CommonMark Spec

Version 0.16 (2015-01-14)
John MacFarlane
Creative
   Commons BY-SA
CommonMark Spec by John MacFarlane is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
This is an older version of the spec. For the most recent version, see http://spec.commonmark.org.

1 Introduction

1.1 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.

1.2 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:

  1. How much indentation is needed for a sublist? The spec says that continuation paragraphs need to be indented four spaces, but is not fully explicit about sublists. It is natural to think that they, too, must be indented four spaces, but Markdown.pl does not require that. This is hardly a "corner case," and divergences between implementations on this issue often lead to surprises for users in real documents. (See this comment by John Gruber.)

  2. 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.)

  3. Is a blank line needed before an indented code block? (Markdown.pl requires it, but this is not mentioned in the documentation, and some implementations do not require it.)

    paragraph
        code?
    
  4. What is the exact rule for determining when list items get wrapped in <p> tags? Can a list be partially "loose" and partially "tight"? What should we do with a list like this?

    1. one
    
    2. two
    3. three
    

    Or this?

    1.  one
        - a
    
        - b
    2.  two
    

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

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

     8. item 1
     9. item 2
    10. item 2a
    
  6. Is this one list with a horizontal rule in its second item, or two lists separated by a horizontal rule?

    * a
    * * * * *
    * b
    
  7. When list markers change from numbers to bullets, do we have two lists or one? (The Markdown syntax description suggests two, but the perl scripts and many other implementations produce one.)

    1. fee
    2. fie
    -  foe
    -  fum
    
  8. What are the precedence rules for the markers of inline structure? For example, is the following a valid link, or does the code span take precedence ?

    [a backtick (`)](/url) and [another backtick (`)](/url).
    
  9. What are the precedence rules for markers of emphasis and strong emphasis? For example, how should the following be parsed?

    *foo *bar* baz*
    
  10. 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`
    
  11. Can list items include section headers? (Markdown.pl does not allow this, but does allow blockquotes to include headers.)

    - # Heading
    
  12. Can list items be empty?

    * a
    *
    * b
    
  13. Can link references be defined inside block quotes or list items?

    > Blockquote [foo].
    >
    > [foo]: /url
    
  14. 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.

1.3 About this document

This document attempts to specify Markdown syntax unambiguously. It contains many examples with side-by-side Markdown and HTML. These are intended to double as conformance tests. An accompanying script spec_tests.py can be used to run the tests against any Markdown program:

python test/spec_tests.py --spec spec.txt --program PROGRAM

Since this document describes how Markdown is to be parsed into an abstract syntax tree, it would have made sense to use an abstract representation of the syntax tree instead of HTML. But HTML is capable of representing the structural distinctions we need to make, and the choice of HTML for the tests makes it possible to run the tests against an implementation without writing an abstract syntax tree renderer.

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.

2 Preliminaries

2.1 Characters and lines

Any sequence of characters is a valid CommonMark document.

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.

A line is a sequence of zero or more characters followed by a line ending or by the end of file.

A line ending is, depending on the platform, a newline (U+000A), carriage return (U+000D), or carriage return + newline.

For security reasons, a conforming parser must strip or replace the Unicode character U+0000.

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), carriage return (U+000D), or newline (U+000A).

Whitespace is a sequence of one or more whitespace characters.

A unicode whitespace character is any code point in the unicode Zs class, 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 non-space character is anything but U+0020.

An ASCII punctuation character is !, ", #, $, %, &, ', (, ), *, +, ,, -, ., /, :, ;, <, =, >, ?, @, [, \, ], ^, _, `, {, |, }, or ~.

A punctuation character is an ASCII punctuation character or anything in the unicode classes Pc, Pd, Pe, Pf, Pi, Po, or Ps.

2.2 Tab expansion

Tabs in lines are expanded to spaces, with a tab stop of 4 characters:

Example 1  (interact)
→foo→baz→→bim
<pre><code>foo baz     bim
</code></pre>
Example 2  (interact)
    a→a
    ὐ→a
<pre><code>a   a
ὐ   a
</code></pre>

3 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.

3.1 Precedence

Indicators of block structure always take precedence over indicators of inline structure. So, for example, the following is a list with two items, not a list with one item containing a code span:

Example 3  (interact)
- `one
- two`
<ul>
<li>`one</li>
<li>two`</li>
</ul>

This means that parsing can proceed in two steps: first, the block structure of the document can be discerned; second, text lines inside paragraphs, 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.

3.2 Container blocks and leaf blocks

We can divide blocks into two types: container blocks, which can contain other blocks, and leaf blocks, which cannot.

4 Leaf blocks

This section describes the different kinds of leaf block that make up a Markdown document.

4.1 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.

Example 4  (interact)
***
---
___
<hr />
<hr />
<hr />

Wrong characters:

Example 5  (interact)
+++
<p>+++</p>
Example 6  (interact)
===
<p>===</p>

Not enough characters:

Example 7  (interact)
--
**
__
<p>--
**
__</p>

One to three spaces indent are allowed:

Example 8  (interact)
 ***
  ***
   ***
<hr />
<hr />
<hr />

Four spaces is too many:

Example 9  (interact)
    ***
<pre><code>***
</code></pre>
Example 10  (interact)
Foo
    ***
<p>Foo
***</p>

More than three characters may be used:

Example 11  (interact)
_____________________________________
<hr />

Spaces are allowed between the characters:

Example 12  (interact)
 - - -
<hr />
Example 13  (interact)
 **  * ** * ** * **
<hr />
Example 14  (interact)
-     -      -      -
<hr />

Spaces are allowed at the end:

Example 15  (interact)
- - - -    
<hr />

However, no other characters may occur in the line:

Example 16  (interact)
_ _ _ _ a

a------

---a---
<p>_ _ _ _ a</p>
<p>a------</p>
<p>---a---</p>

It is required that all of the non-space characters be the same. So, this is not a horizontal rule:

Example 17  (interact)
 *-*
<p><em>-</em></p>

Horizontal rules do not need blank lines before or after:

Example 18  (interact)
- foo
***
- bar
<ul>
<li>foo</li>
</ul>
<hr />
<ul>
<li>bar</li>
</ul>

Horizontal rules can interrupt a paragraph:

Example 19  (interact)
Foo
***
bar
<p>Foo</p>
<hr />
<p>bar</p>

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:

Example 20  (interact)
Foo
---
bar
<h2>Foo</h2>
<p>bar</p>

When both a horizontal rule and a list item are possible interpretations of a line, the horizontal rule takes precedence:

Example 21  (interact)
* Foo
* * *
* Bar
<ul>
<li>Foo</li>
</ul>
<hr />
<ul>
<li>Bar</li>
</ul>

If you want a horizontal rule in a list item, use a different bullet:

Example 22  (interact)
- Foo
- * * *
<ul>
<li>Foo</li>
<li>
<hr />
</li>
</ul>

4.2 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 non-space 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:

Example 23  (interact)
# foo
## foo
### foo
#### foo
##### foo
###### foo
<h1>foo</h1>
<h2>foo</h2>
<h3>foo</h3>
<h4>foo</h4>
<h5>foo</h5>
<h6>foo</h6>

More than six # characters is not a header:

Example 24  (interact)
####### foo
<p>####### foo</p>

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:

Example 25  (interact)
#5 bolt
<p>#5 bolt</p>

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

Example 26  (interact)
\## foo
<p>## foo</p>

Contents are parsed as inlines:

Example 27  (interact)
# foo *bar* \*baz\*
<h1>foo <em>bar</em> *baz*</h1>

Leading and trailing blanks are ignored in parsing inline content:

Example 28  (interact)
#                  foo                     
<h1>foo</h1>

One to three spaces indentation are allowed:

Example 29  (interact)
 ### foo
  ## foo
   # foo
<h3>foo</h3>
<h2>foo</h2>
<h1>foo</h1>

Four spaces are too much:

Example 30  (interact)
    # foo
<pre><code># foo
</code></pre>
Example 31  (interact)
foo
    # bar
<p>foo
# bar</p>

A closing sequence of # characters is optional:

Example 32  (interact)
## foo ##
  ###   bar    ###
<h2>foo</h2>
<h3>bar</h3>

It need not be the same length as the opening sequence:

Example 33  (interact)
# foo ##################################
##### foo ##
<h1>foo</h1>
<h5>foo</h5>

Spaces are allowed after the closing sequence:

Example 34  (interact)
### foo ###     
<h3>foo</h3>

A sequence of # characters with a non-space character following it is not a closing sequence, but counts as part of the contents of the header:

Example 35  (interact)
### foo ### b
<h3>foo ### b</h3>

The closing sequence must be preceded by a space:

Example 36  (interact)
# foo#
<h1>foo#</h1>

Backslash-escaped # characters do not count as part of the closing sequence:

Example 37  (interact)
### foo \###
## foo #\##
# foo \#
<h3>foo ###</h3>
<h2>foo ###</h2>
<h1>foo #</h1>

ATX headers need not be separated from surrounding content by blank lines, and they can interrupt paragraphs:

Example 38  (interact)
****
## foo
****
<hr />
<h2>foo</h2>
<hr />
Example 39  (interact)
Foo bar
# baz
Bar foo
<p>Foo bar</p>
<h1>baz</h1>
<p>Bar foo</p>

ATX headers can be empty:

Example 40  (interact)
## 
#
### ###
<h2></h2>
<h1></h1>
<h3></h3>

4.3 Setext headers

A setext header consists of a line of text, containing at least one non-space 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. 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 header underline.

The header is a level 1 header if = characters are used in the setext header underline, 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:

Example 41  (interact)
Foo *bar*
=========

Foo *bar*
---------
<h1>Foo <em>bar</em></h1>
<h2>Foo <em>bar</em></h2>

The underlining can be any length:

Example 42  (interact)
Foo
-------------------------

Foo
=
<h2>Foo</h2>
<h1>Foo</h1>

The header content can be indented up to three spaces, and need not line up with the underlining:

Example 43  (interact)
   Foo
---

  Foo
-----

  Foo
  ===
<h2>Foo</h2>
<h2>Foo</h2>
<h1>Foo</h1>

Four spaces indent is too much:

Example 44  (interact)
    Foo
    ---

    Foo
---
<pre><code>Foo
---

Foo
</code></pre>
<hr />

The setext header underline can be indented up to three spaces, and may have trailing spaces:

Example 45  (interact)
Foo
   ----      
<h2>Foo</h2>

Four spaces is too much:

Example 46  (interact)
Foo
    ---
<p>Foo
---</p>

The setext header underline cannot contain internal spaces:

Example 47  (interact)
Foo
= =

Foo
--- -
<p>Foo
= =</p>
<p>Foo</p>
<hr />

Trailing spaces in the content line do not cause a line break:

Example 48  (interact)
Foo  
-----
<h2>Foo</h2>

Nor does a backslash at the end:

Example 49  (interact)
Foo\
----
<h2>Foo\</h2>

Since indicators of block structure take precedence over indicators of inline structure, the following are setext headers:

Example 50  (interact)
`Foo
----
`

<a title="a lot
---
of dashes"/>
<h2>`Foo</h2>
<p>`</p>
<h2>&lt;a title=&quot;a lot</h2>
<p>of dashes&quot;/&gt;</p>

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

Example 51  (interact)
> Foo
---
<blockquote>
<p>Foo</p>
</blockquote>
<hr />
Example 52  (interact)
- Foo
---
<ul>
<li>Foo</li>
</ul>
<hr />

A setext header cannot interrupt a paragraph:

Example 53  (interact)
Foo
Bar
---

Foo
Bar
===
<p>Foo
Bar</p>
<hr />
<p>Foo
Bar
===</p>

But in general a blank line is not required before or after:

Example 54  (interact)
---
Foo
---
Bar
---
Baz
<hr />
<h2>Foo</h2>
<h2>Bar</h2>
<p>Baz</p>

Setext headers cannot be empty:

Example 55  (interact)

====
<p>====</p>

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:

Example 56  (interact)
---
---
<hr />
<hr />
Example 57  (interact)
- foo
-----
<ul>
<li>foo</li>
</ul>
<hr />
Example 58  (interact)
    foo
---
<pre><code>foo
</code></pre>
<hr />
Example 59  (interact)
> foo
-----
<blockquote>
<p>foo</p>
</blockquote>
<hr />

If you want a header with > foo as its literal text, you can use backslash escapes:

Example 60  (interact)
\> foo
------
<h2>&gt; foo</h2>

4.4 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. The contents of the code block are the literal contents of the lines, including trailing line endings, minus four spaces of indentation. An indented code block has no info string.

An indented code block cannot interrupt a paragraph, so there must be a blank line between a paragraph and a following indented code block. (A blank line is not needed, however, between a code block and a following paragraph.)

Example 61  (interact)
    a simple
      indented code block
<pre><code>a simple
  indented code block
</code></pre>

The contents are literal text, and do not get parsed as Markdown:

Example 62  (interact)
    <a/>
    *hi*

    - one
<pre><code>&lt;a/&gt;
*hi*

- one
</code></pre>

Here we have three chunks separated by blank lines:

Example 63  (interact)
    chunk1

    chunk2
  
 
 
    chunk3
<pre><code>chunk1

chunk2



chunk3
</code></pre>

Any initial spaces beyond four will be included in the content, even in interior blank lines:

Example 64  (interact)
    chunk1
      
      chunk2
<pre><code>chunk1
  
  chunk2
</code></pre>

An indented code block cannot interrupt a paragraph. (This allows hanging indents and the like.)

Example 65  (interact)
Foo
    bar

<p>Foo
bar</p>

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:

Example 66  (interact)
    foo
bar
<pre><code>foo
</code></pre>
<p>bar</p>

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

Example 67  (interact)
# Header
    foo
Header
------
    foo
----
<h1>Header</h1>
<pre><code>foo
</code></pre>
<h2>Header</h2>
<pre><code>foo
</code></pre>
<hr />

The first line can be indented more than four spaces:

Example 68  (interact)
        foo
    bar
<pre><code>    foo
bar
</code></pre>

Blank lines preceding or following an indented code block are not included in it:

Example 69  (interact)

    
    foo
    

<pre><code>foo
</code></pre>

Trailing spaces are included in the code block's content:

Example 70  (interact)
    foo  
<pre><code>foo  
</code></pre>

4.5 Fenced code blocks

A code fence is a sequence of at least three consecutive backtick characters (`) or tildes (~). (Tildes and backticks cannot be mixed.) A fenced code block begins with a code fence, 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 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:

Example 71  (interact)
```
<
 >
```
<pre><code>&lt;
 &gt;
</code></pre>

With tildes:

Example 72  (interact)
~~~
<
 >
~~~
<pre><code>&lt;
 &gt;
</code></pre>

The closing code fence must use the same character as the opening fence:

Example 73  (interact)
```
aaa
~~~
```
<pre><code>aaa
~~~
</code></pre>
Example 74  (interact)
~~~
aaa
```
~~~
<pre><code>aaa
```
</code></pre>

The closing code fence must be at least as long as the opening fence:

Example 75  (interact)
````
aaa
```
``````
<pre><code>aaa
```
</code></pre>
Example 76  (interact)
~~~~
aaa
~~~
~~~~
<pre><code>aaa
~~~
</code></pre>

Unclosed code blocks are closed by the end of the document:

Example 77  (interact)
```
<pre><code></code></pre>
Example 78  (interact)
`````

```
aaa
<pre><code>
```
aaa
</code></pre>

A code block can have all empty lines as its content:

Example 79  (interact)
```

  
```
<pre><code>
  
</code></pre>

A code block can be empty:

Example 80  (interact)
```
```
<pre><code></code></pre>

Fences can be indented. If the opening fence is indented, content lines will have equivalent opening indentation removed, if present:

Example 81  (interact)
 ```
 aaa
aaa
```
<pre><code>aaa
aaa
</code></pre>
Example 82  (interact)
  ```
aaa
  aaa
aaa
  ```
<pre><code>aaa
aaa
aaa
</code></pre>
Example 83  (interact)
   ```
   aaa
    aaa
  aaa
   ```
<pre><code>aaa
 aaa
aaa
</code></pre>

Four spaces indentation produces an indented code block:

Example 84  (interact)
    ```
    aaa
    ```
<pre><code>```
aaa
```
</code></pre>

Closing fences may be indented by 0-3 spaces, and their indentation need not match that of the opening fence:

Example 85  (interact)
```
aaa
  ```
<pre><code>aaa
</code></pre>
Example 86  (interact)
   ```
aaa
  ```
<pre><code>aaa
</code></pre>

This is not a closing fence, because it is indented 4 spaces:

Example 87  (interact)
```
aaa
    ```
<pre><code>aaa
    ```
</code></pre>

Code fences (opening and closing) cannot contain internal spaces:

Example 88  (interact)
``` ```
aaa
<p><code></code>
aaa</p>
Example 89  (interact)
~~~~~~
aaa
~~~ ~~
<pre><code>aaa
~~~ ~~
</code></pre>

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

Example 90  (interact)
foo
```
bar
```
baz
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>

Other blocks can also occur before and after fenced code blocks without an intervening blank line:

Example 91  (interact)
foo
---
~~~
bar
~~~
# baz
<h2>foo</h2>
<pre><code>bar
</code></pre>
<h1>baz</h1>

An info string can be provided after the opening code fence. Opening and closing spaces will be stripped, and the first word, prefixed with language-, is used as the value for the class attribute of the code element within the enclosing pre element.

Example 92  (interact)
```ruby
def foo(x)
  return 3
end
```
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
Example 93  (interact)
~~~~    ruby startline=3 $%@#$
def foo(x)
  return 3
end
~~~~~~~
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
Example 94  (interact)
````;
````
<pre><code class="language-;"></code></pre>

Info strings for backtick code blocks cannot contain backticks:

Example 95  (interact)
``` aa ```
foo
<p><code>aa</code>
foo</p>

Closing code fences cannot have info strings:

Example 96  (interact)
```
``` aaa
```
<pre><code>``` aaa
</code></pre>

4.6 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:

Example 97  (interact)
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>

okay.
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>
<p>okay.</p>
Example 98  (interact)
 <div>
  *hello*
         <foo><a>
 <div>
  *hello*
         <foo><a>

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

Example 99  (interact)
<DIV CLASS="foo">

*Markdown*

</DIV>
<DIV CLASS="foo">
<p><em>Markdown</em></p>
</DIV>

In the following example, what looks like a Markdown code block is actually part of the HTML block, which continues until a blank line or the end of the document is reached:

Example 100  (interact)
<div></div>
``` c
int x = 33;
```
<div></div>
``` c
int x = 33;
```

A comment:

Example 101  (interact)
<!-- Foo
bar
   baz -->
<!-- Foo
bar
   baz -->

A processing instruction:

Example 102  (interact)
<?php
  echo '>';
?>
<?php
  echo '>';
?>

CDATA:

Example 103  (interact)
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
  {
  return 1;
  }
else
  {
  return 0;
  }
}
]]>
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
  {
  return 1;
  }
else
  {
  return 0;
  }
}
]]>

The opening tag can be indented 1-3 spaces, but not 4:

Example 104  (interact)
  <!-- foo -->

    <!-- foo -->
  <!-- foo -->
<pre><code>&lt;!-- foo --&gt;
</code></pre>

An HTML block can interrupt a paragraph, and need not be preceded by a blank line.

Example 105  (interact)
Foo
<div>
bar
</div>
<p>Foo</p>
<div>
bar
</div>

However, a following blank line is always needed, except at the end of a document:

Example 106  (interact)
<div>
bar
</div>
*foo*
<div>
bar
</div>
*foo*

An incomplete HTML block tag may also start an HTML block:

Example 107  (interact)
<div class
foo
<div class
foo

This rule differs from John Gruber's original Markdown syntax specification, which says:

The only restrictions are that block-level HTML elements — e.g. <div>, <table>, <pre>, <p>, etc. — must be separated from surrounding content by blank lines, and the start and end tags of the block should not be indented with tabs or spaces.

In some ways Gruber's rule is more restrictive than the one given here:

Indeed, most Markdown implementations, including some of Gruber's own perl implementations, do not impose 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:

Example 108  (interact)
<div>

*Emphasized* text.

</div>
<div>
<p><em>Emphasized</em> text.</p>
</div>

Compare:

Example 109  (interact)
<div>
*Emphasized* text.
</div>
<div>
*Emphasized* text.
</div>

Some Markdown implementations have adopted a convention of interpreting content inside tags as text if the open tag has the attribute markdown=1. The rule given above seems a simpler and more elegant way of achieving the same expressive power, which is also much simpler to parse.

The main potential drawback is that one can no longer paste HTML blocks into Markdown documents with 100% reliability. However, in most cases this will work fine, because the blank lines in HTML are usually followed by HTML block tags. For example:

Example 110  (interact)
<table>

<tr>

<td>
Hi
</td>

</tr>

</table>
<table>
<tr>
<td>
Hi
</td>
</tr>
</table>

Moreover, blank lines are usually not necessary and can be deleted. The exception is inside <pre> tags; here, one can replace the blank lines with &#10; entities.

So there is no important loss of expressive power with the new rule.

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-space 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.

Example 111  (interact)
[foo]: /url "title"

[foo]
<p><a href="/url" title="title">foo</a></p>
Example 112  (interact)
   [foo]: 
      /url  
           'the title'  

[foo]
<p><a href="/url" title="the title">foo</a></p>
Example 113  (interact)
[Foo*bar\]]:my_(url) 'title (with parens)'

[Foo*bar\]]
<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
Example 114  (interact)
[Foo bar]:
<my url>
'title'

[Foo bar]
<p><a href="my%20url" title="title">Foo bar</a></p>

The title may be omitted:

Example 115  (interact)
[foo]:
/url

[foo]
<p><a href="/url">foo</a></p>

The link destination may not be omitted:

Example 116  (interact)
[foo]:

[foo]
<p>[foo]:</p>
<p>[foo]</p>

A link can come before its corresponding definition:

Example 117  (interact)
[foo]

[foo]: url
<p><a href="url">foo</a></p>

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

Example 118  (interact)
[foo]

[foo]: first
[foo]: second
<p><a href="first">foo</a></p>

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

Example 119  (interact)
[FOO]: /url

[Foo]
<p><a href="/url">Foo</a></p>
Example 120  (interact)
[ΑΓΩ]: /φου

[αγω]
<p><a href="/%CF%86%CE%BF%CF%85">αγω</a></p>

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

Example 121  (interact)
[foo]: /url

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

Example 122  (interact)
[foo]: /url "title" ok
<p>[foo]: /url &quot;title&quot; ok</p>

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

Example 123  (interact)
    [foo]: /url "title"

[foo]
<pre><code>[foo]: /url &quot;title&quot;
</code></pre>
<p>[foo]</p>

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

Example 124  (interact)
```
[foo]: /url
```

[foo]
<pre><code>[foo]: /url
</code></pre>
<p>[foo]</p>

A link reference definition cannot interrupt a paragraph.

Example 125  (interact)
Foo
[bar]: /baz

[bar]
<p>Foo
[bar]: /baz</p>
<p>[bar]</p>

However, it can directly follow other block elements, such as headers and horizontal rules, and it need not be followed by a blank line.

Example 126  (interact)
# [Foo]
[foo]: /url
> bar
<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>

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

Example 127  (interact)
[foo]: /foo-url "foo"
[bar]: /bar-url
  "bar"
[baz]: /baz-url

[foo],
[bar],
[baz]
<p><a href="/foo-url" title="foo">foo</a>,
<a href="/bar-url" title="bar">bar</a>,
<a href="/baz-url">baz</a></p>

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

Example 128  (interact)
[foo]

> [foo]: /url
<p><a href="/url">foo</a></p>
<blockquote>
</blockquote>

4.8 Paragraphs

A sequence of non-blank lines that cannot be interpreted as other kinds of blocks forms a paragraph. The contents of the paragraph are the result of parsing the paragraph's raw content as inlines. The paragraph's raw content is formed by concatenating the lines and removing initial and final whitespace.

A simple example with two paragraphs:

Example 129  (interact)
aaa

bbb
<p>aaa</p>
<p>bbb</p>

Paragraphs can contain multiple lines, but no blank lines:

Example 130  (interact)
aaa
bbb

ccc
ddd
<p>aaa
bbb</p>
<p>ccc
ddd</p>

Multiple blank lines between paragraph have no effect:

Example 131  (interact)
aaa


bbb
<p>aaa</p>
<p>bbb</p>

Leading spaces are skipped:

Example 132  (interact)
  aaa
 bbb
<p>aaa
bbb</p>

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

Example 133  (interact)
aaa
             bbb
                                       ccc
<p>aaa
bbb
ccc</p>

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

Example 134  (interact)
   aaa
bbb
<p>aaa
bbb</p>
Example 135  (interact)
    aaa
bbb
<pre><code>aaa
</code></pre>
<p>bbb</p>

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:

Example 136  (interact)
aaa     
bbb     
<p>aaa<br />
bbb</p>

4.9 Blank lines

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

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

Example 137  (interact)
  

aaa
  

# aaa

  
<p>aaa</p>
<h1>aaa</h1>

5 Container blocks

A container block is a block that has other blocks as its contents. There are two basic kinds of container blocks: block quotes and list items. Lists are meta-containers for list items.

We define the syntax for container blocks recursively. The general form of the definition is:

If X is a sequence of blocks, then the result of transforming X in such-and-such a way is a container of type Y with these blocks as its content.

So, we explain what counts as a block quote or list item by explaining how these can be generated from their contents. This should suffice to define the syntax, although it does not give a recipe for parsing these constructions. (A recipe is provided below in the section entitled A parsing strategy.)

5.1 Block quotes

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:

  1. Basic case. If a string of lines Ls constitute a sequence of blocks Bs, then the result of prepending a block quote marker to the beginning of each line in Ls is a block quote containing Bs.

  2. 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-space 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.

  3. Consecutiveness. A document cannot contain two block quotes in a row unless there is a blank line between them.

Nothing else counts as a block quote.

Here is a simple example:

Example 138  (interact)
> # Foo
> bar
> baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The spaces after the > characters can be omitted:

Example 139  (interact)
># Foo
>bar
> baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

The > characters can be indented 1-3 spaces:

Example 140  (interact)
   > # Foo
   > bar
 > baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

Four spaces gives us a code block:

Example 141  (interact)
    > # Foo
    > bar
    > baz
<pre><code>&gt; # Foo
&gt; bar
&gt; baz
</code></pre>

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

Example 142  (interact)
> # Foo
> bar
baz
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>

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

Example 143  (interact)
> bar
baz
> foo
<blockquote>
<p>bar
baz
foo</p>
</blockquote>

Laziness only applies to lines that are continuations of paragraphs. Lines containing characters or indentation that indicate block structure cannot be lazy.

Example 144  (interact)
> foo
---
<blockquote>
<p>foo</p>
</blockquote>
<hr />
Example 145  (interact)
> - foo
- bar
<blockquote>
<ul>
<li>foo</li>
</ul>
</blockquote>
<ul>
<li>bar</li>
</ul>
Example 146  (interact)
>     foo
    bar
<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>
Example 147  (interact)
> ```
foo
```
<blockquote>
<pre><code></code></pre>
</blockquote>
<p>foo</p>
<pre><code></code></pre>

A block quote can be empty:

Example 148  (interact)
>
<blockquote>
</blockquote>
Example 149  (interact)
>
>  
> 
<blockquote>
</blockquote>

A block quote can have initial or final blank lines:

Example 150  (interact)
>
> foo
>  
<blockquote>
<p>foo</p>
</blockquote>

A blank line always separates block quotes:

Example 151  (interact)
> foo

> bar
<blockquote>
<p>foo</p>
</blockquote>
<blockquote>
<p>bar</p>
</blockquote>

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

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

Example 152  (interact)
> foo
> bar
<blockquote>
<p>foo
bar</p>
</blockquote>

To get a block quote with two paragraphs, use:

Example 153  (interact)
> foo
>
> bar
<blockquote>
<p>foo</p>
<p>bar</p>
</blockquote>

Block quotes can interrupt paragraphs:

Example 154  (interact)
foo
> bar
<p>foo</p>
<blockquote>
<p>bar</p>
</blockquote>

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

Example 155  (interact)
> aaa
***
> bbb
<blockquote>
<p>aaa</p>
</blockquote>
<hr />
<blockquote>
<p>bbb</p>
</blockquote>

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

Example 156  (interact)
> bar
baz
<blockquote>
<p>bar
baz</p>
</blockquote>
Example 157  (interact)
> bar

baz
<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>
Example 158  (interact)
> bar
>
baz
<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>

It is a consequence of the Laziness rule that any number of initial >s may be omitted on a continuation line of a nested block quote:

Example 159  (interact)
> > > foo
bar
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar</p>
</blockquote>
</blockquote>
</blockquote>
Example 160  (interact)
>>> foo
> bar
>>baz
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar
baz</p>
</blockquote>
</blockquote>
</blockquote>

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

Example 161  (interact)
>     code

>    not code
<blockquote>
<pre><code>code
</code></pre>
</blockquote>
<blockquote>
<p>not code</p>
</blockquote>

5.2 List items

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

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

An ordered list marker is a sequence of one of more digits (0-9), followed by either a . character or a ) character.

The following rules define list items:

  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 < 5 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.

For example, let Ls be the lines

Example 162  (interact)
A paragraph
with two lines.

    indented code

> A block quote.
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>

And let M be the marker 1., and N = 2. Then rule #1 says that the following is an ordered list item with start number 1, and the same contents as Ls:

Example 163  (interact)
1.  A paragraph
    with two lines.

        indented code

    > A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

The most important thing to notice is that the position of the text after the list marker determines how much indentation is needed in subsequent blocks in the list item. If the list marker takes up two spaces, and there are three spaces between the list marker and the next non-space 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:

Example 164  (interact)
- one

 two
<ul>
<li>one</li>
</ul>
<p>two</p>
Example 165  (interact)
- one

  two
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
Example 166  (interact)
 -    one

     two
<ul>
<li>one</li>
</ul>
<pre><code> two
</code></pre>
Example 167  (interact)
 -    one

      two
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>

It is tempting to think of this in terms of columns: the continuation blocks must be indented at least to the column of the first non-space 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:

Example 168  (interact)
   > > 1.  one
>>
>>     two
<blockquote>
<blockquote>
<ol>
<li>
<p>one</p>
<p>two</p>
</li>
</ol>
</blockquote>
</blockquote>

Here two occurs in the same column as the list marker 1., but is actually contained in the list item, because there is 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:

Example 169  (interact)
>>- one
>>
  >  > two
<blockquote>
<blockquote>
<ul>
<li>one</li>
</ul>
<p>two</p>
</blockquote>
</blockquote>

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.

Example 170  (interact)
- foo

  bar

- foo


  bar

- ```
  foo


  bar
  ```

- baz

  + ```
    foo


    bar
    ```
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
<li>
<p>foo</p>
</li>
</ul>
<p>bar</p>
<ul>
<li>
<pre><code>foo


bar
</code></pre>
</li>
<li>
<p>baz</p>
<ul>
<li>
<pre><code>foo


bar
</code></pre>
</li>
</ul>
</li>
</ul>

A list item may contain any kind of block:

Example 171  (interact)
1.  foo

    ```
    bar
    ```

    baz

    > bam
<ol>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>
<blockquote>
<p>bam</p>
</blockquote>
</li>
</ol>
  1. 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:

Example 172  (interact)
- foo

      bar
<ul>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ul>

And in this case it is 11 spaces:

Example 173  (interact)
  10.  foo

           bar
<ol start="10">
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ol>

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

Example 174  (interact)
    indented code

paragraph

    more code
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
Example 175  (interact)
1.     indented code

   paragraph

       more code
<ol>
<li>
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>

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

Example 176  (interact)
1.      indented code

   paragraph

       more code
<ol>
<li>
<pre><code> indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>

Note that rules #1 and #2 only apply to two cases: (a) cases in which the lines to be included in a list item begin with a non-space 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:

Example 177  (interact)
   foo

bar
<p>foo</p>
<p>bar</p>
Example 178  (interact)
-    foo

  bar
<ul>
<li>foo</li>
</ul>
<p>bar</p>

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:

Example 179  (interact)
-  foo

   bar
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
  1. Empty list item. A list marker followed by a line containing only whitespace is a list item with no contents.

Here is an empty bullet list item:

Example 180  (interact)
- foo
-
- bar
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

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

Example 181  (interact)
- foo
-   
- bar
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>

Here is an empty ordered list item:

Example 182  (interact)
1. foo
2.
3. bar
<ol>
<li>foo</li>
<li></li>
<li>bar</li>
</ol>

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

Example 183  (interact)
*
<ul>
<li></li>
</ul>
  1. Indentation. If a sequence of lines Ls constitutes a list item according to rule #1, #2, or #3, 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:

Example 184  (interact)
 1.  A paragraph
     with two lines.

         indented code

     > A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indented two spaces:

Example 185  (interact)
  1.  A paragraph
      with two lines.

          indented code

      > A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indented three spaces:

Example 186  (interact)
   1.  A paragraph
       with two lines.

           indented code

       > A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Four spaces indent gives a code block:

Example 187  (interact)
    1.  A paragraph
        with two lines.

            indented code

        > A block quote.
<pre><code>1.  A paragraph
    with two lines.

        indented code

    &gt; A block quote.
</code></pre>
  1. Laziness. If a string of lines Ls constitute a list item with contents Bs, then the result of deleting some or all of the indentation from one or more lines in which the next non-space character after the indentation is paragraph continuation text is a list item with the same contents and attributes. The unindented lines are called lazy continuation lines.

Here is an example with lazy continuation lines:

Example 188  (interact)
  1.  A paragraph
with two lines.

          indented code

      > A block quote.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>

Indentation can be partially deleted:

Example 189  (interact)
  1.  A paragraph
    with two lines.
<ol>
<li>A paragraph
with two lines.</li>
</ol>

These examples show how laziness can work in nested structures:

Example 190  (interact)
> 1. > Blockquote
continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
Example 191  (interact)
> 1. > Blockquote
> continued here.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
  1. That's all. Nothing that is not counted as a list item by rules #1--5 counts as a list item.

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

So, in this case we need two spaces indent:

Example 192  (interact)
- foo
  - bar
    - baz
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>

One is not enough:

Example 193  (interact)
- foo
 - bar
  - baz
<ul>
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ul>

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

Example 194  (interact)
10) foo
    - bar
<ol start="10">
<li>foo
<ul>
<li>bar</li>
</ul>
</li>
</ol>

Three is not enough:

Example 195  (interact)
10) foo
   - bar
<ol start="10">
<li>foo</li>
</ol>
<ul>
<li>bar</li>
</ul>

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

Example 196  (interact)
- - foo
<ul>
<li>
<ul>
<li>foo</li>
</ul>
</li>
</ul>
Example 197  (interact)
1. - 2. foo
<ol>
<li>
<ul>
<li>
<ol start="2">
<li>foo</li>
</ol>
</li>
</ul>
</li>
</ol>

A list item can contain a header:

Example 198  (interact)
- # Foo
- Bar
  ---
  baz
<ul>
<li>
<h1>Foo</h1>
</li>
<li>
<h2>Bar</h2>
baz</li>
</ul>

5.2.1 Motivation

John Gruber's Markdown spec says the following about list items:

  1. "List markers typically start at the left margin, but may be indented by up to three spaces. List markers must be followed by one or more spaces or a tab."

  2. "To make lists look nice, you can wrap items with hanging indents.... But if you don't want to, you don't have to."

  3. "List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab."

  4. "It looks nice if you indent every line of the subsequent paragraphs, but here again, Markdown will allow you to be lazy."

  5. "To put a blockquote within a list item, the blockquote's > delimiters need to be indented."

  6. "To put a code block within a list item, the code block needs to be indented twice — 8 spaces or two tabs."

These rules specify that a paragraph under a list item must be indented four spaces (presumably, from the left margin, rather than the start of the list marker, but this is not said), and that code under a list item must be indented eight spaces instead of the usual four. They also say that a block quote must be indented, but not by how much; however, the example given has four spaces indentation. Although nothing is said about other kinds of block-level content, it is certainly reasonable to infer that all block elements under a list item, including other lists, must be indented four spaces. This principle has been called the four-space rule.

The four-space rule is clear and principled, and if the reference implementation Markdown.pl had followed it, it probably would have become the standard. However, Markdown.pl allowed paragraphs and sublists to start with only two spaces indentation, at least on the outer level. Worse, its behavior was inconsistent: a sublist of an outer-level list needed two spaces indentation, but a sublist of this sublist needed three spaces. It is not surprising, then, that different implementations of Markdown have developed very different rules for determining what comes under a list item. (Pandoc and python-Markdown, for example, stuck with Gruber's syntax description and the four-space rule, while discount, redcarpet, marked, PHP Markdown, and others followed Markdown.pl's behavior more closely.)

Unfortunately, given the divergences between implementations, there is no way to give a spec for list items that will be guaranteed not to break any existing documents. However, the spec given here should correctly handle lists formatted with either the four-space rule or the more forgiving Markdown.pl behavior, provided they are laid out in a way that is natural for a human to read.

The strategy here is to let the width and indentation of the list marker determine the indentation necessary for blocks to fall under the list item, rather than having a fixed and arbitrary number. The writer can think of the body of the list item as a unit which gets indented to the right enough to fit the list marker (and any indentation on the list marker). (The laziness rule, #5, then allows continuation lines to be unindented if needed.)

This rule is superior, we claim, to any rule requiring a fixed level of indentation from the margin. The four-space rule is clear but unnatural. It is quite unintuitive that

- foo

  bar

  - baz

should be parsed as two lists with an intervening paragraph,

<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>

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

<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

   - one

  two

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

<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>

and similarly

>   - one
>
>  two

as

<blockquote>
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
</blockquote>

This is extremely unintuitive.

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

 10. foo

   bar  

Arguably this text does read like a list item with bar as a subparagraph, which may count in favor of the proposal. However, on this proposal indented code would have to be indented six spaces after the list marker. And this would break a lot of existing Markdown, which has the pattern:

1.  foo

        indented code

where the code is indented eight spaces. The spec above, by contrast, will parse this text as expected, since the code block's indentation is measured from the beginning of foo.

The one case that needs special treatment is a list item that starts with indented code. How much indentation is required in that case, since we don't have a "first paragraph" to measure from? Rule #2 simply stipulates that in such cases, we require one space indentation from the list marker (and then the normal four spaces for the indented code). This will match the four-space rule in cases where the list marker plus its initial indentation takes four spaces (a common case), but diverge in other cases.

5.3 Lists

A list is a sequence of one or more list items of the same type. The list items may be separated by single blank lines, but two blank lines end all containing lists.

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 it any of its constituent list items are separated by blank lines, or if any of its constituent list items directly contain two block-level elements with a blank line between them. Otherwise a list is tight. (The difference in HTML output is that paragraphs in a loose list are wrapped in <p> tags, while paragraphs in a tight list are not.)

Changing the bullet or ordered list delimiter starts a new list:

Example 199  (interact)
- foo
- bar
+ baz
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<ul>
<li>baz</li>
</ul>
Example 200  (interact)
1. foo
2. bar
3) baz
<ol>
<li>foo</li>
<li>bar</li>
</ol>
<ol start="3">
<li>baz</li>
</ol>

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

Example 201  (interact)
Foo
- bar
- baz
<p>Foo</p>
<ul>
<li>bar</li>
<li>baz</li>
</ul>

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

Example 202  (interact)
The number of windows in my house is
14.  The number of doors is 6.
<p>The number of windows in my house is</p>
<ol start="14">
<li>The number of doors is 6.</li>
</ol>

Oddly, Markdown.pl does allow a blockquote to interrupt a paragraph, even though the same considerations might apply. We think that the two cases should be treated the same. Here are two reasons for allowing lists to interrupt paragraphs:

First, it is natural and not uncommon for people to start lists without blank lines:

I need to buy
- new shoes
- a coat
- a plane ticket

Second, we are attracted to a

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

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

  * I need to buy
    - new shoes
    - a coat
    - a plane ticket

is a list item containing a paragraph followed by a nested sublist, as all Markdown implementations agree it is (though the paragraph may be rendered without <p> tags, since the list is "tight"), then

I need to buy
- new shoes
- a coat
- a plane ticket

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

Our adherence to the principle of uniformity thus inclines us to think that there are two coherent packages:

  1. Require blank lines before all lists and blockquotes, including lists that occur as sublists inside other list items.

  2. Require blank lines in none of these places.

reStructuredText takes the first approach, for which there is much to be said. But the second seems more consistent with established practice with Markdown.

There can be blank lines between items, but two blank lines end a list:

Example 203  (interact)
- foo

- bar


- baz
<ul>
<li>
<p>foo</p>
</li>
<li>
<p>bar</p>
</li>
</ul>
<ul>
<li>baz</li>
</ul>

As illustrated above in the section on list items, two blank lines between blocks within a list item will also end a list:

Example 204  (interact)
- foo


  bar
- baz
<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>

Indeed, two blank lines will end all containing lists:

Example 205  (interact)
- foo
  - bar
    - baz


      bim
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>
<pre><code>  bim
</code></pre>

Thus, two blank lines can be used 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:

Example 206  (interact)
- foo
- bar


- baz
- bim
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<ul>
<li>baz</li>
<li>bim</li>
</ul>
Example 207  (interact)
-   foo

    notcode

-   foo


    code
<ul>
<li>
<p>foo</p>
<p>notcode</p>
</li>
<li>
<p>foo</p>
</li>
</ul>
<pre><code>code
</code></pre>

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

Example 208  (interact)
- a
 - b
  - c
   - d
  - e
 - f
- g
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d</li>
<li>e</li>
<li>f</li>
<li>g</li>
</ul>

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

Example 209  (interact)
- a
- b

- c
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ul>

So is this, with a empty second item:

Example 210  (interact)
* a
*

* c
<ul>
<li>
<p>a</p>
</li>
<li></li>
<li>
<p>c</p>
</li>
</ul>

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:

Example 211  (interact)
- a
- b

  c
- d
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
<p>c</p>
</li>
<li>
<p>d</p>
</li>
</ul>
Example 212  (interact)
- a
- b

  [ref]: /url
- d
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>d</p>
</li>
</ul>

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

Example 213  (interact)
- a
- ```
  b


  ```
- c
<ul>
<li>a</li>
<li>
<pre><code>b


</code></pre>
</li>
<li>c</li>
</ul>

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

Example 214  (interact)
- a
  - b

    c
- d
<ul>
<li>a
<ul>
<li>
<p>b</p>
<p>c</p>
</li>
</ul>
</li>
<li>d</li>
</ul>

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

Example 215  (interact)
* a
  > b
  >
* c
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
</li>
<li>c</li>
</ul>

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

Example 216  (interact)
- a
  > b
  ```
  c
  ```
- d
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
<pre><code>c
</code></pre>
</li>
<li>d</li>
</ul>

A single-paragraph list is tight:

Example 217  (interact)
- a
<ul>
<li>a</li>
</ul>
Example 218  (interact)
- a
  - b
<ul>
<li>a
<ul>
<li>b</li>
</ul>
</li>
</ul>

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

Example 219  (interact)
* foo
  * bar

  baz
<ul>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
<p>baz</p>
</li>
</ul>
Example 220  (interact)
- a
  - b
  - c

- d
  - e
  - f
<ul>
<li>
<p>a</p>
<ul>
<li>b</li>
<li>c</li>
</ul>
</li>
<li>
<p>d</p>
<ul>
<li>e</li>
<li>f</li>
</ul>
</li>
</ul>

6 Inlines

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

Example 221  (interact)
`hi`lo`
<p><code>hi</code>lo`</p>

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

6.1 Backslash escapes

Any ASCII punctuation character may be backslash-escaped:

Example 222  (interact)
\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~
<p>!&quot;#$%&amp;'()*+,-./:;&lt;=&gt;?@[\]^_`{|}~</p>

Backslashes before other characters are treated as literal backslashes:

Example 223  (interact)
\→\A\a\ \3\φ\«
<p>\   \A\a\ \3\φ\«</p>

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

Example 224  (interact)
\*not emphasized*
\<br/> not a tag
\[not a link](/foo)
\`not code`
1\. not a list
\* not a list
\# not a header
\[foo]: /url "not a reference"
<p>*not emphasized*
&lt;br/&gt; not a tag
[not a link](/foo)
`not code`
1. not a list
* not a list
# not a header
[foo]: /url &quot;not a reference&quot;</p>

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

Example 225  (interact)
\\*emphasis*
<p>\<em>emphasis</em></p>

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

Example 226  (interact)
foo\
bar
<p>foo<br />
bar</p>

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

Example 227  (interact)
`` \[\` ``
<p><code>\[\`</code></p>
Example 228  (interact)
    \[\]
<pre><code>\[\]
</code></pre>
Example 229  (interact)
~~~
\[\]
~~~
<pre><code>\[\]
</code></pre>
Example 230  (interact)
<http://example.com?find=\*>
<p><a href="http://example.com?find=%5C*">http://example.com?find=\*</a></p>
Example 231  (interact)
<a href="/bar\/)">
<p><a href="/bar\/)"></p>

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

Example 232  (interact)
[foo](/bar\* "ti\*tle")
<p><a href="/bar*" title="ti*tle">foo</a></p>
Example 233  (interact)
[foo]

[foo]: /bar\* "ti\*tle"
<p><a href="/bar*" title="ti*tle">foo</a></p>
Example 234  (interact)
``` foo\+bar
foo
```
<pre><code class="language-foo+bar">foo
</code></pre>

6.2 Entities

With the goal of making this standard as HTML-agnostic as possible, all valid HTML entities (except in code blocks and code spans) are recognized as such and converted into unicode characters before they are stored in the AST. This means that renderers to formats other than HTML need not be HTML-entity aware. HTML renderers may either escape unicode characters as entities or leave them as they are. (However, ", &, <, and > must always be rendered as entities.)

Named entities consist of &

Example 235  (interact)
&nbsp; &amp; &copy; &AElig; &Dcaron; &frac34; &HilbertSpace; &DifferentialD; &ClockwiseContourIntegral;
<p>  &amp; © Æ Ď ¾   ∲</p>

Decimal entities consist of &# + a string of 1--8 arabic digits + ;. Again, these entities need to be recognised and tranformed into their corresponding UTF8 codepoints. Invalid Unicode codepoints will be written as the "unknown codepoint" character (0xFFFD)

Example 236  (interact)
&#35; &#1234; &#992; &#98765432;
<p># Ӓ Ϡ �</p>

Hexadecimal entities consist of &# + either X or x + a string of 1-8 hexadecimal digits

Example 237  (interact)
&#X22; &#XD06; &#xcab;
<p>&quot;  ಫ</p>

Here are some nonentities:

Example 238  (interact)
&nbsp &x; &#; &#x; &ThisIsWayTooLongToBeAnEntityIsntIt; &hi?;
<p>&amp;nbsp &amp;x; &amp;#; &amp;#x; &amp;ThisIsWayTooLongToBeAnEntityIsntIt; &amp;hi?;</p>

Although HTML5 does accept some entities without a trailing semicolon (such as &copy), these are not recognized as entities here, because it makes the grammar too ambiguous:

Example 239  (interact)
&copy
<p>&amp;copy</p>

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

Example 240  (interact)
&MadeUpEntity;
<p>&amp;MadeUpEntity;</p>

Entities are recognized in any context besides code spans or code blocks, including raw HTML, URLs, link titles, and fenced code blocks:

Example 241  (interact)
<a href="&ouml;&ouml;.html">
<p><a href="&ouml;&ouml;.html"></p>
Example 242  (interact)
[foo](/f&ouml;&ouml; "f&ouml;&ouml;")
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
Example 243  (interact)
[foo]

[foo]: /f&ouml;&ouml; "f&ouml;&ouml;"
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
Example 244  (interact)
``` f&ouml;&ouml;
foo
```
<pre><code class="language-föö">foo
</code></pre>

Entities are treated as literal text in code spans and code blocks:

Example 245  (interact)
`f&ouml;&ouml;`
<p><code>f&amp;ouml;&amp;ouml;</code></p>
Example 246  (interact)
    f&ouml;f&ouml;
<pre><code>f&amp;ouml;f&amp;ouml;
</code></pre>

6.3 Code spans

A backtick string is a string of one or more backtick characters (`) that is neither preceded nor followed by a backtick.

A code span begins with a backtick string and ends with a backtick string of equal length. The contents of the code span are the characters between the two backtick strings, with leading and trailing spaces and line endings removed, and whitespace collapsed to single spaces.

This is a simple code span:

Example 247  (interact)
`foo`
<p><code>foo</code></p>

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

Example 248  (interact)
`` foo ` bar  ``
<p><code>foo ` bar</code></p>

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

Example 249  (interact)
` `` `
<p><code>``</code></p>

Line endings are treated like spaces:

Example 250  (interact)
``
foo
``
<p><code>foo</code></p>

Interior spaces and line endings are collapsed into single spaces, just as they would be by a browser:

Example 251  (interact)
`foo   bar
  baz`
<p><code>foo bar baz</code></p>

Q: Why not just leave the spaces, since browsers will collapse them anyway? A: Because we might be targeting a non-HTML format, and we shouldn't rely on HTML-specific rendering assumptions.

(Existing implementations differ in their treatment of internal spaces and line endings. Some, including Markdown.pl and showdown, convert an internal line ending into a <br /> tag. But this makes things difficult for those who like to hard-wrap their paragraphs, since a line break in the midst of a code span will cause an unintended line break in the output. Others just leave internal spaces as they are, which is fine if only HTML is being targeted.)

Example 252  (interact)
`foo `` bar`
<p><code>foo `` bar</code></p>

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

Example 253  (interact)
`foo\`bar`
<p><code>foo\</code>bar`</p>

Backslash escapes are never needed, because one can always choose a string of n backtick characters as delimiters, where the code does not contain any strings of exactly n backtick characters.

Code span backticks have higher precedence than any other inline constructs except HTML tags and autolinks. Thus, for example, this is not parsed as emphasized text, since the second * is part of a code span:

Example 254  (interact)
*foo`*`
<p>*foo<code>*</code></p>

And this is not parsed as a link:

Example 255  (interact)
[not a `link](/foo`)
<p>[not a <code>link](/foo</code>)</p>

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

Example 256  (interact)
`<a href="`">`
<p><code>&lt;a href=&quot;</code>&quot;&gt;`</p>

But this is an HTML tag:

Example 257  (interact)
<a href="`">`
<p><a href="`">`</p>

And this is code:

Example 258  (interact)
`<http://foo.bar.`baz>`
<p><code>&lt;http://foo.bar.</code>baz&gt;`</p>

But this is an autolink:

Example 259  (interact)
<http://foo.bar.`baz>`
<p><a href="http://foo.bar.%60baz">http://foo.bar.`baz</a>`</p>

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

Example 260  (interact)
```foo``
<p>```foo``</p>
Example 261  (interact)
`foo
<p>`foo</p>

6.4 Emphasis and strong emphasis

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 <em> tag; double *'s or _'s will be wrapped with an HTML <strong> tag.

This is enough for most users, but these rules leave much undecided, especially when it comes to nested emphasis. The original Markdown.pl test suite makes it clear that triple *** and ___ delimiters can be used for strong emphasis, and most implementations have also allowed the following patterns:

***strong emph***
***strong** in emph*
***emph* in strong**
**in strong *emph***
*in emph **strong***

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

*emph *with emph* in it*
**strong **with strong** in it**

Many implementations have also restricted intraword emphasis to the * forms, to avoid unwanted emphasis in words containing internal underscores. (It is best practice to put these in code spans, but users often do not.)

internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz

The rules given below capture all of these patterns, while allowing for efficient parsing strategies that do not backtrack.

First, some definitions. A delimiter run is either a sequence of one or more * characters that is not preceded or followed by a * character, or a sequence of one or more _ characters that is not preceded or followed by a _ character.

A left-flanking delimiter run is a delimiter run that is (a) not followed by unicode whitespace, and (b) either not followed by a punctuation character, or preceded by unicode whitespace or a punctuation character.

A right-flanking delimiter run is a delimiter run that is (a) not preceded by unicode whitespace, and (b) either not preceded by a punctuation character, or followed by unicode whitespace or a punctuation character.

Here are some examples of delimiter runs.

(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:

  1. A single * character can open emphasis iff it is part of a left-flanking delimiter run.

  2. A single _ character can open emphasis iff it is part of a left-flanking delimiter run and is not preceded by an ASCII alphanumeric character.

  3. A single * character can close emphasis iff it is part of a right-flanking delimiter run.

  4. A single _ character can close emphasis iff it is part of a right-flanking delimiter run. and it is not followed by an ASCII alphanumeric character.

  5. A double ** can open strong emphasis iff it is part of a left-flanking delimiter run.

  6. A double __ can open strong emphasis iff it is part of a left-flanking delimiter run and is not preceded by an ASCII alphanumeric character.

  7. A double ** can close strong emphasis iff it is part of a right-flanking delimiter run.

  8. A double __ can close strong emphasis iff it is part of a right-flanking delimiter run and is not followed by an ASCII alphanumeric character.

  9. Emphasis begins with a delimiter that can open emphasis and ends with a delimiter that can close emphasis, and that uses the same character (_ or *) as the opening delimiter. There must be a nonempty sequence of inlines between the open delimiter and the closing delimiter; these form the contents of the emphasis inline.

  10. Strong emphasis begins with a delimiter that can open strong emphasis and ends with a delimiter that can close strong emphasis, and that uses the same character (_ or *) as the opening delimiter. There must be a nonempty sequence of inlines between the open delimiter and the closing delimiter; these form the contents of the strong emphasis inline.

  11. A literal * character cannot occur at the beginning or end of *-delimited emphasis or **-delimited strong emphasis, unless it is backslash-escaped.

  12. A literal _ character cannot occur at the beginning or end of _-delimited emphasis or __-delimited strong emphasis, unless it is backslash-escaped.

Where rules 1--12 above are compatible with multiple parsings, the following principles resolve ambiguity:

  1. The number of nestings should be minimized. Thus, for example, an interpretation <strong>...</strong> is always preferred to <em><em>...</em></em>.

  2. An interpretation <strong><em>...</em></strong> is always preferred to <em><strong>..</strong></em>.

  3. When two potential emphasis or strong emphasis spans overlap, so that the second begins before the first ends and ends after the first ends, the first takes precedence. Thus, for example, *foo _bar* baz_ is parsed as <em>foo _bar</em> baz_ rather than *foo <em>bar* baz</em>. For the same reason, **foo*bar** is parsed as <em><em>foo</em>bar</em>* rather than <strong>foo*bar</strong>.

  4. When there are two potential emphasis or strong emphasis spans with the same closing delimiter, the shorter one (the one that opens later) takes precedence. Thus, for example, **foo **bar baz** is parsed as **foo <strong>bar baz</strong> rather than <strong>foo **bar baz</strong>.

  5. Inline code spans, links, images, and HTML tags group more tightly than emphasis. So, when there is a choice between an interpretation that contains one of these elements and one that does not, the former always wins. Thus, for example, *[foo*](bar) is parsed as *<a href="bar">foo*</a> rather than as <em>[foo</em>](bar).

These rules can be illustrated through a series of examples.

Rule 1:

Example 262  (interact)
*foo bar*
<p><em>foo bar</em></p>

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

Example 263  (interact)
a * foo bar*
<p>a * foo bar*</p>

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

Example 264  (interact)
a*"foo"*
<p>a*&quot;foo&quot;*</p>

Unicode nonbreaking spaces count as whitespace, too:

Example 265  (interact)
* a *
<p>* a *</p>

Intraword emphasis with * is permitted:

Example 266  (interact)
foo*bar*
<p>foo<em>bar</em></p>
Example 267  (interact)
5*6*78
<p>5<em>6</em>78</p>

Rule 2:

Example 268  (interact)
_foo bar_
<p><em>foo bar</em></p>

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

Example 269  (interact)
_ foo bar_
<p>_ foo bar_</p>

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

Example 270  (interact)
a_"foo"_
<p>a_&quot;foo&quot;_</p>

Emphasis with _ is not allowed inside ASCII words:

Example 271  (interact)
foo_bar_
<p>foo_bar_</p>
Example 272  (interact)
5_6_78
<p>5_6_78</p>

But it is permitted inside non-ASCII words:

Example 273  (interact)
пристаням_стремятся_
<p>пристаням<em>стремятся</em></p>

Rule 3:

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

Example 274  (interact)
_foo*
<p>_foo*</p>

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

Example 275  (interact)
*foo bar *
<p>*foo bar *</p>

This is not emphasis, because the second * is preceded by punctuation and followed by an alphanumeric (hence it is not part of a right-flanking delimiter run:

Example 276  (interact)
*(*foo)
<p>*(*foo)</p>

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

Example 277  (interact)
*(*foo*)*
<p><em>(<em>foo</em>)</em></p>

Intraword emphasis with * is allowed:

Example 278  (interact)
*foo*bar
<p><em>foo</em>bar</p>

Rule 4:

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

Example 279  (interact)
_foo bar _
<p>_foo bar _</p>

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

Example 280  (interact)
_(_foo)
<p>_(_foo)</p>

This is emphasis within emphasis:

Example 281  (interact)
_(_foo_)_
<p><em>(<em>foo</em>)</em></p>

Intraword emphasis is disallowed for _:

Example 282  (interact)
_foo_bar
<p>_foo_bar</p>
Example 283  (interact)
_пристаням_стремятся
<p><em>пристаням</em>стремятся</p>
Example 284  (interact)
_foo_bar_baz_
<p><em>foo_bar_baz</em></p>

Rule 5:

Example 285  (interact)
**foo bar**
<p><strong>foo bar</strong></p>

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

Example 286  (interact)
** foo bar**
<p>** foo bar**</p>

This is not strong emphasis, because the opening ** is preceded by an alphanumeric and followed by punctuation, and hence not part of a left-flanking delimiter run:

Example 287  (interact)
a**"foo"**
<p>a**&quot;foo&quot;**</p>

Intraword strong emphasis with ** is permitted:

Example 288  (interact)
foo**bar**
<p>foo<strong>bar</strong></p>

Rule 6:

Example 289  (interact)
__foo bar__
<p><strong>foo bar</strong></p>

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

Example 290  (interact)
__ foo bar__
<p>__ foo bar__</p>

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

Example 291  (interact)
a__"foo"__
<p>a__&quot;foo&quot;__</p>

Intraword strong emphasis is forbidden with __:

Example 292  (interact)
foo__bar__
<p>foo__bar__</p>
Example 293  (interact)
5__6__78
<p>5__6__78</p>
Example 294  (interact)
пристаням__стремятся__
<p>пристаням<strong>стремятся</strong></p>
Example 295  (interact)
__foo, __bar__, baz__
<p><strong>foo, <strong>bar</strong>, baz</strong></p>

Rule 7:

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

Example 296  (interact)
**foo bar **
<p>**foo bar **</p>

(Nor can it be interpreted as an emphasized *foo bar *, because of Rule 11.)

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

Example 297  (interact)
**(**foo)
<p>**(**foo)</p>

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

Example 298  (interact)
*(**foo**)*
<p><em>(<strong>foo</strong>)</em></p>
Example 299  (interact)
**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**
<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
<em>Asclepias physocarpa</em>)</strong></p>
Example 300  (interact)
**foo "*bar*" foo**
<p><strong>foo &quot;<em>bar</em>&quot; foo</strong></p>

Intraword emphasis:

Example 301  (interact)
**foo**bar
<p><strong>foo</strong>bar</p>

Rule 8:

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

Example 302  (interact)
__foo bar __
<p>__foo bar __</p>

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

Example 303  (interact)
__(__foo)
<p>__(__foo)</p>

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

Example 304  (interact)
_(__foo__)_
<p><em>(<strong>foo</strong>)</em></p>

Intraword strong emphasis is forbidden with __:

Example 305  (interact)
__foo__bar
<p>__foo__bar</p>
Example 306  (interact)
__пристаням__стремятся
<p><strong>пристаням</strong>стремятся</p>
Example 307  (interact)
__foo__bar__baz__
<p><strong>foo__bar__baz</strong></p>

Rule 9:

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

Example 308  (interact)
*foo [bar](/url)*
<p><em>foo <a href="/url">bar</a></em></p>
Example 309  (interact)
*foo
bar*
<p><em>foo
bar</em></p>

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

Example 310  (interact)
_foo __bar__ baz_
<p><em>foo <strong>bar</strong> baz</em></p>
Example 311  (interact)
_foo _bar_ baz_
<p><em>foo <em>bar</em> baz</em></p>
Example 312  (interact)
__foo_ bar_
<p><em><em>foo</em> bar</em></p>
Example 313  (interact)
*foo *bar**
<p><em>foo <em>bar</em></em></p>
Example 314  (interact)
*foo **bar** baz*
<p><em>foo <strong>bar</strong> baz</em></p>

But note:

Example 315  (interact)
*foo**bar**baz*
<p><em>foo</em><em>bar</em><em>baz</em></p>

The difference is that in the preceding case, the internal delimiters can close emphasis, while in the cases with spaces, they cannot.

Example 316  (interact)
***foo** bar*
<p><em><strong>foo</strong> bar</em></p>
Example 317  (interact)
*foo **bar***
<p><em>foo <strong>bar</strong></em></p>

Note, however, that in the following case we get no strong emphasis, because the opening delimiter is closed by the first * before bar:

Example 318  (interact)
*foo**bar***
<p><em>foo</em><em>bar</em>**</p>

Indefinite levels of nesting are possible:

Example 319  (interact)
*foo **bar *baz* bim** bop*
<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>
Example 320  (interact)
*foo [*bar*](/url)*
<p><em>foo <a href="/url"><em>bar</em></a></em></p>

There can be no empty emphasis or strong emphasis:

Example 321  (interact)
** is not an empty emphasis
<p>** is not an empty emphasis</p>
Example 322  (interact)
**** is not an empty strong emphasis
<p>**** is not an empty strong emphasis</p>

Rule 10:

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

Example 323  (interact)
**foo [bar](/url)**
<p><strong>foo <a href="/url">bar</a></strong></p>
Example 324  (interact)
**foo
bar**
<p><strong>foo
bar</strong></p>

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

Example 325  (interact)
__foo _bar_ baz__
<p><strong>foo <em>bar</em> baz</strong></p>
Example 326  (interact)
__foo __bar__ baz__
<p><strong>foo <strong>bar</strong> baz</strong></p>
Example 327  (interact)
____foo__ bar__
<p><strong><strong>foo</strong> bar</strong></p>
Example 328  (interact)
**foo **bar****
<p><strong>foo <strong>bar</strong></strong></p>
Example 329  (interact)
**foo *bar* baz**
<p><strong>foo <em>bar</em> baz</strong></p>

But note:

Example 330  (interact)
**foo*bar*baz**
<p><em><em>foo</em>bar</em>baz**</p>

The difference is that in the preceding case, the internal delimiters can close emphasis, while in the cases with spaces, they cannot.

Example 331  (interact)
***foo* bar**
<p><strong><em>foo</em> bar</strong></p>
Example 332  (interact)
**foo *bar***
<p><strong>foo <em>bar</em></strong></p>

Indefinite levels of nesting are possible:

Example 333  (interact)
**foo *bar **baz**
bim* bop**
<p><strong>foo <em>bar <strong>baz</strong>
bim</em> bop</strong></p>
Example 334  (interact)
**foo [*bar*](/url)**
<p><strong>foo <a href="/url"><em>bar</em></a></strong></p>

There can be no empty emphasis or strong emphasis:

Example 335  (interact)
__ is not an empty emphasis
<p>__ is not an empty emphasis</p>
Example 336  (interact)
____ is not an empty strong emphasis
<p>____ is not an empty strong emphasis</p>

Rule 11:

Example 337  (interact)
foo ***
<p>foo ***</p>
Example 338  (interact)
foo *\**
<p>foo <em>*</em></p>
Example 339  (interact)
foo *_*
<p>foo <em>_</em></p>
Example 340  (interact)
foo *****
<p>foo *****</p>
Example 341  (interact)
foo **\***
<p>foo <strong>*</strong></p>
Example 342  (interact)
foo **_**
<p>foo <strong>_</strong></p>

Note that when delimiters do not match evenly, Rule 11 determines that the excess literal * characters will appear outside of the emphasis, rather than inside it:

Example 343  (interact)
**foo*
<p>*<em>foo</em></p>
Example 344  (interact)
*foo**
<p><em>foo</em>*</p>
Example 345  (interact)
***foo**
<p>*<strong>foo</strong></p>
Example 346  (interact)
****foo*
<p>***<em>foo</em></p>
Example 347  (interact)
**foo***
<p><strong>foo</strong>*</p>
Example 348  (interact)
*foo****
<p><em>foo</em>***</p>

Rule 12:

Example 349  (interact)
foo ___
<p>foo ___</p>
Example 350  (interact)
foo _\__
<p>foo <em>_</em></p>
Example 351  (interact)
foo _*_
<p>foo <em>*</em></p>
Example 352  (interact)
foo _____
<p>foo _____</p>
Example 353  (interact)
foo __\___
<p>foo <strong>_</strong></p>
Example 354  (interact)
foo __*__
<p>foo <strong>*</strong></p>
Example 355  (interact)
__foo_
<p>_<em>foo</em></p>

Note that when delimiters do not match evenly, Rule 12 determines that the excess literal _ characters will appear outside of the emphasis, rather than inside it:

Example 356  (interact)
_foo__
<p><em>foo</em>_</p>
Example 357  (interact)
___foo__
<p>_<strong>foo</strong></p>
Example 358  (interact)
____foo_
<p>___<em>foo</em></p>
Example 359  (interact)
__foo___
<p><strong>foo</strong>_</p>
Example 360  (interact)
_foo____
<p><em>foo</em>___</p>

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

Example 361  (interact)
**foo**
<p><strong>foo</strong></p>
Example 362  (interact)
*_foo_*
<p><em><em>foo</em></em></p>
Example 363  (interact)
__foo__
<p><strong>foo</strong></p>
Example 364  (interact)
_*foo*_
<p><em><em>foo</em></em></p>

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

Example 365  (interact)
****foo****
<p><strong><strong>foo</strong></strong></p>
Example 366  (interact)
____foo____
<p><strong><strong>foo</strong></strong></p>

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

Example 367  (interact)
******foo******
<p><strong><strong><strong>foo</strong></strong></strong></p>

Rule 14:

Example 368  (interact)
***foo***
<p><strong><em>foo</em></strong></p>
Example 369  (interact)
_____foo_____
<p><strong><strong><em>foo</em></strong></strong></p>

Rule 15:

Example 370  (interact)
*foo _bar* baz_
<p><em>foo _bar</em> baz_</p>
Example 371  (interact)
**foo*bar**
<p><em><em>foo</em>bar</em>*</p>

Rule 16:

Example 372  (interact)
**foo **bar baz**
<p>**foo <strong>bar baz</strong></p>
Example 373  (interact)
*foo *bar baz*
<p>*foo <em>bar baz</em></p>

Rule 17:

Example 374  (interact)
*[bar*](/url)
<p>*<a href="/url">bar*</a></p>
Example 375  (interact)
_foo [bar_](/url)
<p>_foo <a href="/url">bar_</a></p>
Example 376  (interact)
*<img src="foo" title="*"/>
<p>*<img src="foo" title="*"/></p>
Example 377  (interact)
**<a href="**">
<p>**<a href="**"></p>
Example 378  (interact)
__<a href="__">
<p>__<a href="__"></p>
Example 379  (interact)
*a `*`*
<p><em>a <code>*</code></em></p>
Example 380  (interact)
_a `_`_
<p><em>a <code>_</code></em></p>
Example 381  (interact)
**a<http://foo.bar?q=**>
<p>**a<a href="http://foo.bar?q=**">http://foo.bar?q=**</a></p>
Example 382  (interact)
__a<http://foo.bar?q=__>
<p>__a<a href="http://foo.bar?q=__">http://foo.bar?q=__</a></p>

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:

A link destination consists of either

A link title consists of either

An inline link consists of a link text followed immediately by a left parenthesis (, optional whitespace, an optional link destination, an optional link title separated from the link destination by whitespace, optional whitespace, and a right parenthesis ). The link's text consists of the inlines contained in the link text (excluding the enclosing square brackets). The link's URI consists of the link destination, excluding enclosing <...> if present, with backslash-escapes in effect as described above. The link's title consists of the link title, excluding its enclosing delimiters, with backslash-escapes in effect as described above.

Here is a simple inline link:

Example 383  (interact)
[link](/uri "title")
<p><a href="/uri" title="title">link</a></p>

The title may be omitted:

Example 384  (interact)
[link](/uri)
<p><a href="/uri">link</a></p>

Both the title and the destination may be omitted:

Example 385  (interact)
[link]()
<p><a href="">link</a></p>
Example 386  (interact)
[link](<>)
<p><a href="">link</a></p>

If the destination contains spaces, it must be enclosed in pointy braces:

Example 387  (interact)
[link](/my uri)
<p>[link](/my uri)</p>
Example 388  (interact)
[link](</my uri>)
<p><a href="/my%20uri">link</a></p>

The destination cannot contain line breaks, even with pointy braces:

Example 389  (interact)
[link](foo
bar)
<p>[link](foo
bar)</p>

One level of balanced parentheses is allowed without escaping:

Example 390  (interact)
[link]((foo)and(bar))
<p><a href="(foo)and(bar)">link</a></p>

However, if you have parentheses within parentheses, you need to escape or use the <...> form:

Example 391  (interact)
[link](foo(and(bar)))
<p>[link](foo(and(bar)))</p>
Example 392  (interact)
[link](foo(and\(bar\)))
<p><a href="foo(and(bar))">link</a></p>
Example 393  (interact)
[link](<foo(and(bar))>)
<p><a href="foo(and(bar))">link</a></p>

Parentheses and other symbols can also be escaped, as usual in Markdown:

Example 394  (interact)
[link](foo\)\:)
<p><a href="foo):">link</a></p>

URL-escaping should be left alone inside the destination, as all URL-escaped characters are also valid URL characters. HTML entities in the destination will be parsed into their UTF-8 codepoints, as usual, and optionally URL-escaped when written as HTML.

Example 395  (interact)
[link](foo%20b&auml;)
<p><a href="foo%20b%C3%A4">link</a></p>

Note that, because titles can often be parsed as destinations, if you try to omit the destination and keep the title, you'll get unexpected results:

Example 396  (interact)
[link]("title")
<p><a href="%22title%22">link</a></p>

Titles may be in single quotes, double quotes, or parentheses:

Example 397  (interact)
[link](/url "title")
[link](/url 'title')
[link](/url (title))
<p><a href="/url" title="title">link</a>
<a href="/url" title="title">link</a>
<a href="/url" title="title">link</a></p>

Backslash escapes and entities may be used in titles:

Example 398  (interact)
[link](/url "title \"&quot;")
<p><a href="/url" title="title &quot;&quot;">link</a></p>

Nested balanced quotes are not allowed without escaping:

Example 399  (interact)
[link](/url "title "and" title")
<p>[link](/url &quot;title &quot;and&quot; title&quot;)</p>

But it is easy to work around this by using a different quote type:

Example 400  (interact)
[link](/url 'title "and" title')
<p><a href="/url" title="title &quot;and&quot; title">link</a></p>

(Note: Markdown.pl did allow double quotes inside a double-quoted title, and its test suite included a test demonstrating this. But it is hard to see a good rationale for the extra complexity this brings, since there are already many ways---backslash escaping, entities, or using a different quote type for the enclosing title---to write titles containing double quotes. Markdown.pl's handling of titles has a number of other strange features. For example, it allows single-quoted titles in inline links, but not reference links. And, in reference links but not inline links, it allows a title to begin with " and end with ). Markdown.pl 1.0.1 even allows titles with no closing quotation mark, though 1.0.2b8 does not. It seems preferable to adopt a simple, rational rule that works the same way in inline links and link reference definitions.)

Whitespace is allowed around the destination and title:

Example 401  (interact)
[link](   /uri
  "title"  )
<p><a href="/uri" title="title">link</a></p>

But it is not allowed between the link text and the following parenthesis:

Example 402  (interact)
[link] (/uri)
<p>[link] (/uri)</p>

The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:

Example 403  (interact)
[link [foo [bar]]](/uri)
<p><a href="/uri">link [foo [bar]]</a></p>
Example 404  (interact)
[link] bar](/uri)
<p>[link] bar](/uri)</p>
Example 405  (interact)
[link [bar](/uri)
<p>[link <a href="/uri">bar</a></p>
Example 406  (interact)
[link \[bar](/uri)
<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

Example 407  (interact)
[link *foo **bar** `#`*](/uri)
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
Example 408  (interact)
[![moon](moon.jpg)](/uri)
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>

However, links may not contain other links, at any level of nesting.

Example 409  (interact)
[foo [bar](/uri)](/uri)
<p>[foo <a href="/uri">bar</a>](/uri)</p>
Example 410  (interact)
[foo *[bar [baz](/uri)](/uri)*](/uri)
<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>
Example 411  (interact)
![[[foo](uri1)](uri2)](uri3)
<p><img src="uri3" alt="[foo](uri2)" /></p>

These cases illustrate the precedence of link text grouping over emphasis grouping:

Example 412  (interact)
*[foo*](/uri)
<p>*<a href="/uri">foo*</a></p>
Example 413  (interact)
[foo *bar](baz*)
<p><a href="baz*">foo *bar</a></p>

Note that brackets that aren't part of links do not take precedence:

Example 414  (interact)
*foo [bar* baz]
<p><em>foo [bar</em> baz]</p>

These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:

Example 415  (interact)
[foo <bar attr="](baz)">
<p>[foo <bar attr="](baz)"></p>
Example 416  (interact)
[foo`](/uri)`
<p>[foo<code>](/uri)</code></p>
Example 417  (interact)
[foo<http://example.com?search=](uri)>
<p>[foo<a href="http://example.com?search=%5D(uri)">http://example.com?search=](uri)</a></p>

There are three kinds of reference links: full, collapsed, and shortcut.

A full reference link consists of a link text, optional whitespace, and a link label that matches a link reference definition elsewhere in the document.

A link label begins with a left bracket ([) and ends with the first right bracket (]) that is not backslash-escaped. Unescaped square bracket characters are not allowed in link labels. A link label can have at most 999 characters inside the square brackets.

One label matches another just in case their normalized forms are equal. To normalize a label, perform the unicode case fold and collapse consecutive internal whitespace to a single space. If there are multiple matching reference link definitions, the one that comes first in the document is used. (It is desirable in such cases to emit a warning.)

The contents of the first link label are parsed as inlines, which are used as the link's text. The link's URI and title are provided by the matching link reference definition.

Here is a simple example:

Example 418  (interact)
[foo][bar]

[bar]: /url "title"
<p><a href="/url" title="title">foo</a></p>

The rules for the link text are the same as with inline links. Thus:

The link text may contain balanced brackets, but not unbalanced ones, unless they are escaped:

Example 419  (interact)
[link [foo [bar]]][ref]

[ref]: /uri
<p><a href="/uri">link [foo [bar]]</a></p>
Example 420  (interact)
[link \[bar][ref]

[ref]: /uri
<p><a href="/uri">link [bar</a></p>

The link text may contain inline content:

Example 421  (interact)
[link *foo **bar** `#`*][ref]

[ref]: /uri
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
Example 422  (interact)
[![moon](moon.jpg)][ref]

[ref]: /uri
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>

However, links may not contain other links, at any level of nesting.

Example 423  (interact)
[foo [bar](/uri)][ref]

[ref]: /uri
<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>
Example 424  (interact)
[foo *bar [baz][ref]*][ref]

[ref]: /uri
<p>[foo <em>bar <a href="/uri">baz</a></em>]<a href="/uri">ref</a></p>

(In the examples above, we have two shortcut reference links instead of one full reference link.)

The following cases illustrate the precedence of link text grouping over emphasis grouping:

Example 425  (interact)
*[foo*][ref]

[ref]: /uri
<p>*<a href="/uri">foo*</a></p>
Example 426  (interact)
[foo *bar][ref]

[ref]: /uri
<p><a href="/uri">foo *bar</a></p>

These cases illustrate the precedence of HTML tags, code spans, and autolinks over link grouping:

Example 427  (interact)
[foo <bar attr="][ref]">

[ref]: /uri
<p>[foo <bar attr="][ref]"></p>
Example 428  (interact)
[foo`][ref]`

[ref]: /uri
<p>[foo<code>][ref]</code></p>
Example 429  (interact)
[foo<http://example.com?search=][ref]>

[ref]: /uri
<p>[foo<a href="http://example.com?search=%5D%5Bref%5D">http://example.com?search=][ref]</a></p>

Matching is case-insensitive:

Example 430  (interact)
[foo][BaR]

[bar]: /url "title"
<p><a href="/url" title="title">foo</a></p>

Unicode case fold is used:

Example 431  (interact)
[Толпой][Толпой] is a Russian word.

[ТОЛПОЙ]: /url
<p><a href="/url">Толпой</a> is a Russian word.</p>

Consecutive internal whitespace is treated as one space for purposes of determining matching:

Example 432  (interact)
[Foo
  bar]: /url

[Baz][Foo bar]
<p><a href="/url">Baz</a></p>

There can be whitespace between the link text and the link label:

Example 433  (interact)
[foo] [bar]

[bar]: /url "title"
<p><a href="/url" title="title">foo</a></p>
Example 434  (interact)
[foo]
[bar]

[bar]: /url "title"
<p><a href="/url" title="title">foo</a></p>

When there are multiple matching link reference definitions, the first is used:

Example 435  (interact)
[foo]: /url1

[foo]: /url2

[bar][foo]
<p><a href="/url1">bar</a></p>

Note that matching is performed on normalized strings, not parsed inline content. So the following does not match, even though the labels define equivalent inline content:

Example 436  (interact)
[bar][foo\!]

[foo!]: /url
<p>[bar][foo!]</p>

Link labels cannot contain brackets, unless they are backslash-escaped:

Example 437  (interact)
[foo][ref[]

[ref[]: /uri
<p>[foo][ref[]</p>
<p>[ref[]: /uri</p>
Example 438  (interact)
[foo][ref[bar]]

[ref[bar]]: /uri
<p>[foo][ref[bar]]</p>
<p>[ref[bar]]: /uri</p>
Example 439  (interact)
[[[foo]]]

[[[foo]]]: /url
<p>[[[foo]]]</p>
<p>[[[foo]]]: /url</p>
Example 440  (interact)
[foo][ref\[]

[ref\[]: /uri
<p><a href="/uri">foo</a></p>

A collapsed reference link consists of a link label that matches a link reference definition elsewhere in the document, optional whitespace, and the string []. The contents of the first link label are parsed as inlines, which are used as the link's text. The link's URI and title are provided by the matching reference link definition. Thus, [foo][] is equivalent to [foo][foo].

Example 441  (interact)
[foo][]

[foo]: /url "title"
<p><a href="/url" title="title">foo</a></p>
Example 442  (interact)
[*foo* bar][]

[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>

The link labels are case-insensitive:

Example 443  (interact)
[Foo][]

[foo]: /url "title"
<p><a href="/url" title="title">Foo</a></p>

As with full reference links, whitespace is allowed between the two sets of brackets:

Example 444  (interact)
[foo] 
[]

[foo]: /url "title"
<p><a href="/url" title="title">foo</a></p>

A shortcut reference link consists of a link label that matches a link reference definition elsewhere in the document and is not followed by [] or a link label. The contents of the first link label are parsed as inlines, which are used as the link's text. the link's URI and title are provided by the matching link reference definition. Thus, [foo] is equivalent to [foo][].

Example 445  (interact)
[foo]

[foo]: /url "title"
<p><a href="/url" title="title">foo</a></p>
Example 446  (interact)
[*foo* bar]

[*foo* bar]: /url "title"
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
Example 447  (interact)
[[*foo* bar]]

[*foo* bar]: /url "title"
<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>

The link labels are case-insensitive:

Example 448  (interact)
[Foo]

[foo]: /url "title"
<p><a href="/url" title="title">Foo</a></p>

A space after the link text should be preserved:

Example 449  (interact)
[foo] bar

[foo]: /url
<p><a href="/url">foo</a> bar</p>

If you just want bracketed text, you can backslash-escape the opening bracket to avoid links:

Example 450  (interact)
\[foo]

[foo]: /url "title"
<p>[foo]</p>

Note that this is a link, because a link label ends with the first following closing bracket:

Example 451  (interact)
[foo*]: /url

*[foo*]
<p>*<a href="/url">foo*</a></p>

Full references take precedence over shortcut references:

Example 452  (interact)
[foo][bar]

[foo]: /url1
[bar]: /url2
<p><a href="/url2">foo</a></p>

In the following case [bar][baz] is parsed as a reference, [foo] as normal text:

Example 453  (interact)
[foo][bar][baz]

[baz]: /url
<p>[foo]<a href="/url">bar</a></p>

Here, though, [foo][bar] is parsed as a reference, since [bar] is defined:

Example 454  (interact)
[foo][bar][baz]

[baz]: /url1
[bar]: /url2
<p><a href="/url2">foo</a><a href="/url1">baz</a></p>

Here [foo] is not parsed as a shortcut reference, because it is followed by a link label (even though [bar] is not defined):

Example 455  (interact)
[foo][bar][baz]

[baz]: /url1
[foo]: /url2
<p>[foo]<a href="/url1">bar</a></p>

6.6 Images

Syntax for images is like the syntax for links, with one difference. Instead of link text, we have an image description. The rules for this are the same as for link text, except that (a) an image description starts with ![ rather than [, and (b) an image description may contain links. An image description has inline elements as its contents. When an image is rendered to HTML, this is standardly used as the image's alt attribute.

Example 456  (interact)
![foo](/url "title")
<p><img src="/url" alt="foo" title="title" /></p>
Example 457  (interact)
![foo *bar*]

[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 458  (interact)
![foo ![bar](/url)](/url2)
<p><img src="/url2" alt="foo bar" /></p>
Example 459  (interact)
![foo [bar](/url)](/url2)
<p><img src="/url2" alt="foo bar" /></p>

Though this spec is concerned with parsing, not rendering, it is recommended that in rendering to HTML, only the plain string content of the image description be used. Note that in the above example, the alt attribute's value is foo bar, not foo [bar](/url) or foo <a href="/url">bar</a>. Only the plain string content is rendered, without formatting.

Example 460  (interact)
![foo *bar*][]

[foo *bar*]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 461  (interact)
![foo *bar*][foobar]

[FOOBAR]: train.jpg "train & tracks"
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
Example 462  (interact)
![foo](train.jpg)
<p><img src="train.jpg" alt="foo" /></p>
Example 463  (interact)
My ![foo bar](/path/to/train.jpg  "title"   )
<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>
Example 464  (interact)
![foo](<url>)
<p><img src="url" alt="foo" /></p>
Example 465  (interact)
![](/url)
<p><img src="/url" alt="" /></p>

Reference-style:

Example 466  (interact)
![foo] [bar]

[bar]: /url
<p><img src="/url" alt="foo" /></p>
Example 467  (interact)
![foo] [bar]

[BAR]: /url
<p><img src="/url" alt="foo" /></p>

Collapsed:

Example 468  (interact)
![foo][]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" /></p>
Example 469  (interact)
![*foo* bar][]

[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>

The labels are case-insensitive:

Example 470  (interact)
![Foo][]

[foo]: /url "title"
<p><img src="/url" alt="Foo" title="title" /></p>

As with full reference links, whitespace is allowed between the two sets of brackets:

Example 471  (interact)
![foo] 
[]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" /></p>

Shortcut:

Example 472  (interact)
![foo]

[foo]: /url "title"
<p><img src="/url" alt="foo" title="title" /></p>
Example 473  (interact)
![*foo* bar]

[*foo* bar]: /url "title"
<p><img src="/url" alt="foo bar" title="title" /></p>

Note that link labels cannot contain unescaped brackets:

Example 474  (interact)
![[foo]]

[[foo]]: /url "title"
<p>![[foo]]</p>
<p>[[foo]]: /url &quot;title&quot;</p>

The link labels are case-insensitive:

Example 475  (interact)
![Foo]

[foo]: /url "title"
<p><img src="/url" alt="Foo" title="title" /></p>

If you just want bracketed text, you can backslash-escape the opening ! and [:

Example 476  (interact)
\!\[foo]

[foo]: /url "title"
<p>![foo]</p>

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

Example 477  (interact)
\![foo]

[foo]: /url "title"
<p>!<a href="/url" title="title">foo</a></p>

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

A URI autolink consists of <, followed by an absolute URI not containing <, followed by >. It is parsed as a link to the URI, with the URI as the link's label.

An absolute URI, for these purposes, consists of a [scheme] followed by a colon (:) followed by zero or more characters other than ASCII whitespace and control characters, <, and >. If the URI includes these characters, you must use percent-encoding (e.g. %20 for a space).

The following schemes are recognized (case-insensitive): coap, doi, javascript, aaa, aaas, about, acap, cap, cid, crid, data, dav, dict, dns, file, ftp, geo, go, gopher, h323, http, https, iax, icap, im, imap, info, ipp, iris, iris.beep, iris.xpc, iris.xpcs, iris.lwz, ldap, mailto, mid, msrp, msrps, mtqp, mupdate, news, nfs, ni, nih, nntp, opaquelocktoken, pop, pres, rtsp, service, session, shttp, sieve, sip, sips, sms, snmp,soap.beep, soap.beeps, tag, tel, telnet, tftp, thismessage, tn3270, tip, tv, urn, vemmi, ws, wss, xcon, xcon-userid, xmlrpc.beep, xmlrpc.beeps, xmpp, z39.50r, z39.50s, adiumxtra, afp, afs, aim, apt,attachment, aw, beshare, bitcoin, bolo, callto, chrome,chrome-extension, com-eventbrite-attendee, content, cvs,dlna-playsingle, dlna-playcontainer, dtn, dvb, ed2k, facetime, feed, finger, fish, gg, git, gizmoproject, gtalk, hcp, icon, ipn, irc, irc6, ircs, itms, jar, jms, keyparc, lastfm, ldaps, magnet, maps, market,message, mms, ms-help, msnim, mumble, mvn, notes, oid, palm, paparazzi, platform, proxy, psyc, query, res, resource, rmi, rsync, rtmp, secondlife, sftp, sgn, skype, smb, soldat, spotify, ssh, steam, svn, teamspeak, things, udp, unreal, ut2004, ventrilo, view-source, webcal, wtai, wyciwyg, xfire, xri, ymsgr.

Here are some valid autolinks:

Example 478  (interact)
<http://foo.bar.baz>
<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p>
Example 479  (interact)
<http://foo.bar.baz?q=hello&id=22&boolean>
<p><a href="http://foo.bar.baz?q=hello&amp;id=22&amp;boolean">http://foo.bar.baz?q=hello&amp;id=22&amp;boolean</a></p>
Example 480  (interact)
<irc://foo.bar:2233/baz>
<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p>

Uppercase is also fine:

Example 481  (interact)
<MAILTO:FOO@BAR.BAZ>
<p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>

Spaces are not allowed in autolinks:

Example 482  (interact)
<http://foo.bar/baz bim>
<p>&lt;http://foo.bar/baz bim&gt;</p>

An email autolink consists of <, followed by an email address, followed by >. The link's label is the email address, and the URL is mailto: followed by the email address.

An email address, for these purposes, is anything that matches the non-normative regex from the HTML5 spec:

/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?
(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/

Examples of email autolinks:

Example 483  (interact)
<foo@bar.example.com>
<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>
Example 484  (interact)
<foo+special@Bar.baz-bar0.com>
<p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p>

These are not autolinks:

Example 485  (interact)
<>
<p>&lt;&gt;</p>
Example 486  (interact)
<heck://bing.bong>
<p>&lt;heck://bing.bong&gt;</p>
Example 487  (interact)
< http://foo.bar >
<p>&lt; http://foo.bar &gt;</p>
Example 488  (interact)
<foo.bar.baz>
<p>&lt;foo.bar.baz&gt;</p>
Example 489  (interact)
<localhost:5001/foo>
<p>&lt;localhost:5001/foo&gt;</p>
Example 490  (interact)
http://example.com
<p>http://example.com</p>
Example 491  (interact)
foo@bar.example.com
<p>foo@bar.example.com</p>

6.8 Raw HTML

Text between < and > that looks like an HTML tag is parsed as a raw HTML tag and will be rendered in HTML without escaping. Tag and attribute names are not limited to current HTML tags, so custom tags (and even, say, DocBook tags) may be used.

Here is the grammar for tags:

A tag name consists of an ASCII letter followed by zero or more ASCII letters or digits.

An attribute consists of whitespace, an attribute name, and an optional attribute value specification.

An attribute name consists of an ASCII letter, _, or :, followed by zero or more ASCII letters, digits, _, ., :, or -. (Note: This is the XML specification restricted to ASCII. HTML5 is laxer.)

An attribute value specification consists of optional whitespace, a = character, optional whitespace, and an attribute value.

An attribute value consists of an unquoted attribute value, a single-quoted attribute value, or a double-quoted attribute value.

An unquoted attribute value is a nonempty string of characters not including spaces, ", ', =, <, >, or `.

A single-quoted attribute value consists of ', zero or more characters not including ', and a final '.

A double-quoted attribute value consists of ", zero or more characters not including ", and a final ".

An open tag consists of a < character, a tag name, zero or more [attributes], optional whitespace, an optional / character, and a > character.

A closing tag consists of the string </, a tag name, optional whitespace, and the character >.

An HTML comment consists of <!-- + text + -->, where text does not start with > or ->, does not end with -, and does not contain --. (See the HTML5 spec.)

A processing instruction consists of the string <?, a string of characters not including 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 <![CDATA[, a string of characters not including the string ]]>, and the string ]]>.

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

Here are some simple open tags:

Example 492  (interact)
<a><bab><c2c>
<p><a><bab><c2c></p>

Empty elements:

Example 493  (interact)
<a/><b2/>
<p><a/><b2/></p>

Whitespace is allowed:

Example 494  (interact)
<a  /><b2
data="foo" >
<p><a  /><b2
data="foo" ></p>

With attributes:

Example 495  (interact)
<a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 />
<p><a foo="bar" bam = 'baz <em>"</em>'
_boolean zoop:33=zoop:33 /></p>

Illegal tag names, not parsed as HTML:

Example 496  (interact)
<33> <__>
<p>&lt;33&gt; &lt;__&gt;</p>

Illegal attribute names:

Example 497  (interact)
<a h*#ref="hi">
<p>&lt;a h*#ref=&quot;hi&quot;&gt;</p>

Illegal attribute values:

Example 498  (interact)
<a href="hi'> <a href=hi'>
<p>&lt;a href=&quot;hi'&gt; &lt;a href=hi'&gt;</p>

Illegal whitespace:

Example 499  (interact)
< a><
foo><bar/ >
<p>&lt; a&gt;&lt;
foo&gt;&lt;bar/ &gt;</p>

Missing whitespace:

Example 500  (interact)
<a href='bar'title=title>
<p>&lt;a href='bar'title=title&gt;</p>

Closing tags:

Example 501  (interact)
</a>
</foo >
<p></a>
</foo ></p>

Illegal attributes in closing tag:

Example 502  (interact)
</a href="foo">
<p>&lt;/a href=&quot;foo&quot;&gt;</p>

Comments:

Example 503  (interact)
foo <!-- this is a
comment - with hyphen -->
<p>foo <!-- this is a
comment - with hyphen --></p>
Example 504  (interact)
foo <!-- not a comment -- two hyphens -->
<p>foo &lt;!-- not a comment -- two hyphens --&gt;</p>

Not comments:

Example 505  (interact)
foo <!--> foo -->

foo <!-- foo--->
<p>foo &lt;!--&gt; foo --&gt;</p>
<p>foo &lt;!-- foo---&gt;</p>

Processing instructions:

Example 506  (interact)
foo <?php echo $a; ?>
<p>foo <?php echo $a; ?></p>

Declarations:

Example 507  (interact)
foo <!ELEMENT br EMPTY>
<p>foo <!ELEMENT br EMPTY></p>

CDATA sections:

Example 508  (interact)
foo <![CDATA[>&<]]>
<p>foo <![CDATA[>&<]]></p>

Entities are preserved in HTML attributes:

Example 509  (interact)
<a href="&ouml;">
<p><a href="&ouml;"></p>

Backslash escapes do not work in HTML attributes:

Example 510  (interact)
<a href="\*">
<p><a href="\*"></p>
Example 511  (interact)
<a href="\"">
<p>&lt;a href=&quot;&quot;&quot;&gt;</p>

6.9 Hard line breaks

A line break (not in a code span or HTML tag) that is preceded by two or more spaces and does not occur at the end of a block is parsed as a hard line break (rendered in HTML as a <br /> tag):

Example 512  (interact)
foo  
baz
<p>foo<br />
baz</p>

For a more visible alternative, a backslash before the line ending may be used instead of two spaces:

Example 513  (interact)
foo\
baz
<p>foo<br />
baz</p>

More than two spaces can be used:

Example 514  (interact)
foo       
baz
<p>foo<br />
baz</p>

Leading spaces at the beginning of the next line are ignored:

Example 515  (interact)
foo  
     bar
<p>foo<br />
bar</p>
Example 516  (interact)
foo\
     bar
<p>foo<br />
bar</p>

Line breaks can occur inside emphasis, links, and other constructs that allow inline content:

Example 517  (interact)
*foo  
bar*
<p><em>foo<br />
bar</em></p>
Example 518  (interact)
*foo\
bar*
<p><em>foo<br />
bar</em></p>

Line breaks do not occur inside code spans

Example 519  (interact)
`code  
span`
<p><code>code span</code></p>
Example 520  (interact)
`code\
span`
<p><code>code\ span</code></p>

or HTML tags:

Example 521  (interact)
<a href="foo  
bar">
<p><a href="foo  
bar"></p>
Example 522  (interact)
<a href="foo\
bar">
<p><a href="foo\
bar"></p>

Hard line breaks are for separating inline content within a block. Neither syntax for hard line breaks works at the end of a paragraph or other block element:

Example 523  (interact)
foo\
<p>foo\</p>
Example 524  (interact)
foo  
<p>foo</p>
Example 525  (interact)
### foo\
<h3>foo\</h3>
Example 526  (interact)
### foo  
<h3>foo</h3>

6.10 Soft line breaks

A regular line break (not in a code span or HTML tag) that is not preceded by two or more spaces is parsed as a softbreak. (A softbreak may be rendered in HTML either as a line ending or as a space. The result will be the same in browsers. In the examples here, a line ending will be used.)

Example 527  (interact)
foo
baz
<p>foo
baz</p>

Spaces at the end of the line and beginning of the next line are removed:

Example 528  (interact)
foo 
 baz
<p>foo
baz</p>

A conforming parser may render a soft line break in HTML either as a line break or as a space.

A renderer may also provide an option to render soft line breaks as hard line breaks.

6.11 Textual content

Any characters not given an interpretation by the above rules will be parsed as plain textual content.

Example 529  (interact)
hello $.;'there
<p>hello $.;'there</p>
Example 530  (interact)
Foo χρῆν
<p>Foo χρῆν</p>

Internal spaces are preserved verbatim:

Example 531  (interact)
Multiple     spaces
<p>Multiple     spaces</p>

Appendix A: A parsing strategy

Overview

Parsing has two phases:

  1. In the first phase, lines of input are consumed and the block structure of the document---its division into paragraphs, block quotes, list items, and so on---is constructed. Text is assigned to these blocks but not parsed. Link reference definitions are parsed and a map of links is constructed.

  2. In the second phase, the raw text contents of paragraphs and headers are parsed into sequences of Markdown inline elements (strings, code spans, links, emphasis, and so on), using the map of link references constructed in phase 1.

The document tree

At each point in processing, the document is represented as a tree of blocks. The root of the tree is a document block. The document may have any number of other blocks as children. These children may, in turn, have other blocks as children. The last child of a block is normally considered open, meaning that subsequent lines of input can alter its contents. (Blocks that are not open are closed.) Here, for example, is a possible document tree, with the open blocks marked by arrows:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"

How source lines alter the document tree

Each line that is processed has an effect on this tree. The line is analyzed and, depending on its contents, the document may be altered in one or more of the following ways:

  1. One or more open blocks may be closed.
  2. One or more new blocks may be created as children of the last open block.
  3. Text may be added to the last (deepest) open block remaining on the tree.

Once a line has been incorporated into the tree in this way, it can be discarded, so input can be read in a stream.

We can see how this works by considering how the tree above is generated by four lines of Markdown:

> Lorem ipsum dolor
sit amet.
> - Qui *quodsi iracundia*
> - aliquando id

At the outset, our document model is just

-> document

The first line of our text,

> Lorem ipsum dolor

causes a block_quote block to be created as a child of our open document block, and a paragraph block as a child of the block_quote. Then the text is added to the last open block, the paragraph:

-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor"

The next line,

sit amet.

is a "lazy continuation" of the open paragraph, so it gets added to the paragraph's text:

-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor\nsit amet."

The third line,

> - Qui *quodsi iracundia*

causes the paragraph block to be closed, and a new list block opened as a child of the block_quote. A list_item is also added as a child of the list, and a paragraph as a child of the list_item. The text is then added to the new paragraph:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
      -> list_item
        -> paragraph
             "Qui *quodsi iracundia*"

The fourth line,

> - aliquando id

causes the list_item (and its child the paragraph) to be closed, and a new list_item opened up as child of the list. A paragraph is added as a child of the new list_item, to contain the text. We thus obtain the final tree:

-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"

From block structure to the final document

Once all of the input has been parsed, all open blocks are closed.

We then "walk the tree," visiting every node, and parse raw string contents of paragraphs and headers as inlines. At this point we have seen all the link reference definitions, so we can resolve reference links as we go.

document
  block_quote
    paragraph
      str "Lorem ipsum dolor"
      softbreak
      str "sit amet."
    list (type=bullet tight=true bullet_char=-)
      list_item
        paragraph
          str "Qui "
          emph
            str "quodsi iracundia"
      list_item
        paragraph
          str "aliquando id"

Notice how the line ending in the first paragraph has been parsed as a softbreak, and the asterisks in the first list item have become an emph.

The document can be rendered as HTML, or in any other format, given an appropriate renderer.