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diff --git a/tests/test_cmark_spec/spec.md b/tests/test_cmark_spec/spec.md new file mode 100644 index 0000000..2d79f7b --- /dev/null +++ b/tests/test_cmark_spec/spec.md @@ -0,0 +1,9756 @@ +--- +title: CommonMark Spec +author: John MacFarlane +version: 0.30 +date: '2021-06-19' +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 for indicating formatting in email +and usenet posts. It was developed by John Gruber (with +help from Aaron Swartz) and released in 2004 in the form of a +[syntax description](http://daringfireball.net/projects/markdown/syntax) +and a Perl script (`Markdown.pl`) for converting Markdown to +HTML. In the next decade, dozens of implementations were +developed in many languages. Some extended the original +Markdown syntax with conventions for footnotes, tables, and +other document elements. Some allowed Markdown documents to be +rendered in formats other than HTML. Websites like Reddit, +StackOverflow, and GitHub had millions of people using Markdown. +And Markdown started to be used beyond the web, to author books, +articles, slide shows, letters, and lecture notes. + +What distinguishes Markdown from many other lightweight markup +syntaxes, which are often easier to write, is its readability. +As Gruber writes: + +> The overriding design goal for Markdown's formatting syntax is +> to make it as readable as possible. The idea is that a +> Markdown-formatted document should be publishable as-is, as +> plain text, without looking like it's been marked up with tags +> or formatting instructions. +> (<http://daringfireball.net/projects/markdown/>) + +The point can be illustrated by comparing a sample of +[AsciiDoc](http://www.methods.co.nz/asciidoc/) with +an equivalent sample of Markdown. Here is a sample of +AsciiDoc from the AsciiDoc manual: + +``` +1. List item one. ++ +List item one continued with a second paragraph followed by an +Indented block. ++ +................. +$ ls *.sh +$ mv *.sh ~/tmp +................. ++ +List item continued with a third paragraph. + +2. List item two continued with an open block. ++ +-- +This paragraph is part of the preceding list item. + +a. This list is nested and does not require explicit item +continuation. ++ +This paragraph is part of the preceding list item. + +b. List item b. + +This paragraph belongs to item two of the outer list. +-- +``` + +And here is the equivalent in Markdown: +``` +1. List item one. + + List item one continued with a second paragraph followed by an + Indented block. + + $ ls *.sh + $ mv *.sh ~/tmp + + List item continued with a third paragraph. + +2. List item two continued with an open block. + + This paragraph is part of the preceding list item. + + 1. This list is nested and does not require explicit item continuation. + + This paragraph is part of the preceding list item. + + 2. List item b. + + This paragraph belongs to item two of the outer list. +``` + +The AsciiDoc version is, arguably, easier to write. You don't need +to worry about indentation. But the Markdown version is much easier +to read. The nesting of list items is apparent to the eye in the +source, not just in the processed document. + +## Why is a spec needed? + +John Gruber's [canonical description of Markdown's +syntax](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 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](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 thematic break in its second item, + or two lists separated by a thematic break? + + ``` 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 headings? (`Markdown.pl` does not + allow this, but does allow blockquotes to include headings.) + + ``` 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. + +Note that not every feature of the HTML samples is mandated by +the spec. For example, the spec says what counts as a link +destination, but it doesn't mandate that non-ASCII characters in +the URL be percent-encoded. To use the automatic tests, +implementers will need to provide a renderer that conforms to +the expectations of the spec examples (percent-encoding +non-ASCII characters in URLs). But a conforming implementation +can use a different renderer and may choose not to +percent-encode non-ASCII characters in URLs. + +This document is generated from a text file, `spec.txt`, written +in Markdown with a small extension for the side-by-side tests. +The script `tools/makespec.py` can be used to convert `spec.txt` into +HTML or CommonMark (which can then be converted into other formats). + +In the examples, the `→` character is used to represent tabs. + +# Preliminaries + +## Characters and lines + +Any sequence of [characters] is a valid CommonMark +document. + +A [character](@) is a Unicode code point. Although some +code points (for example, combining accents) do not correspond to +characters in an intuitive sense, all code points count as characters +for purposes of this spec. + +This spec does not specify an encoding; it thinks of lines as composed +of [characters] rather than bytes. A conforming parser may be limited +to a certain encoding. + +A [line](@) is a sequence of zero or more [characters] +other than line feed (`U+000A`) or carriage return (`U+000D`), +followed by a [line ending] or by the end of file. + +A [line ending](@) is a line feed (`U+000A`), a carriage return +(`U+000D`) not followed by a line feed, or a carriage return and a +following line feed. + +A line containing no characters, or a line containing only spaces +(`U+0020`) or tabs (`U+0009`), is called a [blank line](@). + +The following definitions of character classes will be used in this spec: + +A [Unicode whitespace character](@) is +any code point in the Unicode `Zs` general category, or a tab (`U+0009`), +line feed (`U+000A`), form feed (`U+000C`), or carriage return (`U+000D`). + +[Unicode whitespace](@) is a sequence of one or more +[Unicode whitespace characters]. + +A [tab](@) is `U+0009`. + +A [space](@) is `U+0020`. + +An [ASCII control character](@) is a character between `U+0000–1F` (both +including) or `U+007F`. + +An [ASCII punctuation character](@) +is `!`, `"`, `#`, `$`, `%`, `&`, `'`, `(`, `)`, +`*`, `+`, `,`, `-`, `.`, `/` (U+0021–2F), +`:`, `;`, `<`, `=`, `>`, `?`, `@` (U+003A–0040), +`[`, `\`, `]`, `^`, `_`, `` ` `` (U+005B–0060), +`{`, `|`, `}`, or `~` (U+007B–007E). + +A [Unicode punctuation character](@) is an [ASCII +punctuation character] or anything in +the general Unicode categories `Pc`, `Pd`, `Pe`, `Pf`, `Pi`, `Po`, or `Ps`. + +## Tabs + +Tabs in lines are not expanded to [spaces]. However, +in contexts where spaces help to define block structure, +tabs behave as if they were replaced by spaces with a tab stop +of 4 characters. + +Thus, for example, a tab can be used instead of four spaces +in an indented code block. (Note, however, that internal +tabs are passed through as literal tabs, not expanded to +spaces.) + +```````````````````````````````` example +→foo→baz→→bim +. +<pre><code>foo→baz→→bim +</code></pre> +```````````````````````````````` + +```````````````````````````````` example + →foo→baz→→bim +. +<pre><code>foo→baz→→bim +</code></pre> +```````````````````````````````` + +```````````````````````````````` example + a→a + ὐ→a +. +<pre><code>a→a +ὐ→a +</code></pre> +```````````````````````````````` + +In the following example, a continuation paragraph of a list +item is indented with a tab; this has exactly the same effect +as indentation with four spaces would: + +```````````````````````````````` example + - foo + +→bar +. +<ul> +<li> +<p>foo</p> +<p>bar</p> +</li> +</ul> +```````````````````````````````` + +```````````````````````````````` example +- foo + +→→bar +. +<ul> +<li> +<p>foo</p> +<pre><code> bar +</code></pre> +</li> +</ul> +```````````````````````````````` + +Normally the `>` that begins a block quote may be followed +optionally by a space, which is not considered part of the +content. In the following case `>` is followed by a tab, +which is treated as if it were expanded into three spaces. +Since one of these spaces is considered part of the +delimiter, `foo` is considered to be indented six spaces +inside the block quote context, so we get an indented +code block starting with two spaces. + +```````````````````````````````` example +>→→foo +. +<blockquote> +<pre><code> foo +</code></pre> +</blockquote> +```````````````````````````````` + +```````````````````````````````` example +-→→foo +. +<ul> +<li> +<pre><code> foo +</code></pre> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example + foo +→bar +. +<pre><code>foo +bar +</code></pre> +```````````````````````````````` + +```````````````````````````````` example + - foo + - bar +→ - baz +. +<ul> +<li>foo +<ul> +<li>bar +<ul> +<li>baz</li> +</ul> +</li> +</ul> +</li> +</ul> +```````````````````````````````` + +```````````````````````````````` example +#→Foo +. +<h1>Foo</h1> +```````````````````````````````` + +```````````````````````````````` example +*→*→*→ +. +<hr /> +```````````````````````````````` + + +## Insecure characters + +For security reasons, the Unicode character `U+0000` must be replaced +with the REPLACEMENT CHARACTER (`U+FFFD`). + + +## Backslash escapes + +Any ASCII punctuation character may be backslash-escaped: + +```````````````````````````````` example +\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~ +. +<p>!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~</p> +```````````````````````````````` + + +Backslashes before other characters are treated as literal +backslashes: + +```````````````````````````````` example +\→\A\a\ \3\φ\« +. +<p>\→\A\a\ \3\φ\«</p> +```````````````````````````````` + + +Escaped characters are treated as regular characters and do +not have their usual Markdown meanings: + +```````````````````````````````` example +\*not emphasized* +\<br/> not a tag +\[not a link](/foo) +\`not code` +1\. not a list +\* not a list +\# not a heading +\[foo]: /url "not a reference" +\ö not a character entity +. +<p>*not emphasized* +<br/> not a tag +[not a link](/foo) +`not code` +1. not a list +* not a list +# not a heading +[foo]: /url "not a reference" +&ouml; not a character entity</p> +```````````````````````````````` + + +If a backslash is itself escaped, the following character is not: + +```````````````````````````````` example +\\*emphasis* +. +<p>\<em>emphasis</em></p> +```````````````````````````````` + + +A backslash at the end of the line is a [hard line break]: + +```````````````````````````````` example +foo\ +bar +. +<p>foo<br /> +bar</p> +```````````````````````````````` + + +Backslash escapes do not work in code blocks, code spans, autolinks, or +raw HTML: + +```````````````````````````````` example +`` \[\` `` +. +<p><code>\[\`</code></p> +```````````````````````````````` + + +```````````````````````````````` example + \[\] +. +<pre><code>\[\] +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +~~~ +\[\] +~~~ +. +<pre><code>\[\] +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +<http://example.com?find=\*> +. +<p><a href="http://example.com?find=%5C*">http://example.com?find=\*</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<a href="/bar\/)"> +. +<a href="/bar\/)"> +```````````````````````````````` + + +But they work in all other contexts, including URLs and link titles, +link references, and [info strings] in [fenced code blocks]: + +```````````````````````````````` example +[foo](/bar\* "ti\*tle") +. +<p><a href="/bar*" title="ti*tle">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo] + +[foo]: /bar\* "ti\*tle" +. +<p><a href="/bar*" title="ti*tle">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +``` foo\+bar +foo +``` +. +<pre><code class="language-foo+bar">foo +</code></pre> +```````````````````````````````` + + +## Entity and numeric character references + +Valid HTML entity references and numeric character references +can be used in place of the corresponding Unicode character, +with the following exceptions: + +- Entity and character references are not recognized in code + blocks and code spans. + +- Entity and character references cannot stand in place of + special characters that define structural elements in + CommonMark. For example, although `*` can be used + in place of a literal `*` character, `*` cannot replace + `*` in emphasis delimiters, bullet list markers, or thematic + breaks. + +Conforming CommonMark parsers need not store information about +whether a particular character was represented in the source +using a Unicode character or an entity reference. + +[Entity references](@) consist of `&` + any of the valid +HTML5 entity names + `;`. The +document <https://html.spec.whatwg.org/entities.json> +is used as an authoritative source for the valid entity +references and their corresponding code points. + +```````````````````````````````` example + & © Æ Ď +¾ ℋ ⅆ +∲ ≧̸ +. +<p> & © Æ Ď +¾ ℋ ⅆ +∲ ≧̸</p> +```````````````````````````````` + + +[Decimal numeric character +references](@) +consist of `&#` + a string of 1--7 arabic digits + `;`. A +numeric character reference is parsed as the corresponding +Unicode character. Invalid Unicode code points will be replaced by +the REPLACEMENT CHARACTER (`U+FFFD`). For security reasons, +the code point `U+0000` will also be replaced by `U+FFFD`. + +```````````````````````````````` example +# Ӓ Ϡ � +. +<p># Ӓ Ϡ �</p> +```````````````````````````````` + + +[Hexadecimal numeric character +references](@) consist of `&#` + +either `X` or `x` + a string of 1-6 hexadecimal digits + `;`. +They too are parsed as the corresponding Unicode character (this +time specified with a hexadecimal numeral instead of decimal). + +```````````````````````````````` example +" ആ ಫ +. +<p>" ആ ಫ</p> +```````````````````````````````` + + +Here are some nonentities: + +```````````````````````````````` example +  &x; &#; &#x; +� +&#abcdef0; +&ThisIsNotDefined; &hi?; +. +<p>&nbsp &x; &#; &#x; +&#87654321; +&#abcdef0; +&ThisIsNotDefined; &hi?;</p> +```````````````````````````````` + + +Although HTML5 does accept some entity references +without a trailing semicolon (such as `©`), these are not +recognized here, because it makes the grammar too ambiguous: + +```````````````````````````````` example +© +. +<p>&copy</p> +```````````````````````````````` + + +Strings that are not on the list of HTML5 named entities are not +recognized as entity references either: + +```````````````````````````````` example +&MadeUpEntity; +. +<p>&MadeUpEntity;</p> +```````````````````````````````` + + +Entity and numeric character references are recognized in any +context besides code spans or code blocks, including +URLs, [link titles], and [fenced code block][] [info strings]: + +```````````````````````````````` example +<a href="öö.html"> +. +<a href="öö.html"> +```````````````````````````````` + + +```````````````````````````````` example +[foo](/föö "föö") +. +<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo] + +[foo]: /föö "föö" +. +<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +``` föö +foo +``` +. +<pre><code class="language-föö">foo +</code></pre> +```````````````````````````````` + + +Entity and numeric character references are treated as literal +text in code spans and code blocks: + +```````````````````````````````` example +`föö` +. +<p><code>f&ouml;&ouml;</code></p> +```````````````````````````````` + + +```````````````````````````````` example + föfö +. +<pre><code>f&ouml;f&ouml; +</code></pre> +```````````````````````````````` + + +Entity and numeric character references cannot be used +in place of symbols indicating structure in CommonMark +documents. + +```````````````````````````````` example +*foo* +*foo* +. +<p>*foo* +<em>foo</em></p> +```````````````````````````````` + +```````````````````````````````` example +* foo + +* foo +. +<p>* foo</p> +<ul> +<li>foo</li> +</ul> +```````````````````````````````` + +```````````````````````````````` example +foo bar +. +<p>foo + +bar</p> +```````````````````````````````` + +```````````````````````````````` example +	foo +. +<p>→foo</p> +```````````````````````````````` + + +```````````````````````````````` example +[a](url "tit") +. +<p>[a](url "tit")</p> +```````````````````````````````` + + + +# Blocks and inlines + +We can think of a document as a sequence of +[blocks](@)---structural elements like paragraphs, block +quotations, lists, headings, rules, and code blocks. Some blocks (like +block quotes and list items) contain other blocks; others (like +headings and paragraphs) contain [inline](@) content---text, +links, emphasized text, images, code spans, and so on. + +## Precedence + +Indicators of block structure always take precedence over indicators +of inline structure. So, for example, the following is a list with +two items, not a list with one item containing a code span: + +```````````````````````````````` example +- `one +- two` +. +<ul> +<li>`one</li> +<li>two`</li> +</ul> +```````````````````````````````` + + +This means that parsing can proceed in two steps: first, the block +structure of the document can be discerned; second, text lines inside +paragraphs, headings, and other block constructs can be parsed for inline +structure. The second step requires information about link reference +definitions that will be available only at the end of the first +step. Note that the first step requires processing lines in sequence, +but the second can be parallelized, since the inline parsing of +one block element does not affect the inline parsing of any other. + +## Container blocks and leaf blocks + +We can divide blocks into two types: +[container blocks](#container-blocks), +which can contain other blocks, and [leaf blocks](#leaf-blocks), +which cannot. + +# Leaf blocks + +This section describes the different kinds of leaf block that make up a +Markdown document. + +## Thematic breaks + +A line consisting of optionally up to three spaces of indentation, followed by a +sequence of three or more matching `-`, `_`, or `*` characters, each followed +optionally by any number of spaces or tabs, forms a +[thematic break](@). + +```````````````````````````````` example +*** +--- +___ +. +<hr /> +<hr /> +<hr /> +```````````````````````````````` + + +Wrong characters: + +```````````````````````````````` example ++++ +. +<p>+++</p> +```````````````````````````````` + + +```````````````````````````````` example +=== +. +<p>===</p> +```````````````````````````````` + + +Not enough characters: + +```````````````````````````````` example +-- +** +__ +. +<p>-- +** +__</p> +```````````````````````````````` + + +Up to three spaces of indentation are allowed: + +```````````````````````````````` example + *** + *** + *** +. +<hr /> +<hr /> +<hr /> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example + *** +. +<pre><code>*** +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +Foo + *** +. +<p>Foo +***</p> +```````````````````````````````` + + +More than three characters may be used: + +```````````````````````````````` example +_____________________________________ +. +<hr /> +```````````````````````````````` + + +Spaces and tabs are allowed between the characters: + +```````````````````````````````` example + - - - +. +<hr /> +```````````````````````````````` + + +```````````````````````````````` example + ** * ** * ** * ** +. +<hr /> +```````````````````````````````` + + +```````````````````````````````` example +- - - - +. +<hr /> +```````````````````````````````` + + +Spaces and tabs are allowed at the end: + +```````````````````````````````` example +- - - - +. +<hr /> +```````````````````````````````` + + +However, no other characters may occur in the line: + +```````````````````````````````` example +_ _ _ _ a + +a------ + +---a--- +. +<p>_ _ _ _ a</p> +<p>a------</p> +<p>---a---</p> +```````````````````````````````` + + +It is required that all of the characters other than spaces or tabs be the same. +So, this is not a thematic break: + +```````````````````````````````` example + *-* +. +<p><em>-</em></p> +```````````````````````````````` + + +Thematic breaks do not need blank lines before or after: + +```````````````````````````````` example +- foo +*** +- bar +. +<ul> +<li>foo</li> +</ul> +<hr /> +<ul> +<li>bar</li> +</ul> +```````````````````````````````` + + +Thematic breaks can interrupt a paragraph: + +```````````````````````````````` example +Foo +*** +bar +. +<p>Foo</p> +<hr /> +<p>bar</p> +```````````````````````````````` + + +If a line of dashes that meets the above conditions for being a +thematic break could also be interpreted as the underline of a [setext +heading], the interpretation as a +[setext heading] takes precedence. Thus, for example, +this is a setext heading, not a paragraph followed by a thematic break: + +```````````````````````````````` example +Foo +--- +bar +. +<h2>Foo</h2> +<p>bar</p> +```````````````````````````````` + + +When both a thematic break and a list item are possible +interpretations of a line, the thematic break takes precedence: + +```````````````````````````````` example +* Foo +* * * +* Bar +. +<ul> +<li>Foo</li> +</ul> +<hr /> +<ul> +<li>Bar</li> +</ul> +```````````````````````````````` + + +If you want a thematic break in a list item, use a different bullet: + +```````````````````````````````` example +- Foo +- * * * +. +<ul> +<li>Foo</li> +<li> +<hr /> +</li> +</ul> +```````````````````````````````` + + +## ATX headings + +An [ATX heading](@) +consists of a string of characters, parsed as inline content, between an +opening sequence of 1--6 unescaped `#` characters and an optional +closing sequence of any number of unescaped `#` characters. +The opening sequence of `#` characters must be followed by spaces or tabs, or +by the end of line. The optional closing sequence of `#`s must be preceded by +spaces or tabs and may be followed by spaces or tabs only. The opening +`#` character may be preceded by up to three spaces of indentation. The raw +contents of the heading are stripped of leading and trailing space or tabs +before being parsed as inline content. The heading level is equal to the number +of `#` characters in the opening sequence. + +Simple headings: + +```````````````````````````````` example +# foo +## foo +### foo +#### foo +##### foo +###### foo +. +<h1>foo</h1> +<h2>foo</h2> +<h3>foo</h3> +<h4>foo</h4> +<h5>foo</h5> +<h6>foo</h6> +```````````````````````````````` + + +More than six `#` characters is not a heading: + +```````````````````````````````` example +####### foo +. +<p>####### foo</p> +```````````````````````````````` + + +At least one space or tab is required between the `#` characters and the +heading's contents, unless the heading is empty. Note that many +implementations currently do not require the space. However, the +space was required by the +[original ATX implementation](http://www.aaronsw.com/2002/atx/atx.py), +and it helps prevent things like the following from being parsed as +headings: + +```````````````````````````````` example +#5 bolt + +#hashtag +. +<p>#5 bolt</p> +<p>#hashtag</p> +```````````````````````````````` + + +This is not a heading, because the first `#` is escaped: + +```````````````````````````````` example +\## foo +. +<p>## foo</p> +```````````````````````````````` + + +Contents are parsed as inlines: + +```````````````````````````````` example +# foo *bar* \*baz\* +. +<h1>foo <em>bar</em> *baz*</h1> +```````````````````````````````` + + +Leading and trailing spaces or tabs are ignored in parsing inline content: + +```````````````````````````````` example +# foo +. +<h1>foo</h1> +```````````````````````````````` + + +Up to three spaces of indentation are allowed: + +```````````````````````````````` example + ### foo + ## foo + # foo +. +<h3>foo</h3> +<h2>foo</h2> +<h1>foo</h1> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example + # foo +. +<pre><code># foo +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +foo + # bar +. +<p>foo +# bar</p> +```````````````````````````````` + + +A closing sequence of `#` characters is optional: + +```````````````````````````````` example +## foo ## + ### bar ### +. +<h2>foo</h2> +<h3>bar</h3> +```````````````````````````````` + + +It need not be the same length as the opening sequence: + +```````````````````````````````` example +# foo ################################## +##### foo ## +. +<h1>foo</h1> +<h5>foo</h5> +```````````````````````````````` + + +Spaces or tabs are allowed after the closing sequence: + +```````````````````````````````` example +### foo ### +. +<h3>foo</h3> +```````````````````````````````` + + +A sequence of `#` characters with anything but spaces or tabs following it +is not a closing sequence, but counts as part of the contents of the +heading: + +```````````````````````````````` example +### foo ### b +. +<h3>foo ### b</h3> +```````````````````````````````` + + +The closing sequence must be preceded by a space or tab: + +```````````````````````````````` example +# foo# +. +<h1>foo#</h1> +```````````````````````````````` + + +Backslash-escaped `#` characters do not count as part +of the closing sequence: + +```````````````````````````````` example +### foo \### +## foo #\## +# foo \# +. +<h3>foo ###</h3> +<h2>foo ###</h2> +<h1>foo #</h1> +```````````````````````````````` + + +ATX headings need not be separated from surrounding content by blank +lines, and they can interrupt paragraphs: + +```````````````````````````````` example +**** +## foo +**** +. +<hr /> +<h2>foo</h2> +<hr /> +```````````````````````````````` + + +```````````````````````````````` example +Foo bar +# baz +Bar foo +. +<p>Foo bar</p> +<h1>baz</h1> +<p>Bar foo</p> +```````````````````````````````` + + +ATX headings can be empty: + +```````````````````````````````` example +## +# +### ### +. +<h2></h2> +<h1></h1> +<h3></h3> +```````````````````````````````` + + +## Setext headings + +A [setext heading](@) consists of one or more +lines of text, not interrupted by a blank line, of which the first line does not +have more than 3 spaces of indentation, followed by +a [setext heading underline]. The lines of text must be such +that, were they not followed by the setext heading underline, +they would be interpreted as a paragraph: they cannot be +interpretable as a [code fence], [ATX heading][ATX headings], +[block quote][block quotes], [thematic break][thematic breaks], +[list item][list items], or [HTML block][HTML blocks]. + +A [setext heading underline](@) is a sequence of +`=` characters or a sequence of `-` characters, with no more than 3 +spaces of indentation and any number of trailing spaces or tabs. If a line +containing a single `-` can be interpreted as an +empty [list items], it should be interpreted this way +and not as a [setext heading underline]. + +The heading is a level 1 heading if `=` characters are used in +the [setext heading underline], and a level 2 heading if `-` +characters are used. The contents of the heading are the result +of parsing the preceding lines of text as CommonMark inline +content. + +In general, a setext heading need not be preceded or followed by a +blank line. However, it cannot interrupt a paragraph, so when a +setext heading comes after a paragraph, a blank line is needed between +them. + +Simple examples: + +```````````````````````````````` example +Foo *bar* +========= + +Foo *bar* +--------- +. +<h1>Foo <em>bar</em></h1> +<h2>Foo <em>bar</em></h2> +```````````````````````````````` + + +The content of the header may span more than one line: + +```````````````````````````````` example +Foo *bar +baz* +==== +. +<h1>Foo <em>bar +baz</em></h1> +```````````````````````````````` + +The contents are the result of parsing the headings's raw +content as inlines. The heading's raw content is formed by +concatenating the lines and removing initial and final +spaces or tabs. + +```````````````````````````````` example + Foo *bar +baz*→ +==== +. +<h1>Foo <em>bar +baz</em></h1> +```````````````````````````````` + + +The underlining can be any length: + +```````````````````````````````` example +Foo +------------------------- + +Foo += +. +<h2>Foo</h2> +<h1>Foo</h1> +```````````````````````````````` + + +The heading content can be preceded by up to three spaces of indentation, and +need not line up with the underlining: + +```````````````````````````````` example + Foo +--- + + Foo +----- + + Foo + === +. +<h2>Foo</h2> +<h2>Foo</h2> +<h1>Foo</h1> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example + Foo + --- + + Foo +--- +. +<pre><code>Foo +--- + +Foo +</code></pre> +<hr /> +```````````````````````````````` + + +The setext heading underline can be preceded by up to three spaces of +indentation, and may have trailing spaces or tabs: + +```````````````````````````````` example +Foo + ---- +. +<h2>Foo</h2> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example +Foo + --- +. +<p>Foo +---</p> +```````````````````````````````` + + +The setext heading underline cannot contain internal spaces or tabs: + +```````````````````````````````` example +Foo += = + +Foo +--- - +. +<p>Foo += =</p> +<p>Foo</p> +<hr /> +```````````````````````````````` + + +Trailing spaces or tabs in the content line do not cause a hard line break: + +```````````````````````````````` example +Foo +----- +. +<h2>Foo</h2> +```````````````````````````````` + + +Nor does a backslash at the end: + +```````````````````````````````` example +Foo\ +---- +. +<h2>Foo\</h2> +```````````````````````````````` + + +Since indicators of block structure take precedence over +indicators of inline structure, the following are setext headings: + +```````````````````````````````` example +`Foo +---- +` + +<a title="a lot +--- +of dashes"/> +. +<h2>`Foo</h2> +<p>`</p> +<h2><a title="a lot</h2> +<p>of dashes"/></p> +```````````````````````````````` + + +The setext heading underline cannot be a [lazy continuation +line] in a list item or block quote: + +```````````````````````````````` example +> Foo +--- +. +<blockquote> +<p>Foo</p> +</blockquote> +<hr /> +```````````````````````````````` + + +```````````````````````````````` example +> foo +bar +=== +. +<blockquote> +<p>foo +bar +===</p> +</blockquote> +```````````````````````````````` + + +```````````````````````````````` example +- Foo +--- +. +<ul> +<li>Foo</li> +</ul> +<hr /> +```````````````````````````````` + + +A blank line is needed between a paragraph and a following +setext heading, since otherwise the paragraph becomes part +of the heading's content: + +```````````````````````````````` example +Foo +Bar +--- +. +<h2>Foo +Bar</h2> +```````````````````````````````` + + +But in general a blank line is not required before or after +setext headings: + +```````````````````````````````` example +--- +Foo +--- +Bar +--- +Baz +. +<hr /> +<h2>Foo</h2> +<h2>Bar</h2> +<p>Baz</p> +```````````````````````````````` + + +Setext headings cannot be empty: + +```````````````````````````````` example + +==== +. +<p>====</p> +```````````````````````````````` + + +Setext heading text lines must not be interpretable as block +constructs other than paragraphs. So, the line of dashes +in these examples gets interpreted as a thematic break: + +```````````````````````````````` example +--- +--- +. +<hr /> +<hr /> +```````````````````````````````` + + +```````````````````````````````` example +- foo +----- +. +<ul> +<li>foo</li> +</ul> +<hr /> +```````````````````````````````` + + +```````````````````````````````` example + foo +--- +. +<pre><code>foo +</code></pre> +<hr /> +```````````````````````````````` + + +```````````````````````````````` example +> foo +----- +. +<blockquote> +<p>foo</p> +</blockquote> +<hr /> +```````````````````````````````` + + +If you want a heading with `> foo` as its literal text, you can +use backslash escapes: + +```````````````````````````````` example +\> foo +------ +. +<h2>> 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 + +``` markdown +Foo +bar +--- +baz +``` + +One can find four different interpretations: + +1. paragraph "Foo", heading "bar", paragraph "baz" +2. paragraph "Foo bar", thematic break, paragraph "baz" +3. paragraph "Foo bar --- baz" +4. heading "Foo bar", paragraph "baz" + +We find interpretation 4 most natural, and interpretation 4 +increases the expressive power of CommonMark, by allowing +multiline headings. Authors who want interpretation 1 can +put a blank line after the first paragraph: + +```````````````````````````````` example +Foo + +bar +--- +baz +. +<p>Foo</p> +<h2>bar</h2> +<p>baz</p> +```````````````````````````````` + + +Authors who want interpretation 2 can put blank lines around +the thematic break, + +```````````````````````````````` example +Foo +bar + +--- + +baz +. +<p>Foo +bar</p> +<hr /> +<p>baz</p> +```````````````````````````````` + + +or use a thematic break that cannot count as a [setext heading +underline], such as + +```````````````````````````````` example +Foo +bar +* * * +baz +. +<p>Foo +bar</p> +<hr /> +<p>baz</p> +```````````````````````````````` + + +Authors who want interpretation 3 can use backslash escapes: + +```````````````````````````````` example +Foo +bar +\--- +baz +. +<p>Foo +bar +--- +baz</p> +```````````````````````````````` + + +## Indented code blocks + +An [indented code block](@) is composed of one or more +[indented chunks] separated by blank lines. +An [indented chunk](@) is a sequence of non-blank lines, +each preceded by four or more spaces of indentation. The contents of the code +block are the literal contents of the lines, including trailing +[line endings], minus four spaces of indentation. +An indented code block has no [info string]. + +An indented code block cannot interrupt a paragraph, so there must be +a blank line between a paragraph and a following indented code block. +(A blank line is not needed, however, between a code block and a following +paragraph.) + +```````````````````````````````` example + a simple + indented code block +. +<pre><code>a simple + indented code block +</code></pre> +```````````````````````````````` + + +If there is any ambiguity between an interpretation of indentation +as a code block and as indicating that material belongs to a [list +item][list items], the list item interpretation takes precedence: + +```````````````````````````````` example + - foo + + bar +. +<ul> +<li> +<p>foo</p> +<p>bar</p> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +1. foo + + - bar +. +<ol> +<li> +<p>foo</p> +<ul> +<li>bar</li> +</ul> +</li> +</ol> +```````````````````````````````` + + + +The contents of a code block are literal text, and do not get parsed +as Markdown: + +```````````````````````````````` example + <a/> + *hi* + + - one +. +<pre><code><a/> +*hi* + +- one +</code></pre> +```````````````````````````````` + + +Here we have three chunks separated by blank lines: + +```````````````````````````````` example + chunk1 + + chunk2 + + + + chunk3 +. +<pre><code>chunk1 + +chunk2 + + + +chunk3 +</code></pre> +```````````````````````````````` + + +Any initial spaces or tabs beyond four spaces of indentation will be included in +the content, even in interior blank lines: + +```````````````````````````````` example + chunk1 + + chunk2 +. +<pre><code>chunk1 + + chunk2 +</code></pre> +```````````````````````````````` + + +An indented code block cannot interrupt a paragraph. (This +allows hanging indents and the like.) + +```````````````````````````````` example +Foo + bar + +. +<p>Foo +bar</p> +```````````````````````````````` + + +However, any non-blank line with fewer than four spaces of indentation ends +the code block immediately. So a paragraph may occur immediately +after indented code: + +```````````````````````````````` example + foo +bar +. +<pre><code>foo +</code></pre> +<p>bar</p> +```````````````````````````````` + + +And indented code can occur immediately before and after other kinds of +blocks: + +```````````````````````````````` example +# Heading + foo +Heading +------ + foo +---- +. +<h1>Heading</h1> +<pre><code>foo +</code></pre> +<h2>Heading</h2> +<pre><code>foo +</code></pre> +<hr /> +```````````````````````````````` + + +The first line can be preceded by more than four spaces of indentation: + +```````````````````````````````` example + foo + bar +. +<pre><code> foo +bar +</code></pre> +```````````````````````````````` + + +Blank lines preceding or following an indented code block +are not included in it: + +```````````````````````````````` example + + + foo + + +. +<pre><code>foo +</code></pre> +```````````````````````````````` + + +Trailing spaces or tabs are included in the code block's content: + +```````````````````````````````` example + foo +. +<pre><code>foo +</code></pre> +```````````````````````````````` + + + +## Fenced code blocks + +A [code fence](@) is a sequence +of at least three consecutive backtick characters (`` ` ``) or +tildes (`~`). (Tildes and backticks cannot be mixed.) +A [fenced code block](@) +begins with a code fence, preceded by up to three spaces of indentation. + +The line with the opening code fence may optionally contain some text +following the code fence; this is trimmed of leading and trailing +spaces or tabs and called the [info string](@). If the [info string] comes +after a backtick fence, it may not contain any backtick +characters. (The reason for this restriction is that otherwise +some inline code would be incorrectly interpreted as the +beginning of a fenced code block.) + +The content of the code block consists of all subsequent lines, until +a closing [code fence] of the same type as the code block +began with (backticks or tildes), and with at least as many backticks +or tildes as the opening code fence. If the leading code fence is +preceded by N spaces of indentation, then up to N spaces of indentation are +removed from each line of the content (if present). (If a content line is not +indented, it is preserved unchanged. If it is indented N spaces or less, all +of the indentation is removed.) + +The closing code fence may be preceded by up to three spaces of indentation, and +may be followed only by spaces or tabs, which are ignored. If the end of the +containing block (or document) is reached and no closing code fence +has been found, the code block contains all of the lines after the +opening code fence until the end of the containing block (or +document). (An alternative spec would require backtracking in the +event that a closing code fence is not found. But this makes parsing +much less efficient, and there seems to be no real down side to the +behavior described here.) + +A fenced code block may interrupt a paragraph, and does not require +a blank line either before or after. + +The content of a code fence is treated as literal text, not parsed +as inlines. The first word of the [info string] is typically used to +specify the language of the code sample, and rendered in the `class` +attribute of the `code` tag. However, this spec does not mandate any +particular treatment of the [info string]. + +Here is a simple example with backticks: + +```````````````````````````````` example +``` +< + > +``` +. +<pre><code>< + > +</code></pre> +```````````````````````````````` + + +With tildes: + +```````````````````````````````` example +~~~ +< + > +~~~ +. +<pre><code>< + > +</code></pre> +```````````````````````````````` + +Fewer than three backticks is not enough: + +```````````````````````````````` example +`` +foo +`` +. +<p><code>foo</code></p> +```````````````````````````````` + +The closing code fence must use the same character as the opening +fence: + +```````````````````````````````` example +``` +aaa +~~~ +``` +. +<pre><code>aaa +~~~ +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +~~~ +aaa +``` +~~~ +. +<pre><code>aaa +``` +</code></pre> +```````````````````````````````` + + +The closing code fence must be at least as long as the opening fence: + +```````````````````````````````` example +```` +aaa +``` +`````` +. +<pre><code>aaa +``` +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +~~~~ +aaa +~~~ +~~~~ +. +<pre><code>aaa +~~~ +</code></pre> +```````````````````````````````` + + +Unclosed code blocks are closed by the end of the document +(or the enclosing [block quote][block quotes] or [list item][list items]): + +```````````````````````````````` example +``` +. +<pre><code></code></pre> +```````````````````````````````` + + +```````````````````````````````` example +````` + +``` +aaa +. +<pre><code> +``` +aaa +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +> ``` +> aaa + +bbb +. +<blockquote> +<pre><code>aaa +</code></pre> +</blockquote> +<p>bbb</p> +```````````````````````````````` + + +A code block can have all empty lines as its content: + +```````````````````````````````` example +``` + + +``` +. +<pre><code> + +</code></pre> +```````````````````````````````` + + +A code block can be empty: + +```````````````````````````````` example +``` +``` +. +<pre><code></code></pre> +```````````````````````````````` + + +Fences can be indented. If the opening fence is indented, +content lines will have equivalent opening indentation removed, +if present: + +```````````````````````````````` example + ``` + aaa +aaa +``` +. +<pre><code>aaa +aaa +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example + ``` +aaa + aaa +aaa + ``` +. +<pre><code>aaa +aaa +aaa +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example + ``` + aaa + aaa + aaa + ``` +. +<pre><code>aaa + aaa +aaa +</code></pre> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example + ``` + aaa + ``` +. +<pre><code>``` +aaa +``` +</code></pre> +```````````````````````````````` + + +Closing fences may be preceded by up to three spaces of indentation, and their +indentation need not match that of the opening fence: + +```````````````````````````````` example +``` +aaa + ``` +. +<pre><code>aaa +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example + ``` +aaa + ``` +. +<pre><code>aaa +</code></pre> +```````````````````````````````` + + +This is not a closing fence, because it is indented 4 spaces: + +```````````````````````````````` example +``` +aaa + ``` +. +<pre><code>aaa + ``` +</code></pre> +```````````````````````````````` + + + +Code fences (opening and closing) cannot contain internal spaces or tabs: + +```````````````````````````````` example +``` ``` +aaa +. +<p><code> </code> +aaa</p> +```````````````````````````````` + + +```````````````````````````````` example +~~~~~~ +aaa +~~~ ~~ +. +<pre><code>aaa +~~~ ~~ +</code></pre> +```````````````````````````````` + + +Fenced code blocks can interrupt paragraphs, and can be followed +directly by paragraphs, without a blank line between: + +```````````````````````````````` example +foo +``` +bar +``` +baz +. +<p>foo</p> +<pre><code>bar +</code></pre> +<p>baz</p> +```````````````````````````````` + + +Other blocks can also occur before and after fenced code blocks +without an intervening blank line: + +```````````````````````````````` example +foo +--- +~~~ +bar +~~~ +# baz +. +<h2>foo</h2> +<pre><code>bar +</code></pre> +<h1>baz</h1> +```````````````````````````````` + + +An [info string] can be provided after the opening code fence. +Although this spec doesn't mandate any particular treatment of +the info string, the first word is typically used to specify +the language of the code block. In HTML output, the language is +normally indicated by adding a class to the `code` element consisting +of `language-` followed by the language name. + +```````````````````````````````` example +```ruby +def foo(x) + return 3 +end +``` +. +<pre><code class="language-ruby">def foo(x) + return 3 +end +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +~~~~ ruby startline=3 $%@#$ +def foo(x) + return 3 +end +~~~~~~~ +. +<pre><code class="language-ruby">def foo(x) + return 3 +end +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +````; +```` +. +<pre><code class="language-;"></code></pre> +```````````````````````````````` + + +[Info strings] for backtick code blocks cannot contain backticks: + +```````````````````````````````` example +``` aa ``` +foo +. +<p><code>aa</code> +foo</p> +```````````````````````````````` + + +[Info strings] for tilde code blocks can contain backticks and tildes: + +```````````````````````````````` example +~~~ aa ``` ~~~ +foo +~~~ +. +<pre><code class="language-aa">foo +</code></pre> +```````````````````````````````` + + +Closing code fences cannot have [info strings]: + +```````````````````````````````` example +``` +``` aaa +``` +. +<pre><code>``` aaa +</code></pre> +```````````````````````````````` + + + +## HTML blocks + +An [HTML block](@) is a group of lines that is treated +as raw HTML (and will not be escaped in HTML output). + +There are seven kinds of [HTML block], which can be defined by their +start and end conditions. The block begins with a line that meets a +[start condition](@) (after up to three optional spaces of indentation). +It ends with the first subsequent line that meets a matching +[end condition](@), or the last line of the document, or the last line of +the [container block](#container-blocks) containing the current HTML +block, if no line is encountered that meets the [end condition]. If +the first line meets both the [start condition] and the [end +condition], the block will contain just that line. + +1. **Start condition:** line begins with the string `<pre`, +`<script`, `<style`, or `<textarea` (case-insensitive), followed by a space, +a tab, the string `>`, or the end of the line.\ +**End condition:** line contains an end tag +`</pre>`, `</script>`, `</style>`, or `</textarea>` (case-insensitive; it +need not match the start tag). + +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 ASCII letter.\ +**End condition:** line contains the character `>`. + +5. **Start condition:** line begins with the string +`<![CDATA[`.\ +**End condition:** line contains the string `]]>`. + +6. **Start condition:** line begins the string `<` or `</` +followed by one of the strings (case-insensitive) `address`, +`article`, `aside`, `base`, `basefont`, `blockquote`, `body`, +`caption`, `center`, `col`, `colgroup`, `dd`, `details`, `dialog`, +`dir`, `div`, `dl`, `dt`, `fieldset`, `figcaption`, `figure`, +`footer`, `form`, `frame`, `frameset`, +`h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `head`, `header`, `hr`, +`html`, `iframe`, `legend`, `li`, `link`, `main`, `menu`, `menuitem`, +`nav`, `noframes`, `ol`, `optgroup`, `option`, `p`, `param`, +`section`, `source`, `summary`, `table`, `tbody`, `td`, +`tfoot`, `th`, `thead`, `title`, `tr`, `track`, `ul`, followed +by a space, a tab, the end of the line, the string `>`, or +the string `/>`.\ +**End condition:** line is followed by a [blank line]. + +7. **Start condition:** line begins with a complete [open tag] +(with any [tag name] other than `pre`, `script`, +`style`, or `textarea`) or a complete [closing tag], +followed by zero or more spaces and tabs, followed by the end of the line.\ +**End condition:** line is followed by a [blank line]. + +HTML blocks continue until they are closed by their appropriate +[end condition], or the last line of the document or other [container +block](#container-blocks). This means any HTML **within an HTML +block** that might otherwise be recognised as a start condition will +be ignored by the parser and passed through as-is, without changing +the parser's state. + +For instance, `<pre>` within an HTML block started by `<table>` will not affect +the parser state; as the HTML block was started in by start condition 6, it +will end at any blank line. This can be surprising: + +```````````````````````````````` example +<table><tr><td> +<pre> +**Hello**, + +_world_. +</pre> +</td></tr></table> +. +<table><tr><td> +<pre> +**Hello**, +<p><em>world</em>. +</pre></p> +</td></tr></table> +```````````````````````````````` + +In this case, the HTML block is terminated by the blank line — the `**Hello**` +text remains verbatim — and regular parsing resumes, with a paragraph, +emphasised `world` and inline and block HTML following. + +All types of [HTML blocks] except type 7 may interrupt +a paragraph. Blocks of type 7 may not interrupt a paragraph. +(This restriction is intended to prevent unwanted interpretation +of long tags inside a wrapped paragraph as starting HTML blocks.) + +Some simple examples follow. Here are some basic HTML blocks +of type 6: + +```````````````````````````````` example +<table> + <tr> + <td> + hi + </td> + </tr> +</table> + +okay. +. +<table> + <tr> + <td> + hi + </td> + </tr> +</table> +<p>okay.</p> +```````````````````````````````` + + +```````````````````````````````` example + <div> + *hello* + <foo><a> +. + <div> + *hello* + <foo><a> +```````````````````````````````` + + +A block can also start with a closing tag: + +```````````````````````````````` example +</div> +*foo* +. +</div> +*foo* +```````````````````````````````` + + +Here we have two HTML blocks with a Markdown paragraph between them: + +```````````````````````````````` example +<DIV CLASS="foo"> + +*Markdown* + +</DIV> +. +<DIV CLASS="foo"> +<p><em>Markdown</em></p> +</DIV> +```````````````````````````````` + + +The tag on the first line can be partial, as long +as it is split where there would be whitespace: + +```````````````````````````````` example +<div id="foo" + class="bar"> +</div> +. +<div id="foo" + class="bar"> +</div> +```````````````````````````````` + + +```````````````````````````````` example +<div id="foo" class="bar + baz"> +</div> +. +<div id="foo" class="bar + baz"> +</div> +```````````````````````````````` + + +An open tag need not be closed: +```````````````````````````````` example +<div> +*foo* + +*bar* +. +<div> +*foo* +<p><em>bar</em></p> +```````````````````````````````` + + + +A partial tag need not even be completed (garbage +in, garbage out): + +```````````````````````````````` example +<div id="foo" +*hi* +. +<div id="foo" +*hi* +```````````````````````````````` + + +```````````````````````````````` example +<div class +foo +. +<div class +foo +```````````````````````````````` + + +The initial tag doesn't even need to be a valid +tag, as long as it starts like one: + +```````````````````````````````` example +<div *???-&&&-<--- +*foo* +. +<div *???-&&&-<--- +*foo* +```````````````````````````````` + + +In type 6 blocks, the initial tag need not be on a line by +itself: + +```````````````````````````````` example +<div><a href="bar">*foo*</a></div> +. +<div><a href="bar">*foo*</a></div> +```````````````````````````````` + + +```````````````````````````````` example +<table><tr><td> +foo +</td></tr></table> +. +<table><tr><td> +foo +</td></tr></table> +```````````````````````````````` + + +Everything until the next blank line or end of document +gets included in the HTML block. So, in the following +example, what looks like a Markdown code block +is actually part of the HTML block, which continues until a blank +line or the end of the document is reached: + +```````````````````````````````` example +<div></div> +``` c +int x = 33; +``` +. +<div></div> +``` c +int x = 33; +``` +```````````````````````````````` + + +To start an [HTML block] with a tag that is *not* in the +list of block-level tags in (6), you must put the tag by +itself on the first line (and it must be complete): + +```````````````````````````````` example +<a href="foo"> +*bar* +</a> +. +<a href="foo"> +*bar* +</a> +```````````````````````````````` + + +In type 7 blocks, the [tag name] can be anything: + +```````````````````````````````` example +<Warning> +*bar* +</Warning> +. +<Warning> +*bar* +</Warning> +```````````````````````````````` + + +```````````````````````````````` example +<i class="foo"> +*bar* +</i> +. +<i class="foo"> +*bar* +</i> +```````````````````````````````` + + +```````````````````````````````` example +</ins> +*bar* +. +</ins> +*bar* +```````````````````````````````` + + +These rules are designed to allow us to work with tags that +can function as either block-level or inline-level tags. +The `<del>` tag is a nice example. We can surround content with +`<del>` tags in three different ways. In this case, we get a raw +HTML block, because the `<del>` tag is on a line by itself: + +```````````````````````````````` example +<del> +*foo* +</del> +. +<del> +*foo* +</del> +```````````````````````````````` + + +In this case, we get a raw HTML block that just includes +the `<del>` tag (because it ends with the following blank +line). So the contents get interpreted as CommonMark: + +```````````````````````````````` example +<del> + +*foo* + +</del> +. +<del> +<p><em>foo</em></p> +</del> +```````````````````````````````` + + +Finally, in this case, the `<del>` tags are interpreted +as [raw HTML] *inside* the CommonMark paragraph. (Because +the tag is not on a line by itself, we get inline HTML +rather than an [HTML block].) + +```````````````````````````````` example +<del>*foo*</del> +. +<p><del><em>foo</em></del></p> +```````````````````````````````` + + +HTML tags designed to contain literal content +(`pre`, `script`, `style`, `textarea`), comments, processing instructions, +and declarations are treated somewhat differently. +Instead of ending at the first blank line, these blocks +end at the first line containing a corresponding end tag. +As a result, these blocks can contain blank lines: + +A pre tag (type 1): + +```````````````````````````````` example +<pre language="haskell"><code> +import Text.HTML.TagSoup + +main :: IO () +main = print $ parseTags tags +</code></pre> +okay +. +<pre language="haskell"><code> +import Text.HTML.TagSoup + +main :: IO () +main = print $ parseTags tags +</code></pre> +<p>okay</p> +```````````````````````````````` + + +A script tag (type 1): + +```````````````````````````````` example +<script type="text/javascript"> +// JavaScript example + +document.getElementById("demo").innerHTML = "Hello JavaScript!"; +</script> +okay +. +<script type="text/javascript"> +// JavaScript example + +document.getElementById("demo").innerHTML = "Hello JavaScript!"; +</script> +<p>okay</p> +```````````````````````````````` + + +A textarea tag (type 1): + +```````````````````````````````` example +<textarea> + +*foo* + +_bar_ + +</textarea> +. +<textarea> + +*foo* + +_bar_ + +</textarea> +```````````````````````````````` + +A style tag (type 1): + +```````````````````````````````` example +<style + type="text/css"> +h1 {color:red;} + +p {color:blue;} +</style> +okay +. +<style + type="text/css"> +h1 {color:red;} + +p {color:blue;} +</style> +<p>okay</p> +```````````````````````````````` + + +If there is no matching end tag, the block will end at the +end of the document (or the enclosing [block quote][block quotes] +or [list item][list items]): + +```````````````````````````````` example +<style + type="text/css"> + +foo +. +<style + type="text/css"> + +foo +```````````````````````````````` + + +```````````````````````````````` example +> <div> +> foo + +bar +. +<blockquote> +<div> +foo +</blockquote> +<p>bar</p> +```````````````````````````````` + + +```````````````````````````````` example +- <div> +- foo +. +<ul> +<li> +<div> +</li> +<li>foo</li> +</ul> +```````````````````````````````` + + +The end tag can occur on the same line as the start tag: + +```````````````````````````````` example +<style>p{color:red;}</style> +*foo* +. +<style>p{color:red;}</style> +<p><em>foo</em></p> +```````````````````````````````` + + +```````````````````````````````` example +<!-- foo -->*bar* +*baz* +. +<!-- foo -->*bar* +<p><em>baz</em></p> +```````````````````````````````` + + +Note that anything on the last line after the +end tag will be included in the [HTML block]: + +```````````````````````````````` example +<script> +foo +</script>1. *bar* +. +<script> +foo +</script>1. *bar* +```````````````````````````````` + + +A comment (type 2): + +```````````````````````````````` example +<!-- Foo + +bar + baz --> +okay +. +<!-- Foo + +bar + baz --> +<p>okay</p> +```````````````````````````````` + + + +A processing instruction (type 3): + +```````````````````````````````` example +<?php + + echo '>'; + +?> +okay +. +<?php + + echo '>'; + +?> +<p>okay</p> +```````````````````````````````` + + +A declaration (type 4): + +```````````````````````````````` example +<!DOCTYPE html> +. +<!DOCTYPE html> +```````````````````````````````` + + +CDATA (type 5): + +```````````````````````````````` example +<![CDATA[ +function matchwo(a,b) +{ + if (a < b && a < 0) then { + return 1; + + } else { + + return 0; + } +} +]]> +okay +. +<![CDATA[ +function matchwo(a,b) +{ + if (a < b && a < 0) then { + return 1; + + } else { + + return 0; + } +} +]]> +<p>okay</p> +```````````````````````````````` + + +The opening tag can be preceded by up to three spaces of indentation, but not +four: + +```````````````````````````````` example + <!-- foo --> + + <!-- foo --> +. + <!-- foo --> +<pre><code><!-- foo --> +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example + <div> + + <div> +. + <div> +<pre><code><div> +</code></pre> +```````````````````````````````` + + +An HTML block of types 1--6 can interrupt a paragraph, and need not be +preceded by a blank line. + +```````````````````````````````` example +Foo +<div> +bar +</div> +. +<p>Foo</p> +<div> +bar +</div> +```````````````````````````````` + + +However, a following blank line is needed, except at the end of +a document, and except for blocks of types 1--5, [above][HTML +block]: + +```````````````````````````````` example +<div> +bar +</div> +*foo* +. +<div> +bar +</div> +*foo* +```````````````````````````````` + + +HTML blocks of type 7 cannot interrupt a paragraph: + +```````````````````````````````` example +Foo +<a href="bar"> +baz +. +<p>Foo +<a href="bar"> +baz</p> +```````````````````````````````` + + +This rule differs from John Gruber's original Markdown syntax +specification, which says: + +> The only restrictions are that block-level HTML elements — +> e.g. `<div>`, `<table>`, `<pre>`, `<p>`, etc. — must be separated from +> surrounding content by blank lines, and the start and end tags of the +> block should not be indented with spaces or tabs. + +In some ways Gruber's rule is more restrictive than the one given +here: + +- It requires that an HTML block be preceded by a blank line. +- It does not allow the start tag to be indented. +- It requires a matching end tag, which it also does not allow to + be indented. + +Most Markdown implementations (including some of Gruber's own) do not +respect all of these restrictions. + +There is one respect, however, in which Gruber's rule is more liberal +than the one given here, since it allows blank lines to occur inside +an HTML block. There are two reasons for disallowing them here. +First, it removes the need to parse balanced tags, which is +expensive and can require backtracking from the end of the document +if no matching end tag is found. Second, it provides a very simple +and flexible way of including Markdown content inside HTML tags: +simply separate the Markdown from the HTML using blank lines: + +Compare: + +```````````````````````````````` example +<div> + +*Emphasized* text. + +</div> +. +<div> +<p><em>Emphasized</em> text.</p> +</div> +```````````````````````````````` + + +```````````````````````````````` example +<div> +*Emphasized* text. +</div> +. +<div> +*Emphasized* text. +</div> +```````````````````````````````` + + +Some Markdown implementations have adopted a convention of +interpreting content inside tags as text if the open tag has +the attribute `markdown=1`. The rule given above seems a simpler and +more elegant way of achieving the same expressive power, which is also +much simpler to parse. + +The main potential drawback is that one can no longer paste HTML +blocks into Markdown documents with 100% reliability. However, +*in most cases* this will work fine, because the blank lines in +HTML are usually followed by HTML block tags. For example: + +```````````````````````````````` example +<table> + +<tr> + +<td> +Hi +</td> + +</tr> + +</table> +. +<table> +<tr> +<td> +Hi +</td> +</tr> +</table> +```````````````````````````````` + + +There are problems, however, if the inner tags are indented +*and* separated by spaces, as then they will be interpreted as +an indented code block: + +```````````````````````````````` example +<table> + + <tr> + + <td> + Hi + </td> + + </tr> + +</table> +. +<table> + <tr> +<pre><code><td> + Hi +</td> +</code></pre> + </tr> +</table> +```````````````````````````````` + + +Fortunately, blank lines are usually not necessary and can be +deleted. The exception is inside `<pre>` tags, but as described +[above][HTML blocks], raw HTML blocks starting with `<pre>` +*can* contain blank lines. + +## Link reference definitions + +A [link reference definition](@) +consists of a [link label], optionally preceded by up to three spaces of +indentation, followed +by a colon (`:`), optional spaces or tabs (including up to one +[line ending]), a [link destination], +optional spaces or tabs (including up to one +[line ending]), and an optional [link +title], which if it is present must be separated +from the [link destination] by spaces or tabs. +No further character may occur. + +A [link reference definition] +does not correspond to a structural element of a document. Instead, it +defines a label which can be used in [reference links] +and reference-style [images] elsewhere in the document. [Link +reference definitions] can come either before or after the links that use +them. + +```````````````````````````````` example +[foo]: /url "title" + +[foo] +. +<p><a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example + [foo]: + /url + 'the title' + +[foo] +. +<p><a href="/url" title="the title">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[Foo*bar\]]:my_(url) 'title (with parens)' + +[Foo*bar\]] +. +<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[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: + +```````````````````````````````` example +[foo]: /url ' +title +line1 +line2 +' + +[foo] +. +<p><a href="/url" title=" +title +line1 +line2 +">foo</a></p> +```````````````````````````````` + + +However, it may not contain a [blank line]: + +```````````````````````````````` example +[foo]: /url 'title + +with blank line' + +[foo] +. +<p>[foo]: /url 'title</p> +<p>with blank line'</p> +<p>[foo]</p> +```````````````````````````````` + + +The title may be omitted: + +```````````````````````````````` example +[foo]: +/url + +[foo] +. +<p><a href="/url">foo</a></p> +```````````````````````````````` + + +The link destination may not be omitted: + +```````````````````````````````` example +[foo]: + +[foo] +. +<p>[foo]:</p> +<p>[foo]</p> +```````````````````````````````` + + However, an empty link destination may be specified using + angle brackets: + +```````````````````````````````` example +[foo]: <> + +[foo] +. +<p><a href="">foo</a></p> +```````````````````````````````` + +The title must be separated from the link destination by +spaces or tabs: + +```````````````````````````````` example +[foo]: <bar>(baz) + +[foo] +. +<p>[foo]: <bar>(baz)</p> +<p>[foo]</p> +```````````````````````````````` + + +Both title and destination can contain backslash escapes +and literal backslashes: + +```````````````````````````````` example +[foo]: /url\bar\*baz "foo\"bar\baz" + +[foo] +. +<p><a href="/url%5Cbar*baz" title="foo"bar\baz">foo</a></p> +```````````````````````````````` + + +A link can come before its corresponding definition: + +```````````````````````````````` example +[foo] + +[foo]: url +. +<p><a href="url">foo</a></p> +```````````````````````````````` + + +If there are several matching definitions, the first one takes +precedence: + +```````````````````````````````` example +[foo] + +[foo]: first +[foo]: second +. +<p><a href="first">foo</a></p> +```````````````````````````````` + + +As noted in the section on [Links], matching of labels is +case-insensitive (see [matches]). + +```````````````````````````````` example +[FOO]: /url + +[Foo] +. +<p><a href="/url">Foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[ΑΓΩ]: /φου + +[αγω] +. +<p><a href="/%CF%86%CE%BF%CF%85">αγω</a></p> +```````````````````````````````` + + +Whether something is a [link reference definition] is +independent of whether the link reference it defines is +used in the document. Thus, for example, the following +document contains just a link reference definition, and +no visible content: + +```````````````````````````````` example +[foo]: /url +. +```````````````````````````````` + + +Here is another one: + +```````````````````````````````` example +[ +foo +]: /url +bar +. +<p>bar</p> +```````````````````````````````` + + +This is not a link reference definition, because there are +characters other than spaces or tabs after the title: + +```````````````````````````````` example +[foo]: /url "title" ok +. +<p>[foo]: /url "title" ok</p> +```````````````````````````````` + + +This is a link reference definition, but it has no title: + +```````````````````````````````` example +[foo]: /url +"title" ok +. +<p>"title" ok</p> +```````````````````````````````` + + +This is not a link reference definition, because it is indented +four spaces: + +```````````````````````````````` example + [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: + +```````````````````````````````` example +``` +[foo]: /url +``` + +[foo] +. +<pre><code>[foo]: /url +</code></pre> +<p>[foo]</p> +```````````````````````````````` + + +A [link reference definition] cannot interrupt a paragraph. + +```````````````````````````````` example +Foo +[bar]: /baz + +[bar] +. +<p>Foo +[bar]: /baz</p> +<p>[bar]</p> +```````````````````````````````` + + +However, it can directly follow other block elements, such as headings +and thematic breaks, and it need not be followed by a blank line. + +```````````````````````````````` example +# [Foo] +[foo]: /url +> bar +. +<h1><a href="/url">Foo</a></h1> +<blockquote> +<p>bar</p> +</blockquote> +```````````````````````````````` + +```````````````````````````````` example +[foo]: /url +bar +=== +[foo] +. +<h1>bar</h1> +<p><a href="/url">foo</a></p> +```````````````````````````````` + +```````````````````````````````` example +[foo]: /url +=== +[foo] +. +<p>=== +<a href="/url">foo</a></p> +```````````````````````````````` + + +Several [link reference definitions] +can occur one after another, without intervening blank lines. + +```````````````````````````````` example +[foo]: /foo-url "foo" +[bar]: /bar-url + "bar" +[baz]: /baz-url + +[foo], +[bar], +[baz] +. +<p><a href="/foo-url" title="foo">foo</a>, +<a href="/bar-url" title="bar">bar</a>, +<a href="/baz-url">baz</a></p> +```````````````````````````````` + + +[Link reference definitions] can occur +inside block containers, like lists and block quotations. They +affect the entire document, not just the container in which they +are defined: + +```````````````````````````````` example +[foo] + +> [foo]: /url +. +<p><a href="/url">foo</a></p> +<blockquote> +</blockquote> +```````````````````````````````` + + +## Paragraphs + +A sequence of non-blank lines that cannot be interpreted as other +kinds of blocks forms a [paragraph](@). +The contents of the paragraph are the result of parsing the +paragraph's raw content as inlines. The paragraph's raw content +is formed by concatenating the lines and removing initial and final +spaces or tabs. + +A simple example with two paragraphs: + +```````````````````````````````` example +aaa + +bbb +. +<p>aaa</p> +<p>bbb</p> +```````````````````````````````` + + +Paragraphs can contain multiple lines, but no blank lines: + +```````````````````````````````` example +aaa +bbb + +ccc +ddd +. +<p>aaa +bbb</p> +<p>ccc +ddd</p> +```````````````````````````````` + + +Multiple blank lines between paragraphs have no effect: + +```````````````````````````````` example +aaa + + +bbb +. +<p>aaa</p> +<p>bbb</p> +```````````````````````````````` + + +Leading spaces or tabs are skipped: + +```````````````````````````````` example + aaa + bbb +. +<p>aaa +bbb</p> +```````````````````````````````` + + +Lines after the first may be indented any amount, since indented +code blocks cannot interrupt paragraphs. + +```````````````````````````````` example +aaa + bbb + ccc +. +<p>aaa +bbb +ccc</p> +```````````````````````````````` + + +However, the first line may be preceded by up to three spaces of indentation. +Four spaces of indentation is too many: + +```````````````````````````````` example + aaa +bbb +. +<p>aaa +bbb</p> +```````````````````````````````` + + +```````````````````````````````` example + aaa +bbb +. +<pre><code>aaa +</code></pre> +<p>bbb</p> +```````````````````````````````` + + +Final spaces or tabs are stripped before inline parsing, so a paragraph +that ends with two or more spaces will not end with a [hard line +break]: + +```````````````````````````````` example +aaa +bbb +. +<p>aaa<br /> +bbb</p> +```````````````````````````````` + + +## Blank lines + +[Blank lines] between block-level elements are ignored, +except for the role they play in determining whether a [list] +is [tight] or [loose]. + +Blank lines at the beginning and end of the document are also ignored. + +```````````````````````````````` example + + +aaa + + +# aaa + + +. +<p>aaa</p> +<h1>aaa</h1> +```````````````````````````````` + + + +# Container blocks + +A [container block](#container-blocks) 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-parsing-strategy).) + +## Block quotes + +A [block quote marker](@), +optionally preceded by up to three spaces of indentation, +consists of (a) the character `>` together with a following space of +indentation, or (b) a single character `>` not followed by a space of +indentation. + +The following rules define [block quotes]: + +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 character other than a space or tab after the + [block quote marker] is [paragraph continuation + text] is a block quote with *Bs* as its content. + [Paragraph continuation text](@) is text + that will be parsed as part of the content of a paragraph, but does + not occur at the beginning of the paragraph. + +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: + +```````````````````````````````` example +> # Foo +> bar +> baz +. +<blockquote> +<h1>Foo</h1> +<p>bar +baz</p> +</blockquote> +```````````````````````````````` + + +The space or tab after the `>` characters can be omitted: + +```````````````````````````````` example +># Foo +>bar +> baz +. +<blockquote> +<h1>Foo</h1> +<p>bar +baz</p> +</blockquote> +```````````````````````````````` + + +The `>` characters can be preceded by up to three spaces of indentation: + +```````````````````````````````` example + > # Foo + > bar + > baz +. +<blockquote> +<h1>Foo</h1> +<p>bar +baz</p> +</blockquote> +```````````````````````````````` + + +Four spaces of indentation is too many: + +```````````````````````````````` example + > # Foo + > bar + > baz +. +<pre><code>> # Foo +> bar +> baz +</code></pre> +```````````````````````````````` + + +The Laziness clause allows us to omit the `>` before +[paragraph continuation text]: + +```````````````````````````````` example +> # Foo +> bar +baz +. +<blockquote> +<h1>Foo</h1> +<p>bar +baz</p> +</blockquote> +```````````````````````````````` + + +A block quote can contain some lazy and some non-lazy +continuation lines: + +```````````````````````````````` example +> 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 + +``` markdown +> foo +> --- +``` + +without changing the meaning: + +```````````````````````````````` example +> foo +--- +. +<blockquote> +<p>foo</p> +</blockquote> +<hr /> +```````````````````````````````` + + +Similarly, if we omit the `> ` in the second line of + +``` markdown +> - foo +> - bar +``` + +then the block quote ends after the first line: + +```````````````````````````````` example +> - foo +- bar +. +<blockquote> +<ul> +<li>foo</li> +</ul> +</blockquote> +<ul> +<li>bar</li> +</ul> +```````````````````````````````` + + +For the same reason, we can't omit the `> ` in front of +subsequent lines of an indented or fenced code block: + +```````````````````````````````` example +> foo + bar +. +<blockquote> +<pre><code>foo +</code></pre> +</blockquote> +<pre><code>bar +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +> ``` +foo +``` +. +<blockquote> +<pre><code></code></pre> +</blockquote> +<p>foo</p> +<pre><code></code></pre> +```````````````````````````````` + + +Note that in the following case, we have a [lazy +continuation line]: + +```````````````````````````````` example +> foo + - bar +. +<blockquote> +<p>foo +- bar</p> +</blockquote> +```````````````````````````````` + + +To see why, note that in + +```markdown +> foo +> - bar +``` + +the `- bar` is indented too far to start a list, and can't +be an indented code block because indented code blocks cannot +interrupt paragraphs, so it is [paragraph continuation text]. + +A block quote can be empty: + +```````````````````````````````` example +> +. +<blockquote> +</blockquote> +```````````````````````````````` + + +```````````````````````````````` example +> +> +> +. +<blockquote> +</blockquote> +```````````````````````````````` + + +A block quote can have initial or final blank lines: + +```````````````````````````````` example +> +> foo +> +. +<blockquote> +<p>foo</p> +</blockquote> +```````````````````````````````` + + +A blank line always separates block quotes: + +```````````````````````````````` example +> foo + +> bar +. +<blockquote> +<p>foo</p> +</blockquote> +<blockquote> +<p>bar</p> +</blockquote> +```````````````````````````````` + + +(Most current Markdown implementations, including John Gruber's +original `Markdown.pl`, will parse this example as a single block quote +with two paragraphs. But it seems better to allow the author to decide +whether two block quotes or one are wanted.) + +Consecutiveness means that if we put these block quotes together, +we get a single block quote: + +```````````````````````````````` example +> foo +> bar +. +<blockquote> +<p>foo +bar</p> +</blockquote> +```````````````````````````````` + + +To get a block quote with two paragraphs, use: + +```````````````````````````````` example +> foo +> +> bar +. +<blockquote> +<p>foo</p> +<p>bar</p> +</blockquote> +```````````````````````````````` + + +Block quotes can interrupt paragraphs: + +```````````````````````````````` example +foo +> bar +. +<p>foo</p> +<blockquote> +<p>bar</p> +</blockquote> +```````````````````````````````` + + +In general, blank lines are not needed before or after block +quotes: + +```````````````````````````````` example +> aaa +*** +> bbb +. +<blockquote> +<p>aaa</p> +</blockquote> +<hr /> +<blockquote> +<p>bbb</p> +</blockquote> +```````````````````````````````` + + +However, because of laziness, a blank line is needed between +a block quote and a following paragraph: + +```````````````````````````````` example +> bar +baz +. +<blockquote> +<p>bar +baz</p> +</blockquote> +```````````````````````````````` + + +```````````````````````````````` example +> bar + +baz +. +<blockquote> +<p>bar</p> +</blockquote> +<p>baz</p> +```````````````````````````````` + + +```````````````````````````````` example +> bar +> +baz +. +<blockquote> +<p>bar</p> +</blockquote> +<p>baz</p> +```````````````````````````````` + + +It is a consequence of the Laziness rule that any number +of initial `>`s may be omitted on a continuation line of a +nested block quote: + +```````````````````````````````` example +> > > foo +bar +. +<blockquote> +<blockquote> +<blockquote> +<p>foo +bar</p> +</blockquote> +</blockquote> +</blockquote> +```````````````````````````````` + + +```````````````````````````````` example +>>> foo +> bar +>>baz +. +<blockquote> +<blockquote> +<blockquote> +<p>foo +bar +baz</p> +</blockquote> +</blockquote> +</blockquote> +```````````````````````````````` + + +When including an indented code block in a block quote, +remember that the [block quote marker] includes +both the `>` and a following space of indentation. So *five spaces* are needed +after the `>`: + +```````````````````````````````` example +> code + +> not code +. +<blockquote> +<pre><code>code +</code></pre> +</blockquote> +<blockquote> +<p>not code</p> +</blockquote> +```````````````````````````````` + + + +## List items + +A [list marker](@) is a +[bullet list marker] or an [ordered list marker]. + +A [bullet list marker](@) +is a `-`, `+`, or `*` character. + +An [ordered list marker](@) +is a sequence of 1--9 arabic digits (`0-9`), followed by either a +`.` character or a `)` character. (The reason for the length +limit is that with 10 digits we start seeing integer overflows +in some browsers.) + +The following rules define [list items]: + +1. **Basic case.** If a sequence of lines *Ls* constitute a sequence of + blocks *Bs* starting with a character other than a space or tab, and *M* is + a list marker of width *W* followed by 1 ≤ *N* ≤ 4 spaces of indentation, + then the result of prepending *M* and the following spaces to the first line + of Ls*, and indenting subsequent lines of *Ls* by *W + N* spaces, is a + list item with *Bs* as its contents. The type of the list item + (bullet or ordered) is determined by the type of its list marker. + If the list item is ordered, then it is also assigned a start + number, based on the ordered list marker. + + Exceptions: + + 1. When the first list item in a [list] interrupts + a paragraph---that is, when it starts on a line that would + otherwise count as [paragraph continuation text]---then (a) + the lines *Ls* must not begin with a blank line, and (b) if + the list item is ordered, the start number must be 1. + 2. If any line is a [thematic break][thematic breaks] then + that line is not a list item. + +For example, let *Ls* be the lines + +```````````````````````````````` example +A paragraph +with two lines. + + indented code + +> A block quote. +. +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +```````````````````````````````` + + +And let *M* be the marker `1.`, and *N* = 2. Then rule #1 says +that the following is an ordered list item with start number 1, +and the same contents as *Ls*: + +```````````````````````````````` example +1. A paragraph + with two lines. + + indented code + + > A block quote. +. +<ol> +<li> +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +The most important thing to notice is that the position of +the text after the list marker determines how much indentation +is needed in subsequent blocks in the list item. If the list +marker takes up two spaces of indentation, and there are three spaces between +the list marker and the next character other than a space or tab, then blocks +must be indented five spaces in order to fall under the list +item. + +Here are some examples showing how far content must be indented to be +put under the list item: + +```````````````````````````````` example +- one + + two +. +<ul> +<li>one</li> +</ul> +<p>two</p> +```````````````````````````````` + + +```````````````````````````````` example +- one + + two +. +<ul> +<li> +<p>one</p> +<p>two</p> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example + - one + + two +. +<ul> +<li>one</li> +</ul> +<pre><code> two +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example + - one + + two +. +<ul> +<li> +<p>one</p> +<p>two</p> +</li> +</ul> +```````````````````````````````` + + +It is tempting to think of this in terms of columns: the continuation +blocks must be indented at least to the column of the first character other than +a space or tab after the list marker. However, that is not quite right. +The spaces of indentation after the list marker determine how much relative +indentation is needed. Which column this indentation reaches will depend on +how the list item is embedded in other constructions, as shown by +this example: + +```````````````````````````````` 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: + +```````````````````````````````` example +>>- one +>> + > > two +. +<blockquote> +<blockquote> +<ul> +<li>one</li> +</ul> +<p>two</p> +</blockquote> +</blockquote> +```````````````````````````````` + + +Note that at least one space or tab is needed between the list marker and +any following content, so these are not list items: + +```````````````````````````````` example +-one + +2.two +. +<p>-one</p> +<p>2.two</p> +```````````````````````````````` + + +A list item may contain blocks that are separated by more than +one blank line. + +```````````````````````````````` example +- foo + + + bar +. +<ul> +<li> +<p>foo</p> +<p>bar</p> +</li> +</ul> +```````````````````````````````` + + +A list item may contain any kind of block: + +```````````````````````````````` example +1. foo + + ``` + bar + ``` + + baz + + > bam +. +<ol> +<li> +<p>foo</p> +<pre><code>bar +</code></pre> +<p>baz</p> +<blockquote> +<p>bam</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +A list item that contains an indented code block will preserve +empty lines within the code block verbatim. + +```````````````````````````````` example +- Foo + + bar + + + baz +. +<ul> +<li> +<p>Foo</p> +<pre><code>bar + + +baz +</code></pre> +</li> +</ul> +```````````````````````````````` + +Note that ordered list start numbers must be nine digits or less: + +```````````````````````````````` example +123456789. ok +. +<ol start="123456789"> +<li>ok</li> +</ol> +```````````````````````````````` + + +```````````````````````````````` example +1234567890. not ok +. +<p>1234567890. not ok</p> +```````````````````````````````` + + +A start number may begin with 0s: + +```````````````````````````````` example +0. ok +. +<ol start="0"> +<li>ok</li> +</ol> +```````````````````````````````` + + +```````````````````````````````` example +003. ok +. +<ol start="3"> +<li>ok</li> +</ol> +```````````````````````````````` + + +A start number may not be negative: + +```````````````````````````````` example +-1. not ok +. +<p>-1. not ok</p> +```````````````````````````````` + + + +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 *M* is a list marker of width *W* followed by + one space of indentation, then the result of prepending *M* and the + following space to the first line of *Ls*, and indenting subsequent lines + of *Ls* by *W + 1* spaces, is a list item with *Bs* as its contents. + If a line is empty, then it need not be indented. The type of the + list item (bullet or ordered) is determined by the type of its list + marker. If the list item is ordered, then it is also assigned a + start number, based on the ordered list marker. + +An indented code block will have to be preceded by four spaces of indentation +beyond the edge of the region where text will be included in the list item. +In the following case that is 6 spaces: + +```````````````````````````````` example +- foo + + bar +. +<ul> +<li> +<p>foo</p> +<pre><code>bar +</code></pre> +</li> +</ul> +```````````````````````````````` + + +And in this case it is 11 spaces: + +```````````````````````````````` example + 10. foo + + bar +. +<ol start="10"> +<li> +<p>foo</p> +<pre><code>bar +</code></pre> +</li> +</ol> +```````````````````````````````` + + +If the *first* block in the list item is an indented code block, +then by rule #2, the contents must be preceded by *one* space of indentation +after the list marker: + +```````````````````````````````` example + indented code + +paragraph + + more code +. +<pre><code>indented code +</code></pre> +<p>paragraph</p> +<pre><code>more code +</code></pre> +```````````````````````````````` + + +```````````````````````````````` example +1. indented code + + paragraph + + more code +. +<ol> +<li> +<pre><code>indented code +</code></pre> +<p>paragraph</p> +<pre><code>more code +</code></pre> +</li> +</ol> +```````````````````````````````` + + +Note that an additional space of indentation is interpreted as space +inside the code block: + +```````````````````````````````` example +1. indented code + + paragraph + + more code +. +<ol> +<li> +<pre><code> indented code +</code></pre> +<p>paragraph</p> +<pre><code>more code +</code></pre> +</li> +</ol> +```````````````````````````````` + + +Note that rules #1 and #2 only apply to two cases: (a) cases +in which the lines to be included in a list item begin with a +character other than a space or tab, and (b) cases in which +they begin with an indented code +block. In a case like the following, where the first block begins with +three spaces of indentation, the rules do not allow us to form a list item by +indenting the whole thing and prepending a list marker: + +```````````````````````````````` example + foo + +bar +. +<p>foo</p> +<p>bar</p> +```````````````````````````````` + + +```````````````````````````````` example +- foo + + bar +. +<ul> +<li>foo</li> +</ul> +<p>bar</p> +```````````````````````````````` + + +This is not a significant restriction, because when a block is preceded by up to +three spaces of indentation, the indentation can always be removed without +a change in interpretation, allowing rule #1 to be applied. So, in +the above case: + +```````````````````````````````` example +- 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*, and *M* is a list marker of width *W*, + then the result of prepending *M* to the first line of *Ls*, and + preceding subsequent lines of *Ls* by *W + 1* spaces of indentation, is a + list item with *Bs* as its contents. + If a line is empty, then it need not be indented. The type of the + list item (bullet or ordered) is determined by the type of its list + marker. If the list item is ordered, then it is also assigned a + start number, based on the ordered list marker. + +Here are some list items that start with a blank line but are not empty: + +```````````````````````````````` example +- + foo +- + ``` + bar + ``` +- + baz +. +<ul> +<li>foo</li> +<li> +<pre><code>bar +</code></pre> +</li> +<li> +<pre><code>baz +</code></pre> +</li> +</ul> +```````````````````````````````` + +When the list item starts with a blank line, the number of spaces +following the list marker doesn't change the required indentation: + +```````````````````````````````` example +- + foo +. +<ul> +<li>foo</li> +</ul> +```````````````````````````````` + + +A list item can begin with at most one blank line. +In the following example, `foo` is not part of the list +item: + +```````````````````````````````` example +- + + foo +. +<ul> +<li></li> +</ul> +<p>foo</p> +```````````````````````````````` + + +Here is an empty bullet list item: + +```````````````````````````````` example +- foo +- +- bar +. +<ul> +<li>foo</li> +<li></li> +<li>bar</li> +</ul> +```````````````````````````````` + + +It does not matter whether there are spaces or tabs following the [list marker]: + +```````````````````````````````` example +- foo +- +- bar +. +<ul> +<li>foo</li> +<li></li> +<li>bar</li> +</ul> +```````````````````````````````` + + +Here is an empty ordered list item: + +```````````````````````````````` example +1. foo +2. +3. bar +. +<ol> +<li>foo</li> +<li></li> +<li>bar</li> +</ol> +```````````````````````````````` + + +A list may start or end with an empty list item: + +```````````````````````````````` example +* +. +<ul> +<li></li> +</ul> +```````````````````````````````` + +However, an empty list item cannot interrupt a paragraph: + +```````````````````````````````` example +foo +* + +foo +1. +. +<p>foo +*</p> +<p>foo +1.</p> +```````````````````````````````` + + +4. **Indentation.** If a sequence of lines *Ls* constitutes a list item + according to rule #1, #2, or #3, then the result of preceding each line + of *Ls* by up to three spaces of indentation (the same for each line) also + constitutes a list item with the same contents and attributes. If a line is + empty, then it need not be indented. + +Indented one space: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +<ol> +<li> +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +Indented two spaces: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +<ol> +<li> +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +Indented three spaces: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +<ol> +<li> +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +Four spaces indent gives a code block: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +<pre><code>1. A paragraph + with two lines. + + indented code + + > A block quote. +</code></pre> +```````````````````````````````` + + + +5. **Laziness.** If a string of lines *Ls* constitute a [list + item](#list-items) with contents *Bs*, then the result of deleting + some or all of the indentation from one or more lines in which the + next character other than a space or tab after the indentation is + [paragraph continuation text] is a + list item with the same contents and attributes. The unindented + lines are called + [lazy continuation line](@)s. + +Here is an example with [lazy continuation lines]: + +```````````````````````````````` example + 1. A paragraph +with two lines. + + indented code + + > A block quote. +. +<ol> +<li> +<p>A paragraph +with two lines.</p> +<pre><code>indented code +</code></pre> +<blockquote> +<p>A block quote.</p> +</blockquote> +</li> +</ol> +```````````````````````````````` + + +Indentation can be partially deleted: + +```````````````````````````````` example + 1. A paragraph + with two lines. +. +<ol> +<li>A paragraph +with two lines.</li> +</ol> +```````````````````````````````` + + +These examples show how laziness can work in nested structures: + +```````````````````````````````` example +> 1. > Blockquote +continued here. +. +<blockquote> +<ol> +<li> +<blockquote> +<p>Blockquote +continued here.</p> +</blockquote> +</li> +</ol> +</blockquote> +```````````````````````````````` + + +```````````````````````````````` example +> 1. > Blockquote +> continued here. +. +<blockquote> +<ol> +<li> +<blockquote> +<p>Blockquote +continued here.</p> +</blockquote> +</li> +</ol> +</blockquote> +```````````````````````````````` + + + +6. **That's all.** Nothing that is not counted as a list item by rules + #1--5 counts as a [list item](#list-items). + +The rules for sublists follow from the general rules +[above][List items]. A sublist must be indented the same number +of spaces of indentation a paragraph would need to be in order to be included +in the list item. + +So, in this case we need two spaces indent: + +```````````````````````````````` example +- foo + - bar + - baz + - boo +. +<ul> +<li>foo +<ul> +<li>bar +<ul> +<li>baz +<ul> +<li>boo</li> +</ul> +</li> +</ul> +</li> +</ul> +</li> +</ul> +```````````````````````````````` + + +One is not enough: + +```````````````````````````````` example +- foo + - bar + - baz + - boo +. +<ul> +<li>foo</li> +<li>bar</li> +<li>baz</li> +<li>boo</li> +</ul> +```````````````````````````````` + + +Here we need four, because the list marker is wider: + +```````````````````````````````` example +10) foo + - bar +. +<ol start="10"> +<li>foo +<ul> +<li>bar</li> +</ul> +</li> +</ol> +```````````````````````````````` + + +Three is not enough: + +```````````````````````````````` example +10) foo + - bar +. +<ol start="10"> +<li>foo</li> +</ol> +<ul> +<li>bar</li> +</ul> +```````````````````````````````` + + +A list may be the first block in a list item: + +```````````````````````````````` example +- - foo +. +<ul> +<li> +<ul> +<li>foo</li> +</ul> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +1. - 2. foo +. +<ol> +<li> +<ul> +<li> +<ol start="2"> +<li>foo</li> +</ol> +</li> +</ul> +</li> +</ol> +```````````````````````````````` + + +A list item can contain a heading: + +```````````````````````````````` example +- # Foo +- Bar + --- + baz +. +<ul> +<li> +<h1>Foo</h1> +</li> +<li> +<h2>Bar</h2> +baz</li> +</ul> +```````````````````````````````` + + +### Motivation + +John Gruber's Markdown spec says the following about list items: + +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 + +``` markdown +- foo + + bar + + - baz +``` + +should be parsed as two lists with an intervening paragraph, + +``` html +<ul> +<li>foo</li> +</ul> +<p>bar</p> +<ul> +<li>baz</li> +</ul> +``` + +as the four-space rule demands, rather than a single list, + +``` html +<ul> +<li> +<p>foo</p> +<p>bar</p> +<ul> +<li>baz</li> +</ul> +</li> +</ul> +``` + +The choice of four spaces is arbitrary. It can be learned, but it is +not likely to be guessed, and it trips up beginners regularly. + +Would it help to adopt a two-space rule? The problem is that such +a rule, together with the rule allowing up to three spaces of indentation for +the initial list marker, allows text that is indented *less than* the +original list marker to be included in the list item. For example, +`Markdown.pl` parses + +``` markdown + - one + + two +``` + +as a single list item, with `two` a continuation paragraph: + +``` html +<ul> +<li> +<p>one</p> +<p>two</p> +</li> +</ul> +``` + +and similarly + +``` markdown +> - one +> +> two +``` + +as + +``` html +<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`: + +``` markdown + 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: + +``` markdown +1. foo + + indented code +``` + +where the code is indented eight spaces. The spec above, by contrast, will +parse this text as expected, since the code block's indentation is measured +from the beginning of `foo`. + +The one case that needs special treatment is a list item that *starts* +with indented code. How much indentation is required in that case, since +we don't have a "first paragraph" to measure from? Rule #2 simply stipulates +that in such cases, we require one space indentation from the list marker +(and then the normal four spaces for the indented code). This will match the +four-space rule in cases where the list marker plus its initial indentation +takes four spaces (a common case), but diverge in other cases. + +## Lists + +A [list](@) is a sequence of one or more +list items [of the same type]. The list items +may be separated by any number of blank lines. + +Two list items are [of the same type](@) +if they begin with a [list marker] of the same type. +Two list markers are of the +same type if (a) they are bullet list markers using the same character +(`-`, `+`, or `*`) or (b) they are ordered list numbers with the same +delimiter (either `.` or `)`). + +A list is an [ordered list](@) +if its constituent list items begin with +[ordered list markers], and a +[bullet list](@) if its constituent list +items begin with [bullet list markers]. + +The [start number](@) +of an [ordered list] is determined by the list number of +its initial list item. The numbers of subsequent list items are +disregarded. + +A list is [loose](@) if any of its constituent +list items are separated by blank lines, or if any of its constituent +list items directly contain two block-level elements with a blank line +between them. Otherwise a list is [tight](@). +(The difference in HTML output is that paragraphs in a loose list are +wrapped in `<p>` tags, while paragraphs in a tight list are not.) + +Changing the bullet or ordered list delimiter starts a new list: + +```````````````````````````````` example +- foo +- bar ++ baz +. +<ul> +<li>foo</li> +<li>bar</li> +</ul> +<ul> +<li>baz</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +1. foo +2. bar +3) baz +. +<ol> +<li>foo</li> +<li>bar</li> +</ol> +<ol start="3"> +<li>baz</li> +</ol> +```````````````````````````````` + + +In CommonMark, a list can interrupt a paragraph. That is, +no blank line is needed to separate a paragraph from a following +list: + +```````````````````````````````` example +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: + +``` markdown +The number of windows in my house is +14. The number of doors is 6. +``` + +Oddly, though, `Markdown.pl` *does* allow a blockquote to +interrupt a paragraph, even though the same considerations might +apply. + +In CommonMark, we do allow lists to interrupt paragraphs, for +two reasons. First, it is natural and not uncommon for people +to start lists without blank lines: + +``` markdown +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 + +``` markdown + * 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 + +``` markdown +I need to buy +- new shoes +- a coat +- a plane ticket +``` + +by itself should be a paragraph followed by a nested sublist. + +Since it is well established Markdown practice to allow lists to +interrupt paragraphs inside list items, the [principle of +uniformity] requires us to allow this outside list items as +well. ([reStructuredText](http://docutils.sourceforge.net/rst.html) +takes a different approach, requiring blank lines before lists +even inside other list items.) + +In order to solve of unwanted lists in paragraphs with +hard-wrapped numerals, we allow only lists starting with `1` to +interrupt paragraphs. Thus, + +```````````````````````````````` example +The number of windows in my house is +14. The number of doors is 6. +. +<p>The number of windows in my house is +14. The number of doors is 6.</p> +```````````````````````````````` + +We may still get an unintended result in cases like + +```````````````````````````````` example +The number of windows in my house is +1. The number of doors is 6. +. +<p>The number of windows in my house is</p> +<ol> +<li>The number of doors is 6.</li> +</ol> +```````````````````````````````` + +but this rule should prevent most spurious list captures. + +There can be any number of blank lines between items: + +```````````````````````````````` example +- foo + +- bar + + +- baz +. +<ul> +<li> +<p>foo</p> +</li> +<li> +<p>bar</p> +</li> +<li> +<p>baz</p> +</li> +</ul> +```````````````````````````````` + +```````````````````````````````` example +- foo + - bar + - baz + + + bim +. +<ul> +<li>foo +<ul> +<li>bar +<ul> +<li> +<p>baz</p> +<p>bim</p> +</li> +</ul> +</li> +</ul> +</li> +</ul> +```````````````````````````````` + + +To separate consecutive lists of the same type, or to separate a +list from an indented code block that would otherwise be parsed +as a subparagraph of the final list item, you can insert a blank HTML +comment: + +```````````````````````````````` example +- foo +- bar + +<!-- --> + +- baz +- bim +. +<ul> +<li>foo</li> +<li>bar</li> +</ul> +<!-- --> +<ul> +<li>baz</li> +<li>bim</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +- foo + + notcode + +- foo + +<!-- --> + + code +. +<ul> +<li> +<p>foo</p> +<p>notcode</p> +</li> +<li> +<p>foo</p> +</li> +</ul> +<!-- --> +<pre><code>code +</code></pre> +```````````````````````````````` + + +List items need not be indented to the same level. The following +list items will be treated as items at the same list level, +since none is indented enough to belong to the previous list +item: + +```````````````````````````````` example +- a + - b + - c + - d + - e + - f +- g +. +<ul> +<li>a</li> +<li>b</li> +<li>c</li> +<li>d</li> +<li>e</li> +<li>f</li> +<li>g</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +1. a + + 2. b + + 3. c +. +<ol> +<li> +<p>a</p> +</li> +<li> +<p>b</p> +</li> +<li> +<p>c</p> +</li> +</ol> +```````````````````````````````` + +Note, however, that list items may not be preceded by more than +three spaces of indentation. Here `- e` is treated as a paragraph continuation +line, because it is indented more than three spaces: + +```````````````````````````````` example +- a + - b + - c + - d + - e +. +<ul> +<li>a</li> +<li>b</li> +<li>c</li> +<li>d +- e</li> +</ul> +```````````````````````````````` + +And here, `3. c` is treated as in indented code block, +because it is indented four spaces and preceded by a +blank line. + +```````````````````````````````` example +1. a + + 2. b + + 3. c +. +<ol> +<li> +<p>a</p> +</li> +<li> +<p>b</p> +</li> +</ol> +<pre><code>3. c +</code></pre> +```````````````````````````````` + + +This is a loose list, because there is a blank line between +two of the list items: + +```````````````````````````````` example +- a +- b + +- c +. +<ul> +<li> +<p>a</p> +</li> +<li> +<p>b</p> +</li> +<li> +<p>c</p> +</li> +</ul> +```````````````````````````````` + + +So is this, with a empty second item: + +```````````````````````````````` example +* a +* + +* c +. +<ul> +<li> +<p>a</p> +</li> +<li></li> +<li> +<p>c</p> +</li> +</ul> +```````````````````````````````` + + +These are loose lists, even though there are no blank lines between the items, +because one of the items directly contains two block-level elements +with a blank line between them: + +```````````````````````````````` example +- a +- b + + c +- d +. +<ul> +<li> +<p>a</p> +</li> +<li> +<p>b</p> +<p>c</p> +</li> +<li> +<p>d</p> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +- a +- b + + [ref]: /url +- d +. +<ul> +<li> +<p>a</p> +</li> +<li> +<p>b</p> +</li> +<li> +<p>d</p> +</li> +</ul> +```````````````````````````````` + + +This is a tight list, because the blank lines are in a code block: + +```````````````````````````````` example +- a +- ``` + b + + + ``` +- c +. +<ul> +<li>a</li> +<li> +<pre><code>b + + +</code></pre> +</li> +<li>c</li> +</ul> +```````````````````````````````` + + +This is a tight list, because the blank line is between two +paragraphs of a sublist. So the sublist is loose while +the outer list is tight: + +```````````````````````````````` example +- a + - b + + c +- d +. +<ul> +<li>a +<ul> +<li> +<p>b</p> +<p>c</p> +</li> +</ul> +</li> +<li>d</li> +</ul> +```````````````````````````````` + + +This is a tight list, because the blank line is inside the +block quote: + +```````````````````````````````` example +* a + > b + > +* c +. +<ul> +<li>a +<blockquote> +<p>b</p> +</blockquote> +</li> +<li>c</li> +</ul> +```````````````````````````````` + + +This list is tight, because the consecutive block elements +are not separated by blank lines: + +```````````````````````````````` example +- a + > b + ``` + c + ``` +- d +. +<ul> +<li>a +<blockquote> +<p>b</p> +</blockquote> +<pre><code>c +</code></pre> +</li> +<li>d</li> +</ul> +```````````````````````````````` + + +A single-paragraph list is tight: + +```````````````````````````````` example +- a +. +<ul> +<li>a</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +- a + - b +. +<ul> +<li>a +<ul> +<li>b</li> +</ul> +</li> +</ul> +```````````````````````````````` + + +This list is loose, because of the blank line between the +two block elements in the list item: + +```````````````````````````````` example +1. ``` + foo + ``` + + bar +. +<ol> +<li> +<pre><code>foo +</code></pre> +<p>bar</p> +</li> +</ol> +```````````````````````````````` + + +Here the outer list is loose, the inner list tight: + +```````````````````````````````` example +* foo + * bar + + baz +. +<ul> +<li> +<p>foo</p> +<ul> +<li>bar</li> +</ul> +<p>baz</p> +</li> +</ul> +```````````````````````````````` + + +```````````````````````````````` example +- a + - b + - c + +- d + - e + - f +. +<ul> +<li> +<p>a</p> +<ul> +<li>b</li> +<li>c</li> +</ul> +</li> +<li> +<p>d</p> +<ul> +<li>e</li> +<li>f</li> +</ul> +</li> +</ul> +```````````````````````````````` + + +# Inlines + +Inlines are parsed sequentially from the beginning of the character +stream to the end (left to right, in left-to-right languages). +Thus, for example, in + +```````````````````````````````` example +`hi`lo` +. +<p><code>hi</code>lo`</p> +```````````````````````````````` + +`hi` is parsed as code, leaving the backtick at the end as a literal +backtick. + + + +## Code spans + +A [backtick string](@) +is a string of one or more backtick characters (`` ` ``) that is neither +preceded nor followed by a backtick. + +A [code span](@) begins with a backtick string and ends with +a backtick string of equal length. The contents of the code span are +the characters between these two backtick strings, normalized in the +following ways: + +- First, [line endings] are converted to [spaces]. +- If the resulting string both begins *and* ends with a [space] + character, but does not consist entirely of [space] + characters, a single [space] character is removed from the + front and back. This allows you to include code that begins + or ends with backtick characters, which must be separated by + whitespace from the opening or closing backtick strings. + +This is a simple code span: + +```````````````````````````````` example +`foo` +. +<p><code>foo</code></p> +```````````````````````````````` + + +Here two backticks are used, because the code contains a backtick. +This example also illustrates stripping of a single leading and +trailing space: + +```````````````````````````````` example +`` foo ` bar `` +. +<p><code>foo ` bar</code></p> +```````````````````````````````` + + +This example shows the motivation for stripping leading and trailing +spaces: + +```````````````````````````````` example +` `` ` +. +<p><code>``</code></p> +```````````````````````````````` + +Note that only *one* space is stripped: + +```````````````````````````````` example +` `` ` +. +<p><code> `` </code></p> +```````````````````````````````` + +The stripping only happens if the space is on both +sides of the string: + +```````````````````````````````` example +` a` +. +<p><code> a</code></p> +```````````````````````````````` + +Only [spaces], and not [unicode whitespace] in general, are +stripped in this way: + +```````````````````````````````` example +` b ` +. +<p><code> b </code></p> +```````````````````````````````` + +No stripping occurs if the code span contains only spaces: + +```````````````````````````````` example +` ` +` ` +. +<p><code> </code> +<code> </code></p> +```````````````````````````````` + + +[Line endings] are treated like spaces: + +```````````````````````````````` example +`` +foo +bar +baz +`` +. +<p><code>foo bar baz</code></p> +```````````````````````````````` + +```````````````````````````````` example +`` +foo +`` +. +<p><code>foo </code></p> +```````````````````````````````` + + +Interior spaces are not collapsed: + +```````````````````````````````` example +`foo bar +baz` +. +<p><code>foo bar baz</code></p> +```````````````````````````````` + +Note that browsers will typically collapse consecutive spaces +when rendering `<code>` elements, so it is recommended that +the following CSS be used: + + code{white-space: pre-wrap;} + + +Note that backslash escapes do not work in code spans. All backslashes +are treated literally: + +```````````````````````````````` example +`foo\`bar` +. +<p><code>foo\</code>bar`</p> +```````````````````````````````` + + +Backslash escapes are never needed, because one can always choose a +string of *n* backtick characters as delimiters, where the code does +not contain any strings of exactly *n* backtick characters. + +```````````````````````````````` example +``foo`bar`` +. +<p><code>foo`bar</code></p> +```````````````````````````````` + +```````````````````````````````` example +` foo `` bar ` +. +<p><code>foo `` bar</code></p> +```````````````````````````````` + + +Code span backticks have higher precedence than any other inline +constructs except HTML tags and autolinks. Thus, for example, this is +not parsed as emphasized text, since the second `*` is part of a code +span: + +```````````````````````````````` example +*foo`*` +. +<p>*foo<code>*</code></p> +```````````````````````````````` + + +And this is not parsed as a link: + +```````````````````````````````` example +[not a `link](/foo`) +. +<p>[not a <code>link](/foo</code>)</p> +```````````````````````````````` + + +Code spans, HTML tags, and autolinks have the same precedence. +Thus, this is code: + +```````````````````````````````` example +`<a href="`">` +. +<p><code><a href="</code>">`</p> +```````````````````````````````` + + +But this is an HTML tag: + +```````````````````````````````` example +<a href="`">` +. +<p><a href="`">`</p> +```````````````````````````````` + + +And this is code: + +```````````````````````````````` example +`<http://foo.bar.`baz>` +. +<p><code><http://foo.bar.</code>baz>`</p> +```````````````````````````````` + + +But this is an autolink: + +```````````````````````````````` example +<http://foo.bar.`baz>` +. +<p><a href="http://foo.bar.%60baz">http://foo.bar.`baz</a>`</p> +```````````````````````````````` + + +When a backtick string is not closed by a matching backtick string, +we just have literal backticks: + +```````````````````````````````` example +```foo`` +. +<p>```foo``</p> +```````````````````````````````` + + +```````````````````````````````` example +`foo +. +<p>`foo</p> +```````````````````````````````` + +The following case also illustrates the need for opening and +closing backtick strings to be equal in length: + +```````````````````````````````` example +`foo``bar`` +. +<p>`foo<code>bar</code></p> +```````````````````````````````` + + +## Emphasis and strong emphasis + +John Gruber's original [Markdown syntax +description](http://daringfireball.net/projects/markdown/syntax#em) says: + +> Markdown treats asterisks (`*`) and underscores (`_`) as indicators of +> emphasis. Text wrapped with one `*` or `_` will be wrapped with an HTML +> `<em>` tag; double `*`'s or `_`'s will be wrapped with an HTML `<strong>` +> tag. + +This is enough for most users, but these rules leave much undecided, +especially when it comes to nested emphasis. The original +`Markdown.pl` test suite makes it clear that triple `***` and +`___` delimiters can be used for strong emphasis, and most +implementations have also allowed the following patterns: + +``` markdown +***strong emph*** +***strong** in emph* +***emph* in strong** +**in strong *emph*** +*in emph **strong*** +``` + +The following patterns are less widely supported, but the intent +is clear and they are useful (especially in contexts like bibliography +entries): + +``` markdown +*emph *with emph* in it* +**strong **with strong** in it** +``` + +Many implementations have also restricted intraword emphasis to +the `*` forms, to avoid unwanted emphasis in words containing +internal underscores. (It is best practice to put these in code +spans, but users often do not.) + +``` markdown +internal emphasis: foo*bar*baz +no emphasis: foo_bar_baz +``` + +The rules given below capture all of these patterns, while allowing +for efficient parsing strategies that do not backtrack. + +First, some definitions. A [delimiter run](@) is either +a sequence of one or more `*` characters that is not preceded or +followed by a non-backslash-escaped `*` character, or a sequence +of one or more `_` characters that is not preceded or followed by +a non-backslash-escaped `_` character. + +A [left-flanking delimiter run](@) is +a [delimiter run] that is (1) not followed by [Unicode whitespace], +and either (2a) not followed by a [Unicode punctuation character], or +(2b) followed by a [Unicode punctuation character] and +preceded by [Unicode whitespace] or a [Unicode punctuation character]. +For purposes of this definition, the beginning and the end of +the line count as Unicode whitespace. + +A [right-flanking delimiter run](@) is +a [delimiter run] that is (1) not preceded by [Unicode whitespace], +and either (2a) not preceded by a [Unicode punctuation character], or +(2b) preceded by a [Unicode punctuation character] and +followed by [Unicode whitespace] or a [Unicode punctuation character]. +For purposes of this definition, the beginning and the end of +the line count as Unicode whitespace. + +Here are some examples of delimiter runs. + + - left-flanking but not right-flanking: + + ``` + ***abc + _abc + **"abc" + _"abc" + ``` + + - right-flanking but not left-flanking: + + ``` + abc*** + abc_ + "abc"** + "abc"_ + ``` + + - Both left and right-flanking: + + ``` + abc***def + "abc"_"def" + ``` + + - Neither left nor right-flanking: + + ``` + abc *** def + a _ b + ``` + +(The idea of distinguishing left-flanking and right-flanking +delimiter runs based on the character before and the character +after comes from Roopesh Chander's +[vfmd](http://www.vfmd.org/vfmd-spec/specification/#procedure-for-identifying-emphasis-tags). +vfmd uses the terminology "emphasis indicator string" instead of "delimiter +run," and its rules for distinguishing left- and right-flanking runs +are a bit more complex than the ones given here.) + +The following rules define emphasis and strong emphasis: + +1. A single `*` character [can open emphasis](@) + iff (if and only if) it is part of a [left-flanking delimiter run]. + +2. A single `_` character [can open emphasis] iff + it is part of a [left-flanking delimiter run] + and either (a) not part of a [right-flanking delimiter run] + or (b) part of a [right-flanking delimiter run] + preceded by a [Unicode punctuation character]. + +3. A single `*` character [can close emphasis](@) + iff it is part of a [right-flanking delimiter run]. + +4. A single `_` character [can close emphasis] iff + it is part of a [right-flanking delimiter run] + and either (a) not part of a [left-flanking delimiter run] + or (b) part of a [left-flanking delimiter run] + followed by a [Unicode punctuation character]. + +5. A double `**` [can open strong emphasis](@) + iff it is part of a [left-flanking delimiter run]. + +6. A double `__` [can open strong emphasis] iff + it is part of a [left-flanking delimiter run] + and either (a) not part of a [right-flanking delimiter run] + or (b) part of a [right-flanking delimiter run] + preceded by a [Unicode punctuation character]. + +7. A double `**` [can close strong emphasis](@) + iff it is part of a [right-flanking delimiter run]. + +8. A double `__` [can close strong emphasis] iff + it is part of a [right-flanking delimiter run] + and either (a) not part of a [left-flanking delimiter run] + or (b) part of a [left-flanking delimiter run] + followed by a [Unicode punctuation character]. + +9. Emphasis begins with a delimiter that [can open emphasis] and ends + with a delimiter that [can close emphasis], and that uses the same + character (`_` or `*`) as the opening delimiter. The + opening and closing delimiters must belong to separate + [delimiter runs]. If one of the delimiters can both + open and close emphasis, then the sum of the lengths of the + delimiter runs containing the opening and closing delimiters + must not be a multiple of 3 unless both lengths are + multiples of 3. + +10. Strong emphasis begins with a delimiter that + [can open strong emphasis] and ends with a delimiter that + [can close strong emphasis], and that uses the same character + (`_` or `*`) as the opening delimiter. The + opening and closing delimiters must belong to separate + [delimiter runs]. If one of the delimiters can both open + and close strong emphasis, then the sum of the lengths of + the delimiter runs containing the opening and closing + delimiters must not be a multiple of 3 unless both lengths + are multiples of 3. + +11. A literal `*` character cannot occur at the beginning or end of + `*`-delimited emphasis or `**`-delimited strong emphasis, unless it + is backslash-escaped. + +12. A literal `_` character cannot occur at the beginning or end of + `_`-delimited emphasis or `__`-delimited strong emphasis, unless it + is backslash-escaped. + +Where rules 1--12 above are compatible with multiple parsings, +the following principles resolve ambiguity: + +13. The number of nestings should be minimized. Thus, for example, + an interpretation `<strong>...</strong>` is always preferred to + `<em><em>...</em></em>`. + +14. An interpretation `<em><strong>...</strong></em>` is always + preferred to `<strong><em>...</em></strong>`. + +15. When two potential emphasis or strong emphasis spans overlap, + so that the second begins before the first ends and ends after + the first ends, the first takes precedence. Thus, for example, + `*foo _bar* baz_` is parsed as `<em>foo _bar</em> baz_` rather + than `*foo <em>bar* baz</em>`. + +16. When there are two potential emphasis or strong emphasis spans + with the same closing delimiter, the shorter one (the one that + opens later) takes precedence. Thus, for example, + `**foo **bar baz**` is parsed as `**foo <strong>bar baz</strong>` + rather than `<strong>foo **bar baz</strong>`. + +17. Inline code spans, links, images, and HTML tags group more tightly + than emphasis. So, when there is a choice between an interpretation + that contains one of these elements and one that does not, the + former always wins. Thus, for example, `*[foo*](bar)` is + parsed as `*<a href="bar">foo*</a>` rather than as + `<em>[foo</em>](bar)`. + +These rules can be illustrated through a series of examples. + +Rule 1: + +```````````````````````````````` example +*foo bar* +. +<p><em>foo bar</em></p> +```````````````````````````````` + + +This is not emphasis, because the opening `*` is followed by +whitespace, and hence not part of a [left-flanking delimiter run]: + +```````````````````````````````` example +a * foo bar* +. +<p>a * foo bar*</p> +```````````````````````````````` + + +This is not emphasis, because the opening `*` is preceded +by an alphanumeric and followed by punctuation, and hence +not part of a [left-flanking delimiter run]: + +```````````````````````````````` example +a*"foo"* +. +<p>a*"foo"*</p> +```````````````````````````````` + + +Unicode nonbreaking spaces count as whitespace, too: + +```````````````````````````````` example +* a * +. +<p>* a *</p> +```````````````````````````````` + + +Intraword emphasis with `*` is permitted: + +```````````````````````````````` example +foo*bar* +. +<p>foo<em>bar</em></p> +```````````````````````````````` + + +```````````````````````````````` example +5*6*78 +. +<p>5<em>6</em>78</p> +```````````````````````````````` + + +Rule 2: + +```````````````````````````````` example +_foo bar_ +. +<p><em>foo bar</em></p> +```````````````````````````````` + + +This is not emphasis, because the opening `_` is followed by +whitespace: + +```````````````````````````````` example +_ foo bar_ +. +<p>_ foo bar_</p> +```````````````````````````````` + + +This is not emphasis, because the opening `_` is preceded +by an alphanumeric and followed by punctuation: + +```````````````````````````````` example +a_"foo"_ +. +<p>a_"foo"_</p> +```````````````````````````````` + + +Emphasis with `_` is not allowed inside words: + +```````````````````````````````` example +foo_bar_ +. +<p>foo_bar_</p> +```````````````````````````````` + + +```````````````````````````````` example +5_6_78 +. +<p>5_6_78</p> +```````````````````````````````` + + +```````````````````````````````` example +пристаням_стремятся_ +. +<p>пристаням_стремятся_</p> +```````````````````````````````` + + +Here `_` does not generate emphasis, because the first delimiter run +is right-flanking and the second left-flanking: + +```````````````````````````````` example +aa_"bb"_cc +. +<p>aa_"bb"_cc</p> +```````````````````````````````` + + +This is emphasis, even though the opening delimiter is +both left- and right-flanking, because it is preceded by +punctuation: + +```````````````````````````````` example +foo-_(bar)_ +. +<p>foo-<em>(bar)</em></p> +```````````````````````````````` + + +Rule 3: + +This is not emphasis, because the closing delimiter does +not match the opening delimiter: + +```````````````````````````````` example +_foo* +. +<p>_foo*</p> +```````````````````````````````` + + +This is not emphasis, because the closing `*` is preceded by +whitespace: + +```````````````````````````````` example +*foo bar * +. +<p>*foo bar *</p> +```````````````````````````````` + + +A line ending also counts as whitespace: + +```````````````````````````````` example +*foo bar +* +. +<p>*foo bar +*</p> +```````````````````````````````` + + +This is not emphasis, because the second `*` is +preceded by punctuation and followed by an alphanumeric +(hence it is not part of a [right-flanking delimiter run]: + +```````````````````````````````` example +*(*foo) +. +<p>*(*foo)</p> +```````````````````````````````` + + +The point of this restriction is more easily appreciated +with this example: + +```````````````````````````````` example +*(*foo*)* +. +<p><em>(<em>foo</em>)</em></p> +```````````````````````````````` + + +Intraword emphasis with `*` is allowed: + +```````````````````````````````` example +*foo*bar +. +<p><em>foo</em>bar</p> +```````````````````````````````` + + + +Rule 4: + +This is not emphasis, because the closing `_` is preceded by +whitespace: + +```````````````````````````````` example +_foo bar _ +. +<p>_foo bar _</p> +```````````````````````````````` + + +This is not emphasis, because the second `_` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +_(_foo) +. +<p>_(_foo)</p> +```````````````````````````````` + + +This is emphasis within emphasis: + +```````````````````````````````` example +_(_foo_)_ +. +<p><em>(<em>foo</em>)</em></p> +```````````````````````````````` + + +Intraword emphasis is disallowed for `_`: + +```````````````````````````````` example +_foo_bar +. +<p>_foo_bar</p> +```````````````````````````````` + + +```````````````````````````````` example +_пристаням_стремятся +. +<p>_пристаням_стремятся</p> +```````````````````````````````` + + +```````````````````````````````` example +_foo_bar_baz_ +. +<p><em>foo_bar_baz</em></p> +```````````````````````````````` + + +This is emphasis, even though the closing delimiter is +both left- and right-flanking, because it is followed by +punctuation: + +```````````````````````````````` example +_(bar)_. +. +<p><em>(bar)</em>.</p> +```````````````````````````````` + + +Rule 5: + +```````````````````````````````` example +**foo bar** +. +<p><strong>foo bar</strong></p> +```````````````````````````````` + + +This is not strong emphasis, because the opening delimiter is +followed by whitespace: + +```````````````````````````````` example +** foo bar** +. +<p>** foo bar**</p> +```````````````````````````````` + + +This is not strong emphasis, because the opening `**` is preceded +by an alphanumeric and followed by punctuation, and hence +not part of a [left-flanking delimiter run]: + +```````````````````````````````` example +a**"foo"** +. +<p>a**"foo"**</p> +```````````````````````````````` + + +Intraword strong emphasis with `**` is permitted: + +```````````````````````````````` example +foo**bar** +. +<p>foo<strong>bar</strong></p> +```````````````````````````````` + + +Rule 6: + +```````````````````````````````` example +__foo bar__ +. +<p><strong>foo bar</strong></p> +```````````````````````````````` + + +This is not strong emphasis, because the opening delimiter is +followed by whitespace: + +```````````````````````````````` example +__ foo bar__ +. +<p>__ foo bar__</p> +```````````````````````````````` + + +A line ending counts as whitespace: +```````````````````````````````` example +__ +foo bar__ +. +<p>__ +foo bar__</p> +```````````````````````````````` + + +This is not strong emphasis, because the opening `__` is preceded +by an alphanumeric and followed by punctuation: + +```````````````````````````````` example +a__"foo"__ +. +<p>a__"foo"__</p> +```````````````````````````````` + + +Intraword strong emphasis is forbidden with `__`: + +```````````````````````````````` example +foo__bar__ +. +<p>foo__bar__</p> +```````````````````````````````` + + +```````````````````````````````` example +5__6__78 +. +<p>5__6__78</p> +```````````````````````````````` + + +```````````````````````````````` example +пристаням__стремятся__ +. +<p>пристаням__стремятся__</p> +```````````````````````````````` + + +```````````````````````````````` example +__foo, __bar__, baz__ +. +<p><strong>foo, <strong>bar</strong>, baz</strong></p> +```````````````````````````````` + + +This is strong emphasis, even though the opening delimiter is +both left- and right-flanking, because it is preceded by +punctuation: + +```````````````````````````````` example +foo-__(bar)__ +. +<p>foo-<strong>(bar)</strong></p> +```````````````````````````````` + + + +Rule 7: + +This is not strong emphasis, because the closing delimiter is preceded +by whitespace: + +```````````````````````````````` example +**foo bar ** +. +<p>**foo bar **</p> +```````````````````````````````` + + +(Nor can it be interpreted as an emphasized `*foo bar *`, because of +Rule 11.) + +This is not strong emphasis, because the second `**` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +**(**foo) +. +<p>**(**foo)</p> +```````````````````````````````` + + +The point of this restriction is more easily appreciated +with these examples: + +```````````````````````````````` example +*(**foo**)* +. +<p><em>(<strong>foo</strong>)</em></p> +```````````````````````````````` + + +```````````````````````````````` example +**Gomphocarpus (*Gomphocarpus physocarpus*, syn. +*Asclepias physocarpa*)** +. +<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn. +<em>Asclepias physocarpa</em>)</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo "*bar*" foo** +. +<p><strong>foo "<em>bar</em>" foo</strong></p> +```````````````````````````````` + + +Intraword emphasis: + +```````````````````````````````` example +**foo**bar +. +<p><strong>foo</strong>bar</p> +```````````````````````````````` + + +Rule 8: + +This is not strong emphasis, because the closing delimiter is +preceded by whitespace: + +```````````````````````````````` example +__foo bar __ +. +<p>__foo bar __</p> +```````````````````````````````` + + +This is not strong emphasis, because the second `__` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +__(__foo) +. +<p>__(__foo)</p> +```````````````````````````````` + + +The point of this restriction is more easily appreciated +with this example: + +```````````````````````````````` example +_(__foo__)_ +. +<p><em>(<strong>foo</strong>)</em></p> +```````````````````````````````` + + +Intraword strong emphasis is forbidden with `__`: + +```````````````````````````````` example +__foo__bar +. +<p>__foo__bar</p> +```````````````````````````````` + + +```````````````````````````````` example +__пристаням__стремятся +. +<p>__пристаням__стремятся</p> +```````````````````````````````` + + +```````````````````````````````` example +__foo__bar__baz__ +. +<p><strong>foo__bar__baz</strong></p> +```````````````````````````````` + + +This is strong emphasis, even though the closing delimiter is +both left- and right-flanking, because it is followed by +punctuation: + +```````````````````````````````` example +__(bar)__. +. +<p><strong>(bar)</strong>.</p> +```````````````````````````````` + + +Rule 9: + +Any nonempty sequence of inline elements can be the contents of an +emphasized span. + +```````````````````````````````` example +*foo [bar](/url)* +. +<p><em>foo <a href="/url">bar</a></em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo +bar* +. +<p><em>foo +bar</em></p> +```````````````````````````````` + + +In particular, emphasis and strong emphasis can be nested +inside emphasis: + +```````````````````````````````` example +_foo __bar__ baz_ +. +<p><em>foo <strong>bar</strong> baz</em></p> +```````````````````````````````` + + +```````````````````````````````` example +_foo _bar_ baz_ +. +<p><em>foo <em>bar</em> baz</em></p> +```````````````````````````````` + + +```````````````````````````````` example +__foo_ bar_ +. +<p><em><em>foo</em> bar</em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo *bar** +. +<p><em>foo <em>bar</em></em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo **bar** baz* +. +<p><em>foo <strong>bar</strong> baz</em></p> +```````````````````````````````` + +```````````````````````````````` example +*foo**bar**baz* +. +<p><em>foo<strong>bar</strong>baz</em></p> +```````````````````````````````` + +Note that in the preceding case, the interpretation + +``` markdown +<p><em>foo</em><em>bar<em></em>baz</em></p> +``` + + +is precluded by the condition that a delimiter that +can both open and close (like the `*` after `foo`) +cannot form emphasis if the sum of the lengths of +the delimiter runs containing the opening and +closing delimiters is a multiple of 3 unless +both lengths are multiples of 3. + + +For the same reason, we don't get two consecutive +emphasis sections in this example: + +```````````````````````````````` example +*foo**bar* +. +<p><em>foo**bar</em></p> +```````````````````````````````` + + +The same condition ensures that the following +cases are all strong emphasis nested inside +emphasis, even when the interior whitespace is +omitted: + + +```````````````````````````````` example +***foo** bar* +. +<p><em><strong>foo</strong> bar</em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo **bar*** +. +<p><em>foo <strong>bar</strong></em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo**bar*** +. +<p><em>foo<strong>bar</strong></em></p> +```````````````````````````````` + + +When the lengths of the interior closing and opening +delimiter runs are *both* multiples of 3, though, +they can match to create emphasis: + +```````````````````````````````` example +foo***bar***baz +. +<p>foo<em><strong>bar</strong></em>baz</p> +```````````````````````````````` + +```````````````````````````````` example +foo******bar*********baz +. +<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p> +```````````````````````````````` + + +Indefinite levels of nesting are possible: + +```````````````````````````````` example +*foo **bar *baz* bim** bop* +. +<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo [*bar*](/url)* +. +<p><em>foo <a href="/url"><em>bar</em></a></em></p> +```````````````````````````````` + + +There can be no empty emphasis or strong emphasis: + +```````````````````````````````` example +** is not an empty emphasis +. +<p>** is not an empty emphasis</p> +```````````````````````````````` + + +```````````````````````````````` example +**** is not an empty strong emphasis +. +<p>**** is not an empty strong emphasis</p> +```````````````````````````````` + + + +Rule 10: + +Any nonempty sequence of inline elements can be the contents of an +strongly emphasized span. + +```````````````````````````````` example +**foo [bar](/url)** +. +<p><strong>foo <a href="/url">bar</a></strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo +bar** +. +<p><strong>foo +bar</strong></p> +```````````````````````````````` + + +In particular, emphasis and strong emphasis can be nested +inside strong emphasis: + +```````````````````````````````` example +__foo _bar_ baz__ +. +<p><strong>foo <em>bar</em> baz</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +__foo __bar__ baz__ +. +<p><strong>foo <strong>bar</strong> baz</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +____foo__ bar__ +. +<p><strong><strong>foo</strong> bar</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo **bar**** +. +<p><strong>foo <strong>bar</strong></strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo *bar* baz** +. +<p><strong>foo <em>bar</em> baz</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo*bar*baz** +. +<p><strong>foo<em>bar</em>baz</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +***foo* bar** +. +<p><strong><em>foo</em> bar</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo *bar*** +. +<p><strong>foo <em>bar</em></strong></p> +```````````````````````````````` + + +Indefinite levels of nesting are possible: + +```````````````````````````````` example +**foo *bar **baz** +bim* bop** +. +<p><strong>foo <em>bar <strong>baz</strong> +bim</em> bop</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo [*bar*](/url)** +. +<p><strong>foo <a href="/url"><em>bar</em></a></strong></p> +```````````````````````````````` + + +There can be no empty emphasis or strong emphasis: + +```````````````````````````````` example +__ is not an empty emphasis +. +<p>__ is not an empty emphasis</p> +```````````````````````````````` + + +```````````````````````````````` example +____ is not an empty strong emphasis +. +<p>____ is not an empty strong emphasis</p> +```````````````````````````````` + + + +Rule 11: + +```````````````````````````````` example +foo *** +. +<p>foo ***</p> +```````````````````````````````` + + +```````````````````````````````` example +foo *\** +. +<p>foo <em>*</em></p> +```````````````````````````````` + + +```````````````````````````````` example +foo *_* +. +<p>foo <em>_</em></p> +```````````````````````````````` + + +```````````````````````````````` example +foo ***** +. +<p>foo *****</p> +```````````````````````````````` + + +```````````````````````````````` example +foo **\*** +. +<p>foo <strong>*</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +foo **_** +. +<p>foo <strong>_</strong></p> +```````````````````````````````` + + +Note that when delimiters do not match evenly, Rule 11 determines +that the excess literal `*` characters will appear outside of the +emphasis, rather than inside it: + +```````````````````````````````` example +**foo* +. +<p>*<em>foo</em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo** +. +<p><em>foo</em>*</p> +```````````````````````````````` + + +```````````````````````````````` example +***foo** +. +<p>*<strong>foo</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +****foo* +. +<p>***<em>foo</em></p> +```````````````````````````````` + + +```````````````````````````````` example +**foo*** +. +<p><strong>foo</strong>*</p> +```````````````````````````````` + + +```````````````````````````````` example +*foo**** +. +<p><em>foo</em>***</p> +```````````````````````````````` + + + +Rule 12: + +```````````````````````````````` example +foo ___ +. +<p>foo ___</p> +```````````````````````````````` + + +```````````````````````````````` example +foo _\__ +. +<p>foo <em>_</em></p> +```````````````````````````````` + + +```````````````````````````````` example +foo _*_ +. +<p>foo <em>*</em></p> +```````````````````````````````` + + +```````````````````````````````` example +foo _____ +. +<p>foo _____</p> +```````````````````````````````` + + +```````````````````````````````` example +foo __\___ +. +<p>foo <strong>_</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +foo __*__ +. +<p>foo <strong>*</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +__foo_ +. +<p>_<em>foo</em></p> +```````````````````````````````` + + +Note that when delimiters do not match evenly, Rule 12 determines +that the excess literal `_` characters will appear outside of the +emphasis, rather than inside it: + +```````````````````````````````` example +_foo__ +. +<p><em>foo</em>_</p> +```````````````````````````````` + + +```````````````````````````````` example +___foo__ +. +<p>_<strong>foo</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +____foo_ +. +<p>___<em>foo</em></p> +```````````````````````````````` + + +```````````````````````````````` example +__foo___ +. +<p><strong>foo</strong>_</p> +```````````````````````````````` + + +```````````````````````````````` example +_foo____ +. +<p><em>foo</em>___</p> +```````````````````````````````` + + +Rule 13 implies that if you want emphasis nested directly inside +emphasis, you must use different delimiters: + +```````````````````````````````` example +**foo** +. +<p><strong>foo</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +*_foo_* +. +<p><em><em>foo</em></em></p> +```````````````````````````````` + + +```````````````````````````````` example +__foo__ +. +<p><strong>foo</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +_*foo*_ +. +<p><em><em>foo</em></em></p> +```````````````````````````````` + + +However, strong emphasis within strong emphasis is possible without +switching delimiters: + +```````````````````````````````` example +****foo**** +. +<p><strong><strong>foo</strong></strong></p> +```````````````````````````````` + + +```````````````````````````````` example +____foo____ +. +<p><strong><strong>foo</strong></strong></p> +```````````````````````````````` + + + +Rule 13 can be applied to arbitrarily long sequences of +delimiters: + +```````````````````````````````` example +******foo****** +. +<p><strong><strong><strong>foo</strong></strong></strong></p> +```````````````````````````````` + + +Rule 14: + +```````````````````````````````` example +***foo*** +. +<p><em><strong>foo</strong></em></p> +```````````````````````````````` + + +```````````````````````````````` example +_____foo_____ +. +<p><em><strong><strong>foo</strong></strong></em></p> +```````````````````````````````` + + +Rule 15: + +```````````````````````````````` example +*foo _bar* baz_ +. +<p><em>foo _bar</em> baz_</p> +```````````````````````````````` + + +```````````````````````````````` example +*foo __bar *baz bim__ bam* +. +<p><em>foo <strong>bar *baz bim</strong> bam</em></p> +```````````````````````````````` + + +Rule 16: + +```````````````````````````````` example +**foo **bar baz** +. +<p>**foo <strong>bar baz</strong></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo *bar baz* +. +<p>*foo <em>bar baz</em></p> +```````````````````````````````` + + +Rule 17: + +```````````````````````````````` example +*[bar*](/url) +. +<p>*<a href="/url">bar*</a></p> +```````````````````````````````` + + +```````````````````````````````` example +_foo [bar_](/url) +. +<p>_foo <a href="/url">bar_</a></p> +```````````````````````````````` + + +```````````````````````````````` example +*<img src="foo" title="*"/> +. +<p>*<img src="foo" title="*"/></p> +```````````````````````````````` + + +```````````````````````````````` example +**<a href="**"> +. +<p>**<a href="**"></p> +```````````````````````````````` + + +```````````````````````````````` example +__<a href="__"> +. +<p>__<a href="__"></p> +```````````````````````````````` + + +```````````````````````````````` example +*a `*`* +. +<p><em>a <code>*</code></em></p> +```````````````````````````````` + + +```````````````````````````````` example +_a `_`_ +. +<p><em>a <code>_</code></em></p> +```````````````````````````````` + + +```````````````````````````````` example +**a<http://foo.bar/?q=**> +. +<p>**a<a href="http://foo.bar/?q=**">http://foo.bar/?q=**</a></p> +```````````````````````````````` + + +```````````````````````````````` example +__a<http://foo.bar/?q=__> +. +<p>__a<a href="http://foo.bar/?q=__">http://foo.bar/?q=__</a></p> +```````````````````````````````` + + + +## Links + +A link contains [link text] (the visible text), a [link destination] +(the URI that is the link destination), and optionally a [link title]. +There are two basic kinds of links in Markdown. In [inline links] the +destination and title are given immediately after the link text. In +[reference links] the destination and title are defined elsewhere in +the document. + +A [link text](@) consists of a sequence of zero or more +inline elements enclosed by square brackets (`[` and `]`). The +following rules apply: + +- Links may not contain other links, at any level of nesting. If + multiple otherwise valid link definitions appear nested inside each + other, the inner-most definition is used. + +- Brackets are allowed in the [link text] only if (a) they + are backslash-escaped or (b) they appear as a matched pair of brackets, + with an open bracket `[`, a sequence of zero or more inlines, and + a close bracket `]`. + +- Backtick [code spans], [autolinks], and raw [HTML tags] bind more tightly + than the brackets in link text. Thus, for example, + `` [foo`]` `` could not be a link text, since the second `]` + is part of a code span. + +- The brackets in link text bind more tightly than markers for + [emphasis and strong emphasis]. Thus, for example, `*[foo*](url)` is a link. + +A [link destination](@) consists of either + +- a sequence of zero or more characters between an opening `<` and a + closing `>` that contains no line endings or unescaped + `<` or `>` characters, or + +- a nonempty sequence of characters that does not start with `<`, + does not include [ASCII control characters][ASCII control character] + or [space] character, and includes parentheses only if (a) they are + backslash-escaped or (b) they are part of a balanced pair of + unescaped parentheses. + (Implementations may impose limits on parentheses nesting to + avoid performance issues, but at least three levels of nesting + should be supported.) + +A [link title](@) consists of either + +- a sequence of zero or more characters between straight double-quote + characters (`"`), including a `"` character only if it is + backslash-escaped, or + +- a sequence of zero or more characters between straight single-quote + characters (`'`), including a `'` character only if it is + backslash-escaped, or + +- a sequence of zero or more characters between matching parentheses + (`(...)`), including a `(` or `)` character only if it is + backslash-escaped. + +Although [link titles] may span multiple lines, they may not contain +a [blank line]. + +An [inline link](@) consists of a [link text] followed immediately +by a left parenthesis `(`, an optional [link destination], an optional +[link title], and a right parenthesis `)`. +These four components may be separated by spaces, tabs, and up to one line +ending. +If both [link destination] and [link title] are present, they *must* be +separated by spaces, tabs, and up to one line ending. + +The link's text consists of the inlines contained +in the [link text] (excluding the enclosing square brackets). +The link's URI consists of the link destination, excluding enclosing +`<...>` if present, with backslash-escapes in effect as described +above. The link's title consists of the link title, excluding its +enclosing delimiters, with backslash-escapes in effect as described +above. + +Here is a simple inline link: + +```````````````````````````````` example +[link](/uri "title") +. +<p><a href="/uri" title="title">link</a></p> +```````````````````````````````` + + +The title, the link text and even +the destination may be omitted: + +```````````````````````````````` example +[link](/uri) +. +<p><a href="/uri">link</a></p> +```````````````````````````````` + +```````````````````````````````` example +[](./target.md) +. +<p><a href="./target.md"></a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link]() +. +<p><a href="">link</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link](<>) +. +<p><a href="">link</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[]() +. +<p><a href=""></a></p> +```````````````````````````````` + +The destination can only contain spaces if it is +enclosed in pointy brackets: + +```````````````````````````````` example +[link](/my uri) +. +<p>[link](/my uri)</p> +```````````````````````````````` + +```````````````````````````````` example +[link](</my uri>) +. +<p><a href="/my%20uri">link</a></p> +```````````````````````````````` + +The destination cannot contain line endings, +even if enclosed in pointy brackets: + +```````````````````````````````` example +[link](foo +bar) +. +<p>[link](foo +bar)</p> +```````````````````````````````` + +```````````````````````````````` example +[link](<foo +bar>) +. +<p>[link](<foo +bar>)</p> +```````````````````````````````` + +The destination can contain `)` if it is enclosed +in pointy brackets: + +```````````````````````````````` example +[a](<b)c>) +. +<p><a href="b)c">a</a></p> +```````````````````````````````` + +Pointy brackets that enclose links must be unescaped: + +```````````````````````````````` example +[link](<foo\>) +. +<p>[link](<foo>)</p> +```````````````````````````````` + +These are not links, because the opening pointy bracket +is not matched properly: + +```````````````````````````````` example +[a](<b)c +[a](<b)c> +[a](<b>c) +. +<p>[a](<b)c +[a](<b)c> +[a](<b>c)</p> +```````````````````````````````` + +Parentheses inside the link destination may be escaped: + +```````````````````````````````` example +[link](\(foo\)) +. +<p><a href="(foo)">link</a></p> +```````````````````````````````` + +Any number of parentheses are allowed without escaping, as long as they are +balanced: + +```````````````````````````````` example +[link](foo(and(bar))) +. +<p><a href="foo(and(bar))">link</a></p> +```````````````````````````````` + +However, if you have unbalanced parentheses, you need to escape or use the +`<...>` form: + +```````````````````````````````` example +[link](foo(and(bar)) +. +<p>[link](foo(and(bar))</p> +```````````````````````````````` + + +```````````````````````````````` example +[link](foo\(and\(bar\)) +. +<p><a href="foo(and(bar)">link</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link](<foo(and(bar)>) +. +<p><a href="foo(and(bar)">link</a></p> +```````````````````````````````` + + +Parentheses and other symbols can also be escaped, as usual +in Markdown: + +```````````````````````````````` example +[link](foo\)\:) +. +<p><a href="foo):">link</a></p> +```````````````````````````````` + + +A link can contain fragment identifiers and queries: + +```````````````````````````````` example +[link](#fragment) + +[link](http://example.com#fragment) + +[link](http://example.com?foo=3#frag) +. +<p><a href="#fragment">link</a></p> +<p><a href="http://example.com#fragment">link</a></p> +<p><a href="http://example.com?foo=3#frag">link</a></p> +```````````````````````````````` + + +Note that a backslash before a non-escapable character is +just a backslash: + +```````````````````````````````` example +[link](foo\bar) +. +<p><a href="foo%5Cbar">link</a></p> +```````````````````````````````` + + +URL-escaping should be left alone inside the destination, as all +URL-escaped characters are also valid URL characters. Entity and +numerical character references in the destination will be parsed +into the corresponding Unicode code points, as usual. These may +be optionally URL-escaped when written as HTML, but this spec +does not enforce any particular policy for rendering URLs in +HTML or other formats. Renderers may make different decisions +about how to escape or normalize URLs in the output. + +```````````````````````````````` example +[link](foo%20bä) +. +<p><a href="foo%20b%C3%A4">link</a></p> +```````````````````````````````` + + +Note that, because titles can often be parsed as destinations, +if you try to omit the destination and keep the title, you'll +get unexpected results: + +```````````````````````````````` example +[link]("title") +. +<p><a href="%22title%22">link</a></p> +```````````````````````````````` + + +Titles may be in single quotes, double quotes, or parentheses: + +```````````````````````````````` example +[link](/url "title") +[link](/url 'title') +[link](/url (title)) +. +<p><a href="/url" title="title">link</a> +<a href="/url" title="title">link</a> +<a href="/url" title="title">link</a></p> +```````````````````````````````` + + +Backslash escapes and entity and numeric character references +may be used in titles: + +```````````````````````````````` example +[link](/url "title \""") +. +<p><a href="/url" title="title """>link</a></p> +```````````````````````````````` + + +Titles must be separated from the link using spaces, tabs, and up to one line +ending. +Other [Unicode whitespace] like non-breaking space doesn't work. + +```````````````````````````````` example +[link](/url "title") +. +<p><a href="/url%C2%A0%22title%22">link</a></p> +```````````````````````````````` + + +Nested balanced quotes are not allowed without escaping: + +```````````````````````````````` example +[link](/url "title "and" title") +. +<p>[link](/url "title "and" title")</p> +```````````````````````````````` + + +But it is easy to work around this by using a different quote type: + +```````````````````````````````` example +[link](/url 'title "and" title') +. +<p><a href="/url" title="title "and" title">link</a></p> +```````````````````````````````` + + +(Note: `Markdown.pl` did allow double quotes inside a double-quoted +title, and its test suite included a test demonstrating this. +But it is hard to see a good rationale for the extra complexity this +brings, since there are already many ways---backslash escaping, +entity and numeric character references, or using a different +quote type for the enclosing title---to write titles containing +double quotes. `Markdown.pl`'s handling of titles has a number +of other strange features. For example, it allows single-quoted +titles in inline links, but not reference links. And, in +reference links but not inline links, it allows a title to begin +with `"` and end with `)`. `Markdown.pl` 1.0.1 even allows +titles with no closing quotation mark, though 1.0.2b8 does not. +It seems preferable to adopt a simple, rational rule that works +the same way in inline links and link reference definitions.) + +Spaces, tabs, and up to one line ending is allowed around the destination and +title: + +```````````````````````````````` example +[link]( /uri + "title" ) +. +<p><a href="/uri" title="title">link</a></p> +```````````````````````````````` + + +But it is not allowed between the link text and the +following parenthesis: + +```````````````````````````````` example +[link] (/uri) +. +<p>[link] (/uri)</p> +```````````````````````````````` + + +The link text may contain balanced brackets, but not unbalanced ones, +unless they are escaped: + +```````````````````````````````` example +[link [foo [bar]]](/uri) +. +<p><a href="/uri">link [foo [bar]]</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link] bar](/uri) +. +<p>[link] bar](/uri)</p> +```````````````````````````````` + + +```````````````````````````````` example +[link [bar](/uri) +. +<p>[link <a href="/uri">bar</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link \[bar](/uri) +. +<p><a href="/uri">link [bar</a></p> +```````````````````````````````` + + +The link text may contain inline content: + +```````````````````````````````` example +[link *foo **bar** `#`*](/uri) +. +<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p> +```````````````````````````````` + + +```````````````````````````````` example +[](/uri) +. +<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p> +```````````````````````````````` + + +However, links may not contain other links, at any level of nesting. + +```````````````````````````````` example +[foo [bar](/uri)](/uri) +. +<p>[foo <a href="/uri">bar</a>](/uri)</p> +```````````````````````````````` + + +```````````````````````````````` example +[foo *[bar [baz](/uri)](/uri)*](/uri) +. +<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p> +```````````````````````````````` + + +```````````````````````````````` example +](uri2)](uri3) +. +<p><img src="uri3" alt="[foo](uri2)" /></p> +```````````````````````````````` + + +These cases illustrate the precedence of link text grouping over +emphasis grouping: + +```````````````````````````````` example +*[foo*](/uri) +. +<p>*<a href="/uri">foo*</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo *bar](baz*) +. +<p><a href="baz*">foo *bar</a></p> +```````````````````````````````` + + +Note that brackets that *aren't* part of links do not take +precedence: + +```````````````````````````````` example +*foo [bar* baz] +. +<p><em>foo [bar</em> baz]</p> +```````````````````````````````` + + +These cases illustrate the precedence of HTML tags, code spans, +and autolinks over link grouping: + +```````````````````````````````` example +[foo <bar attr="](baz)"> +. +<p>[foo <bar attr="](baz)"></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo`](/uri)` +. +<p>[foo<code>](/uri)</code></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo<http://example.com/?search=](uri)> +. +<p>[foo<a href="http://example.com/?search=%5D(uri)">http://example.com/?search=](uri)</a></p> +```````````````````````````````` + + +There are three kinds of [reference link](@)s: +[full](#full-reference-link), [collapsed](#collapsed-reference-link), +and [shortcut](#shortcut-reference-link). + +A [full reference link](@) +consists of a [link text] immediately followed by a [link label] +that [matches] a [link reference definition] elsewhere in the document. + +A [link label](@) begins with a left bracket (`[`) and ends +with the first right bracket (`]`) that is not backslash-escaped. +Between these brackets there must be at least one character that is not a space, +tab, or line ending. +Unescaped square bracket characters are not allowed inside the +opening and closing square brackets of [link labels]. A link +label can have at most 999 characters inside the square +brackets. + +One label [matches](@) +another just in case their normalized forms are equal. To normalize a +label, strip off the opening and closing brackets, +perform the *Unicode case fold*, strip leading and trailing +spaces, tabs, and line endings, and collapse consecutive internal +spaces, tabs, and line endings to a single space. If there are multiple +matching reference link definitions, the one that comes first in the +document is used. (It is desirable in such cases to emit a warning.) + +The link's URI and title are provided by the matching [link +reference definition]. + +Here is a simple example: + +```````````````````````````````` example +[foo][bar] + +[bar]: /url "title" +. +<p><a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +The rules for the [link text] are the same as with +[inline links]. Thus: + +The link text may contain balanced brackets, but not unbalanced ones, +unless they are escaped: + +```````````````````````````````` example +[link [foo [bar]]][ref] + +[ref]: /uri +. +<p><a href="/uri">link [foo [bar]]</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[link \[bar][ref] + +[ref]: /uri +. +<p><a href="/uri">link [bar</a></p> +```````````````````````````````` + + +The link text may contain inline content: + +```````````````````````````````` example +[link *foo **bar** `#`*][ref] + +[ref]: /uri +. +<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p> +```````````````````````````````` + + +```````````````````````````````` example +[][ref] + +[ref]: /uri +. +<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p> +```````````````````````````````` + + +However, links may not contain other links, at any level of nesting. + +```````````````````````````````` example +[foo [bar](/uri)][ref] + +[ref]: /uri +. +<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo *bar [baz][ref]*][ref] + +[ref]: /uri +. +<p>[foo <em>bar <a href="/uri">baz</a></em>]<a href="/uri">ref</a></p> +```````````````````````````````` + + +(In the examples above, we have two [shortcut reference links] +instead of one [full reference link].) + +The following cases illustrate the precedence of link text grouping over +emphasis grouping: + +```````````````````````````````` example +*[foo*][ref] + +[ref]: /uri +. +<p>*<a href="/uri">foo*</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo *bar][ref]* + +[ref]: /uri +. +<p><a href="/uri">foo *bar</a>*</p> +```````````````````````````````` + + +These cases illustrate the precedence of HTML tags, code spans, +and autolinks over link grouping: + +```````````````````````````````` example +[foo <bar attr="][ref]"> + +[ref]: /uri +. +<p>[foo <bar attr="][ref]"></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo`][ref]` + +[ref]: /uri +. +<p>[foo<code>][ref]</code></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo<http://example.com/?search=][ref]> + +[ref]: /uri +. +<p>[foo<a href="http://example.com/?search=%5D%5Bref%5D">http://example.com/?search=][ref]</a></p> +```````````````````````````````` + + +Matching is case-insensitive: + +```````````````````````````````` example +[foo][BaR] + +[bar]: /url "title" +. +<p><a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +Unicode case fold is used: + +```````````````````````````````` example +[ẞ] + +[SS]: /url +. +<p><a href="/url">ẞ</a></p> +```````````````````````````````` + + +Consecutive internal spaces, tabs, and line endings are treated as one space for +purposes of determining matching: + +```````````````````````````````` example +[Foo + bar]: /url + +[Baz][Foo bar] +. +<p><a href="/url">Baz</a></p> +```````````````````````````````` + + +No spaces, tabs, or line endings are allowed between the [link text] and the +[link label]: + +```````````````````````````````` example +[foo] [bar] + +[bar]: /url "title" +. +<p>[foo] <a href="/url" title="title">bar</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[foo] +[bar] + +[bar]: /url "title" +. +<p>[foo] +<a href="/url" title="title">bar</a></p> +```````````````````````````````` + + +This is a departure from John Gruber's original Markdown syntax +description, which explicitly allows whitespace between the link +text and the link label. It brings reference links in line with +[inline links], which (according to both original Markdown and +this spec) cannot have whitespace after the link text. More +importantly, it prevents inadvertent capture of consecutive +[shortcut reference links]. If whitespace is allowed between the +link text and the link label, then in the following we will have +a single reference link, not two shortcut reference links, as +intended: + +``` markdown +[foo] +[bar] + +[foo]: /url1 +[bar]: /url2 +``` + +(Note that [shortcut reference links] were introduced by Gruber +himself in a beta version of `Markdown.pl`, but never included +in the official syntax description. Without shortcut reference +links, it is harmless to allow space between the link text and +link label; but once shortcut references are introduced, it is +too dangerous to allow this, as it frequently leads to +unintended results.) + +When there are multiple matching [link reference definitions], +the first is used: + +```````````````````````````````` example +[foo]: /url1 + +[foo]: /url2 + +[bar][foo] +. +<p><a href="/url1">bar</a></p> +```````````````````````````````` + + +Note that matching is performed on normalized strings, not parsed +inline content. So the following does not match, even though the +labels define equivalent inline content: + +```````````````````````````````` example +[bar][foo\!] + +[foo!]: /url +. +<p>[bar][foo!]</p> +```````````````````````````````` + + +[Link labels] cannot contain brackets, unless they are +backslash-escaped: + +```````````````````````````````` example +[foo][ref[] + +[ref[]: /uri +. +<p>[foo][ref[]</p> +<p>[ref[]: /uri</p> +```````````````````````````````` + + +```````````````````````````````` example +[foo][ref[bar]] + +[ref[bar]]: /uri +. +<p>[foo][ref[bar]]</p> +<p>[ref[bar]]: /uri</p> +```````````````````````````````` + + +```````````````````````````````` example +[[[foo]]] + +[[[foo]]]: /url +. +<p>[[[foo]]]</p> +<p>[[[foo]]]: /url</p> +```````````````````````````````` + + +```````````````````````````````` example +[foo][ref\[] + +[ref\[]: /uri +. +<p><a href="/uri">foo</a></p> +```````````````````````````````` + + +Note that in this example `]` is not backslash-escaped: + +```````````````````````````````` example +[bar\\]: /uri + +[bar\\] +. +<p><a href="/uri">bar\</a></p> +```````````````````````````````` + + +A [link label] must contain at least one character that is not a space, tab, or +line ending: + +```````````````````````````````` example +[] + +[]: /uri +. +<p>[]</p> +<p>[]: /uri</p> +```````````````````````````````` + + +```````````````````````````````` example +[ + ] + +[ + ]: /uri +. +<p>[ +]</p> +<p>[ +]: /uri</p> +```````````````````````````````` + + +A [collapsed reference link](@) +consists of a [link label] that [matches] a +[link reference definition] elsewhere in the +document, followed by the string `[]`. +The contents of the first link label are parsed as inlines, +which are used as the link's text. The link's URI and title are +provided by the matching reference link definition. Thus, +`[foo][]` is equivalent to `[foo][foo]`. + +```````````````````````````````` example +[foo][] + +[foo]: /url "title" +. +<p><a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[*foo* bar][] + +[*foo* bar]: /url "title" +. +<p><a href="/url" title="title"><em>foo</em> bar</a></p> +```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +[Foo][] + +[foo]: /url "title" +. +<p><a href="/url" title="title">Foo</a></p> +```````````````````````````````` + + + +As with full reference links, spaces, tabs, or line endings are not +allowed between the two sets of brackets: + +```````````````````````````````` example +[foo] +[] + +[foo]: /url "title" +. +<p><a href="/url" title="title">foo</a> +[]</p> +```````````````````````````````` + + +A [shortcut reference link](@) +consists of a [link label] that [matches] a +[link reference definition] elsewhere in the +document and is not followed by `[]` or a link label. +The contents of the first link label are parsed as inlines, +which are used as the link's text. The link's URI and title +are provided by the matching link reference definition. +Thus, `[foo]` is equivalent to `[foo][]`. + +```````````````````````````````` example +[foo] + +[foo]: /url "title" +. +<p><a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[*foo* bar] + +[*foo* bar]: /url "title" +. +<p><a href="/url" title="title"><em>foo</em> bar</a></p> +```````````````````````````````` + + +```````````````````````````````` example +[[*foo* bar]] + +[*foo* bar]: /url "title" +. +<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p> +```````````````````````````````` + + +```````````````````````````````` example +[[bar [foo] + +[foo]: /url +. +<p>[[bar <a href="/url">foo</a></p> +```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +[Foo] + +[foo]: /url "title" +. +<p><a href="/url" title="title">Foo</a></p> +```````````````````````````````` + + +A space after the link text should be preserved: + +```````````````````````````````` example +[foo] bar + +[foo]: /url +. +<p><a href="/url">foo</a> bar</p> +```````````````````````````````` + + +If you just want bracketed text, you can backslash-escape the +opening bracket to avoid links: + +```````````````````````````````` example +\[foo] + +[foo]: /url "title" +. +<p>[foo]</p> +```````````````````````````````` + + +Note that this is a link, because a link label ends with the first +following closing bracket: + +```````````````````````````````` example +[foo*]: /url + +*[foo*] +. +<p>*<a href="/url">foo*</a></p> +```````````````````````````````` + + +Full and compact references take precedence over shortcut +references: + +```````````````````````````````` example +[foo][bar] + +[foo]: /url1 +[bar]: /url2 +. +<p><a href="/url2">foo</a></p> +```````````````````````````````` + +```````````````````````````````` example +[foo][] + +[foo]: /url1 +. +<p><a href="/url1">foo</a></p> +```````````````````````````````` + +Inline links also take precedence: + +```````````````````````````````` example +[foo]() + +[foo]: /url1 +. +<p><a href="">foo</a></p> +```````````````````````````````` + +```````````````````````````````` example +[foo](not a link) + +[foo]: /url1 +. +<p><a href="/url1">foo</a>(not a link)</p> +```````````````````````````````` + +In the following case `[bar][baz]` is parsed as a reference, +`[foo]` as normal text: + +```````````````````````````````` example +[foo][bar][baz] + +[baz]: /url +. +<p>[foo]<a href="/url">bar</a></p> +```````````````````````````````` + + +Here, though, `[foo][bar]` is parsed as a reference, since +`[bar]` is defined: + +```````````````````````````````` example +[foo][bar][baz] + +[baz]: /url1 +[bar]: /url2 +. +<p><a href="/url2">foo</a><a href="/url1">baz</a></p> +```````````````````````````````` + + +Here `[foo]` is not parsed as a shortcut reference, because it +is followed by a link label (even though `[bar]` is not defined): + +```````````````````````````````` example +[foo][bar][baz] + +[baz]: /url1 +[foo]: /url2 +. +<p>[foo]<a href="/url1">bar</a></p> +```````````````````````````````` + + + +## Images + +Syntax for images is like the syntax for links, with one +difference. Instead of [link text], we have an +[image description](@). The rules for this are the +same as for [link text], except that (a) an +image description starts with ` +. +<p><img src="/url" alt="foo" title="title" /></p> +```````````````````````````````` + + +```````````````````````````````` example +![foo *bar*] + +[foo *bar*]: train.jpg "train & tracks" +. +<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p> +```````````````````````````````` + + +```````````````````````````````` example +](/url2) +. +<p><img src="/url2" alt="foo bar" /></p> +```````````````````````````````` + + +```````````````````````````````` example +](/url2) +. +<p><img src="/url2" alt="foo bar" /></p> +```````````````````````````````` + + +Though this spec is concerned with parsing, not rendering, it is +recommended that in rendering to HTML, only the plain string content +of the [image description] be used. Note that in +the above example, the alt attribute's value is `foo bar`, not `foo +[bar](/url)` or `foo <a href="/url">bar</a>`. Only the plain string +content is rendered, without formatting. + +```````````````````````````````` example +![foo *bar*][] + +[foo *bar*]: train.jpg "train & tracks" +. +<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p> +```````````````````````````````` + + +```````````````````````````````` example +![foo *bar*][foobar] + +[FOOBAR]: train.jpg "train & tracks" +. +<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p> +```````````````````````````````` + + +```````````````````````````````` example + +. +<p><img src="train.jpg" alt="foo" /></p> +```````````````````````````````` + + +```````````````````````````````` example +My  +. +<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p> +```````````````````````````````` + + +```````````````````````````````` example + +. +<p><img src="url" alt="foo" /></p> +```````````````````````````````` + + +```````````````````````````````` example + +. +<p><img src="/url" alt="" /></p> +```````````````````````````````` + + +Reference-style: + +```````````````````````````````` example +![foo][bar] + +[bar]: /url +. +<p><img src="/url" alt="foo" /></p> +```````````````````````````````` + + +```````````````````````````````` example +![foo][bar] + +[BAR]: /url +. +<p><img src="/url" alt="foo" /></p> +```````````````````````````````` + + +Collapsed: + +```````````````````````````````` example +![foo][] + +[foo]: /url "title" +. +<p><img src="/url" alt="foo" title="title" /></p> +```````````````````````````````` + + +```````````````````````````````` example +![*foo* bar][] + +[*foo* bar]: /url "title" +. +<p><img src="/url" alt="foo bar" title="title" /></p> +```````````````````````````````` + + +The labels are case-insensitive: + +```````````````````````````````` example +![Foo][] + +[foo]: /url "title" +. +<p><img src="/url" alt="Foo" title="title" /></p> +```````````````````````````````` + + +As with reference links, spaces, tabs, and line endings, are not allowed +between the two sets of brackets: + +```````````````````````````````` example +![foo] +[] + +[foo]: /url "title" +. +<p><img src="/url" alt="foo" title="title" /> +[]</p> +```````````````````````````````` + + +Shortcut: + +```````````````````````````````` example +![foo] + +[foo]: /url "title" +. +<p><img src="/url" alt="foo" title="title" /></p> +```````````````````````````````` + + +```````````````````````````````` example +![*foo* bar] + +[*foo* bar]: /url "title" +. +<p><img src="/url" alt="foo bar" title="title" /></p> +```````````````````````````````` + + +Note that link labels cannot contain unescaped brackets: + +```````````````````````````````` example +![[foo]] + +[[foo]]: /url "title" +. +<p>![[foo]]</p> +<p>[[foo]]: /url "title"</p> +```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +![Foo] + +[foo]: /url "title" +. +<p><img src="/url" alt="Foo" title="title" /></p> +```````````````````````````````` + + +If you just want a literal `!` followed by bracketed text, you can +backslash-escape the opening `[`: + +```````````````````````````````` example +!\[foo] + +[foo]: /url "title" +. +<p>![foo]</p> +```````````````````````````````` + + +If you want a link after a literal `!`, backslash-escape the +`!`: + +```````````````````````````````` example +\![foo] + +[foo]: /url "title" +. +<p>!<a href="/url" title="title">foo</a></p> +```````````````````````````````` + + +## Autolinks + +[Autolink](@)s are absolute URIs and email addresses inside +`<` and `>`. They are parsed as links, with the URL or email address +as the link label. + +A [URI autolink](@) consists of `<`, followed by an +[absolute URI] followed by `>`. It is parsed as +a link to the URI, with the URI as the link's label. + +An [absolute URI](@), +for these purposes, consists of a [scheme] followed by a colon (`:`) +followed by zero or more characters other [ASCII control +characters][ASCII control character], [space], `<`, and `>`. +If the URI includes these characters, they must be percent-encoded +(e.g. `%20` for a space). + +For purposes of this spec, a [scheme](@) is any sequence +of 2--32 characters beginning with an ASCII letter and followed +by any combination of ASCII letters, digits, or the symbols plus +("+"), period ("."), or hyphen ("-"). + +Here are some valid autolinks: + +```````````````````````````````` example +<http://foo.bar.baz> +. +<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<http://foo.bar.baz/test?q=hello&id=22&boolean> +. +<p><a href="http://foo.bar.baz/test?q=hello&id=22&boolean">http://foo.bar.baz/test?q=hello&id=22&boolean</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<irc://foo.bar:2233/baz> +. +<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p> +```````````````````````````````` + + +Uppercase is also fine: + +```````````````````````````````` example +<MAILTO:FOO@BAR.BAZ> +. +<p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p> +```````````````````````````````` + + +Note that many strings that count as [absolute URIs] for +purposes of this spec are not valid URIs, because their +schemes are not registered or because of other problems +with their syntax: + +```````````````````````````````` example +<a+b+c:d> +. +<p><a href="a+b+c:d">a+b+c:d</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<made-up-scheme://foo,bar> +. +<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<http://../> +. +<p><a href="http://../">http://../</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<localhost:5001/foo> +. +<p><a href="localhost:5001/foo">localhost:5001/foo</a></p> +```````````````````````````````` + + +Spaces are not allowed in autolinks: + +```````````````````````````````` example +<http://foo.bar/baz bim> +. +<p><http://foo.bar/baz bim></p> +```````````````````````````````` + + +Backslash-escapes do not work inside autolinks: + +```````````````````````````````` example +<http://example.com/\[\> +. +<p><a href="http://example.com/%5C%5B%5C">http://example.com/\[\</a></p> +```````````````````````````````` + + +An [email autolink](@) +consists of `<`, followed by an [email address], +followed by `>`. The link's label is the email address, +and the URL is `mailto:` followed by the email address. + +An [email address](@), +for these purposes, is anything that matches +the [non-normative regex from the HTML5 +spec](https://html.spec.whatwg.org/multipage/forms.html#e-mail-state-(type=email)): + + /^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])? + (?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/ + +Examples of email autolinks: + +```````````````````````````````` example +<foo@bar.example.com> +. +<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p> +```````````````````````````````` + + +```````````````````````````````` example +<foo+special@Bar.baz-bar0.com> +. +<p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p> +```````````````````````````````` + + +Backslash-escapes do not work inside email autolinks: + +```````````````````````````````` example +<foo\+@bar.example.com> +. +<p><foo+@bar.example.com></p> +```````````````````````````````` + + +These are not autolinks: + +```````````````````````````````` example +<> +. +<p><></p> +```````````````````````````````` + + +```````````````````````````````` example +< http://foo.bar > +. +<p>< http://foo.bar ></p> +```````````````````````````````` + + +```````````````````````````````` example +<m:abc> +. +<p><m:abc></p> +```````````````````````````````` + + +```````````````````````````````` example +<foo.bar.baz> +. +<p><foo.bar.baz></p> +```````````````````````````````` + + +```````````````````````````````` example +http://example.com +. +<p>http://example.com</p> +```````````````````````````````` + + +```````````````````````````````` example +foo@bar.example.com +. +<p>foo@bar.example.com</p> +```````````````````````````````` + + +## Raw HTML + +Text between `<` and `>` that looks like an HTML tag is parsed as a +raw HTML tag and will be rendered in HTML without escaping. +Tag and attribute names are not limited to current HTML tags, +so custom tags (and even, say, DocBook tags) may be used. + +Here is the grammar for tags: + +A [tag name](@) consists of an ASCII letter +followed by zero or more ASCII letters, digits, or +hyphens (`-`). + +An [attribute](@) consists of spaces, tabs, and up to one line ending, +an [attribute name], and an optional +[attribute value specification]. + +An [attribute name](@) +consists of an ASCII letter, `_`, or `:`, followed by zero or more ASCII +letters, digits, `_`, `.`, `:`, or `-`. (Note: This is the XML +specification restricted to ASCII. HTML5 is laxer.) + +An [attribute value specification](@) +consists of optional spaces, tabs, and up to one line ending, +a `=` character, optional spaces, tabs, and up to one line ending, +and an [attribute value]. + +An [attribute value](@) +consists of an [unquoted attribute value], +a [single-quoted attribute value], or a [double-quoted attribute value]. + +An [unquoted attribute value](@) +is a nonempty string of characters not +including spaces, tabs, line endings, `"`, `'`, `=`, `<`, `>`, or `` ` ``. + +A [single-quoted attribute value](@) +consists of `'`, zero or more +characters not including `'`, and a final `'`. + +A [double-quoted attribute value](@) +consists of `"`, zero or more +characters not including `"`, and a final `"`. + +An [open tag](@) consists of a `<` character, a [tag name], +zero or more [attributes], optional spaces, tabs, and up to one line ending, +an optional `/` character, and a `>` character. + +A [closing tag](@) consists of the string `</`, a +[tag name], optional spaces, tabs, and up to one line ending, and the character +`>`. + +An [HTML comment](@) consists of `<!--` + *text* + `-->`, +where *text* does not start with `>` or `->`, does not end with `-`, +and does not contain `--`. (See the +[HTML5 spec](http://www.w3.org/TR/html5/syntax.html#comments).) + +A [processing instruction](@) +consists of the string `<?`, a string +of characters not including the string `?>`, and the string +`?>`. + +A [declaration](@) consists of the string `<!`, an ASCII letter, zero or more +characters not including the character `>`, and the character `>`. + +A [CDATA section](@) consists of +the string `<![CDATA[`, a string of characters not including the string +`]]>`, and the string `]]>`. + +An [HTML tag](@) consists of an [open tag], a [closing tag], +an [HTML comment], a [processing instruction], a [declaration], +or a [CDATA section]. + +Here are some simple open tags: + +```````````````````````````````` example +<a><bab><c2c> +. +<p><a><bab><c2c></p> +```````````````````````````````` + + +Empty elements: + +```````````````````````````````` example +<a/><b2/> +. +<p><a/><b2/></p> +```````````````````````````````` + + +Whitespace is allowed: + +```````````````````````````````` example +<a /><b2 +data="foo" > +. +<p><a /><b2 +data="foo" ></p> +```````````````````````````````` + + +With attributes: + +```````````````````````````````` example +<a foo="bar" bam = 'baz <em>"</em>' +_boolean zoop:33=zoop:33 /> +. +<p><a foo="bar" bam = 'baz <em>"</em>' +_boolean zoop:33=zoop:33 /></p> +```````````````````````````````` + + +Custom tag names can be used: + +```````````````````````````````` example +Foo <responsive-image src="foo.jpg" /> +. +<p>Foo <responsive-image src="foo.jpg" /></p> +```````````````````````````````` + + +Illegal tag names, not parsed as HTML: + +```````````````````````````````` example +<33> <__> +. +<p><33> <__></p> +```````````````````````````````` + + +Illegal attribute names: + +```````````````````````````````` example +<a h*#ref="hi"> +. +<p><a h*#ref="hi"></p> +```````````````````````````````` + + +Illegal attribute values: + +```````````````````````````````` example +<a href="hi'> <a href=hi'> +. +<p><a href="hi'> <a href=hi'></p> +```````````````````````````````` + + +Illegal whitespace: + +```````````````````````````````` example +< a>< +foo><bar/ > +<foo bar=baz +bim!bop /> +. +<p>< a>< +foo><bar/ > +<foo bar=baz +bim!bop /></p> +```````````````````````````````` + + +Missing whitespace: + +```````````````````````````````` example +<a href='bar'title=title> +. +<p><a href='bar'title=title></p> +```````````````````````````````` + + +Closing tags: + +```````````````````````````````` example +</a></foo > +. +<p></a></foo ></p> +```````````````````````````````` + + +Illegal attributes in closing tag: + +```````````````````````````````` example +</a href="foo"> +. +<p></a href="foo"></p> +```````````````````````````````` + + +Comments: + +```````````````````````````````` example +foo <!-- this is a +comment - with hyphen --> +. +<p>foo <!-- this is a +comment - with hyphen --></p> +```````````````````````````````` + + +```````````````````````````````` example +foo <!-- not a comment -- two hyphens --> +. +<p>foo <!-- not a comment -- two hyphens --></p> +```````````````````````````````` + + +Not comments: + +```````````````````````````````` example +foo <!--> foo --> + +foo <!-- foo---> +. +<p>foo <!--> foo --></p> +<p>foo <!-- foo---></p> +```````````````````````````````` + + +Processing instructions: + +```````````````````````````````` example +foo <?php echo $a; ?> +. +<p>foo <?php echo $a; ?></p> +```````````````````````````````` + + +Declarations: + +```````````````````````````````` example +foo <!ELEMENT br EMPTY> +. +<p>foo <!ELEMENT br EMPTY></p> +```````````````````````````````` + + +CDATA sections: + +```````````````````````````````` example +foo <![CDATA[>&<]]> +. +<p>foo <![CDATA[>&<]]></p> +```````````````````````````````` + + +Entity and numeric character references are preserved in HTML +attributes: + +```````````````````````````````` example +foo <a href="ö"> +. +<p>foo <a href="ö"></p> +```````````````````````````````` + + +Backslash escapes do not work in HTML attributes: + +```````````````````````````````` example +foo <a href="\*"> +. +<p>foo <a href="\*"></p> +```````````````````````````````` + + +```````````````````````````````` example +<a href="\""> +. +<p><a href="""></p> +```````````````````````````````` + + +## Hard line breaks + +A line ending (not in a code span or HTML tag) that is preceded +by two or more spaces and does not occur at the end of a block +is parsed as a [hard line break](@) (rendered +in HTML as a `<br />` tag): + +```````````````````````````````` example +foo +baz +. +<p>foo<br /> +baz</p> +```````````````````````````````` + + +For a more visible alternative, a backslash before the +[line ending] may be used instead of two or more spaces: + +```````````````````````````````` example +foo\ +baz +. +<p>foo<br /> +baz</p> +```````````````````````````````` + + +More than two spaces can be used: + +```````````````````````````````` example +foo +baz +. +<p>foo<br /> +baz</p> +```````````````````````````````` + + +Leading spaces at the beginning of the next line are ignored: + +```````````````````````````````` example +foo + bar +. +<p>foo<br /> +bar</p> +```````````````````````````````` + + +```````````````````````````````` example +foo\ + bar +. +<p>foo<br /> +bar</p> +```````````````````````````````` + + +Hard line breaks can occur inside emphasis, links, and other constructs +that allow inline content: + +```````````````````````````````` example +*foo +bar* +. +<p><em>foo<br /> +bar</em></p> +```````````````````````````````` + + +```````````````````````````````` example +*foo\ +bar* +. +<p><em>foo<br /> +bar</em></p> +```````````````````````````````` + + +Hard line breaks do not occur inside code spans + +```````````````````````````````` example +`code +span` +. +<p><code>code span</code></p> +```````````````````````````````` + + +```````````````````````````````` example +`code\ +span` +. +<p><code>code\ span</code></p> +```````````````````````````````` + + +or HTML tags: + +```````````````````````````````` example +<a href="foo +bar"> +. +<p><a href="foo +bar"></p> +```````````````````````````````` + + +```````````````````````````````` example +<a href="foo\ +bar"> +. +<p><a href="foo\ +bar"></p> +```````````````````````````````` + + +Hard line breaks are for separating inline content within a block. +Neither syntax for hard line breaks works at the end of a paragraph or +other block element: + +```````````````````````````````` example +foo\ +. +<p>foo\</p> +```````````````````````````````` + + +```````````````````````````````` example +foo +. +<p>foo</p> +```````````````````````````````` + + +```````````````````````````````` example +### foo\ +. +<h3>foo\</h3> +```````````````````````````````` + + +```````````````````````````````` example +### foo +. +<h3>foo</h3> +```````````````````````````````` + + +## Soft line breaks + +A regular line ending (not in a code span or HTML tag) that is not +preceded by two or more spaces or a backslash is parsed as a +[softbreak](@). (A soft line break may be rendered in HTML either as a +[line ending] or as a space. The result will be the same in +browsers. In the examples here, a [line ending] will be used.) + +```````````````````````````````` example +foo +baz +. +<p>foo +baz</p> +```````````````````````````````` + + +Spaces at the end of the line and beginning of the next line are +removed: + +```````````````````````````````` example +foo + baz +. +<p>foo +baz</p> +```````````````````````````````` + + +A conforming parser may render a soft line break in HTML either as a +line ending or as a space. + +A renderer may also provide an option to render soft line breaks +as hard line breaks. + +## Textual content + +Any characters not given an interpretation by the above rules will +be parsed as plain textual content. + +```````````````````````````````` example +hello $.;'there +. +<p>hello $.;'there</p> +```````````````````````````````` + + +```````````````````````````````` example +Foo χρῆν +. +<p>Foo χρῆν</p> +```````````````````````````````` + + +Internal spaces are preserved verbatim: + +```````````````````````````````` example +Multiple spaces +. +<p>Multiple spaces</p> +```````````````````````````````` + + +<!-- END TESTS --> + +# 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: + +``` 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 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 container 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: + +``` markdown +> Lorem ipsum dolor +sit amet. +> - Qui *quodsi iracundia* +> - aliquando id +``` + +At the outset, our document model is just + +``` tree +-> document +``` + +The first line of our text, + +``` markdown +> 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`: + +``` tree +-> document + -> block_quote + -> paragraph + "Lorem ipsum dolor" +``` + +The next line, + +``` markdown +sit amet. +``` + +is a "lazy continuation" of the open `paragraph`, so it gets added +to the paragraph's text: + +``` tree +-> document + -> block_quote + -> paragraph + "Lorem ipsum dolor\nsit amet." +``` + +The third line, + +``` markdown +> - 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`: + +``` tree +-> 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, + +``` markdown +> - 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: + +``` 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. + +``` tree +document + block_quote + paragraph + str "Lorem ipsum dolor" + softbreak + str "sit amet." + list (type=bullet tight=true bullet_char=-) + list_item + paragraph + str "Qui " + emph + str "quodsi iracundia" + list_item + paragraph + str "aliquando id" +``` + +Notice how the [line ending] in the first paragraph has +been parsed as a `softbreak`, and the asterisks in the first list item +have become an `emph`. + +### An algorithm for parsing nested emphasis and links + +By far the trickiest part of inline parsing is handling emphasis, +strong emphasis, links, and images. This is done using the following +algorithm. + +When we're parsing inlines and we hit either + +- a run of `*` or `_` characters, or +- a `[` or `. + +The [delimiter stack] is a doubly linked list. Each +element contains a pointer to a text node, plus information about + +- the type of delimiter (`[`, `![`, `*`, `_`) +- the number of delimiters, +- whether the delimiter is "active" (all are active to start), and +- whether the delimiter is a potential opener, a potential closer, + or both (which depends on what sort of characters precede + and follow the delimiters). + +When we hit a `]` character, we call the *look for link or image* +procedure (see below). + +When we hit the end of the input, we call the *process emphasis* +procedure (see below), with `stack_bottom` = NULL. + +#### *look for link or image* + +Starting at the top of the delimiter stack, we look backwards +through the stack for an opening `[` or `![` delimiter. + +- If we don't find one, we return a literal text node `]`. + +- If we do find one, but it's not *active*, we remove the inactive + delimiter from the stack, and return a literal text node `]`. + +- If we find one and it's active, then we parse ahead to see if + we have an inline link/image, reference link/image, 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 (`*`, `_`), indexed to the length of the closing delimiter run +(modulo 3) and to whether the closing delimiter can also be an +opener. Initialize this to `stack_bottom`. + +Then we repeat the following until we run out of potential +closers: + +- Move `current_position` forward in the delimiter stack (if needed) + until we find the first potential closer with delimiter `*` or `_`. + (This will be the potential closer closest + to the beginning of the input -- the first one in parse order.) + +- Now, look back in the stack (staying above `stack_bottom` and + the `openers_bottom` for this delimiter type) for the + first matching potential opener ("matching" means same delimiter). + +- If one is found: + + + Figure out whether we have emphasis or strong emphasis: + if both closer and opener spans have length >= 2, we have + strong, otherwise regular. + + + Insert an emph or strong emph node accordingly, after + the text node corresponding to the opener. + + + Remove any delimiters between the opener and closer from + the delimiter stack. + + + Remove 1 (for regular emph) or 2 (for strong emph) delimiters + from the opening and closing text nodes. If they become empty + as a result, remove them and remove the corresponding element + of the delimiter stack. If the closing node is removed, reset + `current_position` to the next element in the stack. + +- If none is found: + + + Set `openers_bottom` to the element before `current_position`. + (We know that there are no openers for this kind of closer up to and + including this point, so this puts a lower bound on future searches.) + + + If the closer at `current_position` is not a potential opener, + remove it from the delimiter stack (since we know it can't + be a closer either). + + + Advance `current_position` to the next element in the stack. + +After we're done, we remove all delimiters above `stack_bottom` from the +delimiter stack. |