title: CommonMark Spec
author:
– John MacFarlane
version: 0.11
date: 2014-11-10
…
Introduction
What is Markdown?
Markdown is a plain text format for writing structured documents,
based on conventions used for indicating formatting in email and
usenet posts. It was developed in 2004 by John Gruber, who wrote
the first Markdown-to-HTML converter in perl, and it soon became
widely used in websites. By 2014 there were dozens of
implementations in many languages. Some of them extended basic
Markdown syntax with conventions for footnotes, definition lists,
tables, and other constructs, and some allowed output not just in
HTML but in LaTeX and many other formats.
Why is a spec needed?
John Gruber’s canonical description of Markdown’s
syntax
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 header?
Most implementations do not require the blank line. However,
this can lead to unexpected results in hard-wrapped text, and
also to ambiguities in parsing (note that some implementations
put the header inside the blockquote, while others do not).
(John Gruber has also spoken in favor of requiring the blank
lines.)
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 “ 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 horizontal rule in its second item,
or two lists separated by a horizontal rule?
* 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 headers? (Markdown.pl does not allow this,
but headers can occur in blockquotes.)
- # Heading
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.
About this document
This document attempts to specify Markdown syntax unambiguously.
It contains many examples with side-by-side Markdown and
HTML. These are intended to double as conformance tests. An
accompanying script runtests.pl 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 spec2md.pl can be used to turn spec.txt into pandoc
Markdown, which can then be converted into other formats.
In the examples, the → character is used to represent tabs.
Preprocessing
A line
is a sequence of zero or more characters followed by a
line ending (CR, LF, or CRLF) or by the end of file.
A character is a unicode code point.
This spec does not specify an encoding; it thinks of lines as composed
of characters rather than bytes. A conforming parser may be limited
to a certain encoding.
Tabs in lines are expanded to spaces, with a tab stop of 4 characters:
.
→foo→baz→→bim
.
.
.
a→a
ὐ→a
.
.
Line endings are replaced by newline characters (LF).
A line containing no characters, or a line containing only spaces (after
tab expansion), is called a blank line.
Blocks and inlines
We can think of a document as a sequence of
blocks—structural
elements like paragraphs, block quotations,
lists, headers, rules, and code blocks. Blocks can contain other
blocks, or they can contain inline content:
words, spaces, links, emphasized text, images, and inline code.
Precedence
Indicators of block structure always take precedence over indicators
of inline structure. So, for example, the following is a list with
two items, not a list with one item containing a code span:
.
– one
- two
.
`one
two`
.
This means that parsing can proceed in two steps: first, the block
structure of the document can be discerned; second, text lines inside
paragraphs, headers, and other block constructs can be parsed for inline
structure. The second step requires information about link reference
definitions that will be available only at the end of the first
step. Note that the first step requires processing lines in sequence,
but the second can be parallelized, since the inline parsing of
one block element does not affect the inline parsing of any other.
Container blocks and leaf blocks
We can divide blocks into two types:
container blocks,
which can contain other blocks, and leaf blocks,
which cannot.
Leaf blocks
This section describes the different kinds of leaf block that make up a
Markdown document.
Horizontal rules
A line consisting of 0-3 spaces of indentation, followed by a sequence
of three or more matching -, _, or * characters, each followed
optionally by any number of spaces, forms a horizontal
rule.
.
***
.
.
Wrong characters:
.
+++
.
+++
.
.
.
===
.
Not enough characters:
.
**
__
.
–
**
__
.
One to three spaces indent are allowed:
.
.
.
Four spaces is too many:
.
***
.
.
.
Foo
***
.
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—
.
_ _ _ _ a
a——
—a—
.
It is required that all of the non-space characters be the same.
So, this is not a horizontal rule:
.
-
.
-
.
Horizontal rules do not need blank lines before or after:
.
– foo
bar
.
foo
bar
.
Horizontal rules can interrupt a paragraph:
.
Foo
bar
.
Foo
bar
.
If a line of dashes that meets the above conditions for being a
horizontal rule could also be interpreted as the underline of a setext
header, the interpretation as a
setext-header takes precedence. Thus, for example,
this is a setext header, not a paragraph followed by a horizontal rule:
.
Foo
bar
.
Foo
bar
.
When both a horizontal rule and a list item are possible
interpretations of a line, the horizontal rule is preferred:
.
* Foo
Bar
.
Foo
Bar
.
If you want a horizontal rule in a list item, use a different bullet:
.
– Foo
– * * *
.
Foo
.
ATX headers
An ATX header
consists of a string of characters, parsed as inline content, between an
opening sequence of 1–6 unescaped # characters and an optional
closing sequence of any number of # characters. The opening sequence
of # characters cannot be followed directly by a nonspace character.
The optional closing sequence of #s must be preceded by a space and may be
followed by spaces only. The opening # character may be indented 0-3
spaces. The raw contents of the header are stripped of leading and
trailing spaces before being parsed as inline content. The header level
is equal to the number of # characters in the opening sequence.
Simple headers:
.
foo
foo
foo
foo
foo
foo
.
foo
foo
foo
foo
foo
foo
.
More than six # characters is not a header:
.
####### foo
.
####### foo
.
A space is required between the # characters and the header’s
contents. Note that many implementations currently do not require
the space. However, the space was required by the original ATX
implementation, and it helps
prevent things like the following from being parsed as headers:
.
#5 bolt
.
#5 bolt
.
This is not a header, because the first # is escaped:
.
## foo
.
## foo
.
Contents are parsed as inlines:
.
foo bar *baz*
.
foo bar *baz*
.
Leading and trailing blanks are ignored in parsing inline content:
.
foo
.
foo
.
One to three spaces indentation are allowed:
.
### foo
## foo
# foo
.
foo
foo
foo
.
Four spaces are too much:
.
# foo
.
.
.
foo
# bar
.
foo
# bar
.
A closing sequence of # characters is optional:
.
foo
### bar ###
.
foo
bar
.
It need not be the same length as the opening sequence:
.
foo
foo
.
foo
foo
.
Spaces are allowed after the closing sequence:
.
foo
.
foo
.
A sequence of # characters with a nonspace character following it
is not a closing sequence, but counts as part of the contents of the
header:
.
foo ### b
.
foo ### b
.
The closing sequence must be preceded by a space:
.
foo
.
foo#
.
Backslash-escaped # characters do not count as part
of the closing sequence:
.
foo \
foo #\
foo \
.
foo ###
foo ###
foo #
.
ATX headers need not be separated from surrounding content by blank
lines, and they can interrupt paragraphs:
.
foo
.
foo
.
.
Foo bar
baz
Bar foo
.
Foo bar
baz
Bar foo
.
ATX headers can be empty:
.
#
#
.
.
Setext headers
A setext header
consists of a line of text, containing at least one nonspace character,
with no more than 3 spaces indentation, followed by a setext header
underline. The line of text must be
one that, were it not followed by the setext header underline,
would be interpreted as part of a paragraph: it cannot be a code
block, header, blockquote, horizontal rule, or list. A setext header
underline
is a sequence of = characters or a sequence of - characters, with no
more than 3 spaces indentation and any number of trailing
spaces. The header is a level 1 header if = characters are used, and
a level 2 header if - characters are used. The contents of the header
are the result of parsing the first line as Markdown inline content.
In general, a setext header need not be preceded or followed by a
blank line. However, it cannot interrupt a paragraph, so when a
setext header comes after a paragraph, a blank line is needed between
them.
Simple examples:
.
Foo bar
Foo bar
.
Foo bar
Foo bar
.
The underlining can be any length:
.
Foo
Foo
.
Foo
Foo
.
The header content can be indented up to three spaces, and need
not line up with the underlining:
.
Foo
Foo
Foo
===
.
Foo
Foo
Foo
.
Four spaces indent is too much:
.
Foo
—
Foo
.
.
The setext header underline can be indented up to three spaces, and
may have trailing spaces:
.
Foo
.
Foo
.
Four spaces is too much:
.
Foo
—
.
Foo
—
.
The setext header underline cannot contain internal spaces:
.
Foo
= =
Foo
.
Foo
= =
Foo
.
Trailing spaces in the content line do not cause a line break:
.
Foo
.
Foo
.
Nor does a backslash at the end:
.
Foo\
.
Foo\
.
Since indicators of block structure take precedence over
indicators of inline structure, the following are setext headers:
.
`Foo
`
<a title=”a lot
of dashes”>
.
`Foo
`
<a title="a lot
of dashes"/>
.
The setext header underline cannot be a lazy continuation
line in a list item or block quote:
.
> Foo
.
Foo
.
.
- Foo
.
Foo
.
A setext header cannot interrupt a paragraph:
.
Foo
Bar
Foo
Bar
.
Foo
Bar
Foo
Bar
===
.
But in general a blank line is not required before or after:
.
Foo
Bar
Baz
.
Foo
Bar
Baz
.
Setext headers cannot be empty:
.
====
.
====
.
Setext header text lines must not be interpretable as block
constructs other than paragraphs. So, the line of dashes
in these examples gets interpreted as a horizontal rule:
.
.
.
.
- foo
.
foo
.
.
foo
.
.
.
> foo
.
foo
.
If you want a header with > foo as its literal text, you can
use backslash escapes:
.
\> foo
.
> foo
.
Indented code blocks
An indented code block
is composed of one or more
indented chunks separated by blank lines.
An indented chunk
is a sequence of non-blank lines, each indented four or more
spaces. An indented code block cannot interrupt a paragraph, so
if it occurs before or after a paragraph, there must be an
intervening blank line. The contents of the code block are
the literal contents of the lines, including trailing newlines,
minus four spaces of indentation. An indented code block has no
attributes.
.
a simple
indented code block
.
.
The contents are literal text, and do not get parsed as Markdown:
.
hi
.
.
Here we have three chunks separated by blank lines:
.
chunk1
.
.
Any initial spaces beyond four will be included in the content, even
in interior blank lines:
.
chunk1
.
.
An indented code block cannot interrupt a paragraph. (This
allows hanging indents and the like.)
.
Foo
bar
.
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:
.
foo
bar
.
bar
.
And indented code can occur immediately before and after other kinds of
blocks:
.
Header
Header
foo
.
Header
Header
.
The first line can be indented more than four spaces:
.
foo
bar
.
.
Blank lines preceding or following an indented code block
are not included in it:
.
.
.
Trailing spaces are included in the code block’s content:
.
foo
.
.
Fenced code blocks
A /code 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
spaces and called the info string.
The info string may not contain any backtick
characters. (The reason for this restriction is that otherwise
some inline code would be incorrectly interpreted as the
beginning of a fenced code block.)
The content of the code block consists of all subsequent lines, until
a closing 1/code 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:
.
1
<
>
.
.
With tildes:
.
.
.
The closing code fence must use the same character as the opening
fence:
.
.
<pre><code>aaa
~~~
</code></pre>
.
.
~~~
aaa
```
~~~
.
.
.
~~~~
aaa
~~~
~~~~
.
<pre><code>aaa
~~~
.
Unclosed code blocks are closed by the end of the document:
.
```
.
<pre><code></code></pre>
.
.
`````
```
aaa
.
<pre><code>
```
aaa
.
A code block can have all empty lines as its content:
.
.
.
A code block can be empty:
.
```
.
<pre><code>aaa
aaa
</code></pre>
.
.
aaa
aaa
aaa
.
.
.
aaa
aaa
aaa
.
.
Four spaces indentation produces an indented code block:
.
aaa
.
aaa
```
.
.
.
aaa
.
.
This is not a closing fence, because it is indented 4 spaces:
.
bar
.
.
.
~~~~ ruby startline=3 $%@#$
def foo(x)
return 3
end
~~~~~~~
.
.
.
;
.
.
Info strings for backtick code blocks cannot contain backticks:
.
aa
foo
.
aa
foo
.
Closing code fences cannot have info strings:
.
.
<pre><code>``` aaa
.
HTML blocks
An HTML block tag is
an open tag or closing tag whose tag
name is one of the following (case-insensitive):
article, header, aside, hgroup, blockquote, hr, iframe,
body, li, map, button, object, canvas, ol, caption,
output, col, p, colgroup, pre, dd, progress, div,
section, dl, table, td, dt, tbody, embed, textarea,
fieldset, tfoot, figcaption, th, figure, thead, footer,
tr, form, ul, h1, h2, h3, h4, h5, h6, video,
script, style.
An HTML block begins with an
HTML block tag, HTML comment,
processing instruction,
declaration, or CDATA section.
It ends when a blank line or the end of the
input is encountered. The initial line may be indented up to three
spaces, and subsequent lines may have any indentation. The contents
of the HTML block are interpreted as raw HTML, and will not be escaped
in HTML output.
Some simple examples:
.
hi
okay.
.
hi
okay.
.
.
hello
.
hello
.
Here we have two HTML blocks with a Markdown paragraph between them:
.
Markdown
.
Markdown
.
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:
.
.
.
A comment:
.
.
.
A processing instruction:
.
';
?>
.
';
?>
.
CDATA:
.
<![CDATA[
function matchwo(a,b)
{
if (a < b && a
.
<![CDATA[
function matchwo(a,b)
{
if (a < b && a
.
The opening tag can be indented 1-3 spaces, but not 4:
.
.
.
An HTML block can interrupt a paragraph, and need not be preceded
by a blank line.
.
Foo
bar
.
Foo
bar
.
However, a following blank line is always needed, except at the end of
a document:
.
bar
foo
.
bar
foo
.
An incomplete HTML block tag may also start an HTML block:
.
<div class
foo
.
The only restrictions are that block-level HTML elements —
> e.g. `
`, `
`, `
Hi
.
Hi
.
Moreover, blank lines are usually not necessary and can be
deleted. The exception is inside `
[foo]
.
This is not a link reference definition, because it occurs inside
a code block:
.
```
[foo]: /url
```
[foo]
.
[foo]
.
A [link reference definition](#link-reference-definition) cannot
interrupt a paragraph.
.
Foo
[bar]: /baz
[bar]
.
Foo
[bar]: /baz
[bar]
.
However, it can directly follow other block elements, such as headers
and horizontal rules, and it need not be followed by a blank line.
.
# [Foo]
[foo]: /url
> bar
.
Foo
bar
.
Several [link references](#link-reference) can occur one after another,
without intervening blank lines.
.
[foo]: /foo-url "foo"
[bar]: /bar-url
"bar"
[baz]: /baz-url
[foo],
[bar],
[baz]
.
foo,
bar,
baz
.
[Link reference definitions](#link-reference-definition) can occur
inside block containers, like lists and block quotations. They
affect the entire document, not just the container in which they
are defined:
.
[foo]
> [foo]: /url
.
foo
.
## Paragraphs
A sequence of non-blank lines that cannot be interpreted as other
kinds of blocks forms a [paragraph](@paragraph).
The contents of the paragraph are the result of parsing the
paragraph's raw content as inlines. The paragraph's raw content
is formed by concatenating the lines and removing initial and final
spaces.
A simple example with two paragraphs:
.
aaa
bbb
.
aaa
bbb
.
Paragraphs can contain multiple lines, but no blank lines:
.
aaa
bbb
ccc
ddd
.
aaa
bbb
ccc
ddd
.
Multiple blank lines between paragraph have no effect:
.
aaa
bbb
.
aaa
bbb
.
Leading spaces are skipped:
.
aaa
bbb
.
aaa
bbb
.
Lines after the first may be indented any amount, since indented
code blocks cannot interrupt paragraphs.
.
aaa
bbb
ccc
.
aaa
bbb
ccc
.
However, the first line may be indented at most three spaces,
or an indented code block will be triggered:
.
aaa
bbb
.
aaa
bbb
.
.
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](#hard-line-break):
.
aaa
bbb
.
aaa
bbb
.
## Blank lines
[Blank lines](#blank-line) between block-level elements are ignored,
except for the role they play in determining whether a [list](#list)
is [tight](#tight) or [loose](#loose).
Blank lines at the beginning and end of the document are also ignored.
.
aaa
# aaa
.
aaa
aaa
.
# Container blocks
A [container block](#container-block) is a block that has other
blocks as its contents. There are two basic kinds of container blocks:
[block quotes](#block-quote) and [list items](#list-item).
[Lists](#list) are meta-containers for [list items](#list-item).
We define the syntax for container blocks recursively. The general
form of the definition is:
> If X is a sequence of blocks, then the result of
> transforming X in such-and-such a way is a container of type Y
> with these blocks as its content.
So, we explain what counts as a block quote or list item by explaining
how these can be *generated* from their contents. This should suffice
to define the syntax, although it does not give a recipe for *parsing*
these constructions. (A recipe is provided below in the section entitled
[A parsing strategy](#appendix-a-a-parsing-strategy).)
## Block quotes
A [block quote marker](@block-quote-marker)
consists of 0-3 spaces of initial indent, plus (a) the character `>` together
with a following space, or (b) a single character `>` not followed by a space.
The following rules define [block quotes](@block-quote):
1. **Basic case.** If a string of lines *Ls* constitute a sequence
of blocks *Bs*, then the result of prepending a [block quote
marker](#block-quote-marker) to the beginning of each line in *Ls*
is a [block quote](#block-quote) containing *Bs*.
2. **Laziness.** If a string of lines *Ls* constitute a [block
quote](#block-quote) with contents *Bs*, then the result of deleting
the initial [block quote marker](#block-quote-marker) from one or
more lines in which the next non-space character after the [block
quote marker](#block-quote-marker) is [paragraph continuation
text](#paragraph-continuation-text) is a block quote with *Bs* as
its content.
[Paragraph continuation text](@paragraph-continuation-text) is text
that will be parsed as part of the content of a paragraph, but does
not occur at the beginning of the paragraph.
3. **Consecutiveness.** A document cannot contain two [block
quotes](#block-quote) in a row unless there is a [blank
line](#blank-line) between them.
Nothing else counts as a [block quote](#block-quote).
Here is a simple example:
.
> # Foo
> bar
> baz
.
Foo
bar
baz
.
The spaces after the `>` characters can be omitted:
.
># Foo
>bar
> baz
.
Foo
bar
baz
.
The `>` characters can be indented 1-3 spaces:
.
> # Foo
> bar
> baz
.
Foo
bar
baz
.
Four spaces gives us a code block:
.
> # Foo
> bar
> baz
.
.
The Laziness clause allows us to omit the `>` before a
paragraph continuation line:
.
> # Foo
> bar
baz
.
Foo
bar
baz
.
A block quote can contain some lazy and some non-lazy
continuation lines:
.
> bar
baz
> foo
.
bar
baz
foo
.
Laziness only applies to lines that are continuations of
paragraphs. Lines containing characters or indentation that indicate
block structure cannot be lazy.
.
> foo
---
.
foo
.
.
> - foo
- bar
.
foo
bar
.
.
> foo
bar
.
.
.
> ```
foo
```
.
foo
.
A block quote can be empty:
.
>
.
.
.
>
>
>
.
.
A block quote can have initial or final blank lines:
.
>
> foo
>
.
foo
.
A blank line always separates block quotes:
.
> foo
> bar
.
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
.
foo
bar
.
To get a block quote with two paragraphs, use:
.
> foo
>
> bar
.
foo
bar
.
Block quotes can interrupt paragraphs:
.
foo
> bar
.
foo
bar
.
In general, blank lines are not needed before or after block
quotes:
.
> aaa
***
> bbb
.
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
.
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
.
.
>>> foo
> bar
>>baz
.
foo
bar
baz
.
When including an indented code block in a block quote,
remember that the [block quote marker](#block-quote-marker) includes
both the `>` and a following space. So *five spaces* are needed after
the `>`:
.
> code
> not code
.
not code
.
## List items
A [list marker](@list-marker) is a
[bullet list marker](#bullet-list-marker) or an [ordered list
marker](#ordered-list-marker).
A [bullet list marker](@bullet-list-marker)
is a `-`, `+`, or `*` character.
An [ordered list marker](@ordered-list-marker)
is a sequence of one of more digits (`0-9`), followed by either a
`.` character or a `)` character.
The following rules define [list items](@list-item):
1. **Basic case.** If a sequence of lines *Ls* constitute a sequence of
blocks *Bs* starting with a non-space character and not separated
from each other by more than one blank line, and *M* is a list
marker *M* of width *W* followed by 0 < *N* A block quote.
.
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*:
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
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 nonspace character, then blocks
must be indented five spaces in order to fall under the list
item.
Here are some examples showing how far content must be indented to be
put under the list item:
.
- one
two
.
one
two
.
.
- one
two
.
one
two
.
.
- one
two
.
one
.
.
- 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 nonspace
character after the list marker. However, that is not quite right.
The spaces after the list marker determine how much relative indentation
is needed. Which column this indentation reaches will depend on
how the list item is embedded in other constructions, as shown by
this example:
.
> > 1. one
>>
>> two
.
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
sufficent indentation after the last containing blockquote marker.
The converse is also possible. In the following example, the word `two`
occurs far to the right of the initial text of the list item, `one`, but
it is not considered part of the list item, because it is not indented
far enough past the blockquote marker:
.
>>- one
>>
> > two
.
one
two
.
A list item may not contain blocks that are separated by more than
one blank line. Thus, two blank lines will end a list, unless the
two blanks are contained in a [fenced code block](#fenced-code-block).
.
- foo
bar
- foo
bar
- ```
foo
bar
```
.
foo
bar
foo
bar
.
A list item may contain any kind of block:
.
1. foo
```
bar
```
baz
> bam
.
foo
baz
bam
.
2. **Item starting with indented code.** If a sequence of lines *Ls*
constitute a sequence of blocks *Bs* starting with an indented code
block and not separated from each other by more than one blank line,
and *M* is a list marker *M* of width *W* followed by
one space, then the result of prepending *M* and the following
space to the first line of *Ls*, and indenting subsequent lines of
*Ls* by *W + 1* spaces, is a list item with *Bs* as its contents.
If a line is empty, then it need not be indented. The type of the
list item (bullet or ordered) is determined by the type of its list
marker. If the list item is ordered, then it is also assigned a
start number, based on the ordered list marker.
An indented code block will have to be indented four spaces beyond
the edge of the region where text will be included in the list item.
In the following case that is 6 spaces:
.
- foo
bar
.
foo
.
And in this case it is 11 spaces:
.
10. foo
bar
.
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:
.
indented code
paragraph
more code
.
paragraph
.
.
1. indented code
paragraph
more code
.
paragraph
.
Note that an additional space indent is interpreted as space
inside the code block:
.
1. indented code
paragraph
more code
.
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 nonspace
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
.
foo
bar
.
.
- foo
bar
.
foo
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
.
foo
bar
.
3. **Indentation.** If a sequence of lines *Ls* constitutes a list item
according to rule #1 or #2, then the result of indenting each line
of *L* by 1-3 spaces (the same for each line) also constitutes a
list item with the same contents and attributes. If a line is
empty, then it need not be indented.
Indented one space:
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
A paragraph
with two lines.
A block quote.
.
Indented two spaces:
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
A paragraph
with two lines.
A block quote.
.
Indented three spaces:
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
A paragraph
with two lines.
A block quote.
.
Four spaces indent gives a code block:
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
.
4. **Laziness.** If a string of lines *Ls* constitute a [list
item](#list-item) with contents *Bs*, then the result of deleting
some or all of the indentation from one or more lines in which the
next non-space character after the indentation is
[paragraph continuation text](#paragraph-continuation-text) is a
list item with the same contents and attributes. The unindented
lines are called
[lazy continuation lines](@lazy-continuation-line).
Here is an example with [lazy continuation
lines](#lazy-continuation-line):
.
1. A paragraph
with two lines.
indented code
> A block quote.
.
A paragraph
with two lines.
A block quote.
.
Indentation can be partially deleted:
.
1. A paragraph
with two lines.
.
A paragraph
with two lines.
.
These examples show how laziness can work in nested structures:
.
> 1. > Blockquote
continued here.
.
Blockquote
continued here.
.
.
> 1. > Blockquote
> continued here.
.
Blockquote
continued here.
.
5. **That's all.** Nothing that is not counted as a list item by rules
#1--4 counts as a [list item](#list-item).
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:
.
- foo
- bar
- baz
.
foo
bar
baz
.
One is not enough:
.
- foo
- bar
- baz
.
foo
bar
baz
.
Here we need four, because the list marker is wider:
.
10) foo
- bar
.
foo
bar
.
Three is not enough:
.
10) foo
- bar
.
foo
bar
.
A list may be the first block in a list item:
.
- - foo
.
foo
.
.
1. - 2. foo
.
foo
.
A list item may be empty:
.
- foo
-
- bar
.
foo
bar
.
.
-
.
.
A list item can contain a header:
.
- # Foo
- Bar
---
baz
.
Foo
Bar
baz
.
### Motivation
John Gruber's Markdown spec says the following about list items:
1. "List markers typically start at the left margin, but may be indented
by up to three spaces. List markers must be followed by one or more
spaces or a tab."
2. "To make lists look nice, you can wrap items with hanging indents....
But if you don't want to, you don't have to."
3. "List items may consist of multiple paragraphs. Each subsequent
paragraph in a list item must be indented by either 4 spaces or one
tab."
4. "It looks nice if you indent every line of the subsequent paragraphs,
but here again, Markdown will allow you to be lazy."
5. "To put a blockquote within a list item, the blockquote's `>`
delimiters need to be indented."
6. "To put a code block within a list item, the code block needs to be
indented twice — 8 spaces or two tabs."
These rules specify that a paragraph under a list item must be indented
four spaces (presumably, from the left margin, rather than the start of
the list marker, but this is not said), and that code under a list item
must be indented eight spaces instead of the usual four. They also say
that a block quote must be indented, but not by how much; however, the
example given has four spaces indentation. Although nothing is said
about other kinds of block-level content, it is certainly reasonable to
infer that *all* block elements under a list item, including other
lists, must be indented four spaces. This principle has been called the
*four-space rule*.
The four-space rule is clear and principled, and if the reference
implementation `Markdown.pl` had followed it, it probably would have
become the standard. However, `Markdown.pl` allowed paragraphs and
sublists to start with only two spaces indentation, at least on the
outer level. Worse, its behavior was inconsistent: a sublist of an
outer-level list needed two spaces indentation, but a sublist of this
sublist needed three spaces. It is not surprising, then, that different
implementations of Markdown have developed very different rules for
determining what comes under a list item. (Pandoc and python-Markdown,
for example, stuck with Gruber's syntax description and the four-space
rule, while discount, redcarpet, marked, PHP Markdown, and others
followed `Markdown.pl`'s behavior more closely.)
Unfortunately, given the divergences between implementations, there
is no way to give a spec for list items that will be guaranteed not
to break any existing documents. However, the spec given here should
correctly handle lists formatted with either the four-space rule or
the more forgiving `Markdown.pl` behavior, provided they are laid out
in a way that is natural for a human to read.
The strategy here is to let the width and indentation of the list marker
determine the indentation necessary for blocks to fall under the list
item, rather than having a fixed and arbitrary number. The writer can
think of the body of the list item as a unit which gets indented to the
right enough to fit the list marker (and any indentation on the list
marker). (The laziness rule, #4, then allows continuation lines to be
unindented if needed.)
This rule is superior, we claim, to any rule requiring a fixed level of
indentation from the margin. The four-space rule is clear but
unnatural. It is quite unintuitive that
``` markdown
- foo
bar
- baz
```
should be parsed as two lists with an intervening paragraph,
``` html
<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
```
as the four-space rule demands, rather than a single list,
``` html
<ul>
<li><p>foo</p>
<p>bar</p>
<ul>
<li>baz</li>
</ul></li>
</ul>
```
The choice of four spaces is arbitrary. It can be learned, but it is
not likely to be guessed, and it trips up beginners regularly.
Would it help to adopt a two-space rule? The problem is that such
a rule, together with the rule allowing 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
``` markdown
- one
two
```
as a single list item, with `two` a continuation paragraph:
``` html
<ul>
<li><p>one</p>
<p>two</p></li>
</ul>
```
and similarly
``` markdown
> - one
>
> two
```
as
``` html
<blockquote>
<ul>
<li><p>one</p>
<p>two</p></li>
</ul>
</blockquote>
```
This is extremely unintuitive.
Rather than requiring a fixed indent from the margin, we could require
a fixed indent (say, two spaces, or even one space) from the list marker (which
may itself be indented). This proposal would remove the last