# CommonMark Spec

Version 0.25 (2016-03-24)

This is an older version of the spec. For the most recent version, see http://spec.commonmark.org.

# 1Introduction

## 1.1What is Markdown?

Markdown is a plain text format for writing structured documents, based on conventions used for indicating formatting in email and usenet posts. It was developed in 2004 by John Gruber, who wrote the first Markdown-to-HTML converter in perl, and it soon became widely used in websites. By 2014 there were dozens of implementations in many languages. Some of them extended basic Markdown syntax with conventions for footnotes, definition lists, tables, and other constructs, and some allowed output not just in HTML but in LaTeX and many other formats.

## 1.2Why is a spec needed?

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

1. How much indentation is needed for a sublist? The spec says that continuation paragraphs need to be indented four spaces, but is not fully explicit about sublists. It is natural to think that they, too, must be indented four spaces, but Markdown.pl does not require that. This is hardly a “corner case,” and divergences between implementations on this issue often lead to surprises for users in real documents. (See this comment by John Gruber.)

2. Is a blank line needed before a block quote or heading? Most implementations do not require the blank line. However, this can lead to unexpected results in hard-wrapped text, and also to ambiguities in parsing (note that some implementations put the heading inside the blockquote, while others do not). (John Gruber has also spoken in favor of requiring the blank lines.)

3. Is a blank line needed before an indented code block? (Markdown.pl requires it, but this is not mentioned in the documentation, and some implementations do not require it.)

paragraph
code?

4. What is the exact rule for determining when list items get wrapped in <p> tags? Can a list be partially “loose” and partially “tight”? What should we do with a list like this?

1. one

2. two
3. three


Or this?

1.  one
- a

- b
2.  two


(There are some relevant comments by John Gruber here.)

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

 8. item 1
9. item 2
10. item 2a

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

* a
* * * * *
* b

7. When list markers change from numbers to bullets, do we have two lists or one? (The Markdown syntax description suggests two, but the perl scripts and many other implementations produce one.)

1. fee
2. fie
-  foe
-  fum

8. What are the precedence rules for the markers of inline structure? For example, is the following a valid link, or does the code span take precedence ?

[a backtick ()](/url) and [another backtick ()](/url).

9. What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?

*foo *bar* baz*

10. What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed?

- a long code span can contain a hyphen like this
- and it can screw things up

11. Can list items include section headings? (Markdown.pl does not allow this, but does allow blockquotes to include headings.)

- # Heading

12. Can list items be empty?

* a
*
* b

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

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

14. If there are multiple definitions for the same reference, which takes precedence?

[foo]: /url1
[foo]: /url2

[foo][]


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

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

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

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


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

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

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

# 2Preliminaries

## 2.1Characters and lines

Any sequence of characters is a valid CommonMark document.

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

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

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

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

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

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

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

Whitespace is a sequence of one or more whitespace characters.

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

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

A space is U+0020.

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

An ASCII punctuation character is !, ", #, $, %, &, ', (, ), *, +, ,, -, ., /, :, ;, <, =, >, ?, @, [, \, ], ^, _, , {, |, }, or ~. A punctuation character is an ASCII punctuation character or anything in the Unicode classes Pc, Pd, Pe, Pf, Pi, Po, or Ps. ## 2.2Tabs Tabs in lines are not expanded to spaces. However, in contexts where indentation is significant for the document’s structure, tabs behave as if they were replaced by spaces with a tab stop of 4 characters. →foo→baz→→bim  <pre><code>foo→baz→→bim </code></pre>   →foo→baz→→bim  <pre><code>foo→baz→→bim </code></pre>   a→a ὐ→a  <pre><code>a→a ὐ→a </code></pre>   - foo →bar  <ul> <li> <p>foo</p> <p>bar</p> </li> </ul>  - foo →→bar  <ul> <li> <p>foo</p> <pre><code> bar </code></pre> </li> </ul>  >→→foo  <blockquote> <pre><code> foo </code></pre> </blockquote>  -→→foo  <ul> <li> <pre><code> foo </code></pre> </li> </ul>   foo →bar  <pre><code>foo bar </code></pre>   - foo - bar → - baz  <ul> <li>foo <ul> <li>bar <ul> <li>baz</li> </ul> </li> </ul> </li> </ul>  ## 2.3Insecure characters For security reasons, the Unicode character U+0000 must be replaced with the REPLACEMENT CHARACTER (U+FFFD). # 3Blocks and inlines We can think of a document as a sequence of blocks—structural elements like paragraphs, block quotations, lists, headings, rules, and code blocks. Some blocks (like block quotes and list items) contain other blocks; others (like headings and paragraphs) contain inline content—text, links, emphasized text, images, code, and so on. ## 3.1Precedence Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span: - one - two  <ul> <li>one</li> <li>two</li> </ul>  This means that parsing can proceed in two steps: first, the block structure of the document can be discerned; second, text lines inside paragraphs, headings, and other block constructs can be parsed for inline structure. The second step requires information about link reference definitions that will be available only at the end of the first step. Note that the first step requires processing lines in sequence, but the second can be parallelized, since the inline parsing of one block element does not affect the inline parsing of any other. ## 3.2Container blocks and leaf blocks We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot. # 4Leaf blocks This section describes the different kinds of leaf block that make up a Markdown document. ## 4.1Thematic breaks A line consisting of 0-3 spaces of indentation, followed by a sequence of three or more matching -, _, or * characters, each followed optionally by any number of spaces, forms a thematic break. *** --- ___  <hr /> <hr /> <hr />  Wrong characters: +++  <p>+++</p>  ===  <p>===</p>  Not enough characters: -- ** __  <p>-- ** __</p>  One to three spaces indent are allowed:  *** *** ***  <hr /> <hr /> <hr />  Four spaces is too many:  ***  <pre><code>*** </code></pre>  Foo ***  <p>Foo ***</p>  More than three characters may be used: _____________________________________  <hr />  Spaces are allowed between the characters:  - - -  <hr />   ** * ** * ** * **  <hr />  - - - -  <hr />  Spaces are allowed at the end: - - - -  <hr />  However, no other characters may occur in the line: _ _ _ _ a a------ ---a---  <p>_ _ _ _ a</p> <p>a------</p> <p>---a---</p>  It is required that all of the non-whitespace characters be the same. So, this is not a thematic break:  *-*  <p><em>-</em></p>  Thematic breaks do not need blank lines before or after: - foo *** - bar  <ul> <li>foo</li> </ul> <hr /> <ul> <li>bar</li> </ul>  Thematic breaks can interrupt a paragraph: Foo *** bar  <p>Foo</p> <hr /> <p>bar</p>  If a line of dashes that meets the above conditions for being a thematic break could also be interpreted as the underline of a setext heading, the interpretation as a setext heading takes precedence. Thus, for example, this is a setext heading, not a paragraph followed by a thematic break: Foo --- bar  <h2>Foo</h2> <p>bar</p>  When both a thematic break and a list item are possible interpretations of a line, the thematic break takes precedence: * Foo * * * * Bar  <ul> <li>Foo</li> </ul> <hr /> <ul> <li>Bar</li> </ul>  If you want a thematic break in a list item, use a different bullet: - Foo - * * *  <ul> <li>Foo</li> <li> <hr /> </li> </ul>  ## 4.2ATX headings An ATX heading consists of a string of characters, parsed as inline content, between an opening sequence of 1–6 unescaped # characters and an optional closing sequence of any number of unescaped # characters. The opening sequence of # characters must be followed by a space or by the end of line. The optional closing sequence of #s must be preceded by a space and may be followed by spaces only. The opening # character may be indented 0-3 spaces. The raw contents of the heading are stripped of leading and trailing spaces before being parsed as inline content. The heading level is equal to the number of # characters in the opening sequence. Simple headings: # foo ## foo ### foo #### foo ##### foo ###### foo  <h1>foo</h1> <h2>foo</h2> <h3>foo</h3> <h4>foo</h4> <h5>foo</h5> <h6>foo</h6>  More than six # characters is not a heading: ####### foo  <p>####### foo</p>  At least one space is required between the # characters and the heading’s contents, unless the heading is empty. Note that many implementations currently do not require the space. However, the space was required by the original ATX implementation, and it helps prevent things like the following from being parsed as headings: #5 bolt #hashtag  <p>#5 bolt</p> <p>#hashtag</p>  A tab will not work: #→foo  <p>#→foo</p>  This is not a heading, because the first # is escaped: \## foo  <p>## foo</p>  Contents are parsed as inlines: # foo *bar* \*baz\*  <h1>foo <em>bar</em> *baz*</h1>  Leading and trailing blanks are ignored in parsing inline content: # foo  <h1>foo</h1>  One to three spaces indentation are allowed:  ### foo ## foo # foo  <h3>foo</h3> <h2>foo</h2> <h1>foo</h1>  Four spaces are too much:  # foo  <pre><code># foo </code></pre>  foo # bar  <p>foo # bar</p>  A closing sequence of # characters is optional: ## foo ## ### bar ###  <h2>foo</h2> <h3>bar</h3>  It need not be the same length as the opening sequence: # foo ################################## ##### foo ##  <h1>foo</h1> <h5>foo</h5>  Spaces are allowed after the closing sequence: ### foo ###  <h3>foo</h3>  A sequence of # characters with anything but spaces following it is not a closing sequence, but counts as part of the contents of the heading: ### foo ### b  <h3>foo ### b</h3>  The closing sequence must be preceded by a space: # foo#  <h1>foo#</h1>  Backslash-escaped # characters do not count as part of the closing sequence: ### foo \### ## foo #\## # foo \#  <h3>foo ###</h3> <h2>foo ###</h2> <h1>foo #</h1>  ATX headings need not be separated from surrounding content by blank lines, and they can interrupt paragraphs: **** ## foo ****  <hr /> <h2>foo</h2> <hr />  Foo bar # baz Bar foo  <p>Foo bar</p> <h1>baz</h1> <p>Bar foo</p>  ATX headings can be empty: ## # ### ###  <h2></h2> <h1></h1> <h3></h3>  ## 4.3Setext headings A setext heading consists of one or more lines of text, each containing at least one non-whitespace character, with no more than 3 spaces indentation, followed by a setext heading underline. The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a code fence, ATX heading, block quote, thematic break, list item, or HTML block. A setext heading underline is a sequence of = characters or a sequence of - characters, with no more than 3 spaces indentation and any number of trailing spaces. If a line containing a single - can be interpreted as an empty list items, it should be interpreted this way and not as a setext heading underline. The heading is a level 1 heading if = characters are used in the setext heading underline, and a level 2 heading if - characters are used. The contents of the heading are the result of parsing the preceding lines of text as CommonMark inline content. In general, a setext heading need not be preceded or followed by a blank line. However, it cannot interrupt a paragraph, so when a setext heading comes after a paragraph, a blank line is needed between them. Simple examples: Foo *bar* ========= Foo *bar* ---------  <h1>Foo <em>bar</em></h1> <h2>Foo <em>bar</em></h2>  The content of the header may span more than one line: Foo *bar baz* ====  <h1>Foo <em>bar baz</em></h1>  The underlining can be any length: Foo ------------------------- Foo =  <h2>Foo</h2> <h1>Foo</h1>  The heading content can be indented up to three spaces, and need not line up with the underlining:  Foo --- Foo ----- Foo ===  <h2>Foo</h2> <h2>Foo</h2> <h1>Foo</h1>  Four spaces indent is too much:  Foo --- Foo ---  <pre><code>Foo --- Foo </code></pre> <hr />  The setext heading underline can be indented up to three spaces, and may have trailing spaces: Foo ----  <h2>Foo</h2>  Four spaces is too much: Foo ---  <p>Foo ---</p>  The setext heading underline cannot contain internal spaces: Foo = = Foo --- -  <p>Foo = =</p> <p>Foo</p> <hr />  Trailing spaces in the content line do not cause a line break: Foo -----  <h2>Foo</h2>  Nor does a backslash at the end: Foo\ ----  <h2>Foo\</h2>  Since indicators of block structure take precedence over indicators of inline structure, the following are setext headings: Foo ----  <a title="a lot --- of dashes"/>  <h2>Foo</h2> <p></p> <h2>&lt;a title=&quot;a lot</h2> <p>of dashes&quot;/&gt;</p>  The setext heading underline cannot be a lazy continuation line in a list item or block quote: > Foo ---  <blockquote> <p>Foo</p> </blockquote> <hr />  > foo bar ===  <blockquote> <p>foo bar ===</p> </blockquote>  - Foo ---  <ul> <li>Foo</li> </ul> <hr />  A blank line is needed between a paragraph and a following setext heading, since otherwise the paragraph becomes part of the heading’s content: Foo Bar ---  <h2>Foo Bar</h2>  But in general a blank line is not required before or after setext headings: --- Foo --- Bar --- Baz  <hr /> <h2>Foo</h2> <h2>Bar</h2> <p>Baz</p>  Setext headings cannot be empty:  ====  <p>====</p>  Setext heading text lines must not be interpretable as block constructs other than paragraphs. So, the line of dashes in these examples gets interpreted as a thematic break: --- ---  <hr /> <hr />  - foo -----  <ul> <li>foo</li> </ul> <hr />   foo ---  <pre><code>foo </code></pre> <hr />  > foo -----  <blockquote> <p>foo</p> </blockquote> <hr />  If you want a heading with > foo as its literal text, you can use backslash escapes: \> foo ------  <h2>&gt; foo</h2>  Compatibility note: Most existing Markdown implementations do not allow the text of setext headings to span multiple lines. But there is no consensus about how to interpret Foo bar --- baz  One can find four different interpretations: 1. paragraph “Foo”, heading “bar”, paragraph “baz” 2. paragraph “Foo bar”, thematic break, paragraph “baz” 3. paragraph “Foo bar — baz” 4. heading “Foo bar”, paragraph “baz” We find interpretation 4 most natural, and interpretation 4 increases the expressive power of CommonMark, by allowing multiline headings. Authors who want interpretation 1 can put a blank line after the first paragraph: Foo bar --- baz  <p>Foo</p> <h2>bar</h2> <p>baz</p>  Authors who want interpretation 2 can put blank lines around the thematic break, Foo bar --- baz  <p>Foo bar</p> <hr /> <p>baz</p>  or use a thematic break that cannot count as a setext heading underline, such as Foo bar * * * baz  <p>Foo bar</p> <hr /> <p>baz</p>  Authors who want interpretation 3 can use backslash escapes: Foo bar \--- baz  <p>Foo bar --- baz</p>  ## 4.4Indented code blocks An indented code block is composed of one or more indented chunks separated by blank lines. An indented chunk is a sequence of non-blank lines, each indented four or more spaces. The contents of the code block are the literal contents of the lines, including trailing line endings, minus four spaces of indentation. An indented code block has no info string. An indented code block cannot interrupt a paragraph, so there must be a blank line between a paragraph and a following indented code block. (A blank line is not needed, however, between a code block and a following paragraph.)  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:  - foo bar  <ul> <li> <p>foo</p> <p>bar</p> </li> </ul>  1. foo - bar  <ol> <li> <p>foo</p> <ul> <li>bar</li> </ul> </li> </ol>  The contents of a code block are literal text, and do not get parsed as Markdown:  <a/> *hi* - one  <pre><code>&lt;a/&gt; *hi* - one </code></pre>  Here we have three chunks separated by blank lines:  chunk1 chunk2 chunk3  <pre><code>chunk1 chunk2 chunk3 </code></pre>  Any initial spaces beyond four will be included in the content, even in interior blank lines:  chunk1 chunk2  <pre><code>chunk1 chunk2 </code></pre>  An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.) Foo bar  <p>Foo bar</p>  However, any non-blank line with fewer than four leading spaces ends the code block immediately. So a paragraph may occur immediately after indented code:  foo bar  <pre><code>foo </code></pre> <p>bar</p>  And indented code can occur immediately before and after other kinds of blocks: # Heading foo Heading ------ foo ----  <h1>Heading</h1> <pre><code>foo </code></pre> <h2>Heading</h2> <pre><code>foo </code></pre> <hr />  The first line can be indented more than four spaces:  foo bar  <pre><code> foo bar </code></pre>  Blank lines preceding or following an indented code block are not included in it:  foo  <pre><code>foo </code></pre>  Trailing spaces are included in the code block’s content:  foo  <pre><code>foo </code></pre>  ## 4.5Fenced code blocks A code fence is a sequence of at least three consecutive backtick characters () or tildes (~). (Tildes and backticks cannot be mixed.) A fenced code block begins with a code fence, indented no more than three spaces. The line with the opening code fence may optionally contain some text following the code fence; this is trimmed of leading and trailing spaces and called the info string. The info string may not contain any backtick characters. (The reason for this restriction is that otherwise some inline code would be incorrectly interpreted as the beginning of a fenced code block.) The content of the code block consists of all subsequent lines, until a closing code fence of the same type as the code block began with (backticks or tildes), and with at least as many backticks or tildes as the opening code fence. If the leading code fence is indented N spaces, then up to N spaces of indentation are removed from each line of the content (if present). (If a content line is not indented, it is preserved unchanged. If it is indented less than N spaces, all of the indentation is removed.) The closing code fence may be indented up to three spaces, and may be followed only by spaces, which are ignored. If the end of the containing block (or document) is reached and no closing code fence has been found, the code block contains all of the lines after the opening code fence until the end of the containing block (or document). (An alternative spec would require backtracking in the event that a closing code fence is not found. But this makes parsing much less efficient, and there seems to be no real down side to the behavior described here.) A fenced code block may interrupt a paragraph, and does not require a blank line either before or after. The content of a code fence is treated as literal text, not parsed as inlines. The first word of the info string is typically used to specify the language of the code sample, and rendered in the class attribute of the code tag. However, this spec does not mandate any particular treatment of the info string. Here is a simple example with backticks:  < >   <pre><code>&lt; &gt; </code></pre>  With tildes: ~~~ < > ~~~  <pre><code>&lt; &gt; </code></pre>  The closing code fence must use the same character as the opening fence:  aaa ~~~   <pre><code>aaa ~~~ </code></pre>  ~~~ aaa  ~~~  <pre><code>aaa  </code></pre>  The closing code fence must be at least as long as the opening fence:  aaa    <pre><code>aaa  </code></pre>  ~~~~ aaa ~~~ ~~~~  <pre><code>aaa ~~~ </code></pre>  Unclosed code blocks are closed by the end of the document (or the enclosing block quote or list item):   <pre><code></code></pre>    aaa  <pre><code>  aaa </code></pre>  >  > aaa bbb  <blockquote> <pre><code>aaa </code></pre> </blockquote> <p>bbb</p>  A code block can have all empty lines as its content:    <pre><code> </code></pre>  A code block can be empty:    <pre><code></code></pre>  Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:   aaa aaa   <pre><code>aaa aaa </code></pre>    aaa aaa aaa   <pre><code>aaa aaa aaa </code></pre>    aaa aaa aaa   <pre><code>aaa aaa aaa </code></pre>  Four spaces indentation produces an indented code block:   aaa   <pre><code> aaa  </code></pre>  Closing fences may be indented by 0-3 spaces, and their indentation need not match that of the opening fence:  aaa   <pre><code>aaa </code></pre>    aaa   <pre><code>aaa </code></pre>  This is not a closing fence, because it is indented 4 spaces:  aaa   <pre><code>aaa  </code></pre>  Code fences (opening and closing) cannot contain internal spaces:   aaa  <p><code></code> aaa</p>  ~~~~~~ aaa ~~~ ~~  <pre><code>aaa ~~~ ~~ </code></pre>  Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between: foo  bar  baz  <p>foo</p> <pre><code>bar </code></pre> <p>baz</p>  Other blocks can also occur before and after fenced code blocks without an intervening blank line: foo --- ~~~ bar ~~~ # baz  <h2>foo</h2> <pre><code>bar </code></pre> <h1>baz</h1>  An info string can be provided after the opening code fence. Opening and closing spaces will be stripped, and the first word, prefixed with language-, is used as the value for the class attribute of the code element within the enclosing pre element. ruby def foo(x) return 3 end   <pre><code class="language-ruby">def foo(x) return 3 end </code></pre>  ~~~~ ruby startline=3$%@#$def foo(x) return 3 end ~~~~~~~  <pre><code class="language-ruby">def foo(x) return 3 end </code></pre>  ;   <pre><code class="language-;"></code></pre>  Info strings for backtick code blocks cannot contain backticks:  aa  foo  <p><code>aa</code> foo</p>  Closing code fences cannot have info strings:   aaa   <pre><code> aaa </code></pre>  ## 4.6HTML blocks An HTML block is a group of lines that is treated as raw HTML (and will not be escaped in HTML output). There are seven kinds of HTML block, which can be defined by their start and end conditions. The block begins with a line that meets a start condition (after up to three spaces optional indentation). It ends with the first subsequent line that meets a matching end condition, or the last line of the document, if no line is encountered that meets the end condition. If the first line meets both the start condition and the end condition, the block will contain just that line. 1. Start condition: line begins with the string <script, <pre, or <style (case-insensitive), followed by whitespace, the string >, or the end of the line. End condition: line contains an end tag </script>, </pre>, or </style> (case-insensitive; it need not match the start tag). 2. Start condition: line begins with the string <!--. End condition: line contains the string -->. 3. Start condition: line begins with the string <?. End condition: line contains the string ?>. 4. Start condition: line begins with the string <! followed by an uppercase ASCII letter. End condition: line contains the character >. 5. Start condition: line begins with the string <![CDATA[. End condition: line contains the string ]]>. 6. Start condition: line begins the string < or </ followed by one of the strings (case-insensitive) address, article, aside, base, basefont, blockquote, body, caption, center, col, colgroup, dd, details, dialog, dir, div, dl, dt, fieldset, figcaption, figure, footer, form, frame, frameset, h1, head, header, hr, html, iframe, legend, li, link, main, menu, menuitem, meta, nav, noframes, ol, optgroup, option, p, param, section, source, summary, table, tbody, td, tfoot, th, thead, title, tr, track, ul, followed by whitespace, the end of the line, the string >, or the string />. End condition: line is followed by a blank line. 7. Start condition: line begins with a complete open tag or closing tag (with any tag name other than script, style, or pre) followed only by whitespace or the end of the line. End condition: line is followed by a blank line. All types of HTML blocks except type 7 may interrupt a paragraph. Blocks of type 7 may not interrupt a paragraph. (This restriction is intended to prevent unwanted interpretation of long tags inside a wrapped paragraph as starting HTML blocks.) Some simple examples follow. Here are some basic HTML blocks of type 6: <table> <tr> <td> hi </td> </tr> </table> okay.  <table> <tr> <td> hi </td> </tr> </table> <p>okay.</p>   <div> *hello* <foo><a>   <div> *hello* <foo><a>  A block can also start with a closing tag: </div> *foo*  </div> *foo*  Here we have two HTML blocks with a Markdown paragraph between them: <DIV CLASS="foo"> *Markdown* </DIV>  <DIV CLASS="foo"> <p><em>Markdown</em></p> </DIV>  The tag on the first line can be partial, as long as it is split where there would be whitespace: <div id="foo" class="bar"> </div>  <div id="foo" class="bar"> </div>  <div id="foo" class="bar baz"> </div>  <div id="foo" class="bar baz"> </div>  An open tag need not be closed: <div> *foo* *bar*  <div> *foo* <p><em>bar</em></p>  A partial tag need not even be completed (garbage in, garbage out): <div id="foo" *hi*  <div id="foo" *hi*  <div class foo  <div class foo  The initial tag doesn’t even need to be a valid tag, as long as it starts like one: <div *???-&&&-<--- *foo*  <div *???-&&&-<--- *foo*  In type 6 blocks, the initial tag need not be on a line by itself: <div><a href="bar">*foo*</a></div>  <div><a href="bar">*foo*</a></div>  <table><tr><td> foo </td></tr></table>  <table><tr><td> foo </td></tr></table>  Everything until the next blank line or end of document gets included in the HTML block. So, in the following example, what looks like a Markdown code block is actually part of the HTML block, which continues until a blank line or the end of the document is reached: <div></div>  c int x = 33;   <div></div>  c int x = 33;   To start an HTML block with a tag that is not in the list of block-level tags in (6), you must put the tag by itself on the first line (and it must be complete): <a href="foo"> *bar* </a>  <a href="foo"> *bar* </a>  In type 7 blocks, the tag name can be anything: <Warning> *bar* </Warning>  <Warning> *bar* </Warning>  <i class="foo"> *bar* </i>  <i class="foo"> *bar* </i>  </ins> *bar*  </ins> *bar*  These rules are designed to allow us to work with tags that can function as either block-level or inline-level tags. The <del> tag is a nice example. We can surround content with <del> tags in three different ways. In this case, we get a raw HTML block, because the <del> tag is on a line by itself: <del> *foo* </del>  <del> *foo* </del>  In this case, we get a raw HTML block that just includes the <del> tag (because it ends with the following blank line). So the contents get interpreted as CommonMark: <del> *foo* </del>  <del> <p><em>foo</em></p> </del>  Finally, in this case, the <del> tags are interpreted as raw HTML inside the CommonMark paragraph. (Because the tag is not on a line by itself, we get inline HTML rather than an HTML block.) <del>*foo*</del>  <p><del><em>foo</em></del></p>  HTML tags designed to contain literal content (script, style, pre), comments, processing instructions, and declarations are treated somewhat differently. Instead of ending at the first blank line, these blocks end at the first line containing a corresponding end tag. As a result, these blocks can contain blank lines: A pre tag (type 1): <pre language="haskell"><code> import Text.HTML.TagSoup main :: IO () main = print$ parseTags tags
</code></pre>

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

main :: IO ()
main = print $parseTags tags </code></pre>  A script tag (type 1): <script type="text/javascript"> // JavaScript example document.getElementById("demo").innerHTML = "Hello JavaScript!"; </script>  <script type="text/javascript"> // JavaScript example document.getElementById("demo").innerHTML = "Hello JavaScript!"; </script>  A style tag (type 1): <style type="text/css"> h1 {color:red;} p {color:blue;} </style>  <style type="text/css"> h1 {color:red;} p {color:blue;} </style>  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): <style type="text/css"> foo  <style type="text/css"> foo  > <div> > foo bar  <blockquote> <div> foo </blockquote> <p>bar</p>  - <div> - foo  <ul> <li> <div> </li> <li>foo</li> </ul>  The end tag can occur on the same line as the start tag: <style>p{color:red;}</style> *foo*  <style>p{color:red;}</style> <p><em>foo</em></p>  <!-- foo -->*bar* *baz*  <!-- foo -->*bar* <p><em>baz</em></p>  Note that anything on the last line after the end tag will be included in the HTML block: <script> foo </script>1. *bar*  <script> foo </script>1. *bar*  A comment (type 2): <!-- Foo bar baz -->  <!-- Foo bar baz -->  A processing instruction (type 3): <?php echo '>'; ?>  <?php echo '>'; ?>  A declaration (type 4): <!DOCTYPE html>  <!DOCTYPE html>  CDATA (type 5): <![CDATA[ function matchwo(a,b) { if (a < b && a < 0) then { return 1; } else { return 0; } } ]]>  <![CDATA[ function matchwo(a,b) { if (a < b && a < 0) then { return 1; } else { return 0; } } ]]>  The opening tag can be indented 1-3 spaces, but not 4:  <!-- foo --> <!-- foo -->   <!-- foo --> <pre><code>&lt;!-- foo --&gt; </code></pre>   <div> <div>   <div> <pre><code>&lt;div&gt; </code></pre>  An HTML block of types 1–6 can interrupt a paragraph, and need not be preceded by a blank line. Foo <div> bar </div>  <p>Foo</p> <div> bar </div>  However, a following blank line is needed, except at the end of a document, and except for blocks of types 1–5, above: <div> bar </div> *foo*  <div> bar </div> *foo*  HTML blocks of type 7 cannot interrupt a paragraph: Foo <a href="bar"> baz  <p>Foo <a href="bar"> baz</p>  This rule differs from John Gruber’s original Markdown syntax specification, which says: The only restrictions are that block-level HTML elements — e.g. <div>, <table>, <pre>, <p>, etc. — must be separated from surrounding content by blank lines, and the start and end tags of the block should not be indented with tabs or spaces. In some ways Gruber’s rule is more restrictive than the one given here: • It requires that an HTML block be preceded by a blank line. • It does not allow the start tag to be indented. • It requires a matching end tag, which it also does not allow to be indented. Most Markdown implementations (including some of Gruber’s own) do not respect all of these restrictions. There is one respect, however, in which Gruber’s rule is more liberal than the one given here, since it allows blank lines to occur inside an HTML block. There are two reasons for disallowing them here. First, it removes the need to parse balanced tags, which is expensive and can require backtracking from the end of the document if no matching end tag is found. Second, it provides a very simple and flexible way of including Markdown content inside HTML tags: simply separate the Markdown from the HTML using blank lines: Compare: <div> *Emphasized* text. </div>  <div> <p><em>Emphasized</em> text.</p> </div>  <div> *Emphasized* text. </div>  <div> *Emphasized* text. </div>  Some Markdown implementations have adopted a convention of interpreting content inside tags as text if the open tag has the attribute markdown=1. The rule given above seems a simpler and more elegant way of achieving the same expressive power, which is also much simpler to parse. The main potential drawback is that one can no longer paste HTML blocks into Markdown documents with 100% reliability. However, in most cases this will work fine, because the blank lines in HTML are usually followed by HTML block tags. For example: <table> <tr> <td> Hi </td> </tr> </table>  <table> <tr> <td> Hi </td> </tr> </table>  There are problems, however, if the inner tags are indented and separated by spaces, as then they will be interpreted as an indented code block: <table> <tr> <td> Hi </td> </tr> </table>  <table> <tr> <pre><code>&lt;td&gt; Hi &lt;/td&gt; </code></pre> </tr> </table>  Fortunately, blank lines are usually not necessary and can be deleted. The exception is inside <pre> tags, but as described above, raw HTML blocks starting with <pre> can contain blank lines. A link reference definition consists of a link label, indented up to three spaces, followed by a colon (:), optional whitespace (including up to one line ending), a link destination, optional whitespace (including up to one line ending), and an optional link title, which if it is present must be separated from the link destination by whitespace. No further non-whitespace characters may occur on the line. A link reference definition does not correspond to a structural element of a document. Instead, it defines a label which can be used in reference links and reference-style images elsewhere in the document. Link reference definitions can come either before or after the links that use them. [foo]: /url "title" [foo]  <p><a href="/url" title="title">foo</a></p>   [foo]: /url 'the title' [foo]  <p><a href="/url" title="the title">foo</a></p>  [Foo*bar\]]:my_(url) 'title (with parens)' [Foo*bar\]]  <p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>  [Foo bar]: <my%20url> 'title' [Foo bar]  <p><a href="my%20url" title="title">Foo bar</a></p>  The title may extend over multiple lines: [foo]: /url ' title line1 line2 ' [foo]  <p><a href="/url" title=" title line1 line2 ">foo</a></p>  However, it may not contain a blank line: [foo]: /url 'title with blank line' [foo]  <p>[foo]: /url 'title</p> <p>with blank line'</p> <p>[foo]</p>  The title may be omitted: [foo]: /url [foo]  <p><a href="/url">foo</a></p>  The link destination may not be omitted: [foo]: [foo]  <p>[foo]:</p> <p>[foo]</p>  Both title and destination can contain backslash escapes and literal backslashes: [foo]: /url\bar\*baz "foo\"bar\baz" [foo]  <p><a href="/url%5Cbar*baz" title="foo&quot;bar\baz">foo</a></p>  A link can come before its corresponding definition: [foo] [foo]: url  <p><a href="url">foo</a></p>  If there are several matching definitions, the first one takes precedence: [foo] [foo]: first [foo]: second  <p><a href="first">foo</a></p>  As noted in the section on Links, matching of labels is case-insensitive (see matches). [FOO]: /url [Foo]  <p><a href="/url">Foo</a></p>  [ΑΓΩ]: /φου [αγω]  <p><a href="/%CF%86%CE%BF%CF%85">αγω</a></p>  Here is a link reference definition with no corresponding link. It contributes nothing to the document. [foo]: /url  Here is another one: [ foo ]: /url bar  <p>bar</p>  This is not a link reference definition, because there are non-whitespace characters after the title: [foo]: /url "title" ok  <p>[foo]: /url &quot;title&quot; ok</p>  This is a link reference definition, but it has no title: [foo]: /url "title" ok  <p>&quot;title&quot; ok</p>  This is not a link reference definition, because it is indented four spaces:  [foo]: /url "title" [foo]  <pre><code>[foo]: /url &quot;title&quot; </code></pre> <p>[foo]</p>  This is not a link reference definition, because it occurs inside a code block:  [foo]: /url  [foo]  <pre><code>[foo]: /url </code></pre> <p>[foo]</p>  A link reference definition cannot interrupt a paragraph. Foo [bar]: /baz [bar]  <p>Foo [bar]: /baz</p> <p>[bar]</p>  However, it can directly follow other block elements, such as headings and thematic breaks, and it need not be followed by a blank line. # [Foo] [foo]: /url > bar  <h1><a href="/url">Foo</a></h1> <blockquote> <p>bar</p> </blockquote>  Several link reference definitions can occur one after another, without intervening blank lines. [foo]: /foo-url "foo" [bar]: /bar-url "bar" [baz]: /baz-url [foo], [bar], [baz]  <p><a href="/foo-url" title="foo">foo</a>, <a href="/bar-url" title="bar">bar</a>, <a href="/baz-url">baz</a></p>  Link reference definitions can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined: [foo] > [foo]: /url  <p><a href="/url">foo</a></p> <blockquote> </blockquote>  ## 4.8Paragraphs A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph’s raw content as inlines. The paragraph’s raw content is formed by concatenating the lines and removing initial and final whitespace. A simple example with two paragraphs: aaa bbb  <p>aaa</p> <p>bbb</p>  Paragraphs can contain multiple lines, but no blank lines: aaa bbb ccc ddd  <p>aaa bbb</p> <p>ccc ddd</p>  Multiple blank lines between paragraph have no effect: aaa bbb  <p>aaa</p> <p>bbb</p>  Leading spaces are skipped:  aaa bbb  <p>aaa bbb</p>  Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs. aaa bbb ccc  <p>aaa bbb ccc</p>  However, the first line may be indented at most three spaces, or an indented code block will be triggered:  aaa bbb  <p>aaa bbb</p>   aaa bbb  <pre><code>aaa </code></pre> <p>bbb</p>  Final spaces are stripped before inline parsing, so a paragraph that ends with two or more spaces will not end with a hard line break: aaa bbb  <p>aaa<br /> bbb</p>  ## 4.9Blank lines Blank lines between block-level elements are ignored, except for the role they play in determining whether a list is tight or loose. Blank lines at the beginning and end of the document are also ignored.  aaa # aaa  <p>aaa</p> <h1>aaa</h1>  # 5Container blocks A container block is a block that has other blocks as its contents. There are two basic kinds of container blocks: block quotes and list items. Lists are meta-containers for list items. We define the syntax for container blocks recursively. The general form of the definition is: If X is a sequence of blocks, then the result of transforming X in such-and-such a way is a container of type Y with these blocks as its content. So, we explain what counts as a block quote or list item by explaining how these can be generated from their contents. This should suffice to define the syntax, although it does not give a recipe for parsing these constructions. (A recipe is provided below in the section entitled A parsing strategy.) ## 5.1Block quotes A block quote marker consists of 0-3 spaces of initial indent, plus (a) the character > together with a following space, or (b) a single character > not followed by a space. The following rules define block quotes: 1. Basic case. If a string of lines Ls constitute a sequence of blocks Bs, then the result of prepending a block quote marker to the beginning of each line in Ls is a block quote containing Bs. 2. Laziness. If a string of lines Ls constitute a block quote with contents Bs, then the result of deleting the initial block quote marker from one or more lines in which the next non-whitespace character after the block quote marker is paragraph continuation text is a block quote with Bs as its content. Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph. 3. Consecutiveness. A document cannot contain two block quotes in a row unless there is a blank line between them. Nothing else counts as a block quote. Here is a simple example: > # Foo > bar > baz  <blockquote> <h1>Foo</h1> <p>bar baz</p> </blockquote>  The spaces after the > characters can be omitted: ># Foo >bar > baz  <blockquote> <h1>Foo</h1> <p>bar baz</p> </blockquote>  The > characters can be indented 1-3 spaces:  > # Foo > bar > baz  <blockquote> <h1>Foo</h1> <p>bar baz</p> </blockquote>  Four spaces gives us a code block:  > # Foo > bar > baz  <pre><code>&gt; # Foo &gt; bar &gt; baz </code></pre>  The Laziness clause allows us to omit the > before a paragraph continuation line: > # Foo > bar baz  <blockquote> <h1>Foo</h1> <p>bar baz</p> </blockquote>  A block quote can contain some lazy and some non-lazy continuation lines: > bar baz > foo  <blockquote> <p>bar baz foo</p> </blockquote>  Laziness only applies to lines that would have been continuations of paragraphs had they been prepended with block quote markers. For example, the > cannot be omitted in the second line of > foo > ---  without changing the meaning: > foo ---  <blockquote> <p>foo</p> </blockquote> <hr />  Similarly, if we omit the > in the second line of > - foo > - bar  then the block quote ends after the first line: > - foo - bar  <blockquote> <ul> <li>foo</li> </ul> </blockquote> <ul> <li>bar</li> </ul>  For the same reason, we can’t omit the > in front of subsequent lines of an indented or fenced code block: > foo bar  <blockquote> <pre><code>foo </code></pre> </blockquote> <pre><code>bar </code></pre>  >  foo   <blockquote> <pre><code></code></pre> </blockquote> <p>foo</p> <pre><code></code></pre>  Note that in the following case, we have a paragraph continuation line: > foo - bar  <blockquote> <p>foo - bar</p> </blockquote>  To see why, note that in > foo > - bar  the - bar is indented too far to start a list, and can’t be an indented code block because indented code blocks cannot interrupt paragraphs, so it is a [paragraph continuation line]. A block quote can be empty: >  <blockquote> </blockquote>  > > >  <blockquote> </blockquote>  A block quote can have initial or final blank lines: > > foo >  <blockquote> <p>foo</p> </blockquote>  A blank line always separates block quotes: > foo > bar  <blockquote> <p>foo</p> </blockquote> <blockquote> <p>bar</p> </blockquote>  (Most current Markdown implementations, including John Gruber’s original Markdown.pl, will parse this example as a single block quote with two paragraphs. But it seems better to allow the author to decide whether two block quotes or one are wanted.) Consecutiveness means that if we put these block quotes together, we get a single block quote: > foo > bar  <blockquote> <p>foo bar</p> </blockquote>  To get a block quote with two paragraphs, use: > foo > > bar  <blockquote> <p>foo</p> <p>bar</p> </blockquote>  Block quotes can interrupt paragraphs: foo > bar  <p>foo</p> <blockquote> <p>bar</p> </blockquote>  In general, blank lines are not needed before or after block quotes: > aaa *** > bbb  <blockquote> <p>aaa</p> </blockquote> <hr /> <blockquote> <p>bbb</p> </blockquote>  However, because of laziness, a blank line is needed between a block quote and a following paragraph: > bar baz  <blockquote> <p>bar baz</p> </blockquote>  > bar baz  <blockquote> <p>bar</p> </blockquote> <p>baz</p>  > bar > baz  <blockquote> <p>bar</p> </blockquote> <p>baz</p>  It is a consequence of the Laziness rule that any number of initial >s may be omitted on a continuation line of a nested block quote: > > > foo bar  <blockquote> <blockquote> <blockquote> <p>foo bar</p> </blockquote> </blockquote> </blockquote>  >>> foo > bar >>baz  <blockquote> <blockquote> <blockquote> <p>foo bar baz</p> </blockquote> </blockquote> </blockquote>  When including an indented code block in a block quote, remember that the block quote marker includes both the > and a following space. So five spaces are needed after the >: > code > not code  <blockquote> <pre><code>code </code></pre> </blockquote> <blockquote> <p>not code</p> </blockquote>  ## 5.2List items A bullet list marker is a -, +, or * character. An ordered list marker is a sequence of 1–9 arabic digits (0-9), followed by either a . character or a ) character. (The reason for the length limit is that with 10 digits we start seeing integer overflows in some browsers.) The following rules define list items: 1. Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a non-whitespace character and not separated from each other by more than one blank line, and M is a list marker of width W followed by 0 < N < 5 spaces, then the result of prepending M and the following spaces to the first line of Ls, and indenting subsequent lines of Ls by W + N spaces, is a list item with Bs as its contents. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker. 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, and there are three spaces between the list marker and the next non-whitespace character, then blocks must be indented five spaces in order to fall under the list item. Here are some examples showing how far content must be indented to be put under the list item: - one two  <ul> <li>one</li> </ul> <p>two</p>  - one two  <ul> <li> <p>one</p> <p>two</p> </li> </ul>   - one two  <ul> <li>one</li> </ul> <pre><code> two </code></pre>   - one two  <ul> <li> <p>one</p> <p>two</p> </li> </ul>  It is tempting to think of this in terms of columns: the continuation blocks must be indented at least to the column of the first non-whitespace character after the list marker. However, that is not quite right. The spaces after the list marker determine how much relative indentation is needed. Which column this indentation reaches will depend on how the list item is embedded in other constructions, as shown by this example:  > > 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 is needed between the list marker and any following content, so these are not list items: -one 2.two  <p>-one</p> <p>2.two</p>  A list item may not contain blocks that are separated by more than one blank line. Thus, two blank lines will end a list, unless the two blanks are contained in a fenced code block. - foo bar - foo bar -  foo bar  - baz +  foo bar   <ul> <li> <p>foo</p> <p>bar</p> </li> <li> <p>foo</p> </li> </ul> <p>bar</p> <ul> <li> <pre><code>foo bar </code></pre> </li> <li> <p>baz</p> <ul> <li> <pre><code>foo bar </code></pre> </li> </ul> </li> </ul>  A list item may contain any kind of block: 1. foo  bar  baz > bam  <ol> <li> <p>foo</p> <pre><code>bar </code></pre> <p>baz</p> <blockquote> <p>bam</p> </blockquote> </li> </ol>  A list item that contains an indented code block will preserve empty lines within the code block verbatim, unless there are two or more empty lines in a row (since as described above, two blank lines end the list): - Foo bar baz  <ul> <li> <p>Foo</p> <pre><code>bar baz </code></pre> </li> </ul>  - Foo bar baz  <ul> <li> <p>Foo</p> <pre><code>bar </code></pre> </li> </ul> <pre><code> baz </code></pre>  Note that ordered list start numbers must be nine digits or less: 123456789. ok  <ol start="123456789"> <li>ok</li> </ol>  1234567890. not ok  <p>1234567890. not ok</p>  A start number may begin with 0s: 0. ok  <ol start="0"> <li>ok</li> </ol>  003. ok  <ol start="3"> <li>ok</li> </ol>  A start number may not be negative: -1. not ok  <p>-1. not ok</p>  1. Item starting with indented code. If a sequence of lines Ls constitute a sequence of blocks Bs starting with an indented code block and not separated from each other by more than one blank line, and M is a list marker of width W followed by one space, then the result of prepending M and the following space to the first line of Ls, and indenting subsequent lines of Ls by W + 1 spaces, is a list item with Bs as its contents. If a line is empty, then it need not be indented. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker. An indented code block will have to be indented four spaces beyond the edge of the region where text will be included in the list item. In the following case that is 6 spaces: - foo bar  <ul> <li> <p>foo</p> <pre><code>bar </code></pre> </li> </ul>  And in this case it is 11 spaces:  10. foo bar  <ol start="10"> <li> <p>foo</p> <pre><code>bar </code></pre> </li> </ol>  If the first block in the list item is an indented code block, then by rule #2, the contents must be indented one space 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 indent 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 non-whitespace character, and (b) cases in which they begin with an indented code block. In a case like the following, where the first block begins with a three-space indent, the rules do not allow us to form a list item by indenting the whole thing and prepending a list marker:  foo bar  <p>foo</p> <p>bar</p>  - foo bar  <ul> <li>foo</li> </ul> <p>bar</p>  This is not a significant restriction, because when a block begins with 1-3 spaces indent, the indentation can always be removed without a change in interpretation, allowing rule #1 to be applied. So, in the above case: - foo bar  <ul> <li> <p>foo</p> <p>bar</p> </li> </ul>  1. Item starting with a blank line. If a sequence of lines Ls starting with a single blank line constitute a (possibly empty) sequence of blocks Bs, not separated from each other by more than one blank line, and M is a list marker of width W, then the result of prepending M to the first line of Ls, and indenting subsequent lines of Ls by W + 1 spaces, is a list item with Bs as its contents. If a line is empty, then it need not be indented. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker. Here are some list items that start with a blank line but are not empty: - foo -  bar  - baz  <ul> <li>foo</li> <li> <pre><code>bar </code></pre> </li> <li> <pre><code>baz </code></pre> </li> </ul>  A list item can begin with at most one blank line. In the following example, foo is not part of the list item: - foo  <ul> <li></li> </ul> <p>foo</p>  Here is an empty bullet list item: - foo - - bar  <ul> <li>foo</li> <li></li> <li>bar</li> </ul>  It does not matter whether there are spaces following the list marker: - foo - - bar  <ul> <li>foo</li> <li></li> <li>bar</li> </ul>  Here is an empty ordered list item: 1. foo 2. 3. bar  <ol> <li>foo</li> <li></li> <li>bar</li> </ol>  A list may start or end with an empty list item: *  <ul> <li></li> </ul>  1. Indentation. If a sequence of lines Ls constitutes a list item according to rule #1, #2, or #3, then the result of indenting each line of Ls by 1-3 spaces (the same for each line) also constitutes a list item with the same contents and attributes. If a line is empty, then it need not be indented. Indented one space:  1. A paragraph with two lines. indented code > A block quote.  <ol> <li> <p>A paragraph with two lines.</p> <pre><code>indented code </code></pre> <blockquote> <p>A block quote.</p> </blockquote> </li> </ol>  Indented two spaces:  1. A paragraph with two lines. indented code > A block quote.  <ol> <li> <p>A paragraph with two lines.</p> <pre><code>indented code </code></pre> <blockquote> <p>A block quote.</p> </blockquote> </li> </ol>  Indented three spaces:  1. A paragraph with two lines. indented code > A block quote.  <ol> <li> <p>A paragraph with two lines.</p> <pre><code>indented code </code></pre> <blockquote> <p>A block quote.</p> </blockquote> </li> </ol>  Four spaces indent gives a code block:  1. A paragraph with two lines. indented code > A block quote.  <pre><code>1. A paragraph with two lines. indented code &gt; A block quote. </code></pre>  1. Laziness. If a string of lines Ls constitute a list item with contents Bs, then the result of deleting some or all of the indentation from one or more lines in which the next non-whitespace character after the indentation is paragraph continuation text is a list item with the same contents and attributes. The unindented lines are called lazy continuation lines. Here is an example with lazy continuation lines:  1. A paragraph with two lines. indented code > A block quote.  <ol> <li> <p>A paragraph with two lines.</p> <pre><code>indented code </code></pre> <blockquote> <p>A block quote.</p> </blockquote> </li> </ol>  Indentation can be partially deleted:  1. A paragraph with two lines.  <ol> <li>A paragraph with two lines.</li> </ol>  These examples show how laziness can work in nested structures: > 1. > Blockquote continued here.  <blockquote> <ol> <li> <blockquote> <p>Blockquote continued here.</p> </blockquote> </li> </ol> </blockquote>  > 1. > Blockquote > continued here.  <blockquote> <ol> <li> <blockquote> <p>Blockquote continued here.</p> </blockquote> </li> </ol> </blockquote>  1. That’s all. Nothing that is not counted as a list item by rules #1–5 counts as a list item. The rules for sublists follow from the general rules above. A sublist must be indented the same number of spaces a paragraph would need to be in order to be included in the list item. So, in this case we need two spaces indent: - foo - bar - baz  <ul> <li>foo <ul> <li>bar <ul> <li>baz</li> </ul> </li> </ul> </li> </ul>  One is not enough: - foo - bar - baz  <ul> <li>foo</li> <li>bar</li> <li>baz</li> </ul>  Here we need four, because the list marker is wider: 10) foo - bar  <ol start="10"> <li>foo <ul> <li>bar</li> </ul> </li> </ol>  Three is not enough: 10) foo - bar  <ol start="10"> <li>foo</li> </ol> <ul> <li>bar</li> </ul>  A list may be the first block in a list item: - - foo  <ul> <li> <ul> <li>foo</li> </ul> </li> </ul>  1. - 2. foo  <ol> <li> <ul> <li> <ol start="2"> <li>foo</li> </ol> </li> </ul> </li> </ol>  A list item can contain a heading: - # Foo - Bar --- baz  <ul> <li> <h1>Foo</h1> </li> <li> <h2>Bar</h2> baz</li> </ul>  ### 5.2.1Motivation John Gruber’s Markdown spec says the following about list items: 1. “List markers typically start at the left margin, but may be indented by up to three spaces. List markers must be followed by one or more spaces or a tab.” 2. “To make lists look nice, you can wrap items with hanging indents…. But if you don’t want to, you don’t have to.” 3. “List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab.” 4. “It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy.” 5. “To put a blockquote within a list item, the blockquote’s > delimiters need to be indented.” 6. “To put a code block within a list item, the code block needs to be indented twice — 8 spaces or two tabs.” These rules specify that a paragraph under a list item must be indented four spaces (presumably, from the left margin, rather than the start of the list marker, but this is not said), and that code under a list item must be indented eight spaces instead of the usual four. They also say that a block quote must be indented, but not by how much; however, the example given has four spaces indentation. Although nothing is said about other kinds of block-level content, it is certainly reasonable to infer that all block elements under a list item, including other lists, must be indented four spaces. This principle has been called the four-space rule. The four-space rule is clear and principled, and if the reference implementation Markdown.pl had followed it, it probably would have become the standard. However, Markdown.pl allowed paragraphs and sublists to start with only two spaces indentation, at least on the outer level. Worse, its behavior was inconsistent: a sublist of an outer-level list needed two spaces indentation, but a sublist of this sublist needed three spaces. It is not surprising, then, that different implementations of Markdown have developed very different rules for determining what comes under a list item. (Pandoc and python-Markdown, for example, stuck with Gruber’s syntax description and the four-space rule, while discount, redcarpet, marked, PHP Markdown, and others followed Markdown.pl’s behavior more closely.) Unfortunately, given the divergences between implementations, there is no way to give a spec for list items that will be guaranteed not to break any existing documents. However, the spec given here should correctly handle lists formatted with either the four-space rule or the more forgiving Markdown.pl behavior, provided they are laid out in a way that is natural for a human to read. The strategy here is to let the width and indentation of the list marker determine the indentation necessary for blocks to fall under the list item, rather than having a fixed and arbitrary number. The writer can think of the body of the list item as a unit which gets indented to the right enough to fit the list marker (and any indentation on the list marker). (The laziness rule, #5, then allows continuation lines to be unindented if needed.) This rule is superior, we claim, to any rule requiring a fixed level of indentation from the margin. The four-space rule is clear but unnatural. It is quite unintuitive that - foo bar - baz  should be parsed as two lists with an intervening paragraph, <ul> <li>foo</li> </ul> <p>bar</p> <ul> <li>baz</li> </ul>  as the four-space rule demands, rather than a single list, <ul> <li> <p>foo</p> <p>bar</p> <ul> <li>baz</li> </ul> </li> </ul>  The choice of four spaces is arbitrary. It can be learned, but it is not likely to be guessed, and it trips up beginners regularly. Would it help to adopt a two-space rule? The problem is that such a rule, together with the rule allowing 1–3 spaces indentation of the initial list marker, allows text that is indented less than the original list marker to be included in the list item. For example, Markdown.pl parses  - one two  as a single list item, with two a continuation paragraph: <ul> <li> <p>one</p> <p>two</p> </li> </ul>  and similarly > - one > > two  as <blockquote> <ul> <li> <p>one</p> <p>two</p> </li> </ul> </blockquote>  This is extremely unintuitive. Rather than requiring a fixed indent from the margin, we could require a fixed indent (say, two spaces, or even one space) from the list marker (which may itself be indented). This proposal would remove the last anomaly discussed. Unlike the spec presented above, it would count the following as a list item with a subparagraph, even though the paragraph bar is not indented as far as the first paragraph foo:  10. foo bar  Arguably this text does read like a list item with bar as a subparagraph, which may count in favor of the proposal. However, on this proposal indented code would have to be indented six spaces after the list marker. And this would break a lot of existing Markdown, which has the pattern: 1. foo indented code  where the code is indented eight spaces. The spec above, by contrast, will parse this text as expected, since the code block’s indentation is measured from the beginning of foo. The one case that needs special treatment is a list item that starts with indented code. How much indentation is required in that case, since we don’t have a “first paragraph” to measure from? Rule #2 simply stipulates that in such cases, we require one space indentation from the list marker (and then the normal four spaces for the indented code). This will match the four-space rule in cases where the list marker plus its initial indentation takes four spaces (a common case), but diverge in other cases. ## 5.3Lists A list is a sequence of one or more list items of the same type. The list items may be separated by single blank lines, but two blank lines end all containing lists. Two list items are of the same type if they begin with a list marker of the same type. Two list markers are of the same type if (a) they are bullet list markers using the same character (-, +, or *) or (b) they are ordered list numbers with the same delimiter (either . or )). A list is an ordered list if its constituent list items begin with ordered list markers, and a bullet list if its constituent list items begin with bullet list markers. The start number of an ordered list is determined by the list number of its initial list item. The numbers of subsequent list items are disregarded. A list is loose if any of its constituent list items are separated by blank lines, or if any of its constituent list items directly contain two block-level elements with a blank line between them. Otherwise a list is tight. (The difference in HTML output is that paragraphs in a loose list are wrapped in <p> tags, while paragraphs in a tight list are not.) Changing the bullet or ordered list delimiter starts a new list: - foo - bar + baz  <ul> <li>foo</li> <li>bar</li> </ul> <ul> <li>baz</li> </ul>  1. foo 2. bar 3) baz  <ol> <li>foo</li> <li>bar</li> </ol> <ol start="3"> <li>baz</li> </ol>  In CommonMark, a list can interrupt a paragraph. That is, no blank line is needed to separate a paragraph from a following list: Foo - bar - baz  <p>Foo</p> <ul> <li>bar</li> <li>baz</li> </ul>  Markdown.pl does not allow this, through fear of triggering a list via a numeral in a hard-wrapped line: The number of windows in my house is 14. The number of doors is 6.  <p>The number of windows in my house is</p> <ol start="14"> <li>The number of doors is 6.</li> </ol>  Oddly, Markdown.pl does allow a blockquote to interrupt a paragraph, even though the same considerations might apply. We think that the two cases should be treated the same. Here are two reasons for allowing lists to interrupt paragraphs: 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. Our adherence to the principle of uniformity thus inclines us to think that there are two coherent packages: 1. Require blank lines before all lists and blockquotes, including lists that occur as sublists inside other list items. 2. Require blank lines in none of these places. reStructuredText takes the first approach, for which there is much to be said. But the second seems more consistent with established practice with Markdown. There can be blank lines between items, but two blank lines end a list: - foo - bar - baz  <ul> <li> <p>foo</p> </li> <li> <p>bar</p> </li> </ul> <ul> <li>baz</li> </ul>  As illustrated above in the section on list items, two blank lines between blocks within a list item will also end a list: - foo bar - baz  <ul> <li>foo</li> </ul> <p>bar</p> <ul> <li>baz</li> </ul>  Indeed, two blank lines will end all containing lists: - foo - bar - baz bim  <ul> <li>foo <ul> <li>bar <ul> <li>baz</li> </ul> </li> </ul> </li> </ul> <pre><code> bim </code></pre>  Thus, two blank lines can be used 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: - 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 - h - i  <ul> <li>a</li> <li>b</li> <li>c</li> <li>d</li> <li>e</li> <li>f</li> <li>g</li> <li>h</li> <li>i</li> </ul>  1. a 2. b 3. c  <ol> <li> <p>a</p> </li> <li> <p>b</p> </li> <li> <p>c</p> </li> </ol>  This is a loose list, because there is a blank line between two of the list items: - a - b - c  <ul> <li> <p>a</p> </li> <li> <p>b</p> </li> <li> <p>c</p> </li> </ul>  So is this, with a empty second item: * a * * c  <ul> <li> <p>a</p> </li> <li></li> <li> <p>c</p> </li> </ul>  These are loose lists, even though there is no space between the items, because one of the items directly contains two block-level elements with a blank line between them: - a - b c - d  <ul> <li> <p>a</p> </li> <li> <p>b</p> <p>c</p> </li> <li> <p>d</p> </li> </ul>  - a - b [ref]: /url - d  <ul> <li> <p>a</p> </li> <li> <p>b</p> </li> <li> <p>d</p> </li> </ul>  This is a tight list, because the blank lines are in a code block: - a -  b  - c  <ul> <li>a</li> <li> <pre><code>b </code></pre> </li> <li>c</li> </ul>  This is a tight list, because the blank line is between two paragraphs of a sublist. So the sublist is loose while the outer list is tight: - a - b c - d  <ul> <li>a <ul> <li> <p>b</p> <p>c</p> </li> </ul> </li> <li>d</li> </ul>  This is a tight list, because the blank line is inside the block quote: * a > b > * c  <ul> <li>a <blockquote> <p>b</p> </blockquote> </li> <li>c</li> </ul>  This list is tight, because the consecutive block elements are not separated by blank lines: - a > b  c  - d  <ul> <li>a <blockquote> <p>b</p> </blockquote> <pre><code>c </code></pre> </li> <li>d</li> </ul>  A single-paragraph list is tight: - a  <ul> <li>a</li> </ul>  - a - b  <ul> <li>a <ul> <li>b</li> </ul> </li> </ul>  This list is loose, because of the blank line between the two block elements in the list item: 1.  foo  bar  <ol> <li> <pre><code>foo </code></pre> <p>bar</p> </li> </ol>  Here the outer list is loose, the inner list tight: * foo * bar baz  <ul> <li> <p>foo</p> <ul> <li>bar</li> </ul> <p>baz</p> </li> </ul>  - a - b - c - d - e - f  <ul> <li> <p>a</p> <ul> <li>b</li> <li>c</li> </ul> </li> <li> <p>d</p> <ul> <li>e</li> <li>f</li> </ul> </li> </ul>  # 6Inlines Inlines are parsed sequentially from the beginning of the character stream to the end (left to right, in left-to-right languages). Thus, for example, in hilo  <p><code>hi</code>lo</p>  hi is parsed as code, leaving the backtick at the end as a literal backtick. ## 6.1Backslash escapes Any ASCII punctuation character may be backslash-escaped: \!\"\#\$\%\&\'\*\+\,\-\.\/\:\;\<\=\>\?\@$\\$\^\_\\{\|\}\~

(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/  Examples of email autolinks: <foo@bar.example.com>  <p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>  <foo+special@Bar.baz-bar0.com>  <p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p>  Backslash-escapes do not work inside email autolinks: <foo\+@bar.example.com>  <p>&lt;foo+@bar.example.com&gt;</p>  These are not autolinks: <>  <p>&lt;&gt;</p>  < http://foo.bar >  <p>&lt; http://foo.bar &gt;</p>  <m:abc>  <p>&lt;m:abc&gt;</p>  <foo.bar.baz>  <p>&lt;foo.bar.baz&gt;</p>  http://example.com  <p>http://example.com</p>  foo@bar.example.com  <p>foo@bar.example.com</p>  ## 6.8Raw HTML Text between < and > that looks like an HTML tag is parsed as a raw HTML tag and will be rendered in HTML without escaping. Tag and attribute names are not limited to current HTML tags, so custom tags (and even, say, DocBook tags) may be used. Here is the grammar for tags: A tag name consists of an ASCII letter followed by zero or more ASCII letters, digits, or hyphens (-). An attribute consists of whitespace, an attribute name, and an optional attribute value specification. An attribute name consists of an ASCII letter, _, or :, followed by zero or more ASCII letters, digits, _, ., :, or -. (Note: This is the XML specification restricted to ASCII. HTML5 is laxer.) An attribute value specification consists of optional whitespace, a = character, optional whitespace, and an attribute value. An attribute value consists of an unquoted attribute value, a single-quoted attribute value, or a double-quoted attribute value. An unquoted attribute value is a nonempty string of characters not including spaces, ", ', =, <, >, or . A single-quoted attribute value consists of ', zero or more characters not including ', and a final '. A double-quoted attribute value consists of ", zero or more characters not including ", and a final ". An open tag consists of a < character, a tag name, zero or more attributes, optional whitespace, an optional / character, and a > character. A closing tag consists of the string </, a tag name, optional whitespace, and the character >. An HTML comment consists of <!-- + text + -->, where text does not start with > or ->, does not end with -, and does not contain --. (See the HTML5 spec.) A processing instruction consists of the string <?, a string of characters not including the string ?>, and the string ?>. A declaration consists of the string <!, a name consisting of one or more uppercase ASCII letters, whitespace, a string of characters not including the character >, and the character >. A CDATA section consists of the string <![CDATA[, a string of characters not including the string ]]>, and the string ]]>. An HTML tag consists of an open tag, a closing tag, an HTML comment, a processing instruction, a declaration, or a CDATA section. Here are some simple open tags: <a><bab><c2c>  <p><a><bab><c2c></p>  Empty elements: <a/><b2/>  <p><a/><b2/></p>  Whitespace is allowed: <a /><b2 data="foo" >  <p><a /><b2 data="foo" ></p>  With attributes: <a foo="bar" bam = 'baz <em>"</em>' _boolean zoop:33=zoop:33 />  <p><a foo="bar" bam = 'baz <em>"</em>' _boolean zoop:33=zoop:33 /></p>  Custom tag names can be used: Foo <responsive-image src="foo.jpg" />  <p>Foo <responsive-image src="foo.jpg" /></p>  Illegal tag names, not parsed as HTML: <33> <__>  <p>&lt;33&gt; &lt;__&gt;</p>  Illegal attribute names: <a h*#ref="hi">  <p>&lt;a h*#ref=&quot;hi&quot;&gt;</p>  Illegal attribute values: <a href="hi'> <a href=hi'>  <p>&lt;a href=&quot;hi'&gt; &lt;a href=hi'&gt;</p>  Illegal whitespace: < a>< foo><bar/ >  <p>&lt; a&gt;&lt; foo&gt;&lt;bar/ &gt;</p>  Missing whitespace: <a href='bar'title=title>  <p>&lt;a href='bar'title=title&gt;</p>  Closing tags: </a></foo >  <p></a></foo ></p>  Illegal attributes in closing tag: </a href="foo">  <p>&lt;/a href=&quot;foo&quot;&gt;</p>  Comments: foo <!-- this is a comment - with hyphen -->  <p>foo <!-- this is a comment - with hyphen --></p>  foo <!-- not a comment -- two hyphens -->  <p>foo &lt;!-- not a comment -- two hyphens --&gt;</p>  Not comments: foo <!--> foo --> foo <!-- foo--->  <p>foo &lt;!--&gt; foo --&gt;</p> <p>foo &lt;!-- foo---&gt;</p>  Processing instructions: foo <?php echo$a; ?>

<p>foo <?php echo $a; ?></p>  Declarations: foo <!ELEMENT br EMPTY>  <p>foo <!ELEMENT br EMPTY></p>  CDATA sections: foo <![CDATA[>&<]]>  <p>foo <![CDATA[>&<]]></p>  Entity and numeric character references are preserved in HTML attributes: foo <a href="&ouml;">  <p>foo <a href="&ouml;"></p>  Backslash escapes do not work in HTML attributes: foo <a href="\*">  <p>foo <a href="\*"></p>  <a href="\"">  <p>&lt;a href=&quot;&quot;&quot;&gt;</p>  ## 6.9Hard line breaks A line break (not in a code span or HTML tag) that is preceded by two or more spaces and does not occur at the end of a block is parsed as a hard line break (rendered in HTML as a <br /> tag): foo baz  <p>foo<br /> baz</p>  For a more visible alternative, a backslash before the line ending may be used instead of two spaces: foo\ baz  <p>foo<br /> baz</p>  More than two spaces can be used: foo baz  <p>foo<br /> baz</p>  Leading spaces at the beginning of the next line are ignored: foo bar  <p>foo<br /> bar</p>  foo\ bar  <p>foo<br /> bar</p>  Line breaks can occur inside emphasis, links, and other constructs that allow inline content: *foo bar*  <p><em>foo<br /> bar</em></p>  *foo\ bar*  <p><em>foo<br /> bar</em></p>  Line breaks do not occur inside code spans code span  <p><code>code span</code></p>  code\ span  <p><code>code\ span</code></p>  or HTML tags: <a href="foo bar">  <p><a href="foo bar"></p>  <a href="foo\ bar">  <p><a href="foo\ bar"></p>  Hard line breaks are for separating inline content within a block. Neither syntax for hard line breaks works at the end of a paragraph or other block element: foo\  <p>foo\</p>  foo  <p>foo</p>  ### foo\  <h3>foo\</h3>  ### foo  <h3>foo</h3>  ## 6.10Soft line breaks A regular line break (not in a code span or HTML tag) that is not preceded by two or more spaces or a backslash is parsed as a softbreak. (A softbreak may be rendered in HTML either as a line ending or as a space. The result will be the same in browsers. In the examples here, a line ending will be used.) foo baz  <p>foo baz</p>  Spaces at the end of the line and beginning of the next line are removed: foo baz  <p>foo baz</p>  A conforming parser may render a soft line break in HTML either as a line break or as a space. A renderer may also provide an option to render soft line breaks as hard line breaks. ## 6.11Textual content Any characters not given an interpretation by the above rules will be parsed as plain textual content. hello$.;'there

<p>hello \$.;'there</p>

Foo χρῆν

<p>Foo χρῆν</p>


Internal spaces are preserved verbatim:

Multiple     spaces

<p>Multiple     spaces</p>


# Appendix: A parsing strategy

In this appendix we describe some features of the parsing strategy used in the CommonMark reference implementations.

## Overview

Parsing has two phases:

1. In the first phase, lines of input are consumed and the block structure of the document—its division into paragraphs, block quotes, list items, and so on—is constructed. Text is assigned to these blocks but not parsed. Link reference definitions are parsed and a map of links is constructed.

2. In the second phase, the raw text contents of paragraphs and headings are parsed into sequences of Markdown inline elements (strings, code spans, links, emphasis, and so on), using the map of link references constructed in phase 1.

At each point in processing, the document is represented as a tree of blocks. The root of the tree is a document block. The document may have any number of other blocks as children. These children may, in turn, have other blocks as children. The last child of a block is normally considered open, meaning that subsequent lines of input can alter its contents. (Blocks that are not open are closed.) Here, for example, is a possible document tree, with the open blocks marked by arrows:

-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui *quodsi iracundia*"
-> list_item
-> paragraph
"aliquando id"


## Phase 1: block structure

Each line that is processed has an effect on this tree. The line is analyzed and, depending on its contents, the document may be altered in one or more of the following ways:

1. One or more open blocks may be closed.
2. One or more new blocks may be created as children of the last open block.
3. Text may be added to the last (deepest) open block remaining on the tree.

Once a line has been incorporated into the tree in this way, it can be discarded, so input can be read in a stream.

For each line, we follow this procedure:

1. First we iterate through the open blocks, starting with the root document, and descending through last children down to the last open block. Each block imposes a condition that the line must satisfy if the block is to remain open. For example, a block quote requires a > character. A paragraph requires a non-blank line. In this phase we may match all or just some of the open blocks. But we cannot close unmatched blocks yet, because we may have a lazy continuation line.

2. Next, after consuming the continuation markers for existing blocks, we look for new block starts (e.g. > for a block quote. If we encounter a new block start, we close any blocks unmatched in step 1 before creating the new block as a child of the last matched block.

3. Finally, we look at the remainder of the line (after block markers like >, list markers, and indentation have been consumed). This is text that can be incorporated into the last open block (a paragraph, code block, heading, or raw HTML).

Setext headings are formed when we see a line of a paragraph that is a setext heading underline.

Reference link definitions are detected when a paragraph is closed; the accumulated text lines are parsed to see if they begin with one or more reference link definitions. Any remainder becomes a normal paragraph.

We can see how this works by considering how the tree above is generated by four lines of Markdown:

> Lorem ipsum dolor
sit amet.
> - Qui *quodsi iracundia*
> - aliquando id


At the outset, our document model is just

-> document


The first line of our text,

> Lorem ipsum dolor


causes a block_quote block to be created as a child of our open document block, and a paragraph block as a child of the block_quote. Then the text is added to the last open block, the paragraph:

-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor"


The next line,

sit amet.


is a “lazy continuation” of the open paragraph, so it gets added to the paragraph’s text:

-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor\nsit amet."


The third line,

> - Qui *quodsi iracundia*


causes the paragraph block to be closed, and a new list block opened as a child of the block_quote. A list_item is also added as a child of the list, and a paragraph as a child of the list_item. The text is then added to the new paragraph:

-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
-> list_item
-> paragraph
"Qui *quodsi iracundia*"


The fourth line,

> - aliquando id


causes the list_item (and its child the paragraph) to be closed, and a new list_item opened up as child of the list. A paragraph is added as a child of the new list_item, to contain the text. We thus obtain the final tree:

-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui *quodsi iracundia*"
-> list_item
-> paragraph
"aliquando id"


## Phase 2: inline structure

Once all of the input has been parsed, all open blocks are closed.

We then “walk the tree,” visiting every node, and parse raw string contents of paragraphs and headings as inlines. At this point we have seen all the link reference definitions, so we can resolve reference links as we go.

document
block_quote
paragraph
str "Lorem ipsum dolor"
softbreak
str "sit amet."
list (type=bullet tight=true bullet_char=-)
list_item
paragraph
str "Qui "
emph
str "quodsi iracundia"
list_item
paragraph
str "aliquando id"


Notice how the line ending in the first paragraph has been parsed as a softbreak, and the asterisks in the first list item have become an emph.

By far the trickiest part of inline parsing is handling emphasis, strong emphasis, links, and images. This is done using the following algorithm.

When we’re parsing inlines and we hit either

• a run of * or _ characters, or
• a [ or ![

we insert a text node with these symbols as its literal content, and we add a pointer to this text node to the delimiter stack.

The delimiter stack is a doubly linked list. Each element contains a pointer to a text node, plus information about

• the type of delimiter ([, ![, *, _)
• the number of delimiters,
• whether the delimiter is “active” (all are active to start), and
• whether the delimiter is a potential opener, a potential closer, or both (which depends on what sort of characters precede and follow the delimiters).

When we hit a ] character, we call the look for link or image procedure (see below).

When we hit the end of the input, we call the process emphasis procedure (see below), with stack_bottom = NULL.

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, compact reference link/image, or shortcut reference link/image.

• If we don’t, then we remove the opening delimiter from the delimiter stack and return a literal text node ].

• If we do, then

• We return a link or image node whose children are the inlines after the text node pointed to by the opening delimiter.

• We run process emphasis on these inlines, with the [ opener as stack_bottom.

• We remove the opening delimiter.

• If we have a link (and not an image), we also set all [ delimiters before the opening delimiter to inactive. (This will prevent us from getting links within links.)

#### process emphasis

Parameter stack_bottom sets a lower bound to how far we descend in the delimiter stack. If it is NULL, we can go all the way to the bottom. Otherwise, we stop before visiting stack_bottom.

Let current_position point to the element on the delimiter stack just above stack_bottom (or the first element if stack_bottom is NULL).

We keep track of the openers_bottom for each delimiter type (*, _). 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 in 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.