commonmark

--- title: CommonMark Spec author: John MacFarlane version: 0.18 date: 2015-03-03 license: '[CC-BY-SA 4.0](http://creativecommons.org/licenses/by-sa/4.0/)' ... # Introduction ## What 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. ## Why is a spec needed? John Gruber's [canonical description of Markdown's syntax](http://daringfireball.net/projects/markdown/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](http://article.gmane.org/gmane.text.markdown.general/1997).) 2. Is a blank line needed before a block quote or header? 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 header inside the blockquote, while others do not). (John Gruber has also spoken [in favor of requiring the blank lines](http://article.gmane.org/gmane.text.markdown.general/2146).) 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.) ``` markdown 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? ``` markdown 1. one 2. two 3. three ``` Or this? ``` markdown 1. one - a - b 2. two ``` (There are some relevant comments by John Gruber [here](http://article.gmane.org/gmane.text.markdown.general/2554).) 5. Can list markers be indented? Can ordered list markers be right-aligned? ``` markdown 8. item 1 9. item 2 10. item 2a ``` 6. Is this one list with a horizontal rule in its second item, or two lists separated by a horizontal rule? ``` markdown * 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.) ``` markdown 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 ? ``` markdown [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? ``` markdown *foo *bar* baz* ``` 10. What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed? ``` markdown - `a long code span can contain a hyphen like this - and it can screw things up` ``` 11. Can list items include section headers? (`Markdown.pl` does not allow this, but does allow blockquotes to include headers.) ``` markdown - # Heading ``` 12. Can list items be empty? ``` markdown * a * * b ``` 13. Can link references be defined inside block quotes or list items? ``` markdown > Blockquote [foo]. > > [foo]: /url ``` 14. If there are multiple definitions for the same reference, which takes precedence? ``` markdown [foo]: /url1 [foo]: /url2 [foo][] ``` In the absence of a spec, early implementers consulted `Markdown.pl` to resolve these ambiguities. But `Markdown.pl` was quite buggy, and gave manifestly bad results in many cases, so it was not a satisfactory replacement for a spec. Because there is no unambiguous spec, implementations have diverged considerably. As a result, users are often surprised to find that a document that renders one way on one system (say, a github wiki) renders differently on another (say, converting to docbook using pandoc). To make matters worse, because nothing in Markdown counts as a "syntax error," the divergence often isn't discovered right away. ## About this document This document attempts to specify Markdown syntax unambiguously. It contains many examples with side-by-side Markdown and HTML. These are intended to double as conformance tests. An accompanying script `spec_tests.py` can be used to run the tests against any Markdown program: python test/spec_tests.py --spec spec.txt --program PROGRAM Since this document describes how Markdown is to be parsed into an abstract syntax tree, it would have made sense to use an abstract representation of the syntax tree instead of HTML. But HTML is capable of representing the structural distinctions we need to make, and the choice of HTML for the tests makes it possible to run the tests against an implementation without writing an abstract syntax tree renderer. 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 `spec2md.pl` can be used to turn `spec.txt` into pandoc Markdown, which can then be converted into other formats. In the examples, the `→` character is used to represent tabs. # Preliminaries ## Characters and lines Any sequence of [character]s is a valid CommonMark document. A [character](@character) is a unicode code point. 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](@line) is a sequence of zero or more [character]s followed by a [line ending] or by the end of file. A [line ending](@line-ending) is, depending on the platform, a newline (`U+000A`), carriage return (`U+000D`), or carriage return + newline. For security reasons, a conforming parser must strip or replace the Unicode character `U+0000`. A line containing no characters, or a line containing only spaces (`U+0020`) or tabs (`U+0009`), is called a [blank line](@blank-line). The following definitions of character classes will be used in this spec: A [whitespace character](@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](@whitespace) is a sequence of one or more [whitespace character]s. A [unicode whitespace character](@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](@unicode-whitespace) is a sequence of one or more [unicode whitespace character]s. A [non-space character](@non-space-character) is anything but `U+0020`. An [ASCII punctuation character](@ascii-punctuation-character) is `!`, `"`, `#`, `$`, `%`, `&`, `'`, `(`, `)`, `*`, `+`, `,`, `-`, `.`, `/`, `:`, `;`, `<`, `=`, `>`, `?`, `@`, `[`, `\`, `]`, `^`, `_`, `` ` ``, `{`, `|`, `}`, or `~`. A [punctuation character](@punctuation-character) is an [ASCII punctuation character] or anything in the unicode classes `Pc`, `Pd`, `Pe`, `Pf`, `Pi`, `Po`, or `Ps`. ## Tab expansion Tabs in lines are expanded to spaces, with a tab stop of 4 characters: . →foo→baz→→bim . <pre><code>foo baz bim </code></pre> . . a→a ὐ→a . <pre><code>a a ὐ a </code></pre> . # Blocks and inlines We can think of a document as a sequence of [blocks](@block)---structural elements like paragraphs, block quotations, lists, headers, rules, and code blocks. Blocks can contain other blocks, or they can contain [inline](@inline) content: words, spaces, links, emphasized text, images, and inline code. ## Precedence Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span: . - `one - two` . <ul> <li>`one</li> <li>two`</li> </ul> . This means that parsing can proceed in two steps: first, the block structure of the document can be discerned; second, text lines inside paragraphs, headers, and other block constructs can be parsed for inline structure. The second step requires information about link reference definitions that will be available only at the end of the first step. Note that the first step requires processing lines in sequence, but the second can be parallelized, since the inline parsing of one block element does not affect the inline parsing of any other. ## Container blocks and leaf blocks We can divide blocks into two types: [container block](@container-block)s, which can contain other blocks, and [leaf block](@leaf-block)s, which cannot. # Leaf blocks This section describes the different kinds of leaf block that make up a Markdown document. ## Horizontal rules 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 [horizontal rule](@horizontal-rule). . *** --- ___ . <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-space character]s be the same. So, this is not a horizontal rule: . *-* . <p><em>-</em></p> . Horizontal rules do not need blank lines before or after: . - foo *** - bar . <ul> <li>foo</li> </ul> <hr /> <ul> <li>bar</li> </ul> . Horizontal rules 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 horizontal rule could also be interpreted as the underline of a [setext header], the interpretation as a [setext header] takes precedence. Thus, for example, this is a setext header, not a paragraph followed by a horizontal rule: . Foo --- bar . <h2>Foo</h2> <p>bar</p> . When both a horizontal rule and a list item are possible interpretations of a line, the horizontal rule takes precedence: . * Foo * * * * Bar . <ul> <li>Foo</li> </ul> <hr /> <ul> <li>Bar</li> </ul> . If you want a horizontal rule in a list item, use a different bullet: . - Foo - * * * . <ul> <li>Foo</li> <li> <hr /> </li> </ul> . ## ATX headers An [ATX header](@atx-header) 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 `#` characters. The opening sequence of `#` characters cannot be followed directly by a [non-space character]. 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 header are stripped of leading and trailing spaces before being parsed as inline content. The header level is equal to the number of `#` characters in the opening sequence. Simple headers: . # 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 header: . ####### foo . <p>####### foo</p> . A space is required between the `#` characters and the header's contents. Note that many implementations currently do not require the space. However, the space was required by the [original ATX implementation](http://www.aaronsw.com/2002/atx/atx.py), and it helps prevent things like the following from being parsed as headers: . #5 bolt . <p>#5 bolt</p> . This is not a header, 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 a [non-space character] following it is not a closing sequence, but counts as part of the contents of the header: . ### 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 headers 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 headers can be empty: . ## # ### ### . <h2></h2> <h1></h1> <h3></h3> . ## Setext headers A [setext header](@setext-header) consists of a line of text, containing at least one [non-space character], with no more than 3 spaces indentation, followed by a [setext header underline]. The line of text must be one that, were it not followed by the setext header underline, would be interpreted as part of a paragraph: it cannot be a code block, header, blockquote, horizontal rule, or list. A [setext header underline](@setext-header-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 header underline]. The header is a level 1 header if `=` characters are used in the [setext header underline], and a level 2 header if `-` characters are used. The contents of the header are the result of parsing the first line as Markdown inline content. In general, a setext header need not be preceded or followed by a blank line. However, it cannot interrupt a paragraph, so when a setext header 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 underlining can be any length: . Foo ------------------------- Foo = . <h2>Foo</h2> <h1>Foo</h1> . The header 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 header 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 header 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 headers: . `Foo ---- ` <a title="a lot --- of dashes"/> . <h2>`Foo</h2> <p>`</p> <h2><a title="a lot</h2> <p>of dashes"/></p> . The setext header underline cannot be a [lazy continuation line] in a list item or block quote: . > Foo --- . <blockquote> <p>Foo</p> </blockquote> <hr /> . . - Foo --- . <ul> <li>Foo</li> </ul> <hr /> . A setext header cannot interrupt a paragraph: . Foo Bar --- Foo Bar === . <p>Foo Bar</p> <hr /> <p>Foo Bar ===</p> . But in general a blank line is not required before or after: . --- Foo --- Bar --- Baz . <hr /> <h2>Foo</h2> <h2>Bar</h2> <p>Baz</p> . Setext headers cannot be empty: . ==== . <p>====</p> . Setext header text lines must not be interpretable as block constructs other than paragraphs. So, the line of dashes in these examples gets interpreted as a horizontal rule: . --- --- . <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 header with `> foo` as its literal text, you can use backslash escapes: . \> foo ------ . <h2>> foo</h2> . ## Indented code blocks An [indented code block](@indented-code-block) is composed of one or more [indented chunk]s separated by blank lines. An [indented chunk](@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 ending]s, 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> . The contents are literal text, and do not get parsed as Markdown: . <a/> *hi* - one . <pre><code><a/> *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: . # Header foo Header ------ foo ---- . <h1>Header</h1> <pre><code>foo </code></pre> <h2>Header</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> . ## Fenced code blocks A [code fence](@code-fence) is a sequence of at least three consecutive backtick characters (`` ` ``) or tildes (`~`). (Tildes and backticks cannot be mixed.) A [fenced code block](@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](@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>< > </code></pre> . With tildes: . ~~~ < > ~~~ . <pre><code>< > </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: . ``` . <pre><code></code></pre> . . ````` ``` aaa . <pre><code> ``` aaa </code></pre> . 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 string]s for backtick code blocks cannot contain backticks: . ``` aa ``` foo . <p><code>aa</code> foo</p> . Closing code fences cannot have [info string]s: . ``` ``` aaa ``` . <pre><code>``` aaa </code></pre> . ## HTML blocks An [HTML block tag](@html-block-tag) is an [open tag] or [closing tag] whose tag name is one of the following (case-insensitive): `article`, `header`, `aside`, `hgroup`, `blockquote`, `hr`, `iframe`, `body`, `li`, `map`, `button`, `object`, `canvas`, `ol`, `caption`, `output`, `col`, `p`, `colgroup`, `pre`, `dd`, `progress`, `div`, `section`, `dl`, `table`, `td`, `dt`, `tbody`, `embed`, `textarea`, `fieldset`, `tfoot`, `figcaption`, `th`, `figure`, `thead`, `footer`, `tr`, `form`, `ul`, `h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `video`, `script`, `style`. An [HTML block](@html-block) begins with an [HTML block tag], [HTML comment], [processing instruction], [declaration], or [CDATA section]. It ends when a [blank line] or the end of the input is encountered. The initial line may be indented up to three spaces, and subsequent lines may have any indentation. The contents of the HTML block are interpreted as raw HTML, and will not be escaped in HTML output. Some simple examples: . <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> . 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> . 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; ``` . A comment: .  .  . A processing instruction: . <?php echo '>'; ?> . <?php echo '>'; ?> . CDATA: . <![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: .   .  <pre><code> </code></pre> . An HTML block 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 always needed, except at the end of a document: . <div> bar </div> *foo* . <div> bar </div> *foo* . An incomplete HTML block tag may also start an HTML block: . <div class foo . <div class foo . 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. Indeed, most Markdown implementations, including some of Gruber's own perl implementations, do not impose 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: . <div> *Emphasized* text. </div> . <div> <p><em>Emphasized</em> text.</p> </div> . Compare: . <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> . Moreover, blank lines are usually not necessary and can be deleted. The exception is inside `<pre>` tags; here, one can replace the blank lines with `
` entities. So there is no important loss of expressive power with the new rule. ## Link reference definitions A [link reference definition](@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-space character]s 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 link]s and reference-style [images] elsewhere in the document. [Link reference definitions] can come either before or after the links that use them. . [foo]: /url "title" [foo] . <p><a href="/url" title="title">foo</a></p> . . [foo]: /url 'the title' [foo] . <p><a href="/url" title="the title">foo</a></p> . . [Foo*bar\]]:my_(url) 'title (with parens)' [Foo*bar\]] . <p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p> . . [Foo bar]: <my url> 'title' [Foo bar] . <p><a href="my%20url" title="title">Foo bar</a></p> . The title may extend over multiple lines: . [foo]: /url ' title line1 line2 ' [foo] . <p><a href="/url" title=" title line1 line2 ">foo</a></p> . However, it may not contain a [blank line]: . [foo]: /url 'title with blank line' [foo] . <p>[foo]: /url 'title</p> <p>with blank line'</p> <p>[foo]</p> . The title may be omitted: . [foo]: /url [foo] . <p><a href="/url">foo</a></p> . The link destination may not be omitted: . [foo]: [foo] . <p>[foo]:</p> <p>[foo]</p> . 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-space character]s after the title: . [foo]: /url "title" ok . <p>[foo]: /url "title" ok</p> . This is not a link reference definition, because it is indented four spaces: . [foo]: /url "title" [foo] . <pre><code>[foo]: /url "title" </code></pre> <p>[foo]</p> . This is not a link reference definition, because it occurs inside a code block: . ``` [foo]: /url ``` [foo] . <pre><code>[foo]: /url </code></pre> <p>[foo]</p> . A [link reference definition] cannot interrupt a paragraph. . Foo [bar]: /baz [bar] . <p>Foo [bar]: /baz</p> <p>[bar]</p> . However, it can directly follow other block elements, such as headers and horizontal rules, 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 definition]s 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 definition]s can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined: . [foo] > [foo]: /url . <p><a href="/url">foo</a></p> <blockquote> </blockquote> . ## Paragraphs A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a [paragraph](@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> . ## Blank lines [Blank line]s between block-level elements are ignored, except for the role they play in determining whether a [list] is [tight] or [loose]. Blank lines at the beginning and end of the document are also ignored. . aaa # aaa . <p>aaa</p> <h1>aaa</h1> . # Container blocks A [container block] is a block that has other blocks as its contents. There are two basic kinds of container blocks: [block quotes] and [list items]. [Lists] are meta-containers for [list items]. We define the syntax for container blocks recursively. The general form of the definition is: > If X is a sequence of blocks, then the result of > transforming X in such-and-such a way is a container of type Y > with these blocks as its content. So, we explain what counts as a block quote or list item by explaining how these can be *generated* from their contents. This should suffice to define the syntax, although it does not give a recipe for *parsing* these constructions. (A recipe is provided below in the section entitled [A parsing strategy](#appendix-a-a-parsing-strategy).) ## Block quotes A [block quote marker](@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](#block-quotes) containing *Bs*. 2. **Laziness.** If a string of lines *Ls* constitute a [block quote](#block-quotes) with contents *Bs*, then the result of deleting the initial [block quote marker] from one or more lines in which the next [non-space character] after the [block quote marker] is [paragraph continuation text] is a block quote with *Bs* as its content. [Paragraph continuation text](@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](#block-quotes). 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>> # Foo > bar > 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 are continuations of paragraphs. Lines containing characters or indentation that indicate block structure cannot be lazy. . > foo --- . <blockquote> <p>foo</p> </blockquote> <hr /> . . > - foo - bar . <blockquote> <ul> <li>foo</li> </ul> </blockquote> <ul> <li>bar</li> </ul> . . > 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> . 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> . ## List items A [list marker](@list-marker) is a [bullet list marker] or an [ordered list marker]. A [bullet list marker](@bullet-list-marker) is a `-`, `+`, or `*` character. An [ordered list marker](@ordered-list-marker) is a sequence of one of more digits (`0-9`), followed by either a `.` character or a `)` character. 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-space character] and not separated from each other by more than one blank line, and *M* is a list marker *M* 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-space 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-space 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 sufficent 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> . 2. **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 *M* 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-space 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> . 3. **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 *M* 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> . 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: