions are detected when a paragraph is closed; the accumulated text lines are parsed to see if they begin with one or more reference link definitions. Any remainder becomes a normal paragraph.
We can see how this works by considering how the tree above is generated by four lines of Markdown:
> Lorem ipsum dolor
sit amet.
> - Qui *quodsi iracundia*
> - aliquando id
At the outset, our document model is just
-> document
The first line of our text,
> Lorem ipsum dolor
causes a block_quote block to be created as a child of our
open document block, and a paragraph block as a child of
the block_quote. Then the text is added to the last open
block, the paragraph:
-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor"
The next line,
sit amet.
is a “lazy continuation” of the open paragraph, so it gets added
to the paragraph’s text:
-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor\nsit amet."
The third line,
> - Qui *quodsi iracundia*
causes the paragraph block to be closed, and a new list block
opened as a child of the block_quote. A list_item is also
added as a child of the list, and a paragraph as a child of
the list_item. The text is then added to the new paragraph:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
-> list_item
-> paragraph
"Qui *quodsi iracundia*"
The fourth line,
> - aliquando id
causes the list_item (and its child the paragraph) to be closed,
and a new list_item opened up as child of the list. A paragraph
is added as a child of the new list_item, to contain the text.
We thus obtain the final tree:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui *quodsi iracundia*"
-> list_item
-> paragraph
"aliquando id"
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
* or _ characters, or[ or ![we insert a text node with these symbols as its literal content, and we add a pointer to this text node to the delimiter stack.
The delimiter stack is a doubly linked list. Each element contains a pointer to a text node, plus information about
[, ![, *, _)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.
If we don’t find one, we return a literal text node ].
If we do find one, but it’s not active, we remove the inactive
delimiter from the stack, and return a literal text node ].
If we find one and it’s active, then we parse ahead to see if we have an inline link/image, reference link/image, collapsed reference link/image, or shortcut reference link/image.
If we don’t, then we remove the opening delimiter from the
delimiter stack and return a literal text node ].
If we do, then
We return a link or image node whose children are the inlines after the text node pointed to by the opening delimiter.
We run process emphasis on these inlines, with the [ opener
as stack_bottom.
We remove the opening delimiter.
If we have a link (and not an image), we also set all
[ delimiters before the opening delimiter to inactive. (This
will prevent us from getting links within links.)
Parameter stack_bottom sets a lower bound to how far we
descend in the delimiter stack. If it is NULL, we can
go all the way to the bottom. Otherwise, we stop before
visiting stack_bottom.
Let current_position point to the element on the delimiter stack
just above stack_bottom (or the first element if stack_bottom
is NULL).
We keep track of the openers_bottom for each delimiter
type (*, _), indexed to the length of the closing delimiter run
(modulo 3) and to whether the closing delimiter can also be an
opener. Initialize this to stack_bottom.
Then we repeat the following until we run out of potential closers:
Move current_position forward in the delimiter stack (if needed)
until we find the first potential closer with delimiter * or _.
(This will be the potential closer closest
to the beginning of the input – the first one in parse order.)
Now, look back in the stack (staying above stack_bottom and
the openers_bottom for this delimiter type) for the
first matching potential opener (“matching” means same delimiter).
If one is found:
Figure out whether we have emphasis or strong emphasis: if both closer and opener spans have length >= 2, we have strong, otherwise regular.
Insert an emph or strong emph node accordingly, after the text node corresponding to the opener.
Remove any delimiters between the opener and closer from the delimiter stack.
Remove 1 (for regular emph) or 2 (for strong emph) delimiters
from the opening and closing text nodes. If they become empty
as a result, remove them and remove the corresponding element
of the delimiter stack. If the closing node is removed, reset
current_position to the next element in the stack.
If none is found:
Set openers_bottom to the element before current_position.
(We know that there are no openers for this kind of closer up to and
including this point, so this puts a lower bound on future searches.)
If the closer at current_position is not a potential opener,
remove it from the delimiter stack (since we know it can’t
be a closer either).
Advance current_position to the next element in the stack.
After we’re done, we remove all delimiters above stack_bottom from the
delimiter stack.