GitHub Flavored Markdown, often shortened as GFM, is the dialect of Markdown that is currently supported for user content on GitHub.com and GitHub Enterprise.
This formal specification, based on the CommonMark Spec, defines the syntax and semantics of this dialect.
GFM is a strict superset of CommonMark. All the features which are supported in GitHub user content and that are not specified on the original CommonMark Spec are hence known as extensions, and highlighted as such.
While GFM supports a wide range of inputs, it’s worth noting that GitHub.com and GitHub Enterprise perform additional post-processing and sanitization after GFM is converted to HTML to ensure security and consistency of the website.
Markdown is a plain text format for writing structured documents,
based on conventions for indicating formatting in email
and usenet posts. It was developed by John Gruber (with
help from Aaron Swartz) and released in 2004 in the form of a
syntax description
and a Perl script (Markdown.pl) for converting Markdown to
HTML. In the next decade, dozens of implementations were
developed in many languages. Some extended the original
Markdown syntax with conventions for footnotes, tables, and
other document elements. Some allowed Markdown documents to be
rendered in formats other than HTML. Websites like Reddit,
StackOverflow, and GitHub had millions of people using Markdown.
And Markdown started to be used beyond the web, to author books,
articles, slide shows, letters, and lecture notes.
What distinguishes Markdown from many other lightweight markup syntaxes, which are often easier to write, is its readability. As Gruber writes:
The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions. (http://daringfireball.net/projects/markdown/)
The point can be illustrated by comparing a sample of AsciiDoc with an equivalent sample of Markdown. Here is a sample of AsciiDoc from the AsciiDoc manual:
1. List item one.
+
List item one continued with a second paragraph followed by an
Indented block.
+
.................
$ ls *.sh
$ mv *.sh ~/tmp
.................
+
List item continued with a third paragraph.
2. List item two continued with an open block.
+
--
This paragraph is part of the preceding list item.
a. This list is nested and does not require explicit item
continuation.
+
This paragraph is part of the preceding list item.
b. List item b.
This paragraph belongs to item two of the outer list.
--
And here is the equivalent in Markdown:
1. List item one.
List item one continued with a second paragraph followed by an
Indented block.
$ ls *.sh
$ mv *.sh ~/tmp
List item continued with a third paragraph.
2. List item two continued with an open block.
This paragraph is part of the preceding list item.
1. This list is nested and does not require explicit item continuation.
This paragraph is part of the preceding list item.
2. List item b.
This paragraph belongs to item two of the outer list.
The AsciiDoc version is, arguably, easier to write. You don’t need to worry about indentation. But the Markdown version is much easier to read. The nesting of list items is apparent to the eye in the source, not just in the processed document.
John Gruber’s canonical description of Markdown’s syntax does not specify the syntax unambiguously. Here are some examples of questions it does not answer:
How much indentation is needed for a sublist? The spec says that
continuation paragraphs need to be indented four spaces, but is
not fully explicit about sublists. It is natural to think that
they, too, must be indented four spaces, but Markdown.pl does
not require that. This is hardly a “corner case,” and divergences
between implementations on this issue often lead to surprises for
users in real documents. (See this comment by John
Gruber.)
Is a blank line needed before a block quote or heading? Most implementations do not require the blank line. However, this can lead to unexpected results in hard-wrapped text, and also to ambiguities in parsing (note that some implementations put the heading inside the blockquote, while others do not). (John Gruber has also spoken in favor of requiring the blank lines.)
Is a blank line needed before an indented code block?
(Markdown.pl requires it, but this is not mentioned in the
documentation, and some implementations do not require it.)
paragraph
code?
What is the exact rule for determining when list items get
wrapped in <p> tags? Can a list be partially “loose” and partially
“tight”? What should we do with a list like this?
1. one
2. two
3. three
Or this?
1. one
- a
- b
2. two
(There are some relevant comments by John Gruber here.)
Can list markers be indented? Can ordered list markers be right-aligned?
8. item 1
9. item 2
10. item 2a
Is this one list with a thematic break in its second item, or two lists separated by a thematic break?
* a
* * * * *
* b
When list markers change from numbers to bullets, do we have two lists or one? (The Markdown syntax description suggests two, but the perl scripts and many other implementations produce one.)
1. fee
2. fie
- foe
- fum
What are the precedence rules for the markers of inline structure? For example, is the following a valid link, or does the code span take precedence ?
[a backtick (`)](/url) and [another backtick (`)](/url).
What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?
*foo *bar* baz*
What are the precedence rules between block-level and inline-level structure? For example, how should the following be parsed?
- `a long code span can contain a hyphen like this
- and it can screw things up`
Can list items include section headings? (Markdown.pl does not
allow this, but does allow blockquotes to include headings.)
- # Heading
Can list items be empty?
* a
*
* b
Can link references be defined inside block quotes or list items?
> Blockquote [foo].
>
> [foo]: /url
If there are multiple definitions for the same reference, which takes precedence?
[foo]: /url1
[foo]: /url2
[foo][]
In the absence of a spec, early implementers consulted Markdown.pl
to resolve these ambiguities. But Markdown.pl was quite buggy, and
gave manifestly bad results in many cases, so it was not a
satisfactory replacement for a spec.
Because there is no unambiguous spec, implementations have diverged considerably. As a result, users are often surprised to find that a document that renders one way on one system (say, a GitHub wiki) renders differently on another (say, converting to docbook using pandoc). To make matters worse, because nothing in Markdown counts as a “syntax error,” the divergence often isn’t discovered right away.
This document attempts to specify Markdown syntax unambiguously.
It contains many examples with side-by-side Markdown and
HTML. These are intended to double as conformance tests. An
accompanying script spec_tests.py can be used to run the tests
against any Markdown program:
python test/spec_tests.py --spec spec.txt --program PROGRAM
Since this document describes how Markdown is to be parsed into an abstract syntax tree, it would have made sense to use an abstract representation of the syntax tree instead of HTML. But HTML is capable of representing the structural distinctions we need to make, and the choice of HTML for the tests makes it possible to run the tests against an implementation without writing an abstract syntax tree renderer.
This document is generated from a text file, spec.txt, written
in Markdown with a small extension for the side-by-side tests.
The script tools/makespec.py can be used to convert spec.txt into
HTML or CommonMark (which can then be converted into other formats).
In the examples, the → character is used to represent tabs.
Any sequence of characters is a valid CommonMark document.
A character is a Unicode code point. Although some code points (for example, combining accents) do not correspond to characters in an intuitive sense, all code points count as characters for purposes of this spec.
This spec does not specify an encoding; it thinks of lines as composed of characters rather than bytes. A conforming parser may be limited to a certain encoding.
A line is a sequence of zero or more characters
other than newline (U+000A) or carriage return (U+000D),
followed by a line ending or by the end of file.
A line ending is a newline (U+000A), a carriage return
(U+000D) not followed by a newline, or a carriage return and a
following newline.
A line containing no characters, or a line containing only spaces
(U+0020) or tabs (U+0009), is called a blank line.
The following definitions of character classes will be used in this spec:
A whitespace character is a space
(U+0020), tab (U+0009), newline (U+000A), line tabulation (U+000B),
form feed (U+000C), or carriage return (U+000D).
Whitespace is a sequence of one or more whitespace characters.
A Unicode whitespace character is
any code point in the Unicode Zs general category, or a tab (U+0009),
carriage return (U+000D), newline (U+000A), or form feed
(U+000C).
Unicode whitespace is a sequence of one or more Unicode whitespace characters.
A space is U+0020.
A non-whitespace character is any character that is not a whitespace character.
An ASCII punctuation character
is !, ", #, $, %, &, ', (, ),
*, +, ,, -, ., / (U+0021–2F),
:, ;, <, =, >, ?, @ (U+003A–0040),
[, \, ], ^, _, ` (U+005B–0060),
{, |, }, or ~ (U+007B–007E).
A punctuation character is an ASCII
punctuation character or anything in
the general Unicode categories Pc, Pd, Pe, Pf, Pi, Po, or Ps.
Tabs in lines are not expanded to spaces. However, in contexts where whitespace helps to define block structure, tabs behave as if they were replaced by spaces with a tab stop of 4 characters.
Thus, for example, a tab can be used instead of four spaces in an indented code block. (Note, however, that internal tabs are passed through as literal tabs, not expanded to spaces.)
In the following example, a continuation paragraph of a list item is indented with a tab; this has exactly the same effect as indentation with four spaces would:
Normally the > that begins a block quote may be followed
optionally by a space, which is not considered part of the
content. In the following case > is followed by a tab,
which is treated as if it were expanded into three spaces.
Since one of these spaces is considered part of the
delimiter, foo is considered to be indented six spaces
inside the block quote context, so we get an indented
code block starting with two spaces.
- foo
- bar
→ - baz
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>
For security reasons, the Unicode character U+0000 must be replaced
with the REPLACEMENT CHARACTER (U+FFFD).
We can think of a document as a sequence of blocks—structural elements like paragraphs, block quotations, lists, headings, rules, and code blocks. Some blocks (like block quotes and list items) contain other blocks; others (like headings and paragraphs) contain inline content—text, links, emphasized text, images, code spans, and so on.
Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span:
This means that parsing can proceed in two steps: first, the block structure of the document can be discerned; second, text lines inside paragraphs, headings, and other block constructs can be parsed for inline structure. The second step requires information about link reference definitions that will be available only at the end of the first step. Note that the first step requires processing lines in sequence, but the second can be parallelized, since the inline parsing of one block element does not affect the inline parsing of any other.
We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot.
This section describes the different kinds of leaf block that make up a Markdown document.
A line consisting of 0-3 spaces of indentation, followed by a sequence
of three or more matching -, _, or * characters, each followed
optionally by any number of spaces or tabs, forms a
thematic break.
Wrong characters:
Not enough characters:
One to three spaces indent are allowed:
Four spaces is too many:
More than three characters may be used:
Spaces are allowed between the characters:
Spaces are allowed at the end:
However, no other characters may occur in the line:
It is required that all of the non-whitespace characters be the same. So, this is not a thematic break:
Thematic breaks do not need blank lines before or after:
Thematic breaks can interrupt a paragraph:
If a line of dashes that meets the above conditions for being a thematic break could also be interpreted as the underline of a setext heading, the interpretation as a setext heading takes precedence. Thus, for example, this is a setext heading, not a paragraph followed by a thematic break:
When both a thematic break and a list item are possible interpretations of a line, the thematic break takes precedence:
If you want a thematic break in a list item, use a different bullet:
An ATX heading
consists of a string of characters, parsed as inline content, between an
opening sequence of 1–6 unescaped # characters and an optional
closing sequence of any number of unescaped # characters.
The opening sequence of # characters must be followed by a
space or by the end of line. The optional closing sequence of #s must be
preceded by a space and may be followed by spaces only. The opening
# character may be indented 0-3 spaces. The raw contents of the
heading are stripped of leading and trailing spaces before being parsed
as inline content. The heading level is equal to the number of #
characters in the opening sequence.
Simple headings:
# foo
## foo
### foo
#### foo
##### foo
###### foo
<h1>foo</h1>
<h2>foo</h2>
<h3>foo</h3>
<h4>foo</h4>
<h5>foo</h5>
<h6>foo</h6>
More than six # characters is not a heading:
At least one space is required between the # characters and the
heading’s contents, unless the heading is empty. Note that many
implementations currently do not require the space. However, the
space was required by the
original ATX implementation,
and it helps prevent things like the following from being parsed as
headings:
This is not a heading, because the first # is escaped:
Contents are parsed as inlines:
Leading and trailing whitespace is ignored in parsing inline content:
One to three spaces indentation are allowed:
Four spaces are too much:
A closing sequence of # characters is optional:
It need not be the same length as the opening sequence:
Spaces are allowed after the closing sequence:
A sequence of # characters with anything but spaces following it
is not a closing sequence, but counts as part of the contents of the
heading:
The closing sequence must be preceded by a space:
Backslash-escaped # characters do not count as part
of the closing sequence:
ATX headings need not be separated from surrounding content by blank lines, and they can interrupt paragraphs:
ATX headings can be empty:
A setext heading consists of one or more lines of text, each containing at least one non-whitespace character, with no more than 3 spaces indentation, followed by a setext heading underline. The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a code fence, ATX heading, block quote, thematic break, list item, or HTML block.
A setext heading underline is a sequence of
= characters or a sequence of - characters, with no more than 3
spaces indentation and any number of trailing spaces. If a line
containing a single - can be interpreted as an
empty list items, it should be interpreted this way
and not as a setext heading underline.
The heading is a level 1 heading if = characters are used in
the setext heading underline, and a level 2 heading if -
characters are used. The contents of the heading are the result
of parsing the preceding lines of text as CommonMark inline
content.
In general, a setext heading need not be preceded or followed by a blank line. However, it cannot interrupt a paragraph, so when a setext heading comes after a paragraph, a blank line is needed between them.
Simple examples:
Foo *bar*
=========
Foo *bar*
---------
<h1>Foo <em>bar</em></h1>
<h2>Foo <em>bar</em></h2>
The content of the header may span more than one line:
The contents are the result of parsing the headings’s raw content as inlines. The heading’s raw content is formed by concatenating the lines and removing initial and final whitespace.
The underlining can be any length:
The heading content can be indented up to three spaces, and need not line up with the underlining:
Four spaces indent is too much:
The setext heading underline can be indented up to three spaces, and may have trailing spaces:
Four spaces is too much:
The setext heading underline cannot contain internal spaces:
Trailing spaces in the content line do not cause a line break:
Nor does a backslash at the end:
Since indicators of block structure take precedence over indicators of inline structure, the following are setext headings:
`Foo
----
`
<a title="a lot
---
of dashes"/>
<h2>`Foo</h2>
<p>`</p>
<h2><a title="a lot</h2>
<p>of dashes"/></p>
The setext heading underline cannot be a lazy continuation line in a list item or block quote:
A blank line is needed between a paragraph and a following setext heading, since otherwise the paragraph becomes part of the heading’s content:
But in general a blank line is not required before or after setext headings:
Setext headings cannot be empty:
Setext heading text lines must not be interpretable as block constructs other than paragraphs. So, the line of dashes in these examples gets interpreted as a thematic break:
If you want a heading with > foo as its literal text, you can
use backslash escapes:
Compatibility note: Most existing Markdown implementations do not allow the text of setext headings to span multiple lines. But there is no consensus about how to interpret
Foo
bar
---
baz
One can find four different interpretations:
We find interpretation 4 most natural, and interpretation 4 increases the expressive power of CommonMark, by allowing multiline headings. Authors who want interpretation 1 can put a blank line after the first paragraph:
Authors who want interpretation 2 can put blank lines around the thematic break,
or use a thematic break that cannot count as a setext heading underline, such as
Authors who want interpretation 3 can use backslash escapes:
An indented code block is composed of one or more indented chunks separated by blank lines. An indented chunk is a sequence of non-blank lines, each indented four or more spaces. The contents of the code block are the literal contents of the lines, including trailing line endings, minus four spaces of indentation. An indented code block has no info string.
An indented code block cannot interrupt a paragraph, so there must be a blank line between a paragraph and a following indented code block. (A blank line is not needed, however, between a code block and a following paragraph.)
If there is any ambiguity between an interpretation of indentation as a code block and as indicating that material belongs to a list item, the list item interpretation takes precedence:
The contents of a code block are literal text, and do not get parsed as Markdown:
Here we have three chunks separated by blank lines:
Any initial spaces beyond four will be included in the content, even in interior blank lines:
An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.)
However, any non-blank line with fewer than four leading spaces ends the code block immediately. So a paragraph may occur immediately after indented code:
And indented code can occur immediately before and after other kinds of blocks:
# Heading
foo
Heading
------
foo
----
<h1>Heading</h1>
<pre><code>foo
</code></pre>
<h2>Heading</h2>
<pre><code>foo
</code></pre>
<hr />
The first line can be indented more than four spaces:
Blank lines preceding or following an indented code block are not included in it:
Trailing spaces are included in the code block’s content:
A code fence is a sequence
of at least three consecutive backtick characters (`) or
tildes (~). (Tildes and backticks cannot be mixed.)
A fenced code block
begins with a code fence, indented no more than three spaces.
The line with the opening code fence may optionally contain some text following the code fence; this is trimmed of leading and trailing whitespace 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 indented N spaces, then up to N spaces of indentation are removed from each line of the content (if present). (If a content line is not indented, it is preserved unchanged. If it is indented less than N spaces, all of the indentation is removed.)
The closing code fence may be indented up to three spaces, and may be followed only by spaces, which are ignored. If the end of the containing block (or document) is reached and no closing code fence has been found, the code block contains all of the lines after the opening code fence until the end of the containing block (or document). (An alternative spec would require backtracking in the event that a closing code fence is not found. But this makes parsing much less efficient, and there seems to be no real down side to the behavior described here.)
A fenced code block may interrupt a paragraph, and does not require a blank line either before or after.
The content of a code fence is treated as literal text, not parsed
as inlines. The first word of the info string is typically used to
specify the language of the code sample, and rendered in the class
attribute of the code tag. However, this spec does not mandate any
particular treatment of the info string.
Here is a simple example with backticks:
With tildes:
Fewer than three backticks is not enough:
The closing code fence must use the same character as the opening fence:
The closing code fence must be at least as long as the opening fence:
Unclosed code blocks are closed by the end of the document (or the enclosing block quote or list item):
A code block can have all empty lines as its content:
A code block can be empty:
Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:
Four spaces indentation produces an indented code block:
Closing fences may be indented by 0-3 spaces, and their indentation need not match that of the opening fence:
This is not a closing fence, because it is indented 4 spaces:
Code fences (opening and closing) cannot contain internal spaces:
Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between:
Other blocks can also occur before and after fenced code blocks without an intervening blank line:
An info string can be provided after the opening code fence.
Although this spec doesn’t mandate any particular treatment of
the info string, the first word is typically used to specify
the language of the code block. In HTML output, the language is
normally indicated by adding a class to the code element consisting
of language- followed by the language name.
```ruby
def foo(x)
return 3
end
```
<pre><code class="language-ruby">def foo(x)
return 3
end
</code></pre>
~~~~ ruby startline=3 $%@#$
def foo(x)
return 3
end
~~~~~~~
<pre><code class="language-ruby">def foo(x)
return 3
end
</code></pre>
Info strings for backtick code blocks cannot contain backticks:
Info strings for tilde code blocks can contain backticks and tildes:
Closing code fences cannot have info strings:
An HTML block is a group of lines that is treated as raw HTML (and will not be escaped in HTML output).
There are seven kinds of HTML block, which can be defined by their start and end conditions. The block begins with a line that meets a start condition (after up to three spaces optional 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 <script,
<pre, or <style (case-insensitive), followed by whitespace,
the string >, or the end of the line.
End condition: line contains an end tag
</script>, </pre>, or </style> (case-insensitive; it
need not match the start tag).
Start condition: line begins with the string <!--.
End condition: line contains the string -->.
Start condition: line begins with the string <?.
End condition: line contains the string ?>.
Start condition: line begins with the string <!
followed by an uppercase ASCII letter.
End condition: line contains the character >.
Start condition: line begins with the string
<![CDATA[.
End condition: line contains the string ]]>.
Start condition: line begins 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 whitespace, 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 script,
style, or pre) or a complete closing tag,
followed only by whitespace or the end of the line.
End condition: line is followed by a blank line.
HTML blocks continue until they are closed by their appropriate end condition, or the last line of the document or other container block. This means any HTML within an HTML block that might otherwise be recognised as a start condition will be ignored by the parser and passed through as-is, without changing the parser’s state.
For instance, <pre> within a HTML block started by <table> will not affect
the parser state; as the HTML block was started in by start condition 6, it
will end at any blank line. This can be surprising:
<table><tr><td>
<pre>
**Hello**,
_world_.
</pre>
</td></tr></table>
<table><tr><td>
<pre>
**Hello**,
<p><em>world</em>.
</pre></p>
</td></tr></table>
In this case, the HTML block is terminated by the newline — the **Hello**
text remains verbatim — and regular parsing resumes, with a paragraph,
emphasised world and inline and block HTML following.
All types of HTML blocks except type 7 may interrupt a paragraph. Blocks of type 7 may not interrupt a paragraph. (This restriction is intended to prevent unwanted interpretation of long tags inside a wrapped paragraph as starting HTML blocks.)
Some simple examples follow. Here are some basic HTML blocks of type 6:
<table>
<tr>
<td>
hi
</td>
</tr>
</table>
okay.
<table>
<tr>
<td>
hi
</td>
</tr>
</table>
<p>okay.</p>
A block can also start with a closing tag:
Here we have two HTML blocks with a Markdown paragraph between them:
The tag on the first line can be partial, as long as it is split where there would be whitespace:
An open tag need not be closed:
A partial tag need not even be completed (garbage in, garbage out):
The initial tag doesn’t even need to be a valid tag, as long as it starts like one:
In type 6 blocks, the initial tag need not be on a line by itself:
Everything until the next blank line or end of document gets included in the HTML block. So, in the following example, what looks like a Markdown code block is actually part of the HTML block, which continues until a blank line or the end of the document is reached:
To start an HTML block with a tag that is not in the list of block-level tags in (6), you must put the tag by itself on the first line (and it must be complete):
In type 7 blocks, the tag name can be anything:
These rules are designed to allow us to work with tags that
can function as either block-level or inline-level tags.
The <del> tag is a nice example. We can surround content with
<del> tags in three different ways. In this case, we get a raw
HTML block, because the <del> tag is on a line by itself:
In this case, we get a raw HTML block that just includes
the <del> tag (because it ends with the following blank
line). So the contents get interpreted as CommonMark:
Finally, in this case, the <del> tags are interpreted
as raw HTML inside the CommonMark paragraph. (Because
the tag is not on a line by itself, we get inline HTML
rather than an HTML block.)
HTML tags designed to contain literal content
(script, style, pre), comments, processing instructions,
and declarations are treated somewhat differently.
Instead of ending at the first blank line, these blocks
end at the first line containing a corresponding end tag.
As a result, these blocks can contain blank lines:
A pre tag (type 1):
<pre language="haskell"><code>
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
</code></pre>
okay
<pre language="haskell"><code>
import Text.HTML.TagSoup
main :: IO ()
main = print $ parseTags tags
</code></pre>
<p>okay</p>
A script tag (type 1):
<script type="text/javascript">
// JavaScript example
document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
okay
<script type="text/javascript">
// JavaScript example
document.getElementById("demo").innerHTML = "Hello JavaScript!";
</script>
<p>okay</p>
A style tag (type 1):
<style
type="text/css">
h1 {color:red;}
p {color:blue;}
</style>
okay
<style
type="text/css">
h1 {color:red;}
p {color:blue;}
</style>
<p>okay</p>
If there is no matching end tag, the block will end at the end of the document (or the enclosing block quote or list item):
The end tag can occur on the same line as the start tag:
Note that anything on the last line after the end tag will be included in the HTML block:
A comment (type 2):
A processing instruction (type 3):
A declaration (type 4):
CDATA (type 5):
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then {
return 1;
} else {
return 0;
}
}
]]>
okay
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then {
return 1;
} else {
return 0;
}
}
]]>
<p>okay</p>
The opening tag can be indented 1-3 spaces, but not 4:
An HTML block of types 1–6 can interrupt a paragraph, and need not be preceded by a blank line.
However, a following blank line is needed, except at the end of a document, and except for blocks of types 1–5, above:
HTML blocks of type 7 cannot interrupt a paragraph:
This rule differs from John Gruber’s original Markdown syntax specification, which says:
The only restrictions are that block-level HTML elements — e.g.
<div>,<table>,<pre>,<p>, etc. — must be separated from surrounding content by blank lines, and the start and end tags of the block should not be indented with tabs or spaces.
In some ways Gruber’s rule is more restrictive than the one given here:
Most Markdown implementations (including some of Gruber’s own) do not respect all of these restrictions.
There is one respect, however, in which Gruber’s rule is more liberal than the one given here, since it allows blank lines to occur inside an HTML block. There are two reasons for disallowing them here. First, it removes the need to parse balanced tags, which is expensive and can require backtracking from the end of the document if no matching end tag is found. Second, it provides a very simple and flexible way of including Markdown content inside HTML tags: simply separate the Markdown from the HTML using blank lines:
Compare:
Some Markdown implementations have adopted a convention of
interpreting content inside tags as text if the open tag has
the attribute markdown=1. The rule given above seems a simpler and
more elegant way of achieving the same expressive power, which is also
much simpler to parse.
The main potential drawback is that one can no longer paste HTML blocks into Markdown documents with 100% reliability. However, in most cases this will work fine, because the blank lines in HTML are usually followed by HTML block tags. For example:
There are problems, however, if the inner tags are indented and separated by spaces, as then they will be interpreted as an indented code block:
<table>
<tr>
<td>
Hi
</td>
</tr>
</table>
<table>
<tr>
<pre><code><td>
Hi
</td>
</code></pre>
</tr>
</table>
Fortunately, blank lines are usually not necessary and can be
deleted. The exception is inside <pre> tags, but as described
above, raw HTML blocks starting with <pre>
can contain blank lines.
A link reference definition
consists of a link label, indented up to three spaces, followed
by a colon (:), optional whitespace (including up to one
line ending), a link destination,
optional whitespace (including up to one
line ending), and an optional link
title, which if it is present must be separated
from the link destination by whitespace.
No further non-whitespace characters may occur on the line.
A link reference definition does not correspond to a structural element of a document. Instead, it defines a label which can be used in reference links and reference-style images elsewhere in the document. Link reference definitions can come either before or after the links that use them.
[Foo*bar\]]:my_(url) 'title (with parens)'
[Foo*bar\]]
<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
[Foo bar]:
<my url>
'title'
[Foo bar]
<p><a href="my%20url" title="title">Foo bar</a></p>
The title may extend over multiple lines:
[foo]: /url '
title
line1
line2
'
[foo]
<p><a href="/url" title="
title
line1
line2
">foo</a></p>
However, it may not contain a blank line:
[foo]: /url 'title
with blank line'
[foo]
<p>[foo]: /url 'title</p>
<p>with blank line'</p>
<p>[foo]</p>
The title may be omitted:
The link destination may not be omitted:
However, an empty link destination may be specified using angle brackets:
The title must be separated from the link destination by whitespace:
Both title and destination can contain backslash escapes and literal backslashes:
[foo]: /url\bar\*baz "foo\"bar\baz"
[foo]
<p><a href="/url%5Cbar*baz" title="foo"bar\baz">foo</a></p>
A link can come before its corresponding definition:
If there are several matching definitions, the first one takes precedence:
As noted in the section on Links, matching of labels is case-insensitive (see matches).
Here is a link reference definition with no corresponding link. It contributes nothing to the document.
Here is another one:
This is not a link reference definition, because there are non-whitespace characters after the title:
This is a link reference definition, but it has no title:
This is not a link reference definition, because it is indented four spaces:
[foo]: /url "title"
[foo]
<pre><code>[foo]: /url "title"
</code></pre>
<p>[foo]</p>
This is not a link reference definition, because it occurs inside a code block:
A link reference definition cannot interrupt a paragraph.
However, it can directly follow other block elements, such as headings and thematic breaks, and it need not be followed by a blank line.
# [Foo]
[foo]: /url
> bar
<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>
Several link reference definitions can occur one after another, without intervening blank lines.
[foo]: /foo-url "foo"
[bar]: /bar-url
"bar"
[baz]: /baz-url
[foo],
[bar],
[baz]
<p><a href="/foo-url" title="foo">foo</a>,
<a href="/bar-url" title="bar">bar</a>,
<a href="/baz-url">baz</a></p>
Link reference definitions can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined:
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:
A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph’s raw content as inlines. The paragraph’s raw content is formed by concatenating the lines and removing initial and final whitespace.
A simple example with two paragraphs:
Paragraphs can contain multiple lines, but no blank lines:
Multiple blank lines between paragraph have no effect:
Leading spaces are skipped:
Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs.
However, the first line may be indented at most three spaces, or an indented code block will be triggered:
Final spaces are stripped before inline parsing, so a paragraph that ends with two or more spaces will not end with a hard line break:
Blank lines between block-level elements are ignored, except for the role they play in determining whether a list is tight or loose.
Blank lines at the beginning and end of the document are also ignored.
GFM enables the table extension, where an additional leaf block type is
available.
A table is an arrangement of data with rows and columns, consisting of a single header row, a delimiter row separating the header from the data, and zero or more data rows.
Each row consists of cells containing arbitrary text, in which inlines are
parsed, separated by pipes (|). A leading and trailing pipe is also
recommended for clarity of reading, and if there’s otherwise parsing ambiguity.
Spaces between pipes and cell content are trimmed. Block-level elements cannot
be inserted in a table.
The delimiter row consists of cells whose only content are hyphens (-),
and optionally, a leading or trailing colon (:), or both, to indicate left,
right, or center alignment respectively.
| foo | bar |
| --- | --- |
| baz | bim |
<table>
<thead>
<tr>
<th>foo</th>
<th>bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>baz</td>
<td>bim</td>
</tr>
</tbody>
</table>
Cells in one column don’t need to match length, though it’s easier to read if they are. Likewise, use of leading and trailing pipes may be inconsistent:
| abc | defghi |
:-: | -----------:
bar | baz
<table>
<thead>
<tr>
<th align="center">abc</th>
<th align="right">defghi</th>
</tr>
</thead>
<tbody>
<tr>
<td align="center">bar</td>
<td align="right">baz</td>
</tr>
</tbody>
</table>
Include a pipe in a cell’s content by escaping it, including inside other inline spans:
| f\|oo |
| ------ |
| b `\|` az |
| b **\|** im |
<table>
<thead>
<tr>
<th>f|oo</th>
</tr>
</thead>
<tbody>
<tr>
<td>b <code>|</code> az</td>
</tr>
<tr>
<td>b <strong>|</strong> im</td>
</tr>
</tbody>
</table>
The table is broken at the first empty line, or beginning of another block-level structure:
| abc | def |
| --- | --- |
| bar | baz |
> bar
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>
<blockquote>
<p>bar</p>
</blockquote>
| abc | def |
| --- | --- |
| bar | baz |
bar
bar
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
<tr>
<td>bar</td>
<td></td>
</tr>
</tbody>
</table>
<p>bar</p>
The header row must match the delimiter row in the number of cells. If not, a table will not be recognized:
The remainder of the table’s rows may vary in the number of cells. If there are a number of cells fewer than the number of cells in the header row, empty cells are inserted. If there are greater, the excess is ignored:
| abc | def |
| --- | --- |
| bar |
| bar | baz | boo |
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td></td>
</tr>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>
If there are no rows in the body, no <tbody> is generated in HTML output:
| abc | def |
| --- | --- |
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
</table>
A container block is a block that has other blocks as its contents. There are two basic kinds of container blocks: block quotes and list items. Lists are meta-containers for list items.
We define the syntax for container blocks recursively. The general form of the definition is:
If X is a sequence of blocks, then the result of transforming X in such-and-such a way is a container of type Y with these blocks as its content.
So, we explain what counts as a block quote or list item by explaining how these can be generated from their contents. This should suffice to define the syntax, although it does not give a recipe for parsing these constructions. (A recipe is provided below in the section entitled A parsing strategy.)
A block quote marker
consists of 0-3 spaces of initial indent, plus (a) the character > together
with a following space, or (b) a single character > not followed by a space.
The following rules define block quotes:
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 non-whitespace character after the block quote marker is paragraph continuation text is a block quote with Bs as its content. Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph.
Consecutiveness. A document cannot contain two block quotes in a row unless there is a blank line between them.
Nothing else counts as a block quote.
Here is a simple example:
The spaces after the > characters can be omitted:
The > characters can be indented 1-3 spaces:
Four spaces gives us a code block:
The Laziness clause allows us to omit the > before
paragraph continuation text:
A block quote can contain some lazy and some non-lazy continuation lines:
Laziness only applies to lines that would have been continuations of
paragraphs had they been prepended with block quote markers.
For example, the > cannot be omitted in the second line of
> foo
> ---
without changing the meaning:
Similarly, if we omit the > in the second line of
> - foo
> - bar
then the block quote ends after the first line:
> - foo
- bar
<blockquote>
<ul>
<li>foo</li>
</ul>
</blockquote>
<ul>
<li>bar</li>
</ul>
For the same reason, we can’t omit the > in front of
subsequent lines of an indented or fenced code block:
> foo
bar
<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>