GitHub Flavored Markdown Spec

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:

And here is the equivalent in Markdown:

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.)

What is the exact rule for determining when list items get wrapped in

tags? Can a list be partially “loose” and partially “tight”? What should we do with a list like this?

Or this?

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

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

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

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.)

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 ?

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

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

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

Can list items be empty?

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

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

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:

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:

foo

bar

foo

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.

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:

Foo ***

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:

_ _ _ _ a

a------

---a---

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:

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

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:

More than six # characters is not a heading:

####### foo

At least one space is required between the # characters and the heading’s contents, unless the heading is empty. Note that many implementations currently do not require the space. However, the space was required by the original ATX implementation, and it helps prevent things like the following from being parsed as headings:

#5 bolt

#hashtag

This is not a heading, because the first # is escaped:

## foo

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:

foo # bar

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:

Foo bar

Bar foo

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:

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:

Foo ---

The setext heading underline cannot contain internal spaces:

Foo = =

Foo

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:

`

of dashes;/;

The setext heading underline cannot be a lazy continuation line in a list item or block quote:

Foo

foo bar ===

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:

Baz

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:

foo

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

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:

Foo

baz

Authors who want interpretation 2 can put blank lines around the thematic break,

Foo bar

baz

or use a thematic break that cannot count as a setext heading underline, such as

Foo bar

baz

Authors who want interpretation 3 can use backslash escapes:

Foo bar --- baz

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:

foo

bar

foo

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.)

Foo bar

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:

bar

And indented code can occur immediately before and after other kinds of blocks:

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:

foo

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):

bbb

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:

aaa

Fenced code blocks can interrupt paragraphs, and can be followed directly by paragraphs, without a blank line between:

foo

baz

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.

Info strings for backtick code blocks cannot contain backticks:

aa foo

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 .

Start condition: line begins with 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 . End condition: line contains the string ]]>.

Start condition: line begins 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, within a 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:

world.

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:

okay.

A block can also start with a closing tag:

Here we have two HTML blocks with a Markdown paragraph between them:

Markdown

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:

bar

A partial tag need not even be completed (garbage in, garbage out):

baz

Note that anything on the last line after the end tag will be included in the HTML block:

A comment (type 2):

okay

A processing instruction (type 3):

okay

A declaration (type 4):

CDATA (type 5):

An HTML block of types 1–6 can interrupt a paragraph, and need not be preceded by a blank line.

Foo

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:

Foo baz

This rule differs from John Gruber’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 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:

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:

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:

Fortunately, blank lines are usually not necessary and can be deleted. The exception is inside tags, but as described above, raw HTML blocks starting with 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

foo

Foo*bar]

Foo bar

The title may extend over multiple lines:

foo

However, it may not contain a blank line:

[foo]: /url 'title

with blank line'

[foo]

The title may be omitted:

foo

The link destination may not be omitted:

[foo]:

[foo]

However, an empty link destination may be specified using angle brackets:

foo

The title must be separated from the link destination by whitespace:

[foo]: (baz)

[foo]

Both title and destination can contain backslash escapes and literal backslashes:

foo

A link can come before its corresponding definition:

foo

If there are several matching definitions, the first one takes precedence:

foo

As noted in the section on Links, matching of labels is case-insensitive (see matches).

Foo

αγω

Here is a link reference definition with no corresponding link. It contributes nothing to the document.

Here is another one:

bar

This is not a link reference definition, because there are non-whitespace characters after the title:

[foo]: /url ;title; ok

This is a link reference definition, but it has no title:

;title; ok

This is not a link reference definition, because it is indented four spaces:

[foo]

This is not a link reference definition, because it occurs inside a code block:

[foo]

A link reference definition cannot interrupt a paragraph.

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.

bar

foo

=== foo

Several link reference definitions can occur one after another, without intervening blank lines.

foo, bar, baz

Link reference definitions can occur inside block containers, like lists and block quotations. They affect the entire document, not just the container in which they are defined:

foo

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:

aaa

bbb

Paragraphs can contain multiple lines, but no blank lines:

aaa bbb

ccc ddd

Multiple blank lines between paragraph have no effect:

aaa

bbb

Leading spaces are skipped:

aaa bbb

Lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs.

aaa bbb ccc

However, the first line may be indented at most three spaces, or an indented code block will be triggered:

aaa bbb

bbb

Final spaces are stripped before inline parsing, so a paragraph that ends with two or more spaces will not end with a hard line break:

aaa bbb

Blank lines between block-level elements are ignored, except for the role they play in determining whether a list is tight or loose.

Blank lines at the beginning and end of the document are also ignored.

aaa

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.

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:

Include a pipe in a cell’s content by escaping it, including inside other inline spans:

The table is broken at the first empty line, or beginning of another block-level structure:

bar

bar

The header row must match the delimiter row in the number of cells. If not, a table will not be recognized:

| abc | def | | --- | | bar |

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:

If there are no rows in the body, no is generated in HTML output:

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:

bar baz

The spaces after the > characters can be omitted:

bar baz

The > characters can be indented 1-3 spaces:

bar baz

Four spaces gives us a code block:

The Laziness clause allows us to omit the > before paragraph continuation text:

bar baz

A block quote can contain some lazy and some non-lazy continuation lines:

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 of

without changing the meaning:

foo

Similarly, if we omit the > in the second line of

then the block quote ends after the first line:

For the same reason, we can’t omit the > in front of subsequent lines of an indented or fenced code block:

foo

Note that in the following case, we have a lazy continuation line:

foo - bar

To see why, note that in

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:

A block quote can have initial or final blank lines:

foo

A blank line always separates block quotes:

foo

bar

(Most current Markdown implementations, including John Gruber’s original Markdown.pl, will parse this example as a single block quote with two paragraphs. But it seems better to allow the author to decide whether two block quotes or one are wanted.)

Consecutiveness means that if we put these block quotes together, we get a single block quote:

foo bar

To get a block quote with two paragraphs, use:

foo

bar

Block quotes can interrupt paragraphs:

foo

bar

In general, blank lines are not needed before or after block quotes:

aaa

bbb

However, because of laziness, a blank line is needed between a block quote and a following paragraph:

bar baz

bar

baz

bar

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:

foo bar

foo bar baz

When including an indented code block in a block quote, remember that the block quote marker includes both the > and a following space. So five spaces are needed after the >:

not code

A list marker is a bullet list marker or an ordered list marker.

A bullet list marker is a -, +, or * character.

An ordered list marker is a sequence of 1–9 arabic digits (0-9), followed by either a . character or a ) character. (The reason for the length limit is that with 10 digits we start seeing integer overflows in some browsers.)

The following rules define list items:

Basic case. If a sequence of lines Ls constitute a sequence of blocks Bs starting with a non-whitespace character, and M is a list marker of width W followed by 1 ≤ N ≤ 4 spaces, then the result of prepending M and the following spaces to the first line of Ls, and indenting subsequent lines of Ls by W + N spaces, is a list item with Bs as its contents. The type of the list item (bullet or ordered) is determined by the type of its list marker. If the list item is ordered, then it is also assigned a start number, based on the ordered list marker.

Exceptions:

For example, let Ls be the lines

A paragraph with two lines.

A block quote.

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 paragraph with two lines.

A block quote.

The most important thing to notice is that the position of the text after the list marker determines how much indentation is needed in subsequent blocks in the list item. If the list marker takes up two spaces, and there are three spaces between the list marker and the next non-whitespace character, then blocks must be indented five spaces in order to fall under the list item.

Here are some examples showing how far content must be indented to be put under the list item:

two

one

two

one

two

It is tempting to think of this in terms of columns: the continuation blocks must be indented at least to the column of the first non-whitespace character after the list marker. However, that is not quite right. The spaces after the list marker determine how much relative indentation is needed. Which column this indentation reaches will depend on how the list item is embedded in other constructions, as shown by this example:

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.

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:

two

Note that at least one space is needed between the list marker and any following content, so these are not list items:

-one

2.two

A list item may contain blocks that are separated by more than one blank line.

foo

bar

A list item may contain any kind of block:

foo

baz

bam

A list item that contains an indented code block will preserve empty lines within the code block verbatim.

Foo

Note that ordered list start numbers must be nine digits or less:

1234567890. not ok

A start number may begin with 0s:

A start number may not be negative:

-1. not ok

An indented code block will have to be indented four spaces beyond the edge of the region where text will be included in the list item. In the following case that is 6 spaces:

foo

And in this case it is 11 spaces:

foo

If the first block in the list item is an indented code block, then by rule #2, the contents must be indented one space after the list marker:

paragraph

paragraph

Note that an additional space indent is interpreted as space inside the code block:

paragraph

Note that rules #1 and #2 only apply to two cases: (a) cases in which the lines to be included in a list item begin with a non-whitespace character, and (b) cases in which they begin with an indented code block. In a case like the following, where the first block begins with a three-space indent, the rules do not allow us to form a list item by indenting the whole thing and prepending a list marker:

foo

bar

bar

This is not a significant restriction, because when a block begins with 1-3 spaces indent, the indentation can always be removed without a change in interpretation, allowing rule #1 to be applied. So, in the above case:

foo

bar

Here are some list items that start with a blank line but are not empty:

When the list item starts with a blank line, the number of spaces following the list marker doesn’t change the required indentation:

A list item can begin with at most one blank line. In the following example, foo is not part of the list item:

foo

Here is an empty bullet list item:

It does not matter whether there are spaces following the list marker:

Here is an empty ordered list item:

A list may start or end with an empty list item:

However, an empty list item cannot interrupt a paragraph:

foo *

foo 1.

Indented one space:

A paragraph with two lines.

A block quote.

Indented two spaces:

A paragraph with two lines.

A block quote.

Indented three spaces:

A paragraph with two lines.

A block quote.

Four spaces indent gives a code block:

Here is an example with lazy continuation lines:

A paragraph with two lines.

A block quote.

Indentation can be partially deleted:

These examples show how laziness can work in nested structures:

Blockquote continued here.

Blockquote continued here.

The rules for sublists follow from the general rules above. A sublist must be indented the same number of spaces a paragraph would need to be in order to be included in the list item.

So, in this case we need two spaces indent:

One is not enough:

Here we need four, because the list marker is wider:

Three is not enough:

A list may be the first block in a list item:

A list item can contain a heading:

John Gruber’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

should be parsed as two lists with an intervening paragraph,

bar

as the four-space rule demands, rather than a single list,

foo

bar

The choice of four spaces is arbitrary. It can be learned, but it is not likely to be guessed, and it trips up beginners regularly.

Would it help to adopt a two-space rule? The problem is that such a rule, together with the rule allowing 1–3 spaces indentation of the initial list marker, allows text that is indented less than the original list marker to be included in the list item. For example, Markdown.pl parses

as a single list item, with two a continuation paragraph:

one

two

and similarly

as

one

two

This is extremely unintuitive.

Rather than requiring a fixed indent from the margin, we could require a fixed indent (say, two spaces, or even one space) from the list marker (which may itself be indented). This proposal would remove the last anomaly discussed. Unlike the spec presented above, it would count the following as a list item with a subparagraph, even though the paragraph bar is not indented as far as the first paragraph foo:

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:

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.

GFM enables the tasklist extension, where an additional processing step is performed on list items.

A task list item is a list item where the first block in it is a paragraph which begins with a task list item marker and at least one whitespace character before any other content.

A task list item marker consists of an optional number of spaces, a left bracket ([), either a whitespace character or the letter x in either lowercase or uppercase, and then a right bracket (]).

When rendered, the task list item marker is replaced with a semantic checkbox element; in an HTML output, this would be an element.

If the character between the brackets is a whitespace character, the checkbox is unchecked. Otherwise, the checkbox is checked.

This spec does not define how the checkbox elements are interacted with: in practice, implementors are free to render the checkboxes as disabled or inmutable elements, or they may dynamically handle dynamic interactions (i.e. checking, unchecking) in the final rendered document.

Task lists can be arbitrarily nested:

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:

In CommonMark, a list can interrupt a paragraph. That is, no blank line is needed to separate a paragraph from a following list:

Foo

Markdown.pl does not allow this, through fear of triggering a list via a numeral in a hard-wrapped line:

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:

Second, we are attracted to a

principle of uniformity: if a chunk of text has a certain meaning, it will continue to have the same meaning when put into a container block (such as a list item or blockquote).

(Indeed, the spec for list items and block quotes presupposes this principle.) This principle implies that if

is a list item containing a paragraph followed by a nested sublist, as all Markdown implementations agree it is (though the paragraph may be rendered without

tags, since the list is “tight”), then

by itself should be a paragraph followed by a nested sublist.

Since it is well established Markdown practice to allow lists to interrupt paragraphs inside list items, the principle of uniformity requires us to allow this outside list items as well. (reStructuredText 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,

The number of windows in my house is 14. The number of doors is 6.

We may still get an unintended result in cases like

The number of windows in my house is

but this rule should prevent most spurious list captures.

There can be any number of blank lines between items:

foo

bar

baz

baz

bim

To separate consecutive lists of the same type, or to separate a list from an indented code block that would otherwise be parsed as a subparagraph of the final list item, you can insert a blank HTML comment:

foo

notcode

foo

List items need not be indented to the same level. The following list items will be treated as items at the same list level, since none is indented enough to belong to the previous list item:

a

b

c

Note, however, that list items may not be indented more than three spaces. Here - e is treated as a paragraph continuation line, because it is indented more than three spaces:

And here, 3. c is treated as in indented code block, because it is indented four spaces and preceded by a blank line.

a

b

This is a loose list, because there is a blank line between two of the list items:

a

b

c

So is this, with a empty second item:

a

c

These are loose lists, even though there is no space between the items, because one of the items directly contains two block-level elements with a blank line between them:

a

b

c

d

a

b

d

This is a tight list, because the blank lines are in a code block:

This is a tight list, because the blank line is between two paragraphs of a sublist. So the sublist is loose while the outer list is tight:

b

c

This is a tight list, because the blank line is inside the block quote:

b

This list is tight, because the consecutive block elements are not separated by blank lines:

b

A single-paragraph list is tight:

This list is loose, because of the blank line between the two block elements in the list item:

bar

Here the outer list is loose, the inner list tight:

foo

baz

a

d

Inlines are parsed sequentially from the beginning of the character stream to the end (left to right, in left-to-right languages). Thus, for example, in

hilo`

hi is parsed as code, leaving the backtick at the end as a literal backtick.

Any ASCII punctuation character may be backslash-escaped:

!;#$%;'()*+,-./:;;=;?@[\]^_`{|}~

Backslashes before other characters are treated as literal backslashes:

\→\A\a\ \3\φ\«

Escaped characters are treated as regular characters and do not have their usual Markdown meanings:

*not emphasized* ;br/; not a tag [not a link](/foo) `not code` 1. not a list * not a list # not a heading [foo]: /url ;not a reference; ;ouml; not a character entity

If a backslash is itself escaped, the following character is not:

\emphasis

A backslash at the end of the line is a hard line break:

foo bar

Backslash escapes do not work in code blocks, code spans, autolinks, or raw HTML:

\[\`

http://example.com?find=\*

But they work in all other contexts, including URLs and link titles, link references, and info strings in fenced code blocks:

foo

foo

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/multipage/entities.json is used as an authoritative source for the valid entity references and their corresponding code points.

  ; © 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.

# 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).

; ആ ಫ

Here are some nonentities:

;nbsp ;x; ;#; ;#x; ;#87654321; ;#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:

;copy

Strings that are not on the list of HTML5 named entities are not recognized as entity references either:

;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:

foo

foo

Entity and numeric character references are treated as literal text in code spans and code blocks:

f;ouml;;ouml;

Entity and numeric character references cannot be used in place of symbols indicating structure in CommonMark documents.

*foo* foo

* foo

foo bar

→foo

[a](url ;tit;)

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 the two backtick strings, normalized in the following ways:

This is a simple code span:

foo

Here two backticks are used, because the code contains a backtick. This example also illustrates stripping of a single leading and trailing space:

foo ` bar

This example shows the motivation for stripping leading and trailing spaces:

``

Note that only one space is stripped:

``

The stripping only happens if the space is on both sides of the string:

a

Only spaces, and not unicode whitespace in general, are stripped in this way:

 b 

No stripping occurs if the code span contains only spaces:

Line endings are treated like spaces:

foo bar baz

foo

Interior spaces are not collapsed:

foo bar baz

Note that browsers will typically collapse consecutive spaces when rendering elements, so it is recommended that the following CSS be used:

Note that backslash escapes do not work in code spans. All backslashes are treated literally:

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.

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:

*foo*

And this is not parsed as a link:

[not a link](/foo)

Code spans, HTML tags, and autolinks have the same precedence. Thus, this is code:

;a href=;;;`

But this is an HTML tag:

`

And this is code:

;http://foo.bar.baz;`

But this is an autolink:

http://foo.bar.`baz`

When a backtick string is not closed by a matching backtick string, we just have literal backticks:

```foo``

`foo

The following case also illustrates the need for opening and closing backtick strings to be equal in length:

`foobar

John Gruber’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:

The following patterns are less widely supported, but the intent is clear and they are useful (especially in contexts like bibliography entries):

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.)

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 punctuation character, or (2b) followed by a punctuation character and preceded by Unicode whitespace or a 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 punctuation character, or (2b) preceded by a punctuation character and followed by Unicode whitespace or a 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:

right-flanking but not left-flanking:

Both left and right-flanking:

Neither left nor right-flanking:

(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 punctuation.

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 punctuation.

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 punctuation.

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 punctuation.

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*](bar) is parsed as *foo* rather than as [foo](bar).

These rules can be illustrated through a series of examples.

Rule 1:

foo bar

This is not emphasis, because the opening * is followed by whitespace, and hence not part of a left-flanking delimiter run:

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:

a*;foo;*

Unicode nonbreaking spaces count as whitespace, too:

* a *

Intraword emphasis with * is permitted:

foobar

5678

Rule 2:

foo bar

This is not emphasis, because the opening _ is followed by whitespace:

_ foo bar_

This is not emphasis, because the opening _ is preceded by an alphanumeric and followed by punctuation:

a_;foo;_

Emphasis with _ is not allowed inside words:

foo_bar_

5_6_78

пристаням_стремятся_

Here _ does not generate emphasis, because the first delimiter run is right-flanking and the second left-flanking:

aa_;bb;_cc

This is emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:

foo-(bar)

Rule 3:

This is not emphasis, because the closing delimiter does not match the opening delimiter:

_foo*

This is not emphasis, because the closing * is preceded by whitespace:

*foo bar *

A newline also counts as whitespace:

*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:

*(*foo)

The point of this restriction is more easily appreciated with this example:

(foo)

Intraword emphasis with * is allowed:

foobar

Rule 4:

This is not emphasis, because the closing _ is preceded by whitespace:

_foo bar _

This is not emphasis, because the second _ is preceded by punctuation and followed by an alphanumeric:

_(_foo)

This is emphasis within emphasis:

(foo)

Intraword emphasis is disallowed for _:

_foo_bar

_пристаням_стремятся

foo_bar_baz

This is emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:

(bar).

Rule 5:

foo bar

This is not strong emphasis, because the opening delimiter is followed by whitespace:

** 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:

a**;foo;**

Intraword strong emphasis with ** is permitted:

foobar

Rule 6:

foo bar

This is not strong emphasis, because the opening delimiter is followed by whitespace:

__ foo bar__

A newline counts as whitespace:

__ foo bar__

This is not strong emphasis, because the opening __ is preceded by an alphanumeric and followed by punctuation:

a__;foo;__

Intraword strong emphasis is forbidden with __:

foo__bar__

5__6__78

пристаням__стремятся__

foo, bar, baz

This is strong emphasis, even though the opening delimiter is both left- and right-flanking, because it is preceded by punctuation:

foo-(bar)

Rule 7:

This is not strong emphasis, because the closing delimiter is preceded by whitespace:

**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:

**(**foo)

The point of this restriction is more easily appreciated with these examples:

(foo)

Gomphocarpus (Gomphocarpus physocarpus, syn. Asclepias physocarpa)

foo ;bar; foo

Intraword emphasis:

foobar

Rule 8:

This is not strong emphasis, because the closing delimiter is preceded by whitespace:

__foo bar __

This is not strong emphasis, because the second __ is preceded by punctuation and followed by an alphanumeric:

__(__foo)

The point of this restriction is more easily appreciated with this example:

(foo)

Intraword strong emphasis is forbidden with __:

__foo__bar

__пристаням__стремятся

foo__bar__baz

This is strong emphasis, even though the closing delimiter is both left- and right-flanking, because it is followed by punctuation:

(bar).

Rule 9:

Any nonempty sequence of inline elements can be the contents of an emphasized span.

foo bar

foo bar

In particular, emphasis and strong emphasis can be nested inside emphasis:

foo bar baz

foo bar baz

foo bar

foo bar

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:

foo**bar

The same condition ensures that the following cases are all strong emphasis nested inside emphasis, even when the interior spaces are omitted:

foo bar

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:

foobarbaz

foobar***baz

Indefinite levels of nesting are possible:

foo bar baz bim bop

foo bar

There can be no empty emphasis or strong emphasis:

** is not an empty emphasis

**** is not an empty strong emphasis

Rule 10:

Any nonempty sequence of inline elements can be the contents of an strongly emphasized span.

foo bar

foo bar

In particular, emphasis and strong emphasis can be nested inside strong emphasis:

foo bar baz

foo bar baz

foo bar

foo bar

foo bar baz

foobarbaz

foo bar

foo bar

Indefinite levels of nesting are possible:

foo bar baz bim bop

foo bar

There can be no empty emphasis or strong emphasis:

__ is not an empty emphasis

____ is not an empty strong emphasis

Rule 11:

foo ***

foo *

foo _

foo *****

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:

*foo

foo*

*foo

***foo

foo*

foo***

Rule 12:

foo ___

foo _

foo *

foo _____

foo _

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:

foo_

_foo

___foo

foo_

foo___

Rule 13 implies that if you want emphasis nested directly inside emphasis, you must use different delimiters:

foo

foo

foo

foo

However, strong emphasis within strong emphasis is possible without switching delimiters:

foo

foo

Rule 13 can be applied to arbitrarily long sequences of delimiters:

foo

Rule 14:

foo

foo

Rule 15:

foo _bar baz_

foo bar *baz bim bam

Rule 16:

**foo bar baz

*foo bar baz

Rule 17:

*bar*

_foo bar_

*

**

__

a *

a _

**ahttp://foo.bar/?q=**

__ahttp://foo.bar/?q=__

GFM enables the strikethrough extension, where an additional emphasis type is available.

Strikethrough text is any text wrapped in a matching pair of one or two tildes (~).

Hi Hello, there world!

As with regular emphasis delimiters, a new paragraph will cause strikethrough parsing to cease:

This ~~has a

new paragraph~~.

Three or more tildes do not create a strikethrough:

This will ~~~not~~~ strike.

A link contains link text (the visible text), a link destination (the URI that is the link destination), and optionally a link title. There are two basic kinds of links in Markdown. In inline links the destination and title are given immediately after the link text. In reference links the destination and title are defined elsewhere in the document.

A link text consists of a sequence of zero or more inline elements enclosed by square brackets ([ and ]). The following rules apply:

Links may not contain other links, at any level of nesting. If multiple otherwise valid link definitions appear nested inside each other, the inner-most definition is used.

Brackets are allowed in the link text only if (a) they are backslash-escaped or (b) they appear as a matched pair of brackets, with an open bracket [, a sequence of zero or more inlines, and a close bracket ].

Backtick code spans, autolinks, and raw HTML tags bind more tightly than the brackets in link text. Thus, for example, [foo`]` could not be a link text, since the second ] is part of a code span.

The brackets in link text bind more tightly than markers for emphasis and strong emphasis. Thus, for example, *[foo*](url) is a link.

A link destination consists of either

a sequence of zero or more characters between an opening that contains no line breaks or unescaped characters, or

a nonempty sequence of characters that does not start with

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.

My

Reference-style:

Collapsed:

The labels are case-insensitive:

As with reference links, whitespace is not allowed between the two sets of brackets:

[]

Shortcut:

Note that link labels cannot contain unescaped brackets:

![[foo]]

[[foo]]: /url ;title;

The link labels are case-insensitive:

If you just want a literal ! followed by bracketed text, you can backslash-escape the opening [:

![foo]

If you want a link after a literal !, backslash-escape the !:

!foo

Autolinks are absolute URIs and email addresses inside . They are parsed as links, with the URL or email address as the link label.

A URI autolink consists of . It is parsed as a link to the URI, with the URI as the link’s label.

An absolute URI, for these purposes, consists of a scheme followed by a colon (:) followed by zero or more characters other than ASCII whitespace and control characters, . If the URI includes these characters, they must be percent-encoded (e.g. %20 for a space).

For purposes of this spec, a scheme is any sequence of 2–32 characters beginning with an ASCII letter and followed by any combination of ASCII letters, digits, or the symbols plus (“+”), period (“.”), or hyphen (“-”).

Here are some valid autolinks:

http://foo.bar.baz

http://foo.bar.baz/test?q=hello;id=22;boolean

irc://foo.bar:2233/baz

Uppercase is also fine:

MAILTO:[email protected]

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:

a+b+c:d

made-up-scheme://foo,bar

http://../

localhost:5001/foo

Spaces are not allowed in autolinks:

;http://foo.bar/baz bim;

Backslash-escapes do not work inside autolinks:

http://example.com/\[\

An email autolink consists of . 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:

Examples of email autolinks:

[email protected]

[email protected]

Backslash-escapes do not work inside email autolinks:

;[email protected];

These are not autolinks:

;;

; http://foo.bar ;

;m:abc;

;foo.bar.baz;

http://example.com

[email protected]

GFM enables the autolink extension, where autolinks will be recognised in a greater number of conditions.

Autolinks can also be constructed without requiring the use of to delimit them, although they will be recognized under a smaller set of circumstances. All such recognized autolinks can only come at the beginning of a line, after whitespace, or any of the delimiting characters *, _, ~, and (.

An extended www autolink will be recognized when the text www. is found followed by a valid domain. A valid domain consists of segments of alphanumeric characters, underscores (_) and hyphens (-) separated by periods (.). There must be at least one period, and no underscores may be present in the last two segments of the domain.

The scheme http will be inserted automatically:

www.commonmark.org

After a valid domain, zero or more non-space non-, 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 , a name consisting of one or more uppercase ASCII letters, whitespace, a string of characters not including the character >, and the character >.

A CDATA section consists of the string , a string of characters not including the string ]]>, and the string ]]>.

An HTML tag consists of an open tag, a closing tag, an HTML comment, a processing instruction, a declaration, or a CDATA section.

Here are some simple open tags:

Empty elements:

Whitespace is allowed:

With attributes:

Custom tag names can be used:

Foo

Illegal tag names, not parsed as HTML:

;33; ;__;

Illegal attribute names:

;a h*#ref=;hi;;

Illegal attribute values: