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当前版本 0.30 (2021-06-19) 作者: John MacFarlane 翻译: Yogurt_cry 使用有道翻译
Markdown 是一种用于编写结构化文档的纯文本格式，它基于在电子邮件和 usenet 帖子中指示格式的约定。它是由约翰·格鲁伯 (在Aaron Swartz的帮助下) 开发的，并于 2004 年以语法描述和 Perl 脚本 (Markdown.pl) 的形式发布，用于将 Markdown 转换为 HTML。在接下来的十年中，用多种语言开发了几十种实现。有些人用脚注、表和其他文档元素的约定扩展了原始的 Markdown 语法。有些允许 Markdown 文档以 HTML 以外的格式呈现。像 Reddit、StackOverflow 和 GitHub 这样的网站有数百万人使用 Markdown。Markdown 开始被用于网络之外的领域，如撰写书籍、文章、幻灯片、信件和课堂笔记。
Markdown 与许多其他轻量级标记语法(通常更容易编写)的区别在于其可读性。正如 Gruber 写道:
Markdown 格式语法的首要设计目标是使其尽可能具有可读性。其理念是，Markdown 格式的文档应该是可发布的，即纯文本，而不像它被标记了标记或格式指示。(http://daringfireball.net/projects/markdown/)
这一点可以通过比较 AsciiDoc 的样本和 Markdown 的等效样本来说明。以下是 AsciiDoc 手册中的 AsciiDoc 示例:
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.
--
下面是 Markdown 的示例:
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.
可以说，AsciiDoc 版本更容易编写。你不需要担心缩进。但是 Markdown 版本更容易阅读。列表项的嵌套在源代码中是显而易见的，而不仅仅是在处理过的文档中。
约翰·格鲁伯 对 Markdown 语法的规范描述并没有明确地说明。以下是一些它没有回答的问题:
- 子列表需要多少缩进?规范规定，延续段需要缩进4个空格，但对于子列表并没有完全明确规定。很自然地认为它们也必须缩进四个空格，但是 Markdown.pl 不要求这样做。这不是一个“极端情况”，在这个问题上实现之间的分歧经常会给实际文档中的用户带来意外。(参见约翰·格鲁伯的评论。)
- 在块引号或标题前需要空行吗?大多数实现不需要空行。然而，这可能导致硬包装文本中出现意外的结果，也可能导致解析中的歧义(注意，一些实现将标题放在块引号中，而另一些则没有)。(约翰·格鲁伯也表示支持要求使用空行。)
- 缩进代码块之前需要空行吗?(Markdown.pl 需要它，但在文档中没有提到，一些实现不需要它。)
paragraph code?
- 确定列表项何时被包装在<p>标记中的确切规则是什么?一个列表可以部分“宽松”部分“紧”吗?我们应该如何处理这样的列表?
paragraph code?
one
two
three Or this?
one
a
b
two (There are some relevant comments by 约翰·格鲁伯 here.)
Can list markers be indented? Can ordered list markers be right-aligned?
a backtick ()](/url) and [another backtick ().
What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?
foo bar baz What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed?
`a long code span can contain a hyphen like this
Can list items be empty?
Blockquote foo.
If there are multiple definitions for the same reference, which takes precedence?
foo In the absence of a spec, early implementers consulted Markdown.pl to resolve these ambiguities. But Markdown.pl was quite buggy, and gave manifestly bad results in many cases, so it was not a satisfactory replacement for a spec.
Because there is no unambiguous spec, implementations have diverged considerably. As a result, users are often surprised to find that a document that renders one way on one system (say, a GitHub wiki) renders differently on another (say, converting to docbook using pandoc). To make matters worse, because nothing in Markdown counts as a “syntax error,” the divergence often isn’t discovered right away.
1.3About 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.
2Preliminaries 2.1Characters and lines Any sequence of characters is a valid CommonMark document.
A character is a Unicode code point. Although some code points (for example, combining accents) do not correspond to characters in an intuitive sense, all code points count as characters for purposes of this spec.
This spec does not specify an encoding; it thinks of lines as composed of characters rather than bytes. A conforming parser may be limited to a certain encoding.
A line is a sequence of zero or more characters other than 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 !, ", #, $, %, &, ', (, ), *, +, ,, -, ., / (U+0021–2F), :, ;, <, =, >, ?, @ (U+003A–0040), [, , ], ^, _, ` (U+005B–0060), {, |, }, or ~ (U+007B–007E).
A Unicode punctuation character is an ASCII punctuation character or anything in the general Unicode categories Pc, Pd, Pe, Pf, Pi, Po, or Ps.
2.2Tabs Tabs in lines are not expanded to spaces. However, in contexts where spaces help to define block structure, tabs behave as if they were replaced by spaces with a tab stop of 4 characters.
Thus, for example, a tab can be used instead of four spaces in an indented code block. (Note, however, that internal tabs are passed through as literal tabs, not expanded to spaces.)
Example 1Try It →foo→baz→→bim
foo→baz→→bim
Example 2Try It →foo→baz→→bim
foo→baz→→bim
Example 3Try It a→a ὐ→a
a→a
ὐ→a
In the following example, a continuation paragraph of a list item is indented with a tab; this has exactly the same effect as indentation with four spaces would:
Example 4Try It
→bar
foo
bar
→→bar
foo
bar
Example 6Try It
→→foo
Example 7Try It -→→foofoo
foo
foo
bar
Example 9Try It
2.4Backslash escapes Any ASCII punctuation character may be backslash-escaped:
Example 12Try It !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
Backslashes before other characters are treated as literal backslashes:Example 13Try It \→\A\a\ \3\φ\«
\→\A\a\ \3\φ\«
Escaped characters are treated as regular characters and do not have their usual Markdown meanings:Example 14Try It *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
*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
If a backslash is itself escaped, the following character is not:Example 15Try It \emphasis
\emphasis
A backslash at the end of the line is a hard line break:Example 16Try It
foo
bar
foo
bar
Example 17Try It
\[\`
\[\`
\[\]
Example 19Try It
\[\]
\[\]
Example 20Try It http://example.com?find=\*
Example 21Try It But they work in all other contexts, including URLs and link titles, link references, and info strings in fenced code blocks: Example 23Try It [foo] Example 24Try It ``` foo\+bar foo ```foo
2.5Entity 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 25Try It & © Æ Ď ¾ ℋ ⅆ ∲ ≧̸
& © Æ Ď ¾ ℋ ⅆ ∲ ≧̸
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 26Try It # Ӓ Ϡ �
# Ӓ Ϡ �
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 27Try It " ആ ಫ
" ആ ಫ
Here are some nonentities:Example 28Try It   &x; &#; &#x; � &#abcdef0; &ThisIsNotDefined; &hi?;
  &x; &#; &#x; � &#abcdef0; &ThisIsNotDefined; &hi?;
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 29Try It ©
©
Strings that are not on the list of HTML5 named entities are not recognized as entity references either:Example 30Try It &MadeUpEntity;
&MadeUpEntity;
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 32Try It [foo](/föö "föö") Example 33Try It [foo] Example 34Try It ``` föö foo ```foo
Entity and numeric character references are treated as literal text in code spans and code blocks:
Example 35Try It
föö
föö
föfö
Entity and numeric character references cannot be used in place of symbols indicating structure in CommonMark documents.
Example 37Try It *foo* foo
*foo* foo
Example 38Try It * foo* foo
foo
bar
Example 40Try It foo→foo
Example 41Try It [a](url "tit")[a](url "tit")
3Blocks and inlines We can think of a document as a sequence of blocks—structural elements like paragraphs, block quotations, lists, headings, rules, and code blocks. Some blocks (like block quotes and list items) contain other blocks; others (like headings and paragraphs) contain inline content—text, links, emphasized text, images, code spans, and so on.3.1Precedence Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span:
Example 42Try It
3.2Container blocks and leaf blocks We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot.
4Leaf blocks This section describes the different kinds of leaf block that make up a Markdown document.
4.1Thematic breaks A line consisting of 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 43Try It
Example 44Try It +++
+++
Example 45Try It ======
Not enough characters:** __
-- ** __
Up to three spaces of indentation are allowed:Example 47Try It
Example 48Try It ***
***
Example 49Try It Foo ***
Foo ***
More than three characters may be used:Example 50Try It
Example 51Try It
Example 54Try It
Example 55Try It _ _ _ _ a
a------
---a---
_ _ _ _ a
a------
---a---
It is required that all of the characters other than spaces or tabs be the same. So, this is not a thematic break:Example 56Try It -
-
Thematic breaks do not need blank lines before or after:Example 57Try It
Example 58Try It Foo
bar
Foo
bar
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:bar
bar
When both a thematic break and a list item are possible interpretations of a line, the thematic break takes precedence:Example 60Try It
Example 61Try It
Simple headings:
Example 62Try It
Example 63Try It ####### foo
####### foo
At least one space or tab is required between the # characters and the heading’s contents, unless the heading is empty. Note that many implementations currently do not require the space. However, the space was required by the original ATX implementation, and it helps prevent things like the following from being parsed as headings:Example 64Try It #5 bolt
#hashtag
#5 bolt
#hashtag
This is not a heading, because the first # is escaped:Example 65Try It ## foo
## foo
Contents are parsed as inlines:Example 66Try It
Example 67Try It
Example 68Try It
Example 69Try It # foo
# foo
Example 70Try It foo # bar
foo # bar
A closing sequence of # characters is optional:Example 71Try It
Example 72Try It
Example 73Try It
Example 74Try It
Example 75Try It
Example 76Try It
Example 77Try It
Foo bar
Bar foo
ATX headings can be empty:Example 79Try It
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 85Try It Foo ---
Foo
Foo
---
Foo
Example 87Try It Foo ---
Foo ---
The setext heading underline cannot contain internal spaces or tabs:Example 88Try It Foo = =
Foo
Foo = =
Foo
`
of dashes"/>
`
of dashes"/>
The setext heading underline cannot be a lazy continuation line in a list item or block quote:Example 92Try It
Foo
Foo
Example 94Try It - Foo ---foo bar ===
Baz
Baz
Setext headings cannot be empty:Example 97Try It
====
====
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:foo
foo
baz One can find four different interpretations:
paragraph “Foo”, heading “bar”, paragraph “baz” paragraph “Foo bar”, thematic break, paragraph “baz” paragraph “Foo bar — baz” 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 103Try It Foo
baz
Foo
baz
Authors who want interpretation 2 can put blank lines around the thematic break,Example 104Try It Foo bar
baz
Foo bar
baz
or use a thematic break that cannot count as a setext heading underline, such asExample 105Try It Foo bar
baz
Foo bar
baz
Authors who want interpretation 3 can use backslash escapes:Example 106Try It Foo bar --- baz
Foo bar --- baz
4.4Indented code blocks An indented code block is composed of one or more indented chunks separated by blank lines. An indented chunk is a sequence of non-blank lines, each 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 107Try It a simple indented code block
a simple
indented code block
If there is any ambiguity between an interpretation of indentation as a code block and as indicating that material belongs to a list item, the list item interpretation takes precedence:
Example 108Try It
foo
bar
foo
bar
- bar
foo
Example 110Try It hi
- one
<a/>
*hi*
- one
Here we have three chunks separated by blank lines:
Example 111Try It chunk1
chunk2
chunk3
chunk1
chunk2
chunk3
Any initial spaces or tabs beyond four spaces of indentation will be included in the content, even in interior blank lines:
Example 112Try It chunk1
chunk2
chunk1
chunk2
An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.)
Example 113Try It Foo bar
Foo bar
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 114Try It foo bar
foo
bar
And indented code can occur immediately before and after other kinds of blocks:Example 115Try It
foo
foo
foo
foo
Example 116Try It foo bar
foo
bar
Blank lines preceding or following an indented code block are not included in it:
Example 117Try It
foo
foo
Trailing spaces or tabs are included in the code block’s content:
Example 118Try It foo
foo
4.5Fenced code blocks A code fence is a sequence of at least three consecutive backtick characters (`) or tildes (~). (Tildes and backticks cannot be mixed.) A fenced code block begins with a code fence, 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 119Try It
<
>
<
>
With tildes:
Example 120Try It
<
>
<
>
Fewer than three backticks is not enough:
Example 121Try It
foo
foo
Example 122Try It
aaa
~~~
aaa
~~~
Example 123Try It
aaa
```
aaa
```
The closing code fence must be at least as long as the opening fence:
Example 124Try It
aaa
```
aaa
```
Example 125Try It
aaa
~~~
aaa
~~~
Unclosed code blocks are closed by the end of the document (or the enclosing block quote or list item):
Example 126Try It
<pre><code></code></pre>
Example 127Try It
aaa
<pre><code>
aaa Example 128Try It
aaa
bbb
aaa
bbb
A code block can have all empty lines as its content:Example 129Try It
A code block can be empty:
Example 130Try It
Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:
Example 131Try It
aaa
aaa
aaa
aaa
Example 132Try It
aaa
aaa
aaa
aaa
aaa
aaa
Example 133Try It
aaa
aaa
aaa
aaa
aaa
aaa
Four spaces of indentation is too many:
Example 134Try It
aaa
```
aaa
```
Closing fences may be preceded by up to three spaces of indentation, and their indentation need not match that of the opening fence:
Example 135Try It
aaa
aaa
Example 136Try It
aaa
aaa
This is not a closing fence, because it is indented 4 spaces:
Example 137Try It
aaa
```
<pre><code>aaa
```
</code></pre>
Code fences (opening and closing) cannot contain internal spaces or tabs:
Example 138Try It
aaa
aaa
aaa
~~~ ~~
Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between:
Example 140Try It foo
bar
baz
foo
bar
baz
Other blocks can also occur before and after fenced code blocks without an intervening blank line:bar
bar
Example 142Try It
def foo(x)
return 3
end
def foo(x)
return 3
end
Example 143Try It
def foo(x)
return 3
end
def foo(x)
return 3
end
Example 144Try It
Info strings for backtick code blocks cannot contain backticks:
Example 145Try It
aa
foo
aa
foo
Example 146Try It
foo
foo
Closing code fences cannot have info strings:
Example 147Try It
``` aaa
``` aaa
4.6HTML blocks An HTML block is a group of lines that is treated as raw HTML (and will not be escaped in HTML output).
There are seven kinds of HTML block, which can be defined by their start and end conditions. The block begins with a line that meets a start condition (after up to three optional spaces of indentation). It ends with the first subsequent line that meets a matching end condition, or the last line of the document, or the last line of the container block containing the current HTML block, if no line is encountered that meets the end condition. If the first line meets both the start condition and the end condition, the block will contain just that line.
Start condition: line begins with the string <pre, <script, <style, or <textarea (case-insensitive), followed by a space, a tab, the string >, or the end of the line. End condition: line contains an end tag , , , or (case-insensitive; it need not match the start tag).
Start condition: line begins with the string .
Start condition: line begins with the string .
Start condition: line begins with the string <! followed by an ASCII letter. End condition: line contains the character >.
Start condition: line begins with the string .
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.
Start condition: line begins with a complete open tag (with any tag name other than pre, script, style, or textarea) or a complete closing tag, followed by zero or more spaces and tabs, followed by the end of the line. End condition: line is followed by a blank line.
HTML blocks continue until they are closed by their appropriate end condition, or the last line of the document or other container block. This means any HTML within an HTML block that might otherwise be recognised as a start condition will be ignored by the parser and passed through as-is, without changing the parser’s state.
For instance,
within an HTML block started by 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 148Try It
**Hello**, |
**Hello**, |
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 149Try It
| hi |
okay.
| hi |
okay.
Example 150Try It *foo* Here we have two HTML blocks with a Markdown paragraph between them:Example 152Try It
Markdown
Markdown
Example 153Try It
Example 154Try It An open tag need not be closed:Example 155Try It
bar
bar
A partial tag need not even be completed (garbage in, garbage out):Example 156Try It
| foo |
| foo |
Example 161Try It
``` c int x = 33; ``` ``` 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 162Try It bar
*bar* In type 7 blocks, the tag name can be anything:Example 163Try It bar
*bar* Example 164Try It *bar* *bar* Example 165Try It *bar* *bar* These rules are designed to allow us to work with tags that can function as either block-level or inline-level tags. TheExample 166Try It
foo
Example 167Try It
foo
foo
Example 168Try It
foo
foo
A pre tag (type 1):
Example 169Try It
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
okay
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
okay
A script tag (type 1):Example 170Try It
okay
okay
A textarea tag (type 1):Example 171Try It
*foo* _bar_ *foo* _bar_A style tag (type 1):
Example 172Try It
okay
okay
If there is no matching end tag, the block will end at the end of the document (or the enclosing block quote or list item):Example 173Try It
foo
foo
Example 177Try It *bar* *baz* *bar*baz
Note that anything on the last line after the end tag will be included in the HTML block:Example 178Try It
1. *bar* 1. *bar*A comment (type 2):
Example 179Try It
okay
okay
A processing instruction (type 3):Example 180Try It
'; ?>okay
'; ?>okay
A declaration (type 4):Example 181Try It
CDATA (type 5):
Example 182Try It
okay
okay
The opening tag can be preceded by up to three spaces of indentation, but not four:Example 183Try It
<!-- foo -->
<!-- foo -->
Example 184Try It
<div>
<div>
Example 185Try It Foo
Foo
Example 186Try It
Example 187Try It Foo baz
This rule differs from 约翰·格鲁伯’s original Markdown syntax specification, which says:The only restrictions are that block-level HTML elements — e.g.
,, 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 188Try It
Emphasized text.
Example 189Try ItEmphasized text.
*Emphasized* text.*Emphasized* text.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 190Try It
| Hi |
| Hi |
Example 191Try It
<td>
Hi
</td>
<td>
Hi
</td>
tags, but as described above, raw HTML blocks starting withcan contain blank lines.4.7Link 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 192Try It foo: /url "title"
Example 193Try It [foo]: /url 'the title' Example 194Try It [Foo*bar\]]:my_(url) 'title (with parens)'[Foo*bar]]
Example 195Try It [Foo bar]: 'title'[Foo bar]
The title may extend over multiple lines:Example 196Try It foo: /url ' title line1 line2 '
However, it may not contain a blank line:Example 197Try It foo: /url 'title
with blank line'
[foo]: /url 'title
with blank line'
[foo]
The title may be omitted:Example 198Try It foo: /url
The link destination may not be omitted:Example 199Try It foo:
[foo]:
[foo]
However, an empty link destination may be specified using angle brackets:Example 200Try It foo: <>
The title must be separated from the link destination by spaces or tabs:Example 201Try It foo: (baz)
[foo]: (baz)
[foo]
Both title and destination can contain backslash escapes and literal backslashes:Example 202Try It foo: /url\bar*baz "foo"bar\baz"
A link can come before its corresponding definition:Example 203Try It foo
If there are several matching definitions, the first one takes precedence:Example 204Try It foo
As noted in the section on Links, matching of labels is case-insensitive (see matches).Example 205Try It FOO: /url
Example 206Try It [ΑΓΩ]: /φου[αγω]
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 207Try It foo: /url
Here is another one:
Example 208Try It foo : /url bar
bar
This is not a link reference definition, because there are characters other than spaces or tabs after the title:Example 209Try It foo: /url "title" ok
[foo]: /url "title" ok
This is a link reference definition, but it has no title:Example 210Try It foo: /url "title" ok
"title" ok
This is not a link reference definition, because it is indented four spaces:Example 211Try It foo: /url "title"
[foo]: /url "title"[foo]
This is not a link reference definition, because it occurs inside a code block:Example 212Try It
[foo]: /url[foo]: /url[foo]
A link reference definition cannot interrupt a paragraph.Example 213Try It Foo bar: /baz
Foo [bar]: /baz
[bar]
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 214Try It
Foo
bar
Foo
Example 215Try It [foo]: /url bar === [foo]bar
bar
Example 216Try It [foo]: /url === [foo]=== foo
Several link reference definitions can occur one after another, without intervening blank lines.Example 217Try It foo: /foo-url "foo" bar: /bar-url "bar" baz: /baz-url
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 218Try It foo
4.8Paragraphs A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph’s raw content as inlines. The paragraph’s raw content is formed by concatenating the lines and removing initial and final spaces or tabs.A simple example with two paragraphs:
Example 219Try It aaa
bbb
aaa
bbb
Paragraphs can contain multiple lines, but no blank lines:Example 220Try It aaa bbb
ccc ddd
aaa bbb
ccc ddd
Multiple blank lines between paragraphs have no effect:Example 221Try It aaa
bbb
aaa
bbb
Leading spaces or tabs are skipped:Example 222Try It aaa bbb
aaa bbb
Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs.Example 223Try It aaa bbb ccc
aaa bbb ccc
However, the first line may be preceded by up to three spaces of indentation. Four spaces of indentation is too many:Example 224Try It aaa bbb
aaa bbb
Example 225Try It aaa bbbaaabbb
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 226Try It aaa
bbbaaa
4.9Blank lines Blank lines between block-level elements are ignored, except for the role they play in determining whether a list is tight or loose.
bbbBlank lines at the beginning and end of the document are also ignored.
Example 227Try It
aaa
aaa
aaa
aaa
5Container blocks A container block is a block that has other blocks as its contents. There are two basic kinds of container blocks: block quotes and list items. Lists are meta-containers for list items.We define the syntax for container blocks recursively. The general form of the definition is:
If X is a sequence of blocks, then the result of transforming X in such-and-such a way is a container of type Y with these blocks as its content.
So, we explain what counts as a block quote or list item by explaining how these can be generated from their contents. This should suffice to define the syntax, although it does not give a recipe for parsing these constructions. (A recipe is provided below in the section entitled A parsing strategy.)
5.1Block quotes A block quote marker, optionally preceded by up to three spaces of indentation, consists of (a) the character > together with a following space of indentation, or (b) a single character > not followed by a space of indentation.
The following rules define block quotes:
Basic case. If a string of lines Ls constitute a sequence of blocks Bs, then the result of prepending a block quote marker to the beginning of each line in Ls is a block quote containing Bs.
Laziness. If a string of lines Ls constitute a block quote with contents Bs, then the result of deleting the initial block quote marker from one or more lines in which the next character other than a space or tab after the block quote marker is paragraph continuation text is a block quote with Bs as its content. Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph.
Consecutiveness. A document cannot contain two block quotes in a row unless there is a blank line between them.
Nothing else counts as a block quote.
Here is a simple example:
Example 228Try It
Foo
bar baz
The space or tab after the > characters can be omitted:Foo
bar baz
Example 229Try It
Foo
bar baz
The > characters can be preceded by up to three spaces of indentation:Foo
bar baz
Example 230Try It
Foo
bar baz
Four spaces of indentation is too many:Foo
bar baz
Example 231Try It > # Foo > bar > baz
> # Foo > bar > bazThe Laziness clause allows us to omit the > before paragraph continuation text:
Example 232Try It
Foo
bar baz
A block quote can contain some lazy and some non-lazy continuation lines:Foo
bar baz
Example 233Try It
bar baz foo
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 ofbar baz foo
foo
without changing the meaning:
Example 234Try It
foo
foo
Similarly, if we omit the > in the second line of
- foo
- bar then the block quote ends after the first line:
Example 235Try It
- foo
- foo
Example 236Try It
foo
bar
foo
bar
foo
Example 238Try It
foo - bar
To see why, note that infoo - bar
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 239Try It
Example 240Try It > > >
A block quote can have initial or final blank lines:
Example 241Try It
foo
A blank line always separates block quotes:foo
Example 242Try It
foo
bar
foo
(Most current Markdown implementations, including 约翰·格鲁伯’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.)bar
Consecutiveness means that if we put these block quotes together, we get a single block quote:
Example 243Try It
foo bar
To get a block quote with two paragraphs, use:foo bar
Example 244Try It
foo
bar
Block quotes can interrupt paragraphs:foo
bar
Example 245Try It foo
bar
foo
In general, blank lines are not needed before or after block quotes:bar
Example 246Try It
aaa
bbb
aaa
However, because of laziness, a blank line is needed between a block quote and a following paragraph:bbb
Example 247Try It
bar baz
Example 248Try It > barbar baz
baz
bar
baz
Example 249Try It > bar > bazbar
baz
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 250Try It
foo bar
Example 251Try It >>> foo > bar >>bazfoo bar
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 >:foo bar baz
Example 252Try It
code
not code
code
5.2List items A list marker is a bullet list marker or an ordered list marker.not code
A bullet list marker is a -, +, or * character.
An ordered list marker is a sequence of 1–9 arabic digits (0-9), followed by either a . character or a ) character. (The reason for the length limit is that with 10 digits we start seeing integer overflows in some browsers.)
The following rules define list items:
Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a character other than a space or tab, and M is a list marker of width W followed by 1 ≤ N ≤ 4 spaces of indentation, then the result of prepending M and the following spaces to the first line of Ls*, and indenting subsequent lines of Ls by W + N spaces, is a list item with Bs as its contents. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.
Exceptions:
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. If any line is a thematic break then that line is not a list item. For example, let Ls be the lines
Example 253Try It A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
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:A block quote.
Example 254Try It
A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
A block quote.
Here are some examples showing how far content must be indented to be put under the list item:
Example 255Try It
two
two
Example 256Try It - onetwo
one
two
two
two
two
one
two
Example 259Try It
one
two
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.
one
two
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 260Try It
- one
two
Note that at least one space or tab is needed between the list marker and any following content, so these are not list items:
- one
two
Example 261Try It -one
2.two
-one
2.two
A list item may contain blocks that are separated by more than one blank line.Example 262Try It
foo
bar
foo
bar
Example 263Try It
foo
bar
baz
bam
foo
bar
baz
bam
Example 264Try It
Foo
bar
baz
Foo
bar
baz
Example 265Try It 123456789. ok
1234567890. not ok
A start number may begin with 0s:Example 267Try It 0. ok
Example 269Try It -1. not ok
-1. not ok
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 270Try It
foo
bar
foo
bar
Example 271Try It 10. foo
bar
foo
bar
Example 272Try It indented code
paragraph
more code
indented code
paragraph
more code
paragraph
more code
indented code
paragraph
more code
Example 274Try It
indented code
paragraph
more code
indented code
paragraph
more code
Example 275Try It foo
bar
foo
bar
Example 276Try It - foobar
bar
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 277Try It
foo
bar
foo
bar
bar
baz
bar
baz
foo
foo
foo
Here is an empty bullet list item:Example 281Try It
Example 282Try It
Example 283Try It
Example 284Try It *
Example 285Try It foo *
foo 1.
foo *
foo 1.
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 286Try It
A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
A block quote.
Example 287Try It
A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
A block quote.
Example 288Try It
A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
A block quote.
Example 289Try It 1. A paragraph with two lines.
indented code
> A block quote.
1. A paragraph
with two lines.
indented code
> A block quote.
Laziness. If a string of lines Ls constitute a list item with contents Bs, then the result of deleting some or all of the indentation from one or more lines in which the next 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 lines. Here is an example with lazy continuation lines:
Example 290Try It
A paragraph with two lines.
indented code
A block quote.
A paragraph with two lines.
indented code
A block quote.
Example 291Try It
Example 292Try It
Blockquote continued here.
Example 293Try It > 1. > Blockquote > continued here.
Blockquote continued here.
That’s all. Nothing that is not counted as a list item by rules #1–5 counts as a list item. The rules for sublists follow from the general rules above. A sublist must be indented the same number of spaces of indentation a paragraph would need to be in order to be included in the list item.
Blockquote continued here.
So, in this case we need two spaces indent:
Example 294Try It
Example 295Try It
Example 296Try It 10) foo - bar
Example 297Try It 10) foo
Example 298Try It
Example 300Try It
5.2.1Motivation 约翰·格鲁伯’s Markdown spec says the following about list items:“List markers typically start at the left margin, but may be indented by up to three spaces. List markers must be followed by one or more spaces or a tab.”
“To make lists look nice, you can wrap items with hanging indents…. But if you don’t want to, you don’t have to.”
“List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab.”
“It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy.”
“To put a blockquote within a list item, the blockquote’s > delimiters need to be indented.”
“To put a code block within a list item, the code block needs to be indented twice — 8 spaces or two tabs.”
These rules specify that a paragraph under a list item must be indented four spaces (presumably, from the left margin, rather than the start of the list marker, but this is not said), and that code under a list item must be indented eight spaces instead of the usual four. They also say that a block quote must be indented, but not by how much; however, the example given has four spaces indentation. Although nothing is said about other kinds of block-level content, it is certainly reasonable to infer that all block elements under a list item, including other lists, must be indented four spaces. This principle has been called the four-space rule.
The four-space rule is clear and principled, and if the reference implementation Markdown.pl had followed it, it probably would have become the standard. However, Markdown.pl allowed paragraphs and sublists to start with only two spaces indentation, at least on the outer level. Worse, its behavior was inconsistent: a sublist of an outer-level list needed two spaces indentation, but a sublist of this sublist needed three spaces. It is not surprising, then, that different implementations of Markdown have developed very different rules for determining what comes under a list item. (Pandoc and python-Markdown, for example, stuck with Gruber’s syntax description and the four-space rule, while discount, redcarpet, marked, PHP Markdown, and others followed Markdown.pl’s behavior more closely.)
Unfortunately, given the divergences between implementations, there is no way to give a spec for list items that will be guaranteed not to break any existing documents. However, the spec given here should correctly handle lists formatted with either the four-space rule or the more forgiving Markdown.pl behavior, provided they are laid out in a way that is natural for a human to read.
The strategy here is to let the width and indentation of the list marker determine the indentation necessary for blocks to fall under the list item, rather than having a fixed and arbitrary number. The writer can think of the body of the list item as a unit which gets indented to the right enough to fit the list marker (and any indentation on the list marker). (The laziness rule, #5, then allows continuation lines to be unindented if needed.)
This rule is superior, we claim, to any rule requiring a fixed level of indentation from the margin. The four-space rule is clear but unnatural. It is quite unintuitive that
foo
bar
bar
foo
bar
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
two as a single list item, with two a continuation paragraph:
one
two
- one
two as
This is extremely unintuitive.
one
two
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:
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:
foo
indented code
where the code is indented eight spaces. The spec above, by contrast, will parse this text as expected, since the code block’s indentation is measured from the beginning of foo.
The one case that needs special treatment is a list item that starts with indented code. How much indentation is required in that case, since we don’t have a “first paragraph” to measure from? Rule #2 simply stipulates that in such cases, we require one space indentation from the list marker (and then the normal four spaces for the indented code). This will match the four-space rule in cases where the list marker plus its initial indentation takes four spaces (a common case), but diverge in other cases.
5.3Lists A list is a sequence of one or more list items of the same type. The list items may be separated by 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
tags, while paragraphs in a tight list are not.)
Changing the bullet or ordered list delimiter starts a new list:
Example 301Try It
Example 303Try It Foo
Foo
The number of windows in my house is 14. The number of doors is 6. Oddly, though, Markdown.pl does allow a blockquote to interrupt a paragraph, even though the same considerations might apply.
In CommonMark, we do allow lists to interrupt paragraphs, for two reasons. First, it is natural and not uncommon for people to start lists without blank lines:
I need to buy
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
tags, since the list is “tight”), then
I need to buy
Since it is well established Markdown practice to allow lists to interrupt paragraphs inside list items, the principle of uniformity requires us to allow this outside list items as well. (reStructuredText takes a different approach, requiring blank lines before lists even inside other list items.)
In order to solve of unwanted lists in paragraphs with hard-wrapped numerals, we allow only lists starting with 1 to interrupt paragraphs. Thus,
Example 304Try It The number of windows in my house is 14. The number of doors is 6.
The number of windows in my house is 14. The number of doors is 6.
We may still get an unintended result in cases likeExample 305Try It The number of windows in my house is
The number of windows in my house is
There can be any number of blank lines between items:
Example 306Try It
foo
bar
baz
foo
bar
baz
bim
baz
bim
Example 308Try It
notcode
code
foo
notcode
foo
code
Example 310Try It
b
c
a
b
c
Example 312Try It
Example 313Try It
a
b
3. c
a
b
3. c
Example 314Try It
a
b
c
a
b
c
Example 315Try It
a
c
a
c
Example 316Try It
a
b
c
d
a
b
c
d
a
b
d
Example 318Try It
b
b
Example 319Try It
b
c
b
c
Example 320Try It
b
b
Example 321Try It
b
c
b
c
Example 322Try It
Example 324Try It
foo
bar
foo
bar
Example 325Try It
foo
baz
foo
baz
a
d
Example 327Try It
hilo`
hilo`
6.1Code 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 328Try It
foo
foo
Example 329Try It
foo ` bar
foo ` bar
Example 330Try It
``
``
Example 331Try It
``
``
Example 332Try It
a
a
Example 333Try It
b
b
Example 334Try It
Example 335Try It
foo bar baz
foo bar baz
foo
Example 337Try It
foo bar baz
foo bar baz
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 338Try It
foo\bar`
foo\bar`
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 339Try It
foo`bar
foo`bar
Example 340Try It
` foo `` bar `
foo `` bar
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 341Try It
*foo*
*foo*
And this is not parsed as a link:
Example 342Try It
[not a link](/foo)
[not a link](/foo)
Code spans, HTML tags, and autolinks have the same precedence. Thus, this is code:
Example 343Try It
<a href="">`
<a href="">`
But this is an HTML tag:
Example 344Try It
`
And this is code:
Example 345Try It
<http://foo.bar.baz>`
<http://foo.bar.baz>`
But this is an autolink:
Example 346Try It
http://foo.bar.`baz`
When a backtick string is not closed by a matching backtick string, we just have literal backticks:
Example 347Try It
```foo``
```foo``
Example 348Try It
`foo
`foo
The following case also illustrates the need for opening and closing backtick strings to be equal in length:
Example 349Try It
`foobar
`foobar
6.2Emphasis and strong emphasis
约翰·格鲁伯’s original Markdown syntax description says:
Markdown treats asterisks (*) and underscores (_) as indicators of emphasis. Text wrapped with one * or _ will be wrapped with an HTML tag; double *’s or _’s will be wrapped with an HTML tag.
This is enough for most users, but these rules leave much undecided, especially when it comes to nested emphasis. The original Markdown.pl test suite makes it clear that triple *** and ___ delimiters can be used for strong emphasis, and most implementations have also allowed the following patterns:
strong emph
strong in emph
emph in strong
in strong emph
in emph strong
The following patterns are less widely supported, but the intent is clear and they are useful (especially in contexts like bibliography entries):
emph with emph in it
strong with strong in it
Many implementations have also restricted intraword emphasis to the * forms, to avoid unwanted emphasis in words containing internal underscores. (It is best practice to put these in code spans, but users often do not.)
internal emphasis: foobarbaz
no emphasis: foo_bar_baz
The rules given below capture all of these patterns, while allowing for efficient parsing strategies that do not backtrack.
First, some definitions. A delimiter run is either a sequence of one or more * characters that is not preceded or followed by a non-backslash-escaped * character, or a sequence of one or more _ characters that is not preceded or followed by a non-backslash-escaped _ character.
A left-flanking delimiter run is a delimiter run that is (1) not followed by Unicode whitespace, and either (2a) not followed by a Unicode punctuation character, or (2b) followed by a Unicode punctuation character and preceded by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.
A right-flanking delimiter run is a delimiter run that is (1) not preceded by Unicode whitespace, and either (2a) not preceded by a Unicode punctuation character, or (2b) preceded by a Unicode punctuation character and followed by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.
Here are some examples of delimiter runs.
left-flanking but not right-flanking:
***abc
_abc
**"abc"
_"abc"
right-flanking but not left-flanking:
abc***
abc_
"abc"**
"abc"_
Both left and right-flanking:
abc***def
"abc"_"def"
Neither left nor right-flanking:
abc *** def
a _ b
(The idea of distinguishing left-flanking and right-flanking delimiter runs based on the character before and the character after comes from Roopesh Chander’s vfmd. vfmd uses the terminology “emphasis indicator string” instead of “delimiter run,” and its rules for distinguishing left- and right-flanking runs are a bit more complex than the ones given here.)
The following rules define emphasis and strong emphasis:
A single * character can open emphasis iff (if and only if) it is part of a left-flanking delimiter run.
A single _ character can open emphasis iff it is part of a left-flanking delimiter run and either (a) not part of a right-flanking delimiter run or (b) part of a right-flanking delimiter run preceded by a Unicode punctuation character.
A single * character can close emphasis iff it is part of a right-flanking delimiter run.
A single _ character can close emphasis iff it is part of a right-flanking delimiter run and either (a) not part of a left-flanking delimiter run or (b) part of a left-flanking delimiter run followed by a Unicode punctuation character.
A double ** can open strong emphasis iff it is part of a left-flanking delimiter run.
A double __ can open strong emphasis iff it is part of a left-flanking delimiter run and either (a) not part of a right-flanking delimiter run or (b) part of a right-flanking delimiter run preceded by a Unicode punctuation character.
A double ** can close strong emphasis iff it is part of a right-flanking delimiter run.
A double __ can close strong emphasis iff it is part of a right-flanking delimiter run and either (a) not part of a left-flanking delimiter run or (b) part of a left-flanking delimiter run followed by a Unicode punctuation character.
Emphasis begins with a delimiter that can open emphasis and ends with a delimiter that can close emphasis, and that uses the same character (_ or *) as the opening delimiter. The opening and closing delimiters must belong to separate delimiter runs. If one of the delimiters can both open and close emphasis, then the sum of the lengths of the delimiter runs containing the opening and closing delimiters must not be a multiple of 3 unless both lengths are multiples of 3.
Strong emphasis begins with a delimiter that can open strong emphasis and ends with a delimiter that can close strong emphasis, and that uses the same character (_ or *) as the opening delimiter. The opening and closing delimiters must belong to separate delimiter runs. If one of the delimiters can both open and close strong emphasis, then the sum of the lengths of the delimiter runs containing the opening and closing delimiters must not be a multiple of 3 unless both lengths are multiples of 3.
A literal * character cannot occur at the beginning or end of *-delimited emphasis or **-delimited strong emphasis, unless it is backslash-escaped.
A literal _ character cannot occur at the beginning or end of _-delimited emphasis or __-delimited strong emphasis, unless it is backslash-escaped.
Where rules 1–12 above are compatible with multiple parsings, the following principles resolve ambiguity:
The number of nestings should be minimized. Thus, for example, an interpretation ... is always preferred to ....
An interpretation ... is always preferred to ....
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 foo bar baz rather than foo bar baz.
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 bar baz rather than foo **bar baz.
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* is parsed as foo rather than as foo.
These rules can be illustrated through a series of examples.
Rule 1:
Example 350Try It
foo bar
foo bar
This is not emphasis, because the opening * is followed by whitespace, and hence not part of a left-flanking delimiter run:
Example 351Try It
a * foo bar*
a * foo bar*
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 352Try It
a*"foo"*
a*"foo"*
Unicode nonbreaking spaces count as whitespace, too:
Example 353Try It
- a *
* a *
Intraword emphasis with * is permitted:
Example 354Try It
foobar
foobar
Example 355Try It
5*6*78
5678
Rule 2:
Example 356Try It
foo bar
foo bar
This is not emphasis, because the opening _ is followed by whitespace:
Example 357Try It
_ foo bar_
_ foo bar_
This is not emphasis, because the opening _ is preceded by an alphanumeric and followed by punctuation:
Example 358Try It
a_"foo"_
a_"foo"_
Emphasis with _ is not allowed inside words:
Example 359Try It
foo_bar_
foo_bar_
Example 360Try It
5_6_78
5_6_78
Example 361Try It
пристаням_стремятся_
пристаням_стремятся_
Here _ does not generate emphasis, because the first delimiter run is right-flanking and the second left-flanking:
Example 362Try It
aa_"bb"_cc
aa_"bb"_cc
This is emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:
Example 363Try It
foo-(bar)
foo-(bar)
Rule 3:
This is not emphasis, because the closing delimiter does not match the opening delimiter:
Example 364Try It
_foo*
_foo*
This is not emphasis, because the closing * is preceded by whitespace:
Example 365Try It
*foo bar *
*foo bar *
A line ending also counts as whitespace:
Example 366Try It
*foo bar
*
*foo bar
*
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 367Try It
*(*foo)
*(*foo)
The point of this restriction is more easily appreciated with this example:
Example 368Try It
(foo)
(foo)
Intraword emphasis with * is allowed:
Example 369Try It
foobar
foobar
Rule 4:
This is not emphasis, because the closing _ is preceded by whitespace:
Example 370Try It
_foo bar _
_foo bar _
This is not emphasis, because the second _ is preceded by punctuation and followed by an alphanumeric:
Example 371Try It
_(_foo)
_(_foo)
This is emphasis within emphasis:
Example 372Try It
(foo)
(foo)
Intraword emphasis is disallowed for _:
Example 373Try It
_foo_bar
_foo_bar
Example 374Try It
_пристаням_стремятся
_пристаням_стремятся
Example 375Try It
_foo_bar_baz_
foo_bar_baz
This is emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:
Example 376Try It
(bar).
(bar).
Rule 5:
Example 377Try It
foo bar
foo bar
This is not strong emphasis, because the opening delimiter is followed by whitespace:
Example 378Try It
** foo bar**
** foo bar**
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 379Try It
a**"foo"**
a**"foo"**
Intraword strong emphasis with ** is permitted:
Example 380Try It
foobar
foobar
Rule 6:
Example 381Try It
foo bar
foo bar
This is not strong emphasis, because the opening delimiter is followed by whitespace:
Example 382Try It
__ foo bar__
__ foo bar__
A line ending counts as whitespace:
Example 383Try It
__
foo bar__
__
foo bar__
This is not strong emphasis, because the opening __ is preceded by an alphanumeric and followed by punctuation:
Example 384Try It
a__"foo"__
a__"foo"__
Intraword strong emphasis is forbidden with __:
Example 385Try It
foo__bar__
foo__bar__
Example 386Try It
5__6__78
5__6__78
Example 387Try It
пристаням__стремятся__
пристаням__стремятся__
Example 388Try It
__foo, __bar__, baz__
foo, bar, baz
This is strong emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:
Example 389Try It
foo-(bar)
foo-(bar)
Rule 7:
This is not strong emphasis, because the closing delimiter is preceded by whitespace:
Example 390Try It
**foo bar **
**foo bar **
(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 391Try It
**(**foo)
**(**foo)
The point of this restriction is more easily appreciated with these examples:
Example 392Try It
(foo)
(foo)
Example 393Try It
**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**
Gomphocarpus (Gomphocarpus physocarpus, syn.
Asclepias physocarpa)
Example 394Try It
**foo "*bar*" foo**
foo "bar" foo
Intraword emphasis:
Example 395Try It
foobar
foobar
Rule 8:
This is not strong emphasis, because the closing delimiter is preceded by whitespace:
Example 396Try It
__foo bar __
__foo bar __
This is not strong emphasis, because the second __ is preceded by punctuation and followed by an alphanumeric:
Example 397Try It
__(__foo)
__(__foo)
The point of this restriction is more easily appreciated with this example:
Example 398Try It
(foo)
(foo)
Intraword strong emphasis is forbidden with __:
Example 399Try It
__foo__bar
__foo__bar
Example 400Try It
__пристаням__стремятся
__пристаням__стремятся
Example 401Try It
__foo__bar__baz__
foo__bar__baz
This is strong emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:
Example 402Try It
(bar).
(bar).
Rule 9:
Any nonempty sequence of inline elements can be the contents of an emphasized span.
Example 403Try It
foo bar
foo bar
Example 404Try It
*foo
bar*
foo
bar
In particular, emphasis and strong emphasis can be nested inside emphasis:
Example 405Try It
foo bar baz
foo bar baz
Example 406Try It
_foo _bar_ baz_
foo bar baz
Example 407Try It
__foo_ bar_
foo bar
Example 408Try It
*foo *bar**
foo bar
Example 409Try It
*foo **bar** baz*
foo bar baz
Example 410Try It
*foo**bar**baz*
foobarbaz
Note that in the preceding case, the interpretation
foobarbaz
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 411Try It
foo**bar
foo**bar
The same condition ensures that the following cases are all strong emphasis nested inside emphasis, even when the interior whitespace is omitted:
Example 412Try It
foo bar
foo bar
Example 413Try It
*foo **bar***
foo bar
Example 414Try It
*foo**bar***
foobar
When the lengths of the interior closing and opening delimiter runs are both multiples of 3, though, they can match to create emphasis:
Example 415Try It
foobarbaz
foobarbaz
Example 416Try It
foo******bar*********baz
foobar***baz
Indefinite levels of nesting are possible:
Example 417Try It
foo bar baz bim bop
foo bar baz bim bop
Example 418Try It
*foo [*bar*](/url)*
foo bar
There can be no empty emphasis or strong emphasis:
Example 419Try It
** is not an empty emphasis
** is not an empty emphasis
Example 420Try It
**** is not an empty strong emphasis
**** is not an empty strong emphasis
Rule 10:
Any nonempty sequence of inline elements can be the contents of an strongly emphasized span.
Example 421Try It
foo bar
foo bar
Example 422Try It
**foo
bar**
foo
bar
In particular, emphasis and strong emphasis can be nested inside strong emphasis:
Example 423Try It
foo bar baz
foo bar baz
Example 424Try It
__foo __bar__ baz__
foo bar baz
Example 425Try It
____foo__ bar__
foo bar
Example 426Try It
**foo **bar****
foo bar
Example 427Try It
**foo *bar* baz**
foo bar baz
Example 428Try It
**foo*bar*baz**
foobarbaz
Example 429Try It
***foo* bar**
foo bar
Example 430Try It
**foo *bar***
foo bar
Indefinite levels of nesting are possible:
Example 431Try It
foo bar baz
bim bop
foo bar baz
bim bop
Example 432Try It
**foo [*bar*](/url)**
foo bar
There can be no empty emphasis or strong emphasis:
Example 433Try It
__ is not an empty emphasis
__ is not an empty emphasis
Example 434Try It
____ is not an empty strong emphasis
____ is not an empty strong emphasis
Rule 11:
Example 435Try It
foo ***
foo ***
Example 436Try It
foo *\**
foo *
Example 437Try It
foo *_*
foo _
Example 438Try It
foo *****
foo *****
Example 439Try It
foo **\***
foo *
Example 440Try It
foo **_**
foo _
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 441Try It
*foo
*foo
Example 442Try It
*foo**
foo*
Example 443Try It
***foo**
*foo
Example 444Try It
****foo*
***foo
Example 445Try It
**foo***
foo*
Example 446Try It
*foo****
foo***
Rule 12:
Example 447Try It
foo ___
foo ___
Example 448Try It
foo _\__
foo _
Example 449Try It
foo _*_
foo *
Example 450Try It
foo _____
foo _____
Example 451Try It
foo __\___
foo _
Example 452Try It
foo __*__
foo *
Example 453Try It
__foo_
_foo
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 454Try It
foo_
foo_
Example 455Try It
___foo__
_foo
Example 456Try It
____foo_
___foo
Example 457Try It
__foo___
foo_
Example 458Try It
_foo____
foo___
Rule 13 implies that if you want emphasis nested directly inside emphasis, you must use different delimiters:
Example 459Try It
foo
foo
Example 460Try It
*_foo_*
foo
Example 461Try It
__foo__
foo
Example 462Try It
_*foo*_
foo
However, strong emphasis within strong emphasis is possible without switching delimiters:
Example 463Try It
foo
foo
Example 464Try It
____foo____
foo
Rule 13 can be applied to arbitrarily long sequences of delimiters:
Example 465Try It
foo
foo
Rule 14:
Example 466Try It
foo
foo
Example 467Try It
_____foo_____
foo
Rule 15:
Example 468Try It
foo _bar baz_
foo _bar baz_
Example 469Try It
*foo __bar *baz bim__ bam*
foo bar *baz bim bam
Rule 16:
Example 470Try It
**foo bar baz
**foo bar baz
Example 471Try It
*foo *bar baz*
*foo bar baz
Rule 17:
Example 472Try It
*bar*
*bar*
Example 473Try It
_foo [bar_](/url)
_foo bar_
Example 474Try It
*
*
Example 475Try It
**
Example 476Try It
__
Example 477Try It
*a `*`*
a *
Example 478Try It
_a `_`_
a _
Example 479Try It
**a
Example 480Try It
__a
6.3Links
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* is a link.
A link destination consists of either
a sequence of zero or more characters between an opening < and a closing > that contains no line endings or unescaped < or > characters, or
a nonempty sequence of characters that does not start with <, does not include ASCII control characters or space character, and includes parentheses only if (a) they are backslash-escaped or (b) they are part of a balanced pair of unescaped parentheses. (Implementations may impose limits on parentheses nesting to avoid performance issues, but at least three levels of nesting should be supported.)
A link title consists of either
a sequence of zero or more characters between straight double-quote characters ("), including a " character only if it is backslash-escaped, or
a sequence of zero or more characters between straight single-quote characters ('), including a ' character only if it is backslash-escaped, or
a sequence of zero or more characters between matching parentheses ((...)), including a ( or ) character only if it is backslash-escaped.
Although link titles may span multiple lines, they may not contain a blank line.
An inline link consists of a link text followed immediately by a left parenthesis (, an optional link destination, an optional link title, and a right parenthesis ). These four components may be separated by spaces, tabs, and up to one line ending. If both link destination and link title are present, they must be separated by spaces, tabs, and up to one line ending.
The link’s text consists of the inlines contained in the link text (excluding the enclosing square brackets). The link’s URI consists of the link destination, excluding enclosing <...> if present, with backslash-escapes in effect as described above. The link’s title consists of the link title, excluding its enclosing delimiters, with backslash-escapes in effect as described above.
Here is a simple inline link:
Example 481Try It
link
The title, the link text and even the destination may be omitted:
Example 482Try It
link
Example 483Try It
[](./target.md)
Example 484Try It
[link]()
Example 485Try It
[link](<>)
Example 486Try It
[]()
The destination can only contain spaces if it is enclosed in pointy brackets:
Example 487Try It
[link](/my uri)
[link](/my uri)
Example 488Try It
[link](link
The destination cannot contain line endings, even if enclosed in pointy brackets:
Example 489Try It
[link](foo
bar)
[link](foo
bar)
Example 490Try It
[link]()
[link]()
The destination can contain ) if it is enclosed in pointy brackets:
Example 491Try It
a
Pointy brackets that enclose links must be unescaped:
Example 492Try It
[link](<foo>)
[link](<foo>)
These are not links, because the opening pointy bracket is not matched properly:
Example 493Try It
[a](<b)c
[a](<b)c>
[a](c)
[a](<b)c
[a](<b)c>
[a](c)
Parentheses inside the link destination may be escaped:
Example 494Try It
link
Any number of parentheses are allowed without escaping, as long as they are balanced:
Example 495Try It
link
However, if you have unbalanced parentheses, you need to escape or use the <...> form:
Example 496Try It
[link](foo(and(bar))
[link](foo(and(bar))
Example 497Try It
[link](foo\(and\(bar\))
Example 498Try It
[link]()
Parentheses and other symbols can also be escaped, as usual in Markdown:
Example 499Try It
link
A link can contain fragment identifiers and queries:
Example 500Try It
link
Note that a backslash before a non-escapable character is just a backslash:
Example 501Try It
link
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 502Try It
link
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 503Try It
link
Titles may be in single quotes, double quotes, or parentheses:
Example 504Try It
link
link
link
Backslash escapes and entity and numeric character references may be used in titles:
Example 505Try It
link
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 506Try It
link
Nested balanced quotes are not allowed without escaping:
Example 507Try It
[link](/url "title "and" title")
[link](/url "title "and" title")
But it is easy to work around this by using a different quote type:
Example 508Try It
link
(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 509Try It
link
But it is not allowed between the link text and the following parenthesis:
Example 510Try It
[link] (/uri)
[link] (/uri)
The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:
Example 511Try It
[link [foo bar]](/uri)
Example 512Try It
[link] bar](/uri)
[link] bar](/uri)
Example 513Try It
[link [bar](/uri)
[link bar
Example 514Try It
[link \[bar](/uri)
The link text may contain inline content:
Example 515Try It
link foo bar #
Example 516Try It
[](/uri)
However, links may not contain other links, at any level of nesting.
Example 517Try It
[foo bar](/uri)
[foo bar](/uri)
Example 518Try It
[foo *[bar [baz](/uri)](/uri)*](/uri)
[foo [bar baz](/uri)](/uri)
Example 519Try It
](uri2)](uri3)
](/Yogurt_cry/ccs-md-editor-vue3/raw/master/doc/uri3)
These cases illustrate the precedence of link text grouping over emphasis grouping:
Example 520Try It
*foo*
*foo*
Example 521Try It
[foo *bar](baz*)
Note that brackets that aren’t part of links do not take precedence:
Example 522Try It
foo [bar baz]
foo [bar baz]
These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:
Example 523Try It
[foo
[foo
Example 524Try It
[foo`](/uri)`
[foo](/uri)
Example 525Try It
[foo
[foohttp://example.com/?search=](uri)
There are three kinds of reference links: full, collapsed, and shortcut.
A full reference link consists of a link text immediately followed by a link label that matches a link reference definition elsewhere in the document.
A link label begins with a left bracket ([) and ends with the first right bracket (]) that is not backslash-escaped. Between these brackets there must be at least one character that is not a space, tab, or line ending. Unescaped square bracket characters are not allowed inside the opening and closing square brackets of link labels. A link label can have at most 999 characters inside the square brackets.
One label matches another just in case their normalized forms are equal. To normalize a label, strip off the opening and closing brackets, perform the Unicode case fold, strip leading and trailing spaces, tabs, and line endings, and collapse consecutive internal spaces, tabs, and line endings to a single space. If there are multiple matching reference link definitions, the one that comes first in the document is used. (It is desirable in such cases to emit a warning.)
The link’s URI and title are provided by the matching link reference definition.
Here is a simple example:
Example 526Try It
foo
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 527Try It
[link [foo bar]]ref
Example 528Try It
[link \[bar][ref]
The link text may contain inline content:
Example 529Try It
link foo bar #
Example 530Try It
[][ref]
However, links may not contain other links, at any level of nesting.
Example 531Try It
[foo bar]ref
Example 532Try It
[foo *bar [baz][ref]*][ref]
(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 533Try It
*foo*
*foo*
Example 534Try It
[foo *bar][ref]*
These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:
Example 535Try It
[foo
[foo
Example 536Try It
[foo`][ref]`
[foo][ref]
Example 537Try It
[foo
[foohttp://example.com/?search=][ref]
Matching is case-insensitive:
Example 538Try It
foo
Unicode case fold is used:
Example 539Try It
ẞ
Consecutive internal spaces, tabs, and line endings are treated as one space for purposes of determining matching:
Example 540Try It
[Foo
bar]: /url
[Baz][Foo bar]
No spaces, tabs, or line endings are allowed between the link text and the link label:
[foo] bar
Example 542Try It
[foo]
[bar]
[foo]
bar
This is a departure from 约翰·格鲁伯’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:
When there are multiple matching link reference definitions, the first is used:
Example 543Try It
foo: /url1
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 544Try It
[bar][foo!]
[bar][foo!]
Link labels cannot contain brackets, unless they are backslash-escaped:
Example 545Try It
foo[ref[]
[ref[]: /uri
[foo][ref[]
[ref[]: /uri
Example 546Try It
[foo][ref[bar]]
[refbar]: /uri
[foo][ref[bar]]
[ref[bar]]: /uri
Example 547Try It
[[[foo]]]
[[foo]]: /url
[[[foo]]]
[[[foo]]]: /url
Example 548Try It
[foo][ref\[]
Note that in this example ] is not backslash-escaped:
Example 549Try It
[bar\]: /uri
[bar\]
A link label must contain at least one character that is not a space, tab, or line ending:
Example 550Try It
[]
[]: /uri
[]
[]: /uri
Example 551Try It
[
]
[
]: /uri
[
]
[
]: /uri
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 552Try It
foo
Example 553Try It
[*foo* bar][]
The link labels are case-insensitive:
Example 554Try It
Foo
As with full reference links, spaces, tabs, or line endings are not allowed between the two sets of brackets:
Example 555Try It
foo
[]
foo
[]
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 556Try It
foo
Example 557Try It
[*foo* bar]
Example 558Try It
[[*foo* bar]]
[foo bar]
Example 559Try It
[[bar [foo]
[[bar foo
The link labels are case-insensitive:
Example 560Try It
Foo
A space after the link text should be preserved:
Example 561Try It
foo bar
foo bar
If you just want bracketed text, you can backslash-escape the opening bracket to avoid links:
Example 562Try It
[foo]
[foo]
Note that this is a link, because a link label ends with the first following closing bracket:
Example 563Try It
[foo*]: /url
[foo]
*foo*
Full and compact references take precedence over shortcut references:
Example 564Try It
foo
Example 565Try It
[foo][]
Inline links also take precedence:
Example 566Try It
foo
Example 567Try It
[foo](not a link)
foo(not a link)
In the following case [bar][baz] is parsed as a reference, [foo] as normal text:
[foo]bar
Here, though, [foo][bar] is parsed as a reference, since [bar] is defined:
Here [foo] is not parsed as a shortcut reference, because it is followed by a link label (even though [bar] is not defined):
[foo]bar
6.4Images
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 
Example 572Try It
![foo *bar*]
Example 573Try It
](/url2)
Example 574Try It
](/url2)
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 bar. Only the plain string content is rendered, without formatting.
Example 575Try It
Example 576Try It
![foo *bar*][foobar]
Example 577Try It
Example 578Try It
My 
My 
Example 579Try It
![foo]()
Example 580Try It
Reference-style:
Example 581Try It
Example 582Try It
![foo][bar]
Collapsed:
Example 583Try It
Example 584Try It
![*foo* bar][]
The labels are case-insensitive:
Example 585Try It
As with reference links, spaces, tabs, and line endings, are not allowed between the two sets of brackets:
Example 586Try It
[]
[]
Shortcut:
Example 587Try It
Example 588Try It
![*foo* bar]
Note that link labels cannot contain unescaped brackets:
Example 589Try It
![foo]
[foo]: /url "title"
![[foo]]
[[foo]]: /url "title"
The link labels are case-insensitive:
Example 590Try It
If you just want a literal ! followed by bracketed text, you can backslash-escape the opening [:
Example 591Try It
![foo]
![foo]
If you want a link after a literal !, backslash-escape the !:
Example 592Try It
!foo
!foo
6.5Autolinks
Autolinks are absolute URIs and email addresses inside < and >. They are parsed as links, with the URL or email address as the link label.
A URI autolink consists of <, followed by an absolute URI followed by >. It is parsed as a link to the URI, with the URI as the link’s label.
An absolute URI, for these purposes, consists of a scheme followed by a colon (
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 593Try It
http://foo.bar.baz
Example 594Try It
http://foo.bar.baz/test?q=hello&id=22&boolean
Example 595Try It
Uppercase is also fine:
Example 596Try It
MAILTO:FOO@BAR.BAZ
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 597Try It
a+b+c:d
Example 598Try It
Example 599Try It
Example 600Try It
Spaces are not allowed in autolinks:
Example 601Try It
<http://foo.bar/baz bim>
<http://foo.bar/baz bim>
Backslash-escapes do not work inside autolinks:
Example 602Try It
http://example.com/\[\
An email autolink consists of <, followed by an email address, followed by >. The link’s label is the email address, and the URL is mailto: followed by the email address.
An email address, for these purposes, is anything that matches the non-normative regex from the HTML5 spec:
/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?
(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/
Examples of email autolinks:
Example 603Try It
foo@bar.example.com
Example 604Try It
Backslash-escapes do not work inside email autolinks:
Example 605Try It
<foo+@bar.example.com>
<foo+@bar.example.com>
These are not autolinks:
Example 606Try It
<>
<>
Example 607Try It
< http://foo.bar >
< http://foo.bar >
Example 608Try It
<m:abc>
Example 609Try It
<foo.bar.baz>
Example 610Try It
http://example.com
http://example.com
Example 611Try It
foo@bar.example.com
foo@bar.example.com
6.6Raw 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 , where text does not start with > or ->, does not end with -, and does not contain --. (See the HTML5 spec.)
A processing instruction consists of the string , 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 , and the string ]]>.
An HTML tag consists of an open tag, a closing tag, an HTML comment, a processing instruction, a declaration, or a CDATA section.
Here are some simple open tags:
Empty elements:
Whitespace is allowed:
With attributes:
Custom tag names can be used:
Example 616Try It
Foo
Foo
Illegal tag names, not parsed as HTML:
Example 617Try It
<33> <__>
<33> <__>
Illegal attribute names:
Example 618Try It
<a h*#ref="hi">
<a h*#ref="hi">
Illegal attribute values:
Example 619Try It
<a href="hi'> <a href=hi'>
<a href="hi'> <a href=hi'>
Illegal whitespace:
Example 620Try It
< a><
foo><bar/ >
<foo bar=baz
bim!bop />
< a><
foo><bar/ >
<foo bar=baz
bim!bop />
Missing whitespace:
Example 621Try It
<a href='bar'title=title>
<a href='bar'title=title>
Closing tags:
Example 622Try It
Illegal attributes in closing tag:
Example 623Try It
</a href="foo">
</a href="foo">
Comments:
Example 624Try It
foo
foo
Example 625Try It
foo
foo <!-- not a comment -- two hyphens -->
Not comments:
Example 626Try It
foo foo -->
foo <!-- foo--->
foo <!--> foo -->
foo <!-- foo--->
Processing instructions:
Example 627Try It
foo
foo
Declarations:
Example 628Try It
foo
foo
CDATA sections:
Example 629Try It
foo &<]]>
foo
Entity and numeric character references are preserved in HTML attributes:
Backslash escapes do not work in HTML attributes:
Example 632Try It
<a href=""">
6.7Hard 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
tag):
Example 633Try It
foo
baz
foo
baz
For a more visible alternative, a backslash before the line ending may be used instead of two or more spaces:
Example 634Try It
foo
baz
foo
baz
More than two spaces can be used:
Example 635Try It
foo
baz
foo
baz
Leading spaces at the beginning of the next line are ignored:
Example 636Try It
foo
bar
foo
bar
Example 637Try It
foo\
bar
foo
bar
Hard line breaks can occur inside emphasis, links, and other constructs that allow inline content:
Example 638Try It
foo
bar
foo
bar
Example 639Try It
*foo\
bar*
foo
bar
Hard line breaks do not occur inside code spans
Example 640Try It
code span
code span
Example 641Try It
`code\
span`
code\ span
or HTML tags:
Example 643Try It
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 644Try It
foo\
foo\
Example 645Try It
foo
foo
Example 646Try It
### foo\
foo\
Example 647Try It
### foo
foo
6.8Soft 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 648Try It
foo
baz
foo
baz
Spaces at the end of the line and beginning of the next line are removed:
Example 649Try It
foo
baz
foo
baz
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.
6.9Textual content
Any characters not given an interpretation by the above rules will be parsed as plain textual content.
Example 650Try It
hello $.;'there
hello $.;'there
Example 651Try It
Foo χρῆν
Foo χρῆν
Internal spaces are preserved verbatim:
Example 652Try It
Multiple spaces
Multiple spaces
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:
In the first phase, lines of input are consumed and the block structure of the document—its division into paragraphs, block quotes, list items, and so on—is constructed. Text is assigned to these blocks but not parsed. Link reference definitions are parsed and a map of links is constructed.
In the second phase, the raw text contents of paragraphs and headings are parsed into sequences of Markdown inline elements (strings, code spans, links, emphasis, and so on), using the map of link references constructed in phase 1.
At each point in processing, the document is represented as a tree of blocks. The root of the tree is a document block. The document may have any number of other blocks as children. These children may, in turn, have other blocks as children. The last child of a block is normally considered open, meaning that subsequent lines of input can alter its contents. (Blocks that are not open are closed.) Here, for example, is a possible document tree, with the open blocks marked by arrows:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui quodsi iracundia"
-> list_item
-> paragraph
"aliquando id"
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:
One or more open blocks may be closed.
One or more new blocks may be created as children of the last open block.
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:
First we iterate through the open blocks, starting with the root document, and descending through last children down to the last open block. Each block imposes a condition that the line must satisfy if the block is to remain open. For example, a block quote requires a > character. A paragraph requires a non-blank line. In this phase we may match all or just some of the open blocks. But we cannot close unmatched blocks yet, because we may have a lazy continuation line.
Next, after consuming the continuation markers for existing blocks, we look for new block starts (e.g. > for a block quote). If we encounter a new block start, we close any blocks unmatched in step 1 before creating the new block as a child of the last matched container block.
Finally, we look at the remainder of the line (after block markers like >, list markers, and indentation have been consumed). This is text that can be incorporated into the last open block (a paragraph, code block, heading, or raw HTML).
Setext headings are formed when we see a line of a paragraph that is a setext heading underline.
Reference link definitions are detected when a paragraph is closed; the accumulated text lines are parsed to see if they begin with one or more reference link definitions. Any remainder becomes a normal paragraph.
We can see how this works by considering how the tree above is generated by four lines of Markdown:
Lorem ipsum dolor
sit amet.
- Qui quodsi iracundia
- aliquando id
At the outset, our document model is just
-> document
The first line of our text,
Lorem ipsum dolor
causes a block_quote block to be created as a child of our open document block, and a paragraph block as a child of the block_quote. Then the text is added to the last open block, the paragraph:
-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor"
The next line,
sit amet.
is a “lazy continuation” of the open paragraph, so it gets added to the paragraph’s text:
-> document
-> block_quote
-> paragraph
"Lorem ipsum dolor\nsit amet."
The third line,
- Qui quodsi iracundia
causes the paragraph block to be closed, and a new list block opened as a child of the block_quote. A list_item is also added as a child of the list, and a paragraph as a child of the list_item. The text is then added to the new paragraph:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
-> list_item
-> paragraph
"Qui quodsi iracundia"
The fourth line,
- aliquando id
causes the list_item (and its child the paragraph) to be closed, and a new list_item opened up as child of the list. A paragraph is added as a child of the new list_item, to contain the text. We thus obtain the final tree:
-> document
-> block_quote
paragraph
"Lorem ipsum dolor\nsit amet."
-> list (type=bullet tight=true bullet_char=-)
list_item
paragraph
"Qui quodsi iracundia"
-> list_item
-> paragraph
"aliquando id"
Phase 2: inline structure
Once all of the input has been parsed, all open blocks are closed.
We then “walk the tree,” visiting every node, and parse raw string contents of paragraphs and headings as inlines. At this point we have seen all the link reference definitions, so we can resolve reference links as we go.
document
block_quote
paragraph
str "Lorem ipsum dolor"
softbreak
str "sit amet."
list (type=bullet tight=true bullet_char=-)
list_item
paragraph
str "Qui "
emph
str "quodsi iracundia"
list_item
paragraph
str "aliquando id"
Notice how the line ending in the first paragraph has been parsed as a softbreak, and the asterisks in the first list item have become an emph.
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 ![
we insert a text node with these symbols as its literal content, and we add a pointer to this text node to the delimiter stack.
The delimiter stack is a doubly linked list. Each element contains a pointer to a text node, plus information about
the type of delimiter ([, ![, *, _)
the number of delimiters,
whether the delimiter is “active” (all are active to start), and
whether the delimiter is a potential opener, a potential closer, or both (which depends on what sort of characters precede and follow the delimiters).
When we hit a ] character, we call the look for link or image procedure (see below).
When we hit the end of the input, we call the process emphasis procedure (see below), with stack_bottom = NULL.
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.
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