--- /dev/null
+---
+title: CommonMark Spec
+author: John MacFarlane
+version: 0.25
+date: '2016-03-24'
+license: '[CC-BY-SA 4.0](http://creativecommons.org/licenses/by-sa/4.0/)'
+...
+
+# Introduction
+
+## What is Markdown?
+
+Markdown is a plain text format for writing structured documents,
+based on conventions used for indicating formatting in email and
+usenet posts. It was developed in 2004 by John Gruber, who wrote
+the first Markdown-to-HTML converter in perl, and it soon became
+widely used in websites. By 2014 there were dozens of
+implementations in many languages. Some of them extended basic
+Markdown syntax with conventions for footnotes, definition lists,
+tables, and other constructs, and some allowed output not just in
+HTML but in LaTeX and many other formats.
+
+## Why is a spec needed?
+
+John Gruber's [canonical description of Markdown's
+syntax](http://daringfireball.net/projects/markdown/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](http://article.gmane.org/gmane.text.markdown.general/1997).)
+
+2. Is a blank line needed before a block quote or heading?
+ Most implementations do not require the blank line. However,
+ this can lead to unexpected results in hard-wrapped text, and
+ also to ambiguities in parsing (note that some implementations
+ put the heading inside the blockquote, while others do not).
+ (John Gruber has also spoken [in favor of requiring the blank
+ lines](http://article.gmane.org/gmane.text.markdown.general/2146).)
+
+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.)
+
+ ``` markdown
+ 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?
+
+ ``` markdown
+ 1. one
+
+ 2. two
+ 3. three
+ ```
+
+ Or this?
+
+ ``` markdown
+ 1. one
+ - a
+
+ - b
+ 2. two
+ ```
+
+ (There are some relevant comments by John Gruber
+ [here](http://article.gmane.org/gmane.text.markdown.general/2554).)
+
+5. Can list markers be indented? Can ordered list markers be right-aligned?
+
+ ``` markdown
+ 8. item 1
+ 9. item 2
+ 10. item 2a
+ ```
+
+6. Is this one list with a thematic break in its second item,
+ or two lists separated by a thematic break?
+
+ ``` markdown
+ * 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.)
+
+ ``` markdown
+ 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 ?
+
+ ``` markdown
+ [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?
+
+ ``` markdown
+ *foo *bar* baz*
+ ```
+
+10. What are the precedence rules between block-level and inline-level
+ structure? For example, how should the following be parsed?
+
+ ``` markdown
+ - `a long code span can contain a hyphen like this
+ - and it can screw things up`
+ ```
+
+11. Can list items include section headings? (`Markdown.pl` does not
+ allow this, but does allow blockquotes to include headings.)
+
+ ``` markdown
+ - # Heading
+ ```
+
+12. Can list items be empty?
+
+ ``` markdown
+ * a
+ *
+ * b
+ ```
+
+13. Can link references be defined inside block quotes or list items?
+
+ ``` markdown
+ > Blockquote [foo].
+ >
+ > [foo]: /url
+ ```
+
+14. If there are multiple definitions for the same reference, which takes
+ precedence?
+
+ ``` markdown
+ [foo]: /url1
+ [foo]: /url2
+
+ [foo][]
+ ```
+
+In the absence of a spec, early implementers consulted `Markdown.pl`
+to resolve these ambiguities. But `Markdown.pl` was quite buggy, and
+gave manifestly bad results in many cases, so it was not a
+satisfactory replacement for a spec.
+
+Because there is no unambiguous spec, implementations have diverged
+considerably. As a result, users are often surprised to find that
+a document that renders one way on one system (say, a github wiki)
+renders differently on another (say, converting to docbook using
+pandoc). To make matters worse, because nothing in Markdown counts
+as a "syntax error," the divergence often isn't discovered right away.
+
+## About this document
+
+This document attempts to specify Markdown syntax unambiguously.
+It contains many examples with side-by-side Markdown and
+HTML. These are intended to double as conformance tests. An
+accompanying script `spec_tests.py` can be used to run the tests
+against any Markdown program:
+
+ python test/spec_tests.py --spec spec.txt --program PROGRAM
+
+Since this document describes how Markdown is to be parsed into
+an abstract syntax tree, it would have made sense to use an abstract
+representation of the syntax tree instead of HTML. But HTML is capable
+of representing the structural distinctions we need to make, and the
+choice of HTML for the tests makes it possible to run the tests against
+an implementation without writing an abstract syntax tree renderer.
+
+This document is generated from a text file, `spec.txt`, written
+in Markdown with a small extension for the side-by-side tests.
+The script `tools/makespec.py` can be used to convert `spec.txt` into
+HTML or CommonMark (which can then be converted into other formats).
+
+In the examples, the `→` character is used to represent tabs.
+
+# Preliminaries
+
+## Characters and lines
+
+Any sequence of [characters] is a valid CommonMark
+document.
+
+A [character](@) is a Unicode code point. Although some
+code points (for example, combining accents) do not correspond to
+characters in an intuitive sense, all code points count as characters
+for purposes of this spec.
+
+This spec does not specify an encoding; it thinks of lines as composed
+of [characters] rather than bytes. A conforming parser may be limited
+to a certain encoding.
+
+A [line](@) is a sequence of zero or more [characters]
+other than newline (`U+000A`) or carriage return (`U+000D`),
+followed by a [line ending] or by the end of file.
+
+A [line ending](@) is a newline (`U+000A`), a carriage return
+(`U+000D`) not followed by a newline, or a carriage return and a
+following newline.
+
+A line containing no characters, or a line containing only spaces
+(`U+0020`) or tabs (`U+0009`), is called a [blank line](@).
+
+The following definitions of character classes will be used in this spec:
+
+A [whitespace character](@) is a space
+(`U+0020`), tab (`U+0009`), newline (`U+000A`), line tabulation (`U+000B`),
+form feed (`U+000C`), or carriage return (`U+000D`).
+
+[Whitespace](@) is a sequence of one or more [whitespace
+characters].
+
+A [Unicode whitespace character](@) is
+any code point in the Unicode `Zs` 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 [space](@) is `U+0020`.
+
+A [non-whitespace character](@) is any character
+that is not a [whitespace character].
+
+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`.
+
+## Tabs
+
+Tabs in lines are not expanded to [spaces]. However,
+in contexts where indentation is significant for the
+document's structure, tabs behave as if they were replaced
+by spaces with a tab stop of 4 characters.
+
+```````````````````````````````` example
+→foo→baz→→bim
+.
+<pre><code>foo→baz→→bim
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ →foo→baz→→bim
+.
+<pre><code>foo→baz→→bim
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ a→a
+ ὐ→a
+.
+<pre><code>a→a
+ὐ→a
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ - foo
+
+→bar
+.
+<ul>
+<li>
+<p>foo</p>
+<p>bar</p>
+</li>
+</ul>
+````````````````````````````````
+
+```````````````````````````````` example
+- foo
+
+→→bar
+.
+<ul>
+<li>
+<p>foo</p>
+<pre><code> bar
+</code></pre>
+</li>
+</ul>
+````````````````````````````````
+
+```````````````````````````````` example
+>→→foo
+.
+<blockquote>
+<pre><code> foo
+</code></pre>
+</blockquote>
+````````````````````````````````
+
+```````````````````````````````` example
+-→→foo
+.
+<ul>
+<li>
+<pre><code> foo
+</code></pre>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ foo
+→bar
+.
+<pre><code>foo
+bar
+</code></pre>
+````````````````````````````````
+
+```````````````````````````````` example
+ - foo
+ - bar
+→ - baz
+.
+<ul>
+<li>foo
+<ul>
+<li>bar
+<ul>
+<li>baz</li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+````````````````````````````````
+
+
+
+## Insecure characters
+
+For security reasons, the Unicode character `U+0000` must be replaced
+with the REPLACEMENT CHARACTER (`U+FFFD`).
+
+# Blocks and inlines
+
+We can think of a document as a sequence of
+[blocks](@)---structural elements like paragraphs, block
+quotations, lists, headings, rules, and code blocks. Some blocks (like
+block quotes and list items) contain other blocks; others (like
+headings and paragraphs) contain [inline](@) content---text,
+links, emphasized text, images, code, and so on.
+
+## 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
+- `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, headings, and other block constructs can be parsed for inline
+structure. The second step requires information about link reference
+definitions that will be available only at the end of the first
+step. Note that the first step requires processing lines in sequence,
+but the second can be parallelized, since the inline parsing of
+one block element does not affect the inline parsing of any other.
+
+## Container blocks and leaf blocks
+
+We can divide blocks into two types:
+[container block](@)s,
+which can contain other blocks, and [leaf block](@)s,
+which cannot.
+
+# Leaf blocks
+
+This section describes the different kinds of leaf block that make up a
+Markdown document.
+
+## Thematic breaks
+
+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
+[thematic break](@).
+
+```````````````````````````````` example
+***
+---
+___
+.
+<hr />
+<hr />
+<hr />
+````````````````````````````````
+
+
+Wrong characters:
+
+```````````````````````````````` example
++++
+.
+<p>+++</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+===
+.
+<p>===</p>
+````````````````````````````````
+
+
+Not enough characters:
+
+```````````````````````````````` example
+--
+**
+__
+.
+<p>--
+**
+__</p>
+````````````````````````````````
+
+
+One to three spaces indent are allowed:
+
+```````````````````````````````` example
+ ***
+ ***
+ ***
+.
+<hr />
+<hr />
+<hr />
+````````````````````````````````
+
+
+Four spaces is too many:
+
+```````````````````````````````` example
+ ***
+.
+<pre><code>***
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+Foo
+ ***
+.
+<p>Foo
+***</p>
+````````````````````````````````
+
+
+More than three characters may be used:
+
+```````````````````````````````` example
+_____________________________________
+.
+<hr />
+````````````````````````````````
+
+
+Spaces are allowed between the characters:
+
+```````````````````````````````` example
+ - - -
+.
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+ ** * ** * ** * **
+.
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+- - - -
+.
+<hr />
+````````````````````````````````
+
+
+Spaces are allowed at the end:
+
+```````````````````````````````` example
+- - - -
+.
+<hr />
+````````````````````````````````
+
+
+However, no other characters may occur in the line:
+
+```````````````````````````````` example
+_ _ _ _ a
+
+a------
+
+---a---
+.
+<p>_ _ _ _ a</p>
+<p>a------</p>
+<p>---a---</p>
+````````````````````````````````
+
+
+It is required that all of the [non-whitespace characters] be the same.
+So, this is not a thematic break:
+
+```````````````````````````````` example
+ *-*
+.
+<p><em>-</em></p>
+````````````````````````````````
+
+
+Thematic breaks do not need blank lines before or after:
+
+```````````````````````````````` example
+- foo
+***
+- bar
+.
+<ul>
+<li>foo</li>
+</ul>
+<hr />
+<ul>
+<li>bar</li>
+</ul>
+````````````````````````````````
+
+
+Thematic breaks can interrupt a paragraph:
+
+```````````````````````````````` example
+Foo
+***
+bar
+.
+<p>Foo</p>
+<hr />
+<p>bar</p>
+````````````````````````````````
+
+
+If a line of dashes that meets the above conditions for being a
+thematic break could also be interpreted as the underline of a [setext
+heading], the interpretation as a
+[setext heading] takes precedence. Thus, for example,
+this is a setext heading, not a paragraph followed by a thematic break:
+
+```````````````````````````````` example
+Foo
+---
+bar
+.
+<h2>Foo</h2>
+<p>bar</p>
+````````````````````````````````
+
+
+When both a thematic break and a list item are possible
+interpretations of a line, the thematic break takes precedence:
+
+```````````````````````````````` example
+* Foo
+* * *
+* Bar
+.
+<ul>
+<li>Foo</li>
+</ul>
+<hr />
+<ul>
+<li>Bar</li>
+</ul>
+````````````````````````````````
+
+
+If you want a thematic break in a list item, use a different bullet:
+
+```````````````````````````````` example
+- Foo
+- * * *
+.
+<ul>
+<li>Foo</li>
+<li>
+<hr />
+</li>
+</ul>
+````````````````````````````````
+
+
+## ATX headings
+
+An [ATX heading](@)
+consists of a string of characters, parsed as inline content, between an
+opening sequence of 1--6 unescaped `#` characters and an optional
+closing sequence of any number of unescaped `#` characters.
+The opening sequence of `#` characters must be followed by a
+[space] or by the end of line. The optional closing sequence of `#`s must be
+preceded by a [space] and may be followed by spaces only. The opening
+`#` character may be indented 0-3 spaces. The raw contents of the
+heading are stripped of leading and trailing spaces before being parsed
+as inline content. The heading level is equal to the number of `#`
+characters in the opening sequence.
+
+Simple headings:
+
+```````````````````````````````` example
+# foo
+## foo
+### foo
+#### foo
+##### foo
+###### foo
+.
+<h1>foo</h1>
+<h2>foo</h2>
+<h3>foo</h3>
+<h4>foo</h4>
+<h5>foo</h5>
+<h6>foo</h6>
+````````````````````````````````
+
+
+More than six `#` characters is not a heading:
+
+```````````````````````````````` example
+####### foo
+.
+<p>####### foo</p>
+````````````````````````````````
+
+
+At least one space is required between the `#` characters and the
+heading's contents, unless the heading is empty. Note that many
+implementations currently do not require the space. However, the
+space was required by the
+[original ATX implementation](http://www.aaronsw.com/2002/atx/atx.py),
+and it helps prevent things like the following from being parsed as
+headings:
+
+```````````````````````````````` example
+#5 bolt
+
+#hashtag
+.
+<p>#5 bolt</p>
+<p>#hashtag</p>
+````````````````````````````````
+
+
+A tab will not work:
+
+```````````````````````````````` example
+#→foo
+.
+<p>#→foo</p>
+````````````````````````````````
+
+
+This is not a heading, because the first `#` is escaped:
+
+```````````````````````````````` example
+\## foo
+.
+<p>## foo</p>
+````````````````````````````````
+
+
+Contents are parsed as inlines:
+
+```````````````````````````````` example
+# foo *bar* \*baz\*
+.
+<h1>foo <em>bar</em> *baz*</h1>
+````````````````````````````````
+
+
+Leading and trailing blanks are ignored in parsing inline content:
+
+```````````````````````````````` example
+# foo
+.
+<h1>foo</h1>
+````````````````````````````````
+
+
+One to three spaces indentation are allowed:
+
+```````````````````````````````` example
+ ### foo
+ ## foo
+ # foo
+.
+<h3>foo</h3>
+<h2>foo</h2>
+<h1>foo</h1>
+````````````````````````````````
+
+
+Four spaces are too much:
+
+```````````````````````````````` example
+ # foo
+.
+<pre><code># foo
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo
+ # bar
+.
+<p>foo
+# bar</p>
+````````````````````````````````
+
+
+A closing sequence of `#` characters is optional:
+
+```````````````````````````````` example
+## foo ##
+ ### bar ###
+.
+<h2>foo</h2>
+<h3>bar</h3>
+````````````````````````````````
+
+
+It need not be the same length as the opening sequence:
+
+```````````````````````````````` example
+# foo ##################################
+##### foo ##
+.
+<h1>foo</h1>
+<h5>foo</h5>
+````````````````````````````````
+
+
+Spaces are allowed after the closing sequence:
+
+```````````````````````````````` example
+### foo ###
+.
+<h3>foo</h3>
+````````````````````````````````
+
+
+A sequence of `#` characters with anything but [spaces] following it
+is not a closing sequence, but counts as part of the contents of the
+heading:
+
+```````````````````````````````` example
+### foo ### b
+.
+<h3>foo ### b</h3>
+````````````````````````````````
+
+
+The closing sequence must be preceded by a space:
+
+```````````````````````````````` example
+# foo#
+.
+<h1>foo#</h1>
+````````````````````````````````
+
+
+Backslash-escaped `#` characters do not count as part
+of the closing sequence:
+
+```````````````````````````````` example
+### foo \###
+## foo #\##
+# foo \#
+.
+<h3>foo ###</h3>
+<h2>foo ###</h2>
+<h1>foo #</h1>
+````````````````````````````````
+
+
+ATX headings need not be separated from surrounding content by blank
+lines, and they can interrupt paragraphs:
+
+```````````````````````````````` example
+****
+## foo
+****
+.
+<hr />
+<h2>foo</h2>
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+Foo bar
+# baz
+Bar foo
+.
+<p>Foo bar</p>
+<h1>baz</h1>
+<p>Bar foo</p>
+````````````````````````````````
+
+
+ATX headings can be empty:
+
+```````````````````````````````` example
+##
+#
+### ###
+.
+<h2></h2>
+<h1></h1>
+<h3></h3>
+````````````````````````````````
+
+
+## Setext headings
+
+A [setext heading](@) consists of one or more
+lines of text, each containing at least one [non-whitespace
+character], with no more than 3 spaces indentation, followed by
+a [setext heading underline]. The lines of text must be such
+that, were they not followed by the setext heading underline,
+they would be interpreted as a paragraph: they cannot be
+interpretable as a [code fence], [ATX heading][ATX headings],
+[block quote][block quotes], [thematic break][thematic breaks],
+[list item][list items], or [HTML block][HTML blocks].
+
+A [setext heading underline](@) is a sequence of
+`=` characters or a sequence of `-` characters, with no more than 3
+spaces indentation and any number of trailing spaces. If a line
+containing a single `-` can be interpreted as an
+empty [list items], it should be interpreted this way
+and not as a [setext heading underline].
+
+The heading is a level 1 heading if `=` characters are used in
+the [setext heading underline], and a level 2 heading if `-`
+characters are used. The contents of the heading are the result
+of parsing the preceding lines of text as CommonMark inline
+content.
+
+In general, a setext heading need not be preceded or followed by a
+blank line. However, it cannot interrupt a paragraph, so when a
+setext heading comes after a paragraph, a blank line is needed between
+them.
+
+Simple examples:
+
+```````````````````````````````` example
+Foo *bar*
+=========
+
+Foo *bar*
+---------
+.
+<h1>Foo <em>bar</em></h1>
+<h2>Foo <em>bar</em></h2>
+````````````````````````````````
+
+
+The content of the header may span more than one line:
+
+```````````````````````````````` example
+Foo *bar
+baz*
+====
+.
+<h1>Foo <em>bar
+baz</em></h1>
+````````````````````````````````
+
+
+The underlining can be any length:
+
+```````````````````````````````` example
+Foo
+-------------------------
+
+Foo
+=
+.
+<h2>Foo</h2>
+<h1>Foo</h1>
+````````````````````````````````
+
+
+The heading content can be indented up to three spaces, and need
+not line up with the underlining:
+
+```````````````````````````````` example
+ Foo
+---
+
+ Foo
+-----
+
+ Foo
+ ===
+.
+<h2>Foo</h2>
+<h2>Foo</h2>
+<h1>Foo</h1>
+````````````````````````````````
+
+
+Four spaces indent is too much:
+
+```````````````````````````````` example
+ Foo
+ ---
+
+ Foo
+---
+.
+<pre><code>Foo
+---
+
+Foo
+</code></pre>
+<hr />
+````````````````````````````````
+
+
+The setext heading underline can be indented up to three spaces, and
+may have trailing spaces:
+
+```````````````````````````````` example
+Foo
+ ----
+.
+<h2>Foo</h2>
+````````````````````````````````
+
+
+Four spaces is too much:
+
+```````````````````````````````` example
+Foo
+ ---
+.
+<p>Foo
+---</p>
+````````````````````````````````
+
+
+The setext heading underline cannot contain internal spaces:
+
+```````````````````````````````` example
+Foo
+= =
+
+Foo
+--- -
+.
+<p>Foo
+= =</p>
+<p>Foo</p>
+<hr />
+````````````````````````````````
+
+
+Trailing spaces in the content line do not cause a line break:
+
+```````````````````````````````` example
+Foo
+-----
+.
+<h2>Foo</h2>
+````````````````````````````````
+
+
+Nor does a backslash at the end:
+
+```````````````````````````````` example
+Foo\
+----
+.
+<h2>Foo\</h2>
+````````````````````````````````
+
+
+Since indicators of block structure take precedence over
+indicators of inline structure, the following are setext headings:
+
+```````````````````````````````` example
+`Foo
+----
+`
+
+<a title="a lot
+---
+of dashes"/>
+.
+<h2>`Foo</h2>
+<p>`</p>
+<h2><a title="a lot</h2>
+<p>of dashes"/></p>
+````````````````````````````````
+
+
+The setext heading underline cannot be a [lazy continuation
+line] in a list item or block quote:
+
+```````````````````````````````` example
+> Foo
+---
+.
+<blockquote>
+<p>Foo</p>
+</blockquote>
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+> foo
+bar
+===
+.
+<blockquote>
+<p>foo
+bar
+===</p>
+</blockquote>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- Foo
+---
+.
+<ul>
+<li>Foo</li>
+</ul>
+<hr />
+````````````````````````````````
+
+
+A blank line is needed between a paragraph and a following
+setext heading, since otherwise the paragraph becomes part
+of the heading's content:
+
+```````````````````````````````` example
+Foo
+Bar
+---
+.
+<h2>Foo
+Bar</h2>
+````````````````````````````````
+
+
+But in general a blank line is not required before or after
+setext headings:
+
+```````````````````````````````` example
+---
+Foo
+---
+Bar
+---
+Baz
+.
+<hr />
+<h2>Foo</h2>
+<h2>Bar</h2>
+<p>Baz</p>
+````````````````````````````````
+
+
+Setext headings cannot be empty:
+
+```````````````````````````````` example
+
+====
+.
+<p>====</p>
+````````````````````````````````
+
+
+Setext heading text lines must not be interpretable as block
+constructs other than paragraphs. So, the line of dashes
+in these examples gets interpreted as a thematic break:
+
+```````````````````````````````` example
+---
+---
+.
+<hr />
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+- foo
+-----
+.
+<ul>
+<li>foo</li>
+</ul>
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+ foo
+---
+.
+<pre><code>foo
+</code></pre>
+<hr />
+````````````````````````````````
+
+
+```````````````````````````````` example
+> foo
+-----
+.
+<blockquote>
+<p>foo</p>
+</blockquote>
+<hr />
+````````````````````````````````
+
+
+If you want a heading with `> foo` as its literal text, you can
+use backslash escapes:
+
+```````````````````````````````` example
+\> foo
+------
+.
+<h2>> foo</h2>
+````````````````````````````````
+
+
+**Compatibility note:** Most existing Markdown implementations
+do not allow the text of setext headings to span multiple lines.
+But there is no consensus about how to interpret
+
+``` markdown
+Foo
+bar
+---
+baz
+```
+
+One can find four different interpretations:
+
+1. paragraph "Foo", heading "bar", paragraph "baz"
+2. paragraph "Foo bar", thematic break, paragraph "baz"
+3. paragraph "Foo bar --- baz"
+4. heading "Foo bar", paragraph "baz"
+
+We find interpretation 4 most natural, and interpretation 4
+increases the expressive power of CommonMark, by allowing
+multiline headings. Authors who want interpretation 1 can
+put a blank line after the first paragraph:
+
+```````````````````````````````` example
+Foo
+
+bar
+---
+baz
+.
+<p>Foo</p>
+<h2>bar</h2>
+<p>baz</p>
+````````````````````````````````
+
+
+Authors who want interpretation 2 can put blank lines around
+the thematic break,
+
+```````````````````````````````` example
+Foo
+bar
+
+---
+
+baz
+.
+<p>Foo
+bar</p>
+<hr />
+<p>baz</p>
+````````````````````````````````
+
+
+or use a thematic break that cannot count as a [setext heading
+underline], such as
+
+```````````````````````````````` example
+Foo
+bar
+* * *
+baz
+.
+<p>Foo
+bar</p>
+<hr />
+<p>baz</p>
+````````````````````````````````
+
+
+Authors who want interpretation 3 can use backslash escapes:
+
+```````````````````````````````` example
+Foo
+bar
+\---
+baz
+.
+<p>Foo
+bar
+---
+baz</p>
+````````````````````````````````
+
+
+## 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
+ a simple
+ indented code block
+.
+<pre><code>a simple
+ indented code block
+</code></pre>
+````````````````````````````````
+
+
+If there is any ambiguity between an interpretation of indentation
+as a code block and as indicating that material belongs to a [list
+item][list items], the list item interpretation takes precedence:
+
+```````````````````````````````` example
+ - foo
+
+ bar
+.
+<ul>
+<li>
+<p>foo</p>
+<p>bar</p>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+1. foo
+
+ - bar
+.
+<ol>
+<li>
+<p>foo</p>
+<ul>
+<li>bar</li>
+</ul>
+</li>
+</ol>
+````````````````````````````````
+
+
+
+The contents of a code block are literal text, and do not get parsed
+as Markdown:
+
+```````````````````````````````` example
+ <a/>
+ *hi*
+
+ - one
+.
+<pre><code><a/>
+*hi*
+
+- one
+</code></pre>
+````````````````````````````````
+
+
+Here we have three chunks separated by blank lines:
+
+```````````````````````````````` example
+ 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
+ chunk1
+
+ chunk2
+.
+<pre><code>chunk1
+
+ chunk2
+</code></pre>
+````````````````````````````````
+
+
+An indented code block cannot interrupt a paragraph. (This
+allows hanging indents and the like.)
+
+```````````````````````````````` example
+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
+ foo
+bar
+.
+<pre><code>foo
+</code></pre>
+<p>bar</p>
+````````````````````````````````
+
+
+And indented code can occur immediately before and after other kinds of
+blocks:
+
+```````````````````````````````` example
+# Heading
+ foo
+Heading
+------
+ foo
+----
+.
+<h1>Heading</h1>
+<pre><code>foo
+</code></pre>
+<h2>Heading</h2>
+<pre><code>foo
+</code></pre>
+<hr />
+````````````````````````````````
+
+
+The first line can be indented more than four spaces:
+
+```````````````````````````````` example
+ foo
+ bar
+.
+<pre><code> foo
+bar
+</code></pre>
+````````````````````````````````
+
+
+Blank lines preceding or following an indented code block
+are not included in it:
+
+```````````````````````````````` example
+
+
+ foo
+
+
+.
+<pre><code>foo
+</code></pre>
+````````````````````````````````
+
+
+Trailing spaces are included in the code block's content:
+
+```````````````````````````````` example
+ foo
+.
+<pre><code>foo
+</code></pre>
+````````````````````````````````
+
+
+
+## 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
+```
+<
+ >
+```
+.
+<pre><code><
+ >
+</code></pre>
+````````````````````````````````
+
+
+With tildes:
+
+```````````````````````````````` example
+~~~
+<
+ >
+~~~
+.
+<pre><code><
+ >
+</code></pre>
+````````````````````````````````
+
+
+The closing code fence must use the same character as the opening
+fence:
+
+```````````````````````````````` example
+```
+aaa
+~~~
+```
+.
+<pre><code>aaa
+~~~
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+~~~
+aaa
+```
+~~~
+.
+<pre><code>aaa
+```
+</code></pre>
+````````````````````````````````
+
+
+The closing code fence must be at least as long as the opening fence:
+
+```````````````````````````````` example
+````
+aaa
+```
+``````
+.
+<pre><code>aaa
+```
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+~~~~
+aaa
+~~~
+~~~~
+.
+<pre><code>aaa
+~~~
+</code></pre>
+````````````````````````````````
+
+
+Unclosed code blocks are closed by the end of the document
+(or the enclosing [block quote][block quotes] or [list item][list items]):
+
+```````````````````````````````` example
+```
+.
+<pre><code></code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+`````
+
+```
+aaa
+.
+<pre><code>
+```
+aaa
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+> ```
+> aaa
+
+bbb
+.
+<blockquote>
+<pre><code>aaa
+</code></pre>
+</blockquote>
+<p>bbb</p>
+````````````````````````````````
+
+
+A code block can have all empty lines as its content:
+
+```````````````````````````````` example
+```
+
+
+```
+.
+<pre><code>
+
+</code></pre>
+````````````````````````````````
+
+
+A code block can be empty:
+
+```````````````````````````````` example
+```
+```
+.
+<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
+ ```
+ aaa
+aaa
+```
+.
+<pre><code>aaa
+aaa
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ ```
+aaa
+ aaa
+aaa
+ ```
+.
+<pre><code>aaa
+aaa
+aaa
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ ```
+ aaa
+ aaa
+ aaa
+ ```
+.
+<pre><code>aaa
+ aaa
+aaa
+</code></pre>
+````````````````````````````````
+
+
+Four spaces indentation produces an indented code block:
+
+```````````````````````````````` example
+ ```
+ 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
+```
+aaa
+ ```
+.
+<pre><code>aaa
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ ```
+aaa
+ ```
+.
+<pre><code>aaa
+</code></pre>
+````````````````````````````````
+
+
+This is not a closing fence, because it is indented 4 spaces:
+
+```````````````````````````````` example
+```
+aaa
+ ```
+.
+<pre><code>aaa
+ ```
+</code></pre>
+````````````````````````````````
+
+
+
+Code fences (opening and closing) cannot contain internal spaces:
+
+```````````````````````````````` example
+``` ```
+aaa
+.
+<p><code></code>
+aaa</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+~~~~~~
+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
+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
+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
+```ruby
+def foo(x)
+ return 3
+end
+```
+.
+<pre><code class="language-ruby">def foo(x)
+ return 3
+end
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+~~~~ ruby startline=3 $%@#$
+def foo(x)
+ return 3
+end
+~~~~~~~
+.
+<pre><code class="language-ruby">def foo(x)
+ return 3
+end
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+````;
+````
+.
+<pre><code class="language-;"></code></pre>
+````````````````````````````````
+
+
+[Info strings] for backtick code blocks cannot contain backticks:
+
+```````````````````````````````` example
+``` aa ```
+foo
+.
+<p><code>aa</code>
+foo</p>
+````````````````````````````````
+
+
+Closing code fences cannot have [info strings]:
+
+```````````````````````````````` example
+```
+``` aaa
+```
+.
+<pre><code>``` aaa
+</code></pre>
+````````````````````````````````
+
+
+
+## HTML blocks
+
+An [HTML block](@) is a group of lines that is treated
+as raw HTML (and will not be escaped in HTML output).
+
+There are seven kinds of [HTML block], which can be defined
+by their start and end conditions. The block begins with a line that
+meets a [start condition](@) (after up to three spaces
+optional indentation). It ends with the first subsequent line that
+meets a matching [end condition](@), or the last line of
+the document, if no line is encountered that meets the
+[end condition]. If the first line meets both the [start condition]
+and the [end condition], the block will contain just that line.
+
+1. **Start condition:** line begins with the string `<script`,
+`<pre`, or `<style` (case-insensitive), followed by whitespace,
+the string `>`, or the end of the line.\
+**End condition:** line contains an end tag
+`</script>`, `</pre>`, or `</style>` (case-insensitive; it
+need not match the start tag).
+
+2. **Start condition:** line begins with the string `<!--`.\
+**End condition:** line contains the string `-->`.
+
+3. **Start condition:** line begins with the string `<?`.\
+**End condition:** line contains the string `?>`.
+
+4. **Start condition:** line begins with the string `<!`
+followed by an uppercase ASCII letter.\
+**End condition:** line contains the character `>`.
+
+5. **Start condition:** line begins with the string
+`<![CDATA[`.\
+**End condition:** line contains the string `]]>`.
+
+6. **Start condition:** line begins the string `<` or `</`
+followed by one of the strings (case-insensitive) `address`,
+`article`, `aside`, `base`, `basefont`, `blockquote`, `body`,
+`caption`, `center`, `col`, `colgroup`, `dd`, `details`, `dialog`,
+`dir`, `div`, `dl`, `dt`, `fieldset`, `figcaption`, `figure`,
+`footer`, `form`, `frame`, `frameset`, `h1`, `head`, `header`, `hr`,
+`html`, `iframe`, `legend`, `li`, `link`, `main`, `menu`, `menuitem`,
+`meta`, `nav`, `noframes`, `ol`, `optgroup`, `option`, `p`, `param`,
+`section`, `source`, `summary`, `table`, `tbody`, `td`,
+`tfoot`, `th`, `thead`, `title`, `tr`, `track`, `ul`, followed
+by [whitespace], the end of the line, the string `>`, or
+the string `/>`.\
+**End condition:** line is followed by a [blank line].
+
+7. **Start condition:** line begins with a complete [open tag]
+or [closing tag] (with any [tag name] other than `script`,
+`style`, or `pre`) followed only by [whitespace]
+or the end of the line.\
+**End condition:** line is followed by a [blank line].
+
+All types of [HTML blocks] except type 7 may interrupt
+a paragraph. Blocks of type 7 may not interrupt a paragraph.
+(This restriction is intended to prevent unwanted interpretation
+of long tags inside a wrapped paragraph as starting HTML blocks.)
+
+Some simple examples follow. Here are some basic HTML blocks
+of type 6:
+
+```````````````````````````````` example
+<table>
+ <tr>
+ <td>
+ hi
+ </td>
+ </tr>
+</table>
+
+okay.
+.
+<table>
+ <tr>
+ <td>
+ hi
+ </td>
+ </tr>
+</table>
+<p>okay.</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ <div>
+ *hello*
+ <foo><a>
+.
+ <div>
+ *hello*
+ <foo><a>
+````````````````````````````````
+
+
+A block can also start with a closing tag:
+
+```````````````````````````````` example
+</div>
+*foo*
+.
+</div>
+*foo*
+````````````````````````````````
+
+
+Here we have two HTML blocks with a Markdown paragraph between them:
+
+```````````````````````````````` example
+<DIV CLASS="foo">
+
+*Markdown*
+
+</DIV>
+.
+<DIV CLASS="foo">
+<p><em>Markdown</em></p>
+</DIV>
+````````````````````````````````
+
+
+The tag on the first line can be partial, as long
+as it is split where there would be whitespace:
+
+```````````````````````````````` example
+<div id="foo"
+ class="bar">
+</div>
+.
+<div id="foo"
+ class="bar">
+</div>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<div id="foo" class="bar
+ baz">
+</div>
+.
+<div id="foo" class="bar
+ baz">
+</div>
+````````````````````````````````
+
+
+An open tag need not be closed:
+```````````````````````````````` example
+<div>
+*foo*
+
+*bar*
+.
+<div>
+*foo*
+<p><em>bar</em></p>
+````````````````````````````````
+
+
+
+A partial tag need not even be completed (garbage
+in, garbage out):
+
+```````````````````````````````` example
+<div id="foo"
+*hi*
+.
+<div id="foo"
+*hi*
+````````````````````````````````
+
+
+```````````````````````````````` example
+<div class
+foo
+.
+<div class
+foo
+````````````````````````````````
+
+
+The initial tag doesn't even need to be a valid
+tag, as long as it starts like one:
+
+```````````````````````````````` example
+<div *???-&&&-<---
+*foo*
+.
+<div *???-&&&-<---
+*foo*
+````````````````````````````````
+
+
+In type 6 blocks, the initial tag need not be on a line by
+itself:
+
+```````````````````````````````` example
+<div><a href="bar">*foo*</a></div>
+.
+<div><a href="bar">*foo*</a></div>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<table><tr><td>
+foo
+</td></tr></table>
+.
+<table><tr><td>
+foo
+</td></tr></table>
+````````````````````````````````
+
+
+Everything until the next blank line or end of document
+gets included in the HTML block. So, in the following
+example, what looks like a Markdown code block
+is actually part of the HTML block, which continues until a blank
+line or the end of the document is reached:
+
+```````````````````````````````` example
+<div></div>
+``` c
+int x = 33;
+```
+.
+<div></div>
+``` c
+int x = 33;
+```
+````````````````````````````````
+
+
+To start an [HTML block] with a tag that is *not* in the
+list of block-level tags in (6), you must put the tag by
+itself on the first line (and it must be complete):
+
+```````````````````````````````` example
+<a href="foo">
+*bar*
+</a>
+.
+<a href="foo">
+*bar*
+</a>
+````````````````````````````````
+
+
+In type 7 blocks, the [tag name] can be anything:
+
+```````````````````````````````` example
+<Warning>
+*bar*
+</Warning>
+.
+<Warning>
+*bar*
+</Warning>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<i class="foo">
+*bar*
+</i>
+.
+<i class="foo">
+*bar*
+</i>
+````````````````````````````````
+
+
+```````````````````````````````` example
+</ins>
+*bar*
+.
+</ins>
+*bar*
+````````````````````````````````
+
+
+These rules are designed to allow us to work with tags that
+can function as either block-level or inline-level tags.
+The `<del>` tag is a nice example. We can surround content with
+`<del>` tags in three different ways. In this case, we get a raw
+HTML block, because the `<del>` tag is on a line by itself:
+
+```````````````````````````````` example
+<del>
+*foo*
+</del>
+.
+<del>
+*foo*
+</del>
+````````````````````````````````
+
+
+In this case, we get a raw HTML block that just includes
+the `<del>` tag (because it ends with the following blank
+line). So the contents get interpreted as CommonMark:
+
+```````````````````````````````` example
+<del>
+
+*foo*
+
+</del>
+.
+<del>
+<p><em>foo</em></p>
+</del>
+````````````````````````````````
+
+
+Finally, in this case, the `<del>` tags are interpreted
+as [raw HTML] *inside* the CommonMark paragraph. (Because
+the tag is not on a line by itself, we get inline HTML
+rather than an [HTML block].)
+
+```````````````````````````````` example
+<del>*foo*</del>
+.
+<p><del><em>foo</em></del></p>
+````````````````````````````````
+
+
+HTML tags designed to contain literal content
+(`script`, `style`, `pre`), comments, processing instructions,
+and declarations are treated somewhat differently.
+Instead of ending at the first blank line, these blocks
+end at the first line containing a corresponding end tag.
+As a result, these blocks can contain blank lines:
+
+A pre tag (type 1):
+
+```````````````````````````````` example
+<pre language="haskell"><code>
+import Text.HTML.TagSoup
+
+main :: IO ()
+main = print $ parseTags tags
+</code></pre>
+.
+<pre language="haskell"><code>
+import Text.HTML.TagSoup
+
+main :: IO ()
+main = print $ parseTags tags
+</code></pre>
+````````````````````````````````
+
+
+A script tag (type 1):
+
+```````````````````````````````` example
+<script type="text/javascript">
+// JavaScript example
+
+document.getElementById("demo").innerHTML = "Hello JavaScript!";
+</script>
+.
+<script type="text/javascript">
+// JavaScript example
+
+document.getElementById("demo").innerHTML = "Hello JavaScript!";
+</script>
+````````````````````````````````
+
+
+A style tag (type 1):
+
+```````````````````````````````` example
+<style
+ type="text/css">
+h1 {color:red;}
+
+p {color:blue;}
+</style>
+.
+<style
+ type="text/css">
+h1 {color:red;}
+
+p {color:blue;}
+</style>
+````````````````````````````````
+
+
+If there is no matching end tag, the block will end at the
+end of the document (or the enclosing [block quote][block quotes]
+or [list item][list items]):
+
+```````````````````````````````` example
+<style
+ type="text/css">
+
+foo
+.
+<style
+ type="text/css">
+
+foo
+````````````````````````````````
+
+
+```````````````````````````````` example
+> <div>
+> foo
+
+bar
+.
+<blockquote>
+<div>
+foo
+</blockquote>
+<p>bar</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- <div>
+- foo
+.
+<ul>
+<li>
+<div>
+</li>
+<li>foo</li>
+</ul>
+````````````````````````````````
+
+
+The end tag can occur on the same line as the start tag:
+
+```````````````````````````````` example
+<style>p{color:red;}</style>
+*foo*
+.
+<style>p{color:red;}</style>
+<p><em>foo</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<!-- foo -->*bar*
+*baz*
+.
+<!-- foo -->*bar*
+<p><em>baz</em></p>
+````````````````````````````````
+
+
+Note that anything on the last line after the
+end tag will be included in the [HTML block]:
+
+```````````````````````````````` example
+<script>
+foo
+</script>1. *bar*
+.
+<script>
+foo
+</script>1. *bar*
+````````````````````````````````
+
+
+A comment (type 2):
+
+```````````````````````````````` example
+<!-- Foo
+
+bar
+ baz -->
+.
+<!-- Foo
+
+bar
+ baz -->
+````````````````````````````````
+
+
+
+A processing instruction (type 3):
+
+```````````````````````````````` example
+<?php
+
+ echo '>';
+
+?>
+.
+<?php
+
+ echo '>';
+
+?>
+````````````````````````````````
+
+
+A declaration (type 4):
+
+```````````````````````````````` example
+<!DOCTYPE html>
+.
+<!DOCTYPE html>
+````````````````````````````````
+
+
+CDATA (type 5):
+
+```````````````````````````````` example
+<![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
+ <!-- foo -->
+
+ <!-- foo -->
+.
+ <!-- foo -->
+<pre><code><!-- foo -->
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ <div>
+
+ <div>
+.
+ <div>
+<pre><code><div>
+</code></pre>
+````````````````````````````````
+
+
+An HTML block of types 1--6 can interrupt a paragraph, and need not be
+preceded by a blank line.
+
+```````````````````````````````` example
+Foo
+<div>
+bar
+</div>
+.
+<p>Foo</p>
+<div>
+bar
+</div>
+````````````````````````````````
+
+
+However, a following blank line is needed, except at the end of
+a document, and except for blocks of types 1--5, above:
+
+```````````````````````````````` example
+<div>
+bar
+</div>
+*foo*
+.
+<div>
+bar
+</div>
+*foo*
+````````````````````````````````
+
+
+HTML blocks of type 7 cannot interrupt a paragraph:
+
+```````````````````````````````` example
+Foo
+<a href="bar">
+baz
+.
+<p>Foo
+<a href="bar">
+baz</p>
+````````````````````````````````
+
+
+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:
+
+- It requires that an HTML block be preceded by a blank line.
+- It does not allow the start tag to be indented.
+- It requires a matching end tag, which it also does not allow to
+ be indented.
+
+Most Markdown implementations (including some of Gruber's own) do not
+respect all of these restrictions.
+
+There is one respect, however, in which Gruber's rule is more liberal
+than the one given here, since it allows blank lines to occur inside
+an HTML block. There are two reasons for disallowing them here.
+First, it removes the need to parse balanced tags, which is
+expensive and can require backtracking from the end of the document
+if no matching end tag is found. Second, it provides a very simple
+and flexible way of including Markdown content inside HTML tags:
+simply separate the Markdown from the HTML using blank lines:
+
+Compare:
+
+```````````````````````````````` example
+<div>
+
+*Emphasized* text.
+
+</div>
+.
+<div>
+<p><em>Emphasized</em> text.</p>
+</div>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<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
+<table>
+
+<tr>
+
+<td>
+Hi
+</td>
+
+</tr>
+
+</table>
+.
+<table>
+<tr>
+<td>
+Hi
+</td>
+</tr>
+</table>
+````````````````````````````````
+
+
+There are problems, however, if the inner tags are indented
+*and* separated by spaces, as then they will be interpreted as
+an indented code block:
+
+```````````````````````````````` example
+<table>
+
+ <tr>
+
+ <td>
+ Hi
+ </td>
+
+ </tr>
+
+</table>
+.
+<table>
+ <tr>
+<pre><code><td>
+ Hi
+</td>
+</code></pre>
+ </tr>
+</table>
+````````````````````````````````
+
+
+Fortunately, blank lines are usually not necessary and can be
+deleted. The exception is inside `<pre>` tags, but as described
+above, raw HTML blocks starting with `<pre>` *can* contain blank
+lines.
+
+## Link reference definitions
+
+A [link reference definition](@)
+consists of a [link label], indented up to three spaces, followed
+by a colon (`:`), optional [whitespace] (including up to one
+[line ending]), a [link destination],
+optional [whitespace] (including up to one
+[line ending]), and an optional [link
+title], which if it is present must be separated
+from the [link destination] by [whitespace].
+No further [non-whitespace characters] may occur on the line.
+
+A [link reference definition]
+does not correspond to a structural element of a document. Instead, it
+defines a label which can be used in [reference links]
+and reference-style [images] elsewhere in the document. [Link
+reference definitions] can come either before or after the links that use
+them.
+
+```````````````````````````````` example
+[foo]: /url "title"
+
+[foo]
+.
+<p><a href="/url" title="title">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ [foo]:
+ /url
+ 'the title'
+
+[foo]
+.
+<p><a href="/url" title="the title">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[Foo*bar\]]:my_(url) 'title (with parens)'
+
+[Foo*bar\]]
+.
+<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[Foo bar]:
+<my%20url>
+'title'
+
+[Foo bar]
+.
+<p><a href="my%20url" title="title">Foo bar</a></p>
+````````````````````````````````
+
+
+The title may extend over multiple lines:
+
+```````````````````````````````` example
+[foo]: /url '
+title
+line1
+line2
+'
+
+[foo]
+.
+<p><a href="/url" title="
+title
+line1
+line2
+">foo</a></p>
+````````````````````````````````
+
+
+However, it may not contain a [blank line]:
+
+```````````````````````````````` example
+[foo]: /url 'title
+
+with blank line'
+
+[foo]
+.
+<p>[foo]: /url 'title</p>
+<p>with blank line'</p>
+<p>[foo]</p>
+````````````````````````````````
+
+
+The title may be omitted:
+
+```````````````````````````````` example
+[foo]:
+/url
+
+[foo]
+.
+<p><a href="/url">foo</a></p>
+````````````````````````````````
+
+
+The link destination may not be omitted:
+
+```````````````````````````````` example
+[foo]:
+
+[foo]
+.
+<p>[foo]:</p>
+<p>[foo]</p>
+````````````````````````````````
+
+
+Both title and destination can contain backslash escapes
+and literal backslashes:
+
+```````````````````````````````` example
+[foo]: /url\bar\*baz "foo\"bar\baz"
+
+[foo]
+.
+<p><a href="/url%5Cbar*baz" title="foo"bar\baz">foo</a></p>
+````````````````````````````````
+
+
+A link can come before its corresponding definition:
+
+```````````````````````````````` example
+[foo]
+
+[foo]: url
+.
+<p><a href="url">foo</a></p>
+````````````````````````````````
+
+
+If there are several matching definitions, the first one takes
+precedence:
+
+```````````````````````````````` example
+[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
+[FOO]: /url
+
+[Foo]
+.
+<p><a href="/url">Foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+[foo]: /url
+.
+````````````````````````````````
+
+
+Here is another one:
+
+```````````````````````````````` example
+[
+foo
+]: /url
+bar
+.
+<p>bar</p>
+````````````````````````````````
+
+
+This is not a link reference definition, because there are
+[non-whitespace characters] after the title:
+
+```````````````````````````````` example
+[foo]: /url "title" ok
+.
+<p>[foo]: /url "title" ok</p>
+````````````````````````````````
+
+
+This is a link reference definition, but it has no title:
+
+```````````````````````````````` example
+[foo]: /url
+"title" ok
+.
+<p>"title" ok</p>
+````````````````````````````````
+
+
+This is not a link reference definition, because it is indented
+four spaces:
+
+```````````````````````````````` example
+ [foo]: /url "title"
+
+[foo]
+.
+<pre><code>[foo]: /url "title"
+</code></pre>
+<p>[foo]</p>
+````````````````````````````````
+
+
+This is not a link reference definition, because it occurs inside
+a code block:
+
+```````````````````````````````` example
+```
+[foo]: /url
+```
+
+[foo]
+.
+<pre><code>[foo]: /url
+</code></pre>
+<p>[foo]</p>
+````````````````````````````````
+
+
+A [link reference definition] cannot interrupt a paragraph.
+
+```````````````````````````````` example
+Foo
+[bar]: /baz
+
+[bar]
+.
+<p>Foo
+[bar]: /baz</p>
+<p>[bar]</p>
+````````````````````````````````
+
+
+However, it can directly follow other block elements, such as headings
+and thematic breaks, and it need not be followed by a blank line.
+
+```````````````````````````````` example
+# [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
+[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
+[foo]
+
+> [foo]: /url
+.
+<p><a href="/url">foo</a></p>
+<blockquote>
+</blockquote>
+````````````````````````````````
+
+
+
+## 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
+aaa
+
+bbb
+.
+<p>aaa</p>
+<p>bbb</p>
+````````````````````````````````
+
+
+Paragraphs can contain multiple lines, but no blank lines:
+
+```````````````````````````````` example
+aaa
+bbb
+
+ccc
+ddd
+.
+<p>aaa
+bbb</p>
+<p>ccc
+ddd</p>
+````````````````````````````````
+
+
+Multiple blank lines between paragraph have no effect:
+
+```````````````````````````````` example
+aaa
+
+
+bbb
+.
+<p>aaa</p>
+<p>bbb</p>
+````````````````````````````````
+
+
+Leading spaces are skipped:
+
+```````````````````````````````` example
+ aaa
+ bbb
+.
+<p>aaa
+bbb</p>
+````````````````````````````````
+
+
+Lines after the first may be indented any amount, since indented
+code blocks cannot interrupt paragraphs.
+
+```````````````````````````````` example
+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
+ aaa
+bbb
+.
+<p>aaa
+bbb</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ 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
+aaa
+bbb
+.
+<p>aaa<br />
+bbb</p>
+````````````````````````````````
+
+
+## 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
+
+
+aaa
+
+
+# aaa
+
+
+.
+<p>aaa</p>
+<h1>aaa</h1>
+````````````````````````````````
+
+
+
+# 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](#appendix-a-parsing-strategy).)
+
+## 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](#block-quotes) containing *Bs*.
+
+2. **Laziness.** If a string of lines *Ls* constitute a [block
+ quote](#block-quotes) with contents *Bs*, then the result of deleting
+ the initial [block quote marker] from one or
+ more lines in which the next [non-whitespace character] after the [block
+ quote marker] is [paragraph continuation
+ text] is a block quote with *Bs* as its content.
+ [Paragraph continuation text](@) is text
+ that will be parsed as part of the content of a paragraph, but does
+ not occur at the beginning of the paragraph.
+
+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](#block-quotes).
+
+Here is a simple example:
+
+```````````````````````````````` example
+> # Foo
+> bar
+> baz
+.
+<blockquote>
+<h1>Foo</h1>
+<p>bar
+baz</p>
+</blockquote>
+````````````````````````````````
+
+
+The spaces after the `>` characters can be omitted:
+
+```````````````````````````````` example
+># Foo
+>bar
+> baz
+.
+<blockquote>
+<h1>Foo</h1>
+<p>bar
+baz</p>
+</blockquote>
+````````````````````````````````
+
+
+The `>` characters can be indented 1-3 spaces:
+
+```````````````````````````````` example
+ > # Foo
+ > bar
+ > baz
+.
+<blockquote>
+<h1>Foo</h1>
+<p>bar
+baz</p>
+</blockquote>
+````````````````````````````````
+
+
+Four spaces gives us a code block:
+
+```````````````````````````````` example
+ > # Foo
+ > bar
+ > baz
+.
+<pre><code>> # Foo
+> bar
+> baz
+</code></pre>
+````````````````````````````````
+
+
+The Laziness clause allows us to omit the `>` before a
+paragraph continuation line:
+
+```````````````````````````````` example
+> # 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
+> bar
+baz
+> foo
+.
+<blockquote>
+<p>bar
+baz
+foo</p>
+</blockquote>
+````````````````````````````````
+
+
+Laziness only applies to lines that would have been continuations of
+paragraphs had they been prepended with [block quote markers].
+For example, the `> ` cannot be omitted in the second line of
+
+``` markdown
+> foo
+> ---
+```
+
+without changing the meaning:
+
+```````````````````````````````` example
+> foo
+---
+.
+<blockquote>
+<p>foo</p>
+</blockquote>
+<hr />
+````````````````````````````````
+
+
+Similarly, if we omit the `> ` in the second line of
+
+``` markdown
+> - foo
+> - bar
+```
+
+then the block quote ends after the first line:
+
+```````````````````````````````` example
+> - foo
+- bar
+.
+<blockquote>
+<ul>
+<li>foo</li>
+</ul>
+</blockquote>
+<ul>
+<li>bar</li>
+</ul>
+````````````````````````````````
+
+
+For the same reason, we can't omit the `> ` in front of
+subsequent lines of an indented or fenced code block:
+
+```````````````````````````````` example
+> foo
+ bar
+.
+<blockquote>
+<pre><code>foo
+</code></pre>
+</blockquote>
+<pre><code>bar
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+> ```
+foo
+```
+.
+<blockquote>
+<pre><code></code></pre>
+</blockquote>
+<p>foo</p>
+<pre><code></code></pre>
+````````````````````````````````
+
+
+Note that in the following case, we have a paragraph
+continuation line:
+
+```````````````````````````````` example
+> foo
+ - bar
+.
+<blockquote>
+<p>foo
+- bar</p>
+</blockquote>
+````````````````````````````````
+
+
+To see why, note that in
+
+```markdown
+> foo
+> - bar
+```
+
+the `- bar` is indented too far to start a list, and can't
+be an indented code block because indented code blocks cannot
+interrupt paragraphs, so it is a [paragraph continuation line].
+
+A block quote can be empty:
+
+```````````````````````````````` example
+>
+.
+<blockquote>
+</blockquote>
+````````````````````````````````
+
+
+```````````````````````````````` example
+>
+>
+>
+.
+<blockquote>
+</blockquote>
+````````````````````````````````
+
+
+A block quote can have initial or final blank lines:
+
+```````````````````````````````` example
+>
+> foo
+>
+.
+<blockquote>
+<p>foo</p>
+</blockquote>
+````````````````````````````````
+
+
+A blank line always separates block quotes:
+
+```````````````````````````````` example
+> 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
+> foo
+> bar
+.
+<blockquote>
+<p>foo
+bar</p>
+</blockquote>
+````````````````````````````````
+
+
+To get a block quote with two paragraphs, use:
+
+```````````````````````````````` example
+> foo
+>
+> bar
+.
+<blockquote>
+<p>foo</p>
+<p>bar</p>
+</blockquote>
+````````````````````````````````
+
+
+Block quotes can interrupt paragraphs:
+
+```````````````````````````````` example
+foo
+> bar
+.
+<p>foo</p>
+<blockquote>
+<p>bar</p>
+</blockquote>
+````````````````````````````````
+
+
+In general, blank lines are not needed before or after block
+quotes:
+
+```````````````````````````````` example
+> 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
+> bar
+baz
+.
+<blockquote>
+<p>bar
+baz</p>
+</blockquote>
+````````````````````````````````
+
+
+```````````````````````````````` example
+> bar
+
+baz
+.
+<blockquote>
+<p>bar</p>
+</blockquote>
+<p>baz</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+> 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
+> > > foo
+bar
+.
+<blockquote>
+<blockquote>
+<blockquote>
+<p>foo
+bar</p>
+</blockquote>
+</blockquote>
+</blockquote>
+````````````````````````````````
+
+
+```````````````````````````````` example
+>>> 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
+> code
+
+> not code
+.
+<blockquote>
+<pre><code>code
+</code></pre>
+</blockquote>
+<blockquote>
+<p>not code</p>
+</blockquote>
+````````````````````````````````
+
+
+
+## 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 1--9 arabic digits (`0-9`), followed by either a
+`.` character or a `)` character. (The reason for the length
+limit is that with 10 digits we start seeing integer overflows
+in some browsers.)
+
+The following rules define [list items]:
+
+1. **Basic case.** If a sequence of lines *Ls* constitute a sequence of
+ blocks *Bs* starting with a [non-whitespace character] and not separated
+ from each other by more than one blank line, and *M* is a list
+ marker 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
+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
+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-whitespace character], then blocks
+must be indented five spaces in order to fall under the list
+item.
+
+Here are some examples showing how far content must be indented to be
+put under the list item:
+
+```````````````````````````````` example
+- one
+
+ two
+.
+<ul>
+<li>one</li>
+</ul>
+<p>two</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- one
+
+ two
+.
+<ul>
+<li>
+<p>one</p>
+<p>two</p>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ - one
+
+ two
+.
+<ul>
+<li>one</li>
+</ul>
+<pre><code> two
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ - 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-whitespace character] after the list marker. However, that is not quite right.
+The spaces after the list marker determine how much relative indentation
+is needed. Which column this indentation reaches will depend on
+how the list item is embedded in other constructions, as shown by
+this example:
+
+```````````````````````````````` example
+ > > 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
+sufficient indentation after the last containing blockquote marker.
+
+The converse is also possible. In the following example, the word `two`
+occurs far to the right of the initial text of the list item, `one`, but
+it is not considered part of the list item, because it is not indented
+far enough past the blockquote marker:
+
+```````````````````````````````` example
+>>- one
+>>
+ > > two
+.
+<blockquote>
+<blockquote>
+<ul>
+<li>one</li>
+</ul>
+<p>two</p>
+</blockquote>
+</blockquote>
+````````````````````````````````
+
+
+Note that at least one space is needed between the list marker and
+any following content, so these are not list items:
+
+```````````````````````````````` example
+-one
+
+2.two
+.
+<p>-one</p>
+<p>2.two</p>
+````````````````````````````````
+
+
+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
+- 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
+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>
+````````````````````````````````
+
+
+A list item that contains an indented code block will preserve
+empty lines within the code block verbatim, unless there are two
+or more empty lines in a row (since as described above, two
+blank lines end the list):
+
+```````````````````````````````` example
+- Foo
+
+ bar
+
+ baz
+.
+<ul>
+<li>
+<p>Foo</p>
+<pre><code>bar
+
+baz
+</code></pre>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- Foo
+
+ bar
+
+
+ baz
+.
+<ul>
+<li>
+<p>Foo</p>
+<pre><code>bar
+</code></pre>
+</li>
+</ul>
+<pre><code> baz
+</code></pre>
+````````````````````````````````
+
+
+Note that ordered list start numbers must be nine digits or less:
+
+```````````````````````````````` example
+123456789. ok
+.
+<ol start="123456789">
+<li>ok</li>
+</ol>
+````````````````````````````````
+
+
+```````````````````````````````` example
+1234567890. not ok
+.
+<p>1234567890. not ok</p>
+````````````````````````````````
+
+
+A start number may begin with 0s:
+
+```````````````````````````````` example
+0. ok
+.
+<ol start="0">
+<li>ok</li>
+</ol>
+````````````````````````````````
+
+
+```````````````````````````````` example
+003. ok
+.
+<ol start="3">
+<li>ok</li>
+</ol>
+````````````````````````````````
+
+
+A start number may not be negative:
+
+```````````````````````````````` example
+-1. not ok
+.
+<p>-1. not ok</p>
+````````````````````````````````
+
+
+
+2. **Item starting with indented code.** If a sequence of lines *Ls*
+ constitute a sequence of blocks *Bs* starting with an indented code
+ block and not separated from each other by more than one blank line,
+ and *M* is a list marker 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
+- foo
+
+ bar
+.
+<ul>
+<li>
+<p>foo</p>
+<pre><code>bar
+</code></pre>
+</li>
+</ul>
+````````````````````````````````
+
+
+And in this case it is 11 spaces:
+
+```````````````````````````````` example
+ 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
+ indented code
+
+paragraph
+
+ more code
+.
+<pre><code>indented code
+</code></pre>
+<p>paragraph</p>
+<pre><code>more code
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+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
+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-whitespace character], and (b) cases in which
+they begin with an indented code
+block. In a case like the following, where the first block begins with
+a three-space indent, the rules do not allow us to form a list item by
+indenting the whole thing and prepending a list marker:
+
+```````````````````````````````` example
+ foo
+
+bar
+.
+<p>foo</p>
+<p>bar</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- 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
+- foo
+
+ bar
+.
+<ul>
+<li>
+<p>foo</p>
+<p>bar</p>
+</li>
+</ul>
+````````````````````````````````
+
+
+3. **Item starting with a blank line.** If a sequence of lines *Ls*
+ starting with a single [blank line] constitute a (possibly empty)
+ sequence of blocks *Bs*, not separated from each other by more than
+ one blank line, and *M* is a list marker of width *W*,
+ then the result of prepending *M* 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.
+
+Here are some list items that start with a blank line but are not empty:
+
+```````````````````````````````` example
+-
+ foo
+-
+ ```
+ bar
+ ```
+-
+ baz
+.
+<ul>
+<li>foo</li>
+<li>
+<pre><code>bar
+</code></pre>
+</li>
+<li>
+<pre><code>baz
+</code></pre>
+</li>
+</ul>
+````````````````````````````````
+
+When the list item starts with a blank line, the number of spaces
+following the list marker doesn't change the required indentation:
+
+```````````````````````````````` example
+-
+ foo
+.
+<ul>
+<li>foo</li>
+</ul>
+````````````````````````````````
+
+
+A list item can begin with at most one blank line.
+In the following example, `foo` is not part of the list
+item:
+
+```````````````````````````````` example
+-
+
+ foo
+.
+<ul>
+<li></li>
+</ul>
+<p>foo</p>
+````````````````````````````````
+
+
+Here is an empty bullet list item:
+
+```````````````````````````````` example
+- 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
+- foo
+-
+- bar
+.
+<ul>
+<li>foo</li>
+<li></li>
+<li>bar</li>
+</ul>
+````````````````````````````````
+
+
+Here is an empty ordered list item:
+
+```````````````````````````````` example
+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
+*
+.
+<ul>
+<li></li>
+</ul>
+````````````````````````````````
+
+
+
+4. **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 *Ls* 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
+ 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
+ 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
+ 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
+ 1. A paragraph
+ with two lines.
+
+ indented code
+
+ > A block quote.
+.
+<pre><code>1. A paragraph
+ with two lines.
+
+ indented code
+
+ > A block quote.
+</code></pre>
+````````````````````````````````
+
+
+
+5. **Laziness.** If a string of lines *Ls* constitute a [list
+ item](#list-items) with contents *Bs*, then the result of deleting
+ some or all of the indentation from one or more lines in which the
+ next [non-whitespace 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 line](@)s.
+
+Here is an example with [lazy continuation lines]:
+
+```````````````````````````````` example
+ 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
+ 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
+> 1. > Blockquote
+continued here.
+.
+<blockquote>
+<ol>
+<li>
+<blockquote>
+<p>Blockquote
+continued here.</p>
+</blockquote>
+</li>
+</ol>
+</blockquote>
+````````````````````````````````
+
+
+```````````````````````````````` example
+> 1. > Blockquote
+> continued here.
+.
+<blockquote>
+<ol>
+<li>
+<blockquote>
+<p>Blockquote
+continued here.</p>
+</blockquote>
+</li>
+</ol>
+</blockquote>
+````````````````````````````````
+
+
+
+6. **That's all.** Nothing that is not counted as a list item by rules
+ #1--5 counts as a [list item](#list-items).
+
+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
+- foo
+ - bar
+ - baz
+.
+<ul>
+<li>foo
+<ul>
+<li>bar
+<ul>
+<li>baz</li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+````````````````````````````````
+
+
+One is not enough:
+
+```````````````````````````````` example
+- 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
+10) foo
+ - bar
+.
+<ol start="10">
+<li>foo
+<ul>
+<li>bar</li>
+</ul>
+</li>
+</ol>
+````````````````````````````````
+
+
+Three is not enough:
+
+```````````````````````````````` example
+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
+- - foo
+.
+<ul>
+<li>
+<ul>
+<li>foo</li>
+</ul>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+1. - 2. foo
+.
+<ol>
+<li>
+<ul>
+<li>
+<ol start="2">
+<li>foo</li>
+</ol>
+</li>
+</ul>
+</li>
+</ol>
+````````````````````````````````
+
+
+A list item can contain a heading:
+
+```````````````````````````````` example
+- # Foo
+- Bar
+ ---
+ baz
+.
+<ul>
+<li>
+<h1>Foo</h1>
+</li>
+<li>
+<h2>Bar</h2>
+baz</li>
+</ul>
+````````````````````````````````
+
+
+### 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
+
+``` markdown
+- foo
+
+ bar
+
+ - baz
+```
+
+should be parsed as two lists with an intervening paragraph,
+
+``` html
+<ul>
+<li>foo</li>
+</ul>
+<p>bar</p>
+<ul>
+<li>baz</li>
+</ul>
+```
+
+as the four-space rule demands, rather than a single list,
+
+``` html
+<ul>
+<li>
+<p>foo</p>
+<p>bar</p>
+<ul>
+<li>baz</li>
+</ul>
+</li>
+</ul>
+```
+
+The choice of four spaces is arbitrary. It can be learned, but it is
+not likely to be guessed, and it trips up beginners regularly.
+
+Would it help to adopt a two-space rule? The problem is that such
+a rule, together with the rule allowing 1--3 spaces indentation of the
+initial list marker, allows text that is indented *less than* the
+original list marker to be included in the list item. For example,
+`Markdown.pl` parses
+
+``` markdown
+ - one
+
+ two
+```
+
+as a single list item, with `two` a continuation paragraph:
+
+``` html
+<ul>
+<li>
+<p>one</p>
+<p>two</p>
+</li>
+</ul>
+```
+
+and similarly
+
+``` markdown
+> - one
+>
+> two
+```
+
+as
+
+``` html
+<blockquote>
+<ul>
+<li>
+<p>one</p>
+<p>two</p>
+</li>
+</ul>
+</blockquote>
+```
+
+This is extremely unintuitive.
+
+Rather than requiring a fixed indent from the margin, we could require
+a fixed indent (say, two spaces, or even one space) from the list marker (which
+may itself be indented). This proposal would remove the last 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`:
+
+``` markdown
+ 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:
+
+``` markdown
+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.
+
+## 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 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
+- foo
+- bar
++ baz
+.
+<ul>
+<li>foo</li>
+<li>bar</li>
+</ul>
+<ul>
+<li>baz</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+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
+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
+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](http://docutils.sourceforge.net/rst.html) 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
+- 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
+- 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
+- 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
+- foo
+- bar
+
+
+- baz
+- bim
+.
+<ul>
+<li>foo</li>
+<li>bar</li>
+</ul>
+<ul>
+<li>baz</li>
+<li>bim</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- 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
+- a
+ - b
+ - c
+ - d
+ - e
+ - f
+ - g
+ - h
+- i
+.
+<ul>
+<li>a</li>
+<li>b</li>
+<li>c</li>
+<li>d</li>
+<li>e</li>
+<li>f</li>
+<li>g</li>
+<li>h</li>
+<li>i</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+1. a
+
+ 2. b
+
+ 3. c
+.
+<ol>
+<li>
+<p>a</p>
+</li>
+<li>
+<p>b</p>
+</li>
+<li>
+<p>c</p>
+</li>
+</ol>
+````````````````````````````````
+
+
+This is a loose list, because there is a blank line between
+two of the list items:
+
+```````````````````````````````` example
+- 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
+* 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
+- 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
+- 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
+- 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
+- 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
+* 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
+- 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
+- a
+.
+<ul>
+<li>a</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- a
+ - b
+.
+<ul>
+<li>a
+<ul>
+<li>b</li>
+</ul>
+</li>
+</ul>
+````````````````````````````````
+
+
+This list is loose, because of the blank line between the
+two block elements in the list item:
+
+```````````````````````````````` example
+1. ```
+ foo
+ ```
+
+ bar
+.
+<ol>
+<li>
+<pre><code>foo
+</code></pre>
+<p>bar</p>
+</li>
+</ol>
+````````````````````````````````
+
+
+Here the outer list is loose, the inner list tight:
+
+```````````````````````````````` example
+* foo
+ * bar
+
+ baz
+.
+<ul>
+<li>
+<p>foo</p>
+<ul>
+<li>bar</li>
+</ul>
+<p>baz</p>
+</li>
+</ul>
+````````````````````````````````
+
+
+```````````````````````````````` example
+- 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>
+````````````````````````````````
+
+
+# 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
+`hi`lo`
+.
+<p><code>hi</code>lo`</p>
+````````````````````````````````
+
+
+`hi` is parsed as code, leaving the backtick at the end as a literal
+backtick.
+
+## Backslash escapes
+
+Any ASCII punctuation character may be backslash-escaped:
+
+```````````````````````````````` example
+\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~
+.
+<p>!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~</p>
+````````````````````````````````
+
+
+Backslashes before other characters are treated as literal
+backslashes:
+
+```````````````````````````````` example
+\→\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
+\*not emphasized*
+\<br/> not a tag
+\[not a link](/foo)
+\`not code`
+1\. not a list
+\* not a list
+\# not a heading
+\[foo]: /url "not a reference"
+.
+<p>*not emphasized*
+<br/> not a tag
+[not a link](/foo)
+`not code`
+1. not a list
+* not a list
+# not a heading
+[foo]: /url "not a reference"</p>
+````````````````````````````````
+
+
+If a backslash is itself escaped, the following character is not:
+
+```````````````````````````````` example
+\\*emphasis*
+.
+<p>\<em>emphasis</em></p>
+````````````````````````````````
+
+
+A backslash at the end of the line is a [hard line break]:
+
+```````````````````````````````` example
+foo\
+bar
+.
+<p>foo<br />
+bar</p>
+````````````````````````````````
+
+
+Backslash escapes do not work in code blocks, code spans, autolinks, or
+raw HTML:
+
+```````````````````````````````` example
+`` \[\` ``
+.
+<p><code>\[\`</code></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ \[\]
+.
+<pre><code>\[\]
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+~~~
+\[\]
+~~~
+.
+<pre><code>\[\]
+</code></pre>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<http://example.com?find=\*>
+.
+<p><a href="http://example.com?find=%5C*">http://example.com?find=\*</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<a href="/bar\/)">
+.
+<a href="/bar\/)">
+````````````````````````````````
+
+
+But they work in all other contexts, including URLs and link titles,
+link references, and [info strings] in [fenced code blocks]:
+
+```````````````````````````````` example
+[foo](/bar\* "ti\*tle")
+.
+<p><a href="/bar*" title="ti*tle">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo]
+
+[foo]: /bar\* "ti\*tle"
+.
+<p><a href="/bar*" title="ti*tle">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+``` foo\+bar
+foo
+```
+.
+<pre><code class="language-foo+bar">foo
+</code></pre>
+````````````````````````````````
+
+
+
+## Entity and numeric character references
+
+All valid HTML entity references and numeric character
+references, except those occuring in code blocks and code spans,
+are recognized as such and treated as equivalent to the
+corresponding Unicode characters. Conforming CommonMark parsers
+need not store information about whether a particular character
+was represented in the source using a Unicode character or
+an entity reference.
+
+[Entity references](@) consist of `&` + any of the valid
+HTML5 entity names + `;`. The
+document <https://html.spec.whatwg.org/multipage/entities.json>
+is used as an authoritative source for the valid entity
+references and their corresponding code points.
+
+```````````````````````````````` example
+ & © Æ Ď
+¾ ℋ ⅆ
+∲ ≧̸
+.
+<p> & © Æ Ď
+¾ ℋ ⅆ
+∲ ≧̸</p>
+````````````````````````````````
+
+
+[Decimal numeric character
+references](@)
+consist of `&#` + a string of 1--8 arabic digits + `;`. A
+numeric character reference is parsed as the corresponding
+Unicode character. Invalid Unicode code points will be replaced by
+the REPLACEMENT CHARACTER (`U+FFFD`). For security reasons,
+the code point `U+0000` will also be replaced by `U+FFFD`.
+
+```````````````````````````````` example
+# Ӓ Ϡ � �
+.
+<p># Ӓ Ϡ � �</p>
+````````````````````````````````
+
+
+[Hexadecimal numeric character
+references](@) consist of `&#` +
+either `X` or `x` + a string of 1-8 hexadecimal digits + `;`.
+They too are parsed as the corresponding Unicode character (this
+time specified with a hexadecimal numeral instead of decimal).
+
+```````````````````````````````` example
+" ആ ಫ
+.
+<p>" ആ ಫ</p>
+````````````````````````````````
+
+
+Here are some nonentities:
+
+```````````````````````````````` example
+  &x; &#; &#x;
+&ThisIsNotDefined; &hi?;
+.
+<p>&nbsp &x; &#; &#x;
+&ThisIsNotDefined; &hi?;</p>
+````````````````````````````````
+
+
+Although HTML5 does accept some entity references
+without a trailing semicolon (such as `©`), these are not
+recognized here, because it makes the grammar too ambiguous:
+
+```````````````````````````````` example
+©
+.
+<p>&copy</p>
+````````````````````````````````
+
+
+Strings that are not on the list of HTML5 named entities are not
+recognized as entity references either:
+
+```````````````````````````````` example
+&MadeUpEntity;
+.
+<p>&MadeUpEntity;</p>
+````````````````````````````````
+
+
+Entity and numeric character references are recognized in any
+context besides code spans or code blocks, including
+URLs, [link titles], and [fenced code block][] [info strings]:
+
+```````````````````````````````` example
+<a href="öö.html">
+.
+<a href="öö.html">
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo](/föö "föö")
+.
+<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo]
+
+[foo]: /föö "föö"
+.
+<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+``` föö
+foo
+```
+.
+<pre><code class="language-föö">foo
+</code></pre>
+````````````````````````````````
+
+
+Entity and numeric character references are treated as literal
+text in code spans and code blocks:
+
+```````````````````````````````` example
+`föö`
+.
+<p><code>f&ouml;&ouml;</code></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+ föfö
+.
+<pre><code>f&ouml;f&ouml;
+</code></pre>
+````````````````````````````````
+
+
+## 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
+`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
+`` foo ` bar ``
+.
+<p><code>foo ` bar</code></p>
+````````````````````````````````
+
+
+This example shows the motivation for stripping leading and trailing
+spaces:
+
+```````````````````````````````` example
+` `` `
+.
+<p><code>``</code></p>
+````````````````````````````````
+
+
+[Line endings] are treated like spaces:
+
+```````````````````````````````` example
+``
+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
+`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
+`foo `` bar`
+.
+<p><code>foo `` bar</code></p>
+````````````````````````````````
+
+
+Note that backslash escapes do not work in code spans. All backslashes
+are treated literally:
+
+```````````````````````````````` example
+`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
+*foo`*`
+.
+<p>*foo<code>*</code></p>
+````````````````````````````````
+
+
+And this is not parsed as a link:
+
+```````````````````````````````` example
+[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
+`<a href="`">`
+.
+<p><code><a href="</code>">`</p>
+````````````````````````````````
+
+
+But this is an HTML tag:
+
+```````````````````````````````` example
+<a href="`">`
+.
+<p><a href="`">`</p>
+````````````````````````````````
+
+
+And this is code:
+
+```````````````````````````````` example
+`<http://foo.bar.`baz>`
+.
+<p><code><http://foo.bar.</code>baz>`</p>
+````````````````````````````````
+
+
+But this is an autolink:
+
+```````````````````````````````` example
+<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
+```foo``
+.
+<p>```foo``</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+`foo
+.
+<p>`foo</p>
+````````````````````````````````
+
+
+## Emphasis and strong emphasis
+
+John Gruber's original [Markdown syntax
+description](http://daringfireball.net/projects/markdown/syntax#em) 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:
+
+``` markdown
+***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):
+
+``` markdown
+*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.)
+
+``` markdown
+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].
+For purposes of this definition, the beginning and the end of
+the line count as Unicode whitespace.
+
+A [right-flanking delimiter run](@) is
+a [delimiter run] that is (a) not preceded by [Unicode whitespace],
+and (b) either not preceded by a [punctuation character], or
+followed by [Unicode whitespace] or a [punctuation character].
+For purposes of this definition, the beginning and the end of
+the line count as Unicode whitespace.
+
+Here are some examples of delimiter runs.
+
+ - left-flanking but not right-flanking:
+
+ ```
+ ***abc
+ _abc
+ **"abc"
+ _"abc"
+ ```
+
+ - right-flanking but not left-flanking:
+
+ ```
+ abc***
+ abc_
+ "abc"**
+ "abc"_
+ ```
+
+ - Both left and right-flanking:
+
+ ```
+ abc***def
+ "abc"_"def"
+ ```
+
+ - Neither left nor right-flanking:
+
+ ```
+ abc *** def
+ a _ b
+ ```
+
+(The idea of distinguishing left-flanking and right-flanking
+delimiter runs based on the character before and the character
+after comes from Roopesh Chander's
+[vfmd](http://www.vfmd.org/vfmd-spec/specification/#procedure-for-identifying-emphasis-tags).
+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 (if and only if) 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 either (a) not part of a [right-flanking delimiter run]
+ or (b) part of a [right-flanking delimiter run]
+ preceded by punctuation.
+
+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 either (a) not part of a [left-flanking delimiter run]
+ or (b) part of a [left-flanking delimiter run]
+ followed by punctuation.
+
+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 either (a) not part of a [right-flanking delimiter run]
+ or (b) part of a [right-flanking delimiter run]
+ preceded by punctuation.
+
+7. A double `**` [can close strong emphasis](@)
+ iff it is part of a [right-flanking delimiter run].
+
+8. A double `__` [can close strong emphasis]
+ it is part of a [right-flanking delimiter run]
+ and either (a) not part of a [left-flanking delimiter run]
+ or (b) part of a [left-flanking delimiter run]
+ followed by punctuation.
+
+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:
+
+13. The number of nestings should be minimized. Thus, for example,
+ an interpretation `<strong>...</strong>` is always preferred to
+ `<em><em>...</em></em>`.
+
+14. An interpretation `<strong><em>...</em></strong>` is always
+ preferred to `<em><strong>..</strong></em>`.
+
+15. 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>`.
+
+16. 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>`.
+
+17. 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
+*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
+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
+a*"foo"*
+.
+<p>a*"foo"*</p>
+````````````````````````````````
+
+
+Unicode nonbreaking spaces count as whitespace, too:
+
+```````````````````````````````` example
+* a *
+.
+<p>* a *</p>
+````````````````````````````````
+
+
+Intraword emphasis with `*` is permitted:
+
+```````````````````````````````` example
+foo*bar*
+.
+<p>foo<em>bar</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+5*6*78
+.
+<p>5<em>6</em>78</p>
+````````````````````````````````
+
+
+Rule 2:
+
+```````````````````````````````` example
+_foo bar_
+.
+<p><em>foo bar</em></p>
+````````````````````````````````
+
+
+This is not emphasis, because the opening `_` is followed by
+whitespace:
+
+```````````````````````````````` example
+_ foo bar_
+.
+<p>_ foo bar_</p>
+````````````````````````````````
+
+
+This is not emphasis, because the opening `_` is preceded
+by an alphanumeric and followed by punctuation:
+
+```````````````````````````````` example
+a_"foo"_
+.
+<p>a_"foo"_</p>
+````````````````````````````````
+
+
+Emphasis with `_` is not allowed inside words:
+
+```````````````````````````````` example
+foo_bar_
+.
+<p>foo_bar_</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+5_6_78
+.
+<p>5_6_78</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+пристаням_стремятся_
+.
+<p>пристаням_стремятся_</p>
+````````````````````````````````
+
+
+Here `_` does not generate emphasis, because the first delimiter run
+is right-flanking and the second left-flanking:
+
+```````````````````````````````` example
+aa_"bb"_cc
+.
+<p>aa_"bb"_cc</p>
+````````````````````````````````
+
+
+This is emphasis, even though the opening delimiter is
+both left- and right-flanking, because it is preceded by
+punctuation:
+
+```````````````````````````````` example
+foo-_(bar)_
+.
+<p>foo-<em>(bar)</em></p>
+````````````````````````````````
+
+
+Rule 3:
+
+This is not emphasis, because the closing delimiter does
+not match the opening delimiter:
+
+```````````````````````````````` example
+_foo*
+.
+<p>_foo*</p>
+````````````````````````````````
+
+
+This is not emphasis, because the closing `*` is preceded by
+whitespace:
+
+```````````````````````````````` example
+*foo bar *
+.
+<p>*foo bar *</p>
+````````````````````````````````
+
+
+A newline also counts as whitespace:
+
+```````````````````````````````` example
+*foo bar
+*
+.
+<p>*foo bar</p>
+<ul>
+<li></li>
+</ul>
+````````````````````````````````
+
+
+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
+*(*foo)
+.
+<p>*(*foo)</p>
+````````````````````````````````
+
+
+The point of this restriction is more easily appreciated
+with this example:
+
+```````````````````````````````` example
+*(*foo*)*
+.
+<p><em>(<em>foo</em>)</em></p>
+````````````````````````````````
+
+
+Intraword emphasis with `*` is allowed:
+
+```````````````````````````````` example
+*foo*bar
+.
+<p><em>foo</em>bar</p>
+````````````````````````````````
+
+
+
+Rule 4:
+
+This is not emphasis, because the closing `_` is preceded by
+whitespace:
+
+```````````````````````````````` example
+_foo bar _
+.
+<p>_foo bar _</p>
+````````````````````````````````
+
+
+This is not emphasis, because the second `_` is
+preceded by punctuation and followed by an alphanumeric:
+
+```````````````````````````````` example
+_(_foo)
+.
+<p>_(_foo)</p>
+````````````````````````````````
+
+
+This is emphasis within emphasis:
+
+```````````````````````````````` example
+_(_foo_)_
+.
+<p><em>(<em>foo</em>)</em></p>
+````````````````````````````````
+
+
+Intraword emphasis is disallowed for `_`:
+
+```````````````````````````````` example
+_foo_bar
+.
+<p>_foo_bar</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_пристаням_стремятся
+.
+<p>_пристаням_стремятся</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_foo_bar_baz_
+.
+<p><em>foo_bar_baz</em></p>
+````````````````````````````````
+
+
+This is emphasis, even though the closing delimiter is
+both left- and right-flanking, because it is followed by
+punctuation:
+
+```````````````````````````````` example
+_(bar)_.
+.
+<p><em>(bar)</em>.</p>
+````````````````````````````````
+
+
+Rule 5:
+
+```````````````````````````````` example
+**foo bar**
+.
+<p><strong>foo bar</strong></p>
+````````````````````````````````
+
+
+This is not strong emphasis, because the opening delimiter is
+followed by whitespace:
+
+```````````````````````````````` example
+** 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
+a**"foo"**
+.
+<p>a**"foo"**</p>
+````````````````````````````````
+
+
+Intraword strong emphasis with `**` is permitted:
+
+```````````````````````````````` example
+foo**bar**
+.
+<p>foo<strong>bar</strong></p>
+````````````````````````````````
+
+
+Rule 6:
+
+```````````````````````````````` example
+__foo bar__
+.
+<p><strong>foo bar</strong></p>
+````````````````````````````````
+
+
+This is not strong emphasis, because the opening delimiter is
+followed by whitespace:
+
+```````````````````````````````` example
+__ foo bar__
+.
+<p>__ foo bar__</p>
+````````````````````````````````
+
+
+A newline counts as whitespace:
+```````````````````````````````` example
+__
+foo bar__
+.
+<p>__
+foo bar__</p>
+````````````````````````````````
+
+
+This is not strong emphasis, because the opening `__` is preceded
+by an alphanumeric and followed by punctuation:
+
+```````````````````````````````` example
+a__"foo"__
+.
+<p>a__"foo"__</p>
+````````````````````````````````
+
+
+Intraword strong emphasis is forbidden with `__`:
+
+```````````````````````````````` example
+foo__bar__
+.
+<p>foo__bar__</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+5__6__78
+.
+<p>5__6__78</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+пристаням__стремятся__
+.
+<p>пристаням__стремятся__</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo, __bar__, baz__
+.
+<p><strong>foo, <strong>bar</strong>, baz</strong></p>
+````````````````````````````````
+
+
+This is strong emphasis, even though the opening delimiter is
+both left- and right-flanking, because it is preceded by
+punctuation:
+
+```````````````````````````````` example
+foo-__(bar)__
+.
+<p>foo-<strong>(bar)</strong></p>
+````````````````````````````````
+
+
+
+Rule 7:
+
+This is not strong emphasis, because the closing delimiter is preceded
+by whitespace:
+
+```````````````````````````````` example
+**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
+**(**foo)
+.
+<p>**(**foo)</p>
+````````````````````````````````
+
+
+The point of this restriction is more easily appreciated
+with these examples:
+
+```````````````````````````````` example
+*(**foo**)*
+.
+<p><em>(<strong>foo</strong>)</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
+*Asclepias physocarpa*)**
+.
+<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
+<em>Asclepias physocarpa</em>)</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo "*bar*" foo**
+.
+<p><strong>foo "<em>bar</em>" foo</strong></p>
+````````````````````````````````
+
+
+Intraword emphasis:
+
+```````````````````````````````` example
+**foo**bar
+.
+<p><strong>foo</strong>bar</p>
+````````````````````````````````
+
+
+Rule 8:
+
+This is not strong emphasis, because the closing delimiter is
+preceded by whitespace:
+
+```````````````````````````````` example
+__foo bar __
+.
+<p>__foo bar __</p>
+````````````````````````````````
+
+
+This is not strong emphasis, because the second `__` is
+preceded by punctuation and followed by an alphanumeric:
+
+```````````````````````````````` example
+__(__foo)
+.
+<p>__(__foo)</p>
+````````````````````````````````
+
+
+The point of this restriction is more easily appreciated
+with this example:
+
+```````````````````````````````` example
+_(__foo__)_
+.
+<p><em>(<strong>foo</strong>)</em></p>
+````````````````````````````````
+
+
+Intraword strong emphasis is forbidden with `__`:
+
+```````````````````````````````` example
+__foo__bar
+.
+<p>__foo__bar</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__пристаням__стремятся
+.
+<p>__пристаням__стремятся</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo__bar__baz__
+.
+<p><strong>foo__bar__baz</strong></p>
+````````````````````````````````
+
+
+This is strong emphasis, even though the closing delimiter is
+both left- and right-flanking, because it is followed by
+punctuation:
+
+```````````````````````````````` example
+__(bar)__.
+.
+<p><strong>(bar)</strong>.</p>
+````````````````````````````````
+
+
+Rule 9:
+
+Any nonempty sequence of inline elements can be the contents of an
+emphasized span.
+
+```````````````````````````````` example
+*foo [bar](/url)*
+.
+<p><em>foo <a href="/url">bar</a></em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo
+bar*
+.
+<p><em>foo
+bar</em></p>
+````````````````````````````````
+
+
+In particular, emphasis and strong emphasis can be nested
+inside emphasis:
+
+```````````````````````````````` example
+_foo __bar__ baz_
+.
+<p><em>foo <strong>bar</strong> baz</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_foo _bar_ baz_
+.
+<p><em>foo <em>bar</em> baz</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo_ bar_
+.
+<p><em><em>foo</em> bar</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo *bar**
+.
+<p><em>foo <em>bar</em></em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo **bar** baz*
+.
+<p><em>foo <strong>bar</strong> baz</em></p>
+````````````````````````````````
+
+
+But note:
+
+```````````````````````````````` example
+*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
+***foo** bar*
+.
+<p><em><strong>foo</strong> bar</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*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
+*foo**bar***
+.
+<p><em>foo</em><em>bar</em>**</p>
+````````````````````````````````
+
+
+
+Indefinite levels of nesting are possible:
+
+```````````````````````````````` example
+*foo **bar *baz* bim** bop*
+.
+<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*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
+** is not an empty emphasis
+.
+<p>** is not an empty emphasis</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**** 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
+**foo [bar](/url)**
+.
+<p><strong>foo <a href="/url">bar</a></strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo
+bar**
+.
+<p><strong>foo
+bar</strong></p>
+````````````````````````````````
+
+
+In particular, emphasis and strong emphasis can be nested
+inside strong emphasis:
+
+```````````````````````````````` example
+__foo _bar_ baz__
+.
+<p><strong>foo <em>bar</em> baz</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo __bar__ baz__
+.
+<p><strong>foo <strong>bar</strong> baz</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+____foo__ bar__
+.
+<p><strong><strong>foo</strong> bar</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo **bar****
+.
+<p><strong>foo <strong>bar</strong></strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo *bar* baz**
+.
+<p><strong>foo <em>bar</em> baz</strong></p>
+````````````````````````````````
+
+
+But note:
+
+```````````````````````````````` example
+**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
+***foo* bar**
+.
+<p><strong><em>foo</em> bar</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo *bar***
+.
+<p><strong>foo <em>bar</em></strong></p>
+````````````````````````````````
+
+
+Indefinite levels of nesting are possible:
+
+```````````````````````````````` example
+**foo *bar **baz**
+bim* bop**
+.
+<p><strong>foo <em>bar <strong>baz</strong>
+bim</em> bop</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**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
+__ is not an empty emphasis
+.
+<p>__ is not an empty emphasis</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+____ is not an empty strong emphasis
+.
+<p>____ is not an empty strong emphasis</p>
+````````````````````````````````
+
+
+
+Rule 11:
+
+```````````````````````````````` example
+foo ***
+.
+<p>foo ***</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo *\**
+.
+<p>foo <em>*</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo *_*
+.
+<p>foo <em>_</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo *****
+.
+<p>foo *****</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo **\***
+.
+<p>foo <strong>*</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+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
+**foo*
+.
+<p>*<em>foo</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo**
+.
+<p><em>foo</em>*</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+***foo**
+.
+<p>*<strong>foo</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+****foo*
+.
+<p>***<em>foo</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo***
+.
+<p><strong>foo</strong>*</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo****
+.
+<p><em>foo</em>***</p>
+````````````````````````````````
+
+
+
+Rule 12:
+
+```````````````````````````````` example
+foo ___
+.
+<p>foo ___</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo _\__
+.
+<p>foo <em>_</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo _*_
+.
+<p>foo <em>*</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo _____
+.
+<p>foo _____</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo __\___
+.
+<p>foo <strong>_</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo __*__
+.
+<p>foo <strong>*</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__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
+_foo__
+.
+<p><em>foo</em>_</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+___foo__
+.
+<p>_<strong>foo</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+____foo_
+.
+<p>___<em>foo</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo___
+.
+<p><strong>foo</strong>_</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_foo____
+.
+<p><em>foo</em>___</p>
+````````````````````````````````
+
+
+Rule 13 implies that if you want emphasis nested directly inside
+emphasis, you must use different delimiters:
+
+```````````````````````````````` example
+**foo**
+.
+<p><strong>foo</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*_foo_*
+.
+<p><em><em>foo</em></em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__foo__
+.
+<p><strong>foo</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_*foo*_
+.
+<p><em><em>foo</em></em></p>
+````````````````````````````````
+
+
+However, strong emphasis within strong emphasis is possible without
+switching delimiters:
+
+```````````````````````````````` example
+****foo****
+.
+<p><strong><strong>foo</strong></strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+____foo____
+.
+<p><strong><strong>foo</strong></strong></p>
+````````````````````````````````
+
+
+
+Rule 13 can be applied to arbitrarily long sequences of
+delimiters:
+
+```````````````````````````````` example
+******foo******
+.
+<p><strong><strong><strong>foo</strong></strong></strong></p>
+````````````````````````````````
+
+
+Rule 14:
+
+```````````````````````````````` example
+***foo***
+.
+<p><strong><em>foo</em></strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_____foo_____
+.
+<p><strong><strong><em>foo</em></strong></strong></p>
+````````````````````````````````
+
+
+Rule 15:
+
+```````````````````````````````` example
+*foo _bar* baz_
+.
+<p><em>foo _bar</em> baz_</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**foo*bar**
+.
+<p><em><em>foo</em>bar</em>*</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo __bar *baz bim__ bam*
+.
+<p><em>foo <strong>bar *baz bim</strong> bam</em></p>
+````````````````````````````````
+
+
+Rule 16:
+
+```````````````````````````````` example
+**foo **bar baz**
+.
+<p>**foo <strong>bar baz</strong></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo *bar baz*
+.
+<p>*foo <em>bar baz</em></p>
+````````````````````````````````
+
+
+Rule 17:
+
+```````````````````````````````` example
+*[bar*](/url)
+.
+<p>*<a href="/url">bar*</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_foo [bar_](/url)
+.
+<p>_foo <a href="/url">bar_</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*<img src="foo" title="*"/>
+.
+<p>*<img src="foo" title="*"/></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**<a href="**">
+.
+<p>**<a href="**"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__<a href="__">
+.
+<p>__<a href="__"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*a `*`*
+.
+<p><em>a <code>*</code></em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+_a `_`_
+.
+<p><em>a <code>_</code></em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+**a<http://foo.bar/?q=**>
+.
+<p>**a<a href="http://foo.bar/?q=**">http://foo.bar/?q=**</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+__a<http://foo.bar/?q=__>
+.
+<p>__a<a href="http://foo.bar/?q=__">http://foo.bar/?q=__</a></p>
+````````````````````````````````
+
+
+
+## Links
+
+A link contains [link text] (the visible text), a [link destination]
+(the URI that is the link destination), and optionally a [link title].
+There are two basic kinds of links in Markdown. In [inline links] the
+destination and title are given immediately after the link text. In
+[reference links] the destination and title are defined elsewhere in
+the document.
+
+A [link text](@) consists of a sequence of zero or more
+inline elements enclosed by square brackets (`[` and `]`). The
+following rules apply:
+
+- Links may not contain other links, at any level of nesting. If
+ multiple otherwise valid link definitions appear nested inside each
+ other, the inner-most definition is used.
+
+- Brackets are allowed in the [link text] only if (a) they
+ are backslash-escaped or (b) they appear as a matched pair of brackets,
+ with an open bracket `[`, a sequence of zero or more inlines, and
+ a close bracket `]`.
+
+- Backtick [code spans], [autolinks], and raw [HTML tags] bind more tightly
+ than the brackets in link text. Thus, for example,
+ `` [foo`]` `` could not be a link text, since the second `]`
+ is part of a code span.
+
+- The brackets in link text bind more tightly than markers for
+ [emphasis and strong emphasis]. Thus, for example, `*[foo*](url)` is a link.
+
+A [link destination](@) consists of either
+
+- a sequence of zero or more characters between an opening `<` and a
+ closing `>` that contains no spaces, line breaks, or unescaped
+ `<` or `>` characters, or
+
+- a nonempty sequence of characters that does not include
+ ASCII space or control characters, and includes parentheses
+ only if (a) they are backslash-escaped or (b) they are part of
+ a balanced pair of unescaped parentheses that is not itself
+ inside a balanced pair of unescaped parentheses.
+
+A [link title](@) consists of either
+
+- a sequence of zero or more characters between straight double-quote
+ characters (`"`), including a `"` character only if it is
+ backslash-escaped, or
+
+- a sequence of zero or more characters between straight single-quote
+ characters (`'`), including a `'` character only if it is
+ backslash-escaped, or
+
+- a sequence of zero or more characters between matching parentheses
+ (`(...)`), including a `)` character only if it is backslash-escaped.
+
+Although [link titles] may span multiple lines, they may not contain
+a [blank line].
+
+An [inline link](@) consists of a [link text] followed immediately
+by a left parenthesis `(`, 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
+[link](/uri "title")
+.
+<p><a href="/uri" title="title">link</a></p>
+````````````````````````````````
+
+
+The title may be omitted:
+
+```````````````````````````````` example
+[link](/uri)
+.
+<p><a href="/uri">link</a></p>
+````````````````````````````````
+
+
+Both the title and the destination may be omitted:
+
+```````````````````````````````` example
+[link]()
+.
+<p><a href="">link</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link](<>)
+.
+<p><a href="">link</a></p>
+````````````````````````````````
+
+
+The destination cannot contain spaces or line breaks,
+even if enclosed in pointy brackets:
+
+```````````````````````````````` example
+[link](/my uri)
+.
+<p>[link](/my uri)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link](</my uri>)
+.
+<p>[link](</my uri>)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link](foo
+bar)
+.
+<p>[link](foo
+bar)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link](<foo
+bar>)
+.
+<p>[link](<foo
+bar>)</p>
+````````````````````````````````
+
+Parentheses inside the link destination may be escaped:
+
+```````````````````````````````` example
+[link](\(foo\))
+.
+<p><a href="(foo)">link</a></p>
+````````````````````````````````
+
+One level of balanced parentheses is allowed without escaping:
+
+```````````````````````````````` example
+[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
+[link](foo(and(bar)))
+.
+<p>[link](foo(and(bar)))</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link](foo(and\(bar\)))
+.
+<p><a href="foo(and(bar))">link</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+[link](foo\)\:)
+.
+<p><a href="foo):">link</a></p>
+````````````````````````````````
+
+
+A link can contain fragment identifiers and queries:
+
+```````````````````````````````` example
+[link](#fragment)
+
+[link](http://example.com#fragment)
+
+[link](http://example.com?foo=3#frag)
+.
+<p><a href="#fragment">link</a></p>
+<p><a href="http://example.com#fragment">link</a></p>
+<p><a href="http://example.com?foo=3#frag">link</a></p>
+````````````````````````````````
+
+
+Note that a backslash before a non-escapable character is
+just a backslash:
+
+```````````````````````````````` example
+[link](foo\bar)
+.
+<p><a href="foo%5Cbar">link</a></p>
+````````````````````````````````
+
+
+URL-escaping should be left alone inside the destination, as all
+URL-escaped characters are also valid URL characters. Entity and
+numerical character references in the destination will be parsed
+into the corresponding Unicode code points, as usual. These may
+be optionally URL-escaped when written as HTML, but this spec
+does not enforce any particular policy for rendering URLs in
+HTML or other formats. Renderers may make different decisions
+about how to escape or normalize URLs in the output.
+
+```````````````````````````````` example
+[link](foo%20bä)
+.
+<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
+[link]("title")
+.
+<p><a href="%22title%22">link</a></p>
+````````````````````````````````
+
+
+Titles may be in single quotes, double quotes, or parentheses:
+
+```````````````````````````````` example
+[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 entity and numeric character references
+may be used in titles:
+
+```````````````````````````````` example
+[link](/url "title \""")
+.
+<p><a href="/url" title="title """>link</a></p>
+````````````````````````````````
+
+
+Nested balanced quotes are not allowed without escaping:
+
+```````````````````````````````` example
+[link](/url "title "and" title")
+.
+<p>[link](/url "title "and" title")</p>
+````````````````````````````````
+
+
+But it is easy to work around this by using a different quote type:
+
+```````````````````````````````` example
+[link](/url 'title "and" title')
+.
+<p><a href="/url" title="title "and" 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,
+entity and numeric character references, or using a different
+quote type for the enclosing title---to write titles containing
+double quotes. `Markdown.pl`'s handling of titles has a number
+of other strange features. For example, it allows single-quoted
+titles in inline links, but not reference links. And, in
+reference links but not inline links, it allows a title to begin
+with `"` and end with `)`. `Markdown.pl` 1.0.1 even allows
+titles with no closing quotation mark, though 1.0.2b8 does not.
+It seems preferable to adopt a simple, rational rule that works
+the same way in inline links and link reference definitions.)
+
+[Whitespace] is allowed around the destination and title:
+
+```````````````````````````````` example
+[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
+[link] (/uri)
+.
+<p>[link] (/uri)</p>
+````````````````````````````````
+
+
+The link text may contain balanced brackets, but not unbalanced ones,
+unless they are escaped:
+
+```````````````````````````````` example
+[link [foo [bar]]](/uri)
+.
+<p><a href="/uri">link [foo [bar]]</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link] bar](/uri)
+.
+<p>[link] bar](/uri)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link [bar](/uri)
+.
+<p>[link <a href="/uri">bar</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link \[bar](/uri)
+.
+<p><a href="/uri">link [bar</a></p>
+````````````````````````````````
+
+
+The link text may contain inline content:
+
+```````````````````````````````` example
+[link *foo **bar** `#`*](/uri)
+.
+<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[](/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
+[foo [bar](/uri)](/uri)
+.
+<p>[foo <a href="/uri">bar</a>](/uri)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo *[bar [baz](/uri)](/uri)*](/uri)
+.
+<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+](uri2)](uri3)
+.
+<p><img src="uri3" alt="[foo](uri2)" /></p>
+````````````````````````````````
+
+
+These cases illustrate the precedence of link text grouping over
+emphasis grouping:
+
+```````````````````````````````` example
+*[foo*](/uri)
+.
+<p>*<a href="/uri">foo*</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+*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
+[foo <bar attr="](baz)">
+.
+<p>[foo <bar attr="](baz)"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo`](/uri)`
+.
+<p>[foo<code>](/uri)</code></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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 link](@)s:
+[full](#full-reference-link), [collapsed](#collapsed-reference-link),
+and [shortcut](#shortcut-reference-link).
+
+A [full reference link](@)
+consists of a [link text] immediately followed by a [link label]
+that [matches] a [link reference definition] elsewhere in the document.
+
+A [link label](@) begins with a left bracket (`[`) and ends
+with the first right bracket (`]`) that is not backslash-escaped.
+Between these brackets there must be at least one [non-whitespace character].
+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
+[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
+[link [foo [bar]]][ref]
+
+[ref]: /uri
+.
+<p><a href="/uri">link [foo [bar]]</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[link \[bar][ref]
+
+[ref]: /uri
+.
+<p><a href="/uri">link [bar</a></p>
+````````````````````````````````
+
+
+The link text may contain inline content:
+
+```````````````````````````````` example
+[link *foo **bar** `#`*][ref]
+
+[ref]: /uri
+.
+<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[][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
+[foo [bar](/uri)][ref]
+
+[ref]: /uri
+.
+<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+*[foo*][ref]
+
+[ref]: /uri
+.
+<p>*<a href="/uri">foo*</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+[foo <bar attr="][ref]">
+
+[ref]: /uri
+.
+<p>[foo <bar attr="][ref]"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo`][ref]`
+
+[ref]: /uri
+.
+<p>[foo<code>][ref]</code></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[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
+[foo][BaR]
+
+[bar]: /url "title"
+.
+<p><a href="/url" title="title">foo</a></p>
+````````````````````````````````
+
+
+Unicode case fold is used:
+
+```````````````````````````````` example
+[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
+[Foo
+ bar]: /url
+
+[Baz][Foo bar]
+.
+<p><a href="/url">Baz</a></p>
+````````````````````````````````
+
+
+No [whitespace] is allowed between the [link text] and the
+[link label]:
+
+```````````````````````````````` example
+[foo] [bar]
+
+[bar]: /url "title"
+.
+<p>[foo] <a href="/url" title="title">bar</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo]
+[bar]
+
+[bar]: /url "title"
+.
+<p>[foo]
+<a href="/url" title="title">bar</a></p>
+````````````````````````````````
+
+
+This is a departure from John Gruber's original Markdown syntax
+description, which explicitly allows whitespace between the link
+text and the link label. It brings reference links in line with
+[inline links], which (according to both original Markdown and
+this spec) cannot have whitespace after the link text. More
+importantly, it prevents inadvertent capture of consecutive
+[shortcut reference links]. If whitespace is allowed between the
+link text and the link label, then in the following we will have
+a single reference link, not two shortcut reference links, as
+intended:
+
+``` markdown
+[foo]
+[bar]
+
+[foo]: /url1
+[bar]: /url2
+```
+
+(Note that [shortcut reference links] were introduced by Gruber
+himself in a beta version of `Markdown.pl`, but never included
+in the official syntax description. Without shortcut reference
+links, it is harmless to allow space between the link text and
+link label; but once shortcut references are introduced, it is
+too dangerous to allow this, as it frequently leads to
+unintended results.)
+
+When there are multiple matching [link reference definitions],
+the first is used:
+
+```````````````````````````````` example
+[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
+[bar][foo\!]
+
+[foo!]: /url
+.
+<p>[bar][foo!]</p>
+````````````````````````````````
+
+
+[Link labels] cannot contain brackets, unless they are
+backslash-escaped:
+
+```````````````````````````````` example
+[foo][ref[]
+
+[ref[]: /uri
+.
+<p>[foo][ref[]</p>
+<p>[ref[]: /uri</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo][ref[bar]]
+
+[ref[bar]]: /uri
+.
+<p>[foo][ref[bar]]</p>
+<p>[ref[bar]]: /uri</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[[[foo]]]
+
+[[[foo]]]: /url
+.
+<p>[[[foo]]]</p>
+<p>[[[foo]]]: /url</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[foo][ref\[]
+
+[ref\[]: /uri
+.
+<p><a href="/uri">foo</a></p>
+````````````````````````````````
+
+
+Note that in this example `]` is not backslash-escaped:
+
+```````````````````````````````` example
+[bar\\]: /uri
+
+[bar\\]
+.
+<p><a href="/uri">bar\</a></p>
+````````````````````````````````
+
+
+A [link label] must contain at least one [non-whitespace character]:
+
+```````````````````````````````` example
+[]
+
+[]: /uri
+.
+<p>[]</p>
+<p>[]: /uri</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[
+ ]
+
+[
+ ]: /uri
+.
+<p>[
+]</p>
+<p>[
+]: /uri</p>
+````````````````````````````````
+
+
+A [collapsed reference link](@)
+consists of a [link label] that [matches] a
+[link reference definition] elsewhere in the
+document, followed by the string `[]`.
+The contents of the first link label are parsed as inlines,
+which are used as the link's text. The link's URI and title are
+provided by the matching reference link definition. Thus,
+`[foo][]` is equivalent to `[foo][foo]`.
+
+```````````````````````````````` example
+[foo][]
+
+[foo]: /url "title"
+.
+<p><a href="/url" title="title">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[*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
+[Foo][]
+
+[foo]: /url "title"
+.
+<p><a href="/url" title="title">Foo</a></p>
+````````````````````````````````
+
+
+
+As with full reference links, [whitespace] is not
+allowed between the two sets of brackets:
+
+```````````````````````````````` example
+[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
+[foo]
+
+[foo]: /url "title"
+.
+<p><a href="/url" title="title">foo</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[*foo* bar]
+
+[*foo* bar]: /url "title"
+.
+<p><a href="/url" title="title"><em>foo</em> bar</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[[*foo* bar]]
+
+[*foo* bar]: /url "title"
+.
+<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+[[bar [foo]
+
+[foo]: /url
+.
+<p>[[bar <a href="/url">foo</a></p>
+````````````````````````````````
+
+
+The link labels are case-insensitive:
+
+```````````````````````````````` example
+[Foo]
+
+[foo]: /url "title"
+.
+<p><a href="/url" title="title">Foo</a></p>
+````````````````````````````````
+
+
+A space after the link text should be preserved:
+
+```````````````````````````````` example
+[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
+\[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
+[foo*]: /url
+
+*[foo*]
+.
+<p>*<a href="/url">foo*</a></p>
+````````````````````````````````
+
+
+Full references take precedence over shortcut references:
+
+```````````````````````````````` example
+[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
+[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
+[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
+[foo][bar][baz]
+
+[baz]: /url1
+[foo]: /url2
+.
+<p>[foo]<a href="/url1">bar</a></p>
+````````````````````````````````
+
+
+
+## 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 `
+.
+<p><img src="/url" alt="foo" title="title" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+![foo *bar*]
+
+[foo *bar*]: train.jpg "train & tracks"
+.
+<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+](/url2)
+.
+<p><img src="/url2" alt="foo bar" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+](/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
+![foo *bar*][]
+
+[foo *bar*]: train.jpg "train & tracks"
+.
+<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+![foo *bar*][foobar]
+
+[FOOBAR]: train.jpg "train & tracks"
+.
+<p><img src="train.jpg" alt="foo bar" title="train & tracks" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+
+.
+<p><img src="train.jpg" alt="foo" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+My 
+.
+<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+
+.
+<p><img src="url" alt="foo" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+
+.
+<p><img src="/url" alt="" /></p>
+````````````````````````````````
+
+
+Reference-style:
+
+```````````````````````````````` example
+![foo][bar]
+
+[bar]: /url
+.
+<p><img src="/url" alt="foo" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+![foo][bar]
+
+[BAR]: /url
+.
+<p><img src="/url" alt="foo" /></p>
+````````````````````````````````
+
+
+Collapsed:
+
+```````````````````````````````` example
+![foo][]
+
+[foo]: /url "title"
+.
+<p><img src="/url" alt="foo" title="title" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+![*foo* bar][]
+
+[*foo* bar]: /url "title"
+.
+<p><img src="/url" alt="foo bar" title="title" /></p>
+````````````````````````````````
+
+
+The labels are case-insensitive:
+
+```````````````````````````````` example
+![Foo][]
+
+[foo]: /url "title"
+.
+<p><img src="/url" alt="Foo" title="title" /></p>
+````````````````````````````````
+
+
+As with reference links, [whitespace] is not allowed
+between the two sets of brackets:
+
+```````````````````````````````` example
+![foo]
+[]
+
+[foo]: /url "title"
+.
+<p><img src="/url" alt="foo" title="title" />
+[]</p>
+````````````````````````````````
+
+
+Shortcut:
+
+```````````````````````````````` example
+![foo]
+
+[foo]: /url "title"
+.
+<p><img src="/url" alt="foo" title="title" /></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+![*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
+![[foo]]
+
+[[foo]]: /url "title"
+.
+<p>![[foo]]</p>
+<p>[[foo]]: /url "title"</p>
+````````````````````````````````
+
+
+The link labels are case-insensitive:
+
+```````````````````````````````` example
+![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
+\!\[foo]
+
+[foo]: /url "title"
+.
+<p>![foo]</p>
+````````````````````````````````
+
+
+If you want a link after a literal `!`, backslash-escape the
+`!`:
+
+```````````````````````````````` example
+\![foo]
+
+[foo]: /url "title"
+.
+<p>!<a href="/url" title="title">foo</a></p>
+````````````````````````````````
+
+
+## Autolinks
+
+[Autolink](@)s 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, they must be percent-encoded
+(e.g. `%20` for a space).
+
+For purposes of this spec, a [scheme](@) is any sequence
+of 2--32 characters beginning with an ASCII letter and followed
+by any combination of ASCII letters, digits, or the symbols plus
+("+"), period ("."), or hyphen ("-").
+
+Here are some valid autolinks:
+
+```````````````````````````````` example
+<http://foo.bar.baz>
+.
+<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<http://foo.bar.baz/test?q=hello&id=22&boolean>
+.
+<p><a href="http://foo.bar.baz/test?q=hello&id=22&boolean">http://foo.bar.baz/test?q=hello&id=22&boolean</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<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
+<MAILTO:FOO@BAR.BAZ>
+.
+<p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>
+````````````````````````````````
+
+
+Note that many strings that count as [absolute URIs] for
+purposes of this spec are not valid URIs, because their
+schemes are not registered or because of other problems
+with their syntax:
+
+```````````````````````````````` example
+<a+b+c:d>
+.
+<p><a href="a+b+c:d">a+b+c:d</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<made-up-scheme://foo,bar>
+.
+<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<http://../>
+.
+<p><a href="http://../">http://../</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<localhost:5001/foo>
+.
+<p><a href="localhost:5001/foo">localhost:5001/foo</a></p>
+````````````````````````````````
+
+
+Spaces are not allowed in autolinks:
+
+```````````````````````````````` example
+<http://foo.bar/baz bim>
+.
+<p><http://foo.bar/baz bim></p>
+````````````````````````````````
+
+
+Backslash-escapes do not work inside autolinks:
+
+```````````````````````````````` example
+<http://example.com/\[\>
+.
+<p><a href="http://example.com/%5C%5B%5C">http://example.com/\[\</a></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](https://html.spec.whatwg.org/multipage/forms.html#e-mail-state-(type=email)):
+
+ /^[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
+<foo@bar.example.com>
+.
+<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<foo+special@Bar.baz-bar0.com>
+.
+<p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p>
+````````````````````````````````
+
+
+Backslash-escapes do not work inside email autolinks:
+
+```````````````````````````````` example
+<foo\+@bar.example.com>
+.
+<p><foo+@bar.example.com></p>
+````````````````````````````````
+
+
+These are not autolinks:
+
+```````````````````````````````` example
+<>
+.
+<p><></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+< http://foo.bar >
+.
+<p>< http://foo.bar ></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<m:abc>
+.
+<p><m:abc></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<foo.bar.baz>
+.
+<p><foo.bar.baz></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+http://example.com
+.
+<p>http://example.com</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo@bar.example.com
+.
+<p>foo@bar.example.com</p>
+````````````````````````````````
+
+
+## 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, digits, or
+hyphens (`-`).
+
+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](http://www.w3.org/TR/html5/syntax.html#comments).)
+
+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
+<a><bab><c2c>
+.
+<p><a><bab><c2c></p>
+````````````````````````````````
+
+
+Empty elements:
+
+```````````````````````````````` example
+<a/><b2/>
+.
+<p><a/><b2/></p>
+````````````````````````````````
+
+
+[Whitespace] is allowed:
+
+```````````````````````````````` example
+<a /><b2
+data="foo" >
+.
+<p><a /><b2
+data="foo" ></p>
+````````````````````````````````
+
+
+With attributes:
+
+```````````````````````````````` example
+<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>
+````````````````````````````````
+
+
+Custom tag names can be used:
+
+```````````````````````````````` example
+Foo <responsive-image src="foo.jpg" />
+.
+<p>Foo <responsive-image src="foo.jpg" /></p>
+````````````````````````````````
+
+
+Illegal tag names, not parsed as HTML:
+
+```````````````````````````````` example
+<33> <__>
+.
+<p><33> <__></p>
+````````````````````````````````
+
+
+Illegal attribute names:
+
+```````````````````````````````` example
+<a h*#ref="hi">
+.
+<p><a h*#ref="hi"></p>
+````````````````````````````````
+
+
+Illegal attribute values:
+
+```````````````````````````````` example
+<a href="hi'> <a href=hi'>
+.
+<p><a href="hi'> <a href=hi'></p>
+````````````````````````````````
+
+
+Illegal [whitespace]:
+
+```````````````````````````````` example
+< a><
+foo><bar/ >
+.
+<p>< a><
+foo><bar/ ></p>
+````````````````````````````````
+
+
+Missing [whitespace]:
+
+```````````````````````````````` example
+<a href='bar'title=title>
+.
+<p><a href='bar'title=title></p>
+````````````````````````````````
+
+
+Closing tags:
+
+```````````````````````````````` example
+</a></foo >
+.
+<p></a></foo ></p>
+````````````````````````````````
+
+
+Illegal attributes in closing tag:
+
+```````````````````````````````` example
+</a href="foo">
+.
+<p></a href="foo"></p>
+````````````````````````````````
+
+
+Comments:
+
+```````````````````````````````` example
+foo <!-- this is a
+comment - with hyphen -->
+.
+<p>foo <!-- this is a
+comment - with hyphen --></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo <!-- not a comment -- two hyphens -->
+.
+<p>foo <!-- not a comment -- two hyphens --></p>
+````````````````````````````````
+
+
+Not comments:
+
+```````````````````````````````` example
+foo <!--> foo -->
+
+foo <!-- foo--->
+.
+<p>foo <!--> foo --></p>
+<p>foo <!-- foo---></p>
+````````````````````````````````
+
+
+Processing instructions:
+
+```````````````````````````````` example
+foo <?php echo $a; ?>
+.
+<p>foo <?php echo $a; ?></p>
+````````````````````````````````
+
+
+Declarations:
+
+```````````````````````````````` example
+foo <!ELEMENT br EMPTY>
+.
+<p>foo <!ELEMENT br EMPTY></p>
+````````````````````````````````
+
+
+CDATA sections:
+
+```````````````````````````````` example
+foo <![CDATA[>&<]]>
+.
+<p>foo <![CDATA[>&<]]></p>
+````````````````````````````````
+
+
+Entity and numeric character references are preserved in HTML
+attributes:
+
+```````````````````````````````` example
+foo <a href="ö">
+.
+<p>foo <a href="ö"></p>
+````````````````````````````````
+
+
+Backslash escapes do not work in HTML attributes:
+
+```````````````````````````````` example
+foo <a href="\*">
+.
+<p>foo <a href="\*"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<a href="\"">
+.
+<p><a href="""></p>
+````````````````````````````````
+
+
+## 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
+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
+foo\
+baz
+.
+<p>foo<br />
+baz</p>
+````````````````````````````````
+
+
+More than two spaces can be used:
+
+```````````````````````````````` example
+foo
+baz
+.
+<p>foo<br />
+baz</p>
+````````````````````````````````
+
+
+Leading spaces at the beginning of the next line are ignored:
+
+```````````````````````````````` example
+foo
+ bar
+.
+<p>foo<br />
+bar</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo\
+ bar
+.
+<p>foo<br />
+bar</p>
+````````````````````````````````
+
+
+Line breaks can occur inside emphasis, links, and other constructs
+that allow inline content:
+
+```````````````````````````````` example
+*foo
+bar*
+.
+<p><em>foo<br />
+bar</em></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+*foo\
+bar*
+.
+<p><em>foo<br />
+bar</em></p>
+````````````````````````````````
+
+
+Line breaks do not occur inside code spans
+
+```````````````````````````````` example
+`code
+span`
+.
+<p><code>code span</code></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+`code\
+span`
+.
+<p><code>code\ span</code></p>
+````````````````````````````````
+
+
+or HTML tags:
+
+```````````````````````````````` example
+<a href="foo
+bar">
+.
+<p><a href="foo
+bar"></p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+<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
+foo\
+.
+<p>foo\</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+foo
+.
+<p>foo</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+### foo\
+.
+<h3>foo\</h3>
+````````````````````````````````
+
+
+```````````````````````````````` example
+### foo
+.
+<h3>foo</h3>
+````````````````````````````````
+
+
+## Soft line breaks
+
+A regular line break (not in a code span or HTML tag) that is not
+preceded by two or more spaces or a backslash is parsed as a
+softbreak. (A 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
+foo
+baz
+.
+<p>foo
+baz</p>
+````````````````````````````````
+
+
+Spaces at the end of the line and beginning of the next line are
+removed:
+
+```````````````````````````````` example
+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.
+
+## Textual content
+
+Any characters not given an interpretation by the above rules will
+be parsed as plain textual content.
+
+```````````````````````````````` example
+hello $.;'there
+.
+<p>hello $.;'there</p>
+````````````````````````````````
+
+
+```````````````````````````````` example
+Foo χρῆν
+.
+<p>Foo χρῆν</p>
+````````````````````````````````
+
+
+Internal spaces are preserved verbatim:
+
+```````````````````````````````` example
+Multiple spaces
+.
+<p>Multiple spaces</p>
+````````````````````````````````
+
+
+<!-- END TESTS -->
+
+# Appendix: A parsing strategy
+
+In this appendix we describe some features of the parsing strategy
+used in the CommonMark reference implementations.
+
+## Overview
+
+Parsing has two phases:
+
+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 headings
+are parsed into sequences of Markdown inline elements (strings,
+code spans, links, emphasis, and so on), using the map of link
+references constructed in phase 1.
+
+At each point in processing, the document is represented as a tree of
+**blocks**. The root of the tree is a `document` block. The `document`
+may have any number of other blocks as **children**. These children
+may, in turn, have other blocks as children. The last child of a block
+is normally considered **open**, meaning that subsequent lines of input
+can alter its contents. (Blocks that are not open are **closed**.)
+Here, for example, is a possible document tree, with the open blocks
+marked by arrows:
+
+``` tree
+-> document
+ -> block_quote
+ paragraph
+ "Lorem ipsum dolor\nsit amet."
+ -> list (type=bullet tight=true bullet_char=-)
+ list_item
+ paragraph
+ "Qui *quodsi iracundia*"
+ -> list_item
+ -> paragraph
+ "aliquando id"
+```
+
+## Phase 1: block structure
+
+Each line that is processed has an effect on this tree. The line is
+analyzed and, depending on its contents, the document may be altered
+in one or more of the following ways:
+
+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.
+
+For each line, we follow this procedure:
+
+1. First we iterate through the open blocks, starting with the
+root document, and descending through last children down to the last
+open block. Each block imposes a condition that the line must satisfy
+if the block is to remain open. For example, a block quote requires a
+`>` character. A paragraph requires a non-blank line.
+In this phase we may match all or just some of the open
+blocks. But we cannot close unmatched blocks yet, because we may have a
+[lazy continuation line].
+
+2. Next, after consuming the continuation markers for existing
+blocks, we look for new block starts (e.g. `>` for a block quote.
+If we encounter a new block start, we close any blocks unmatched
+in step 1 before creating the new block as a child of the last
+matched block.
+
+3. Finally, we look at the remainder of the line (after block
+markers like `>`, list markers, and indentation have been consumed).
+This is text that can be incorporated into the last open
+block (a paragraph, code block, heading, or raw HTML).
+
+Setext headings are formed when we see a line of a paragraph
+that is a [setext heading underline].
+
+Reference link definitions are detected when a paragraph is closed;
+the accumulated text lines are parsed to see if they begin with
+one or more reference link definitions. Any remainder becomes a
+normal paragraph.
+
+We can see how this works by considering how the tree above is
+generated by four lines of Markdown:
+
+``` markdown
+> Lorem ipsum dolor
+sit amet.
+> - Qui *quodsi iracundia*
+> - aliquando id
+```
+
+At the outset, our document model is just
+
+``` tree
+-> document
+```
+
+The first line of our text,
+
+``` markdown
+> 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`:
+
+``` tree
+-> document
+ -> block_quote
+ -> paragraph
+ "Lorem ipsum dolor"
+```
+
+The next line,
+
+``` markdown
+sit amet.
+```
+
+is a "lazy continuation" of the open `paragraph`, so it gets added
+to the paragraph's text:
+
+``` tree
+-> document
+ -> block_quote
+ -> paragraph
+ "Lorem ipsum dolor\nsit amet."
+```
+
+The third line,
+
+``` markdown
+> - 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`:
+
+``` tree
+-> 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,
+
+``` markdown
+> - 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:
+
+``` tree
+-> document
+ -> block_quote
+ paragraph
+ "Lorem ipsum dolor\nsit amet."
+ -> list (type=bullet tight=true bullet_char=-)
+ list_item
+ paragraph
+ "Qui *quodsi iracundia*"
+ -> list_item
+ -> paragraph
+ "aliquando id"
+```
+
+## Phase 2: inline structure
+
+Once all of the input has been parsed, all open blocks are closed.
+
+We then "walk the tree," visiting every node, and parse raw
+string contents of paragraphs and headings as inlines. At this
+point we have seen all the link reference definitions, so we can
+resolve reference links as we go.
+
+``` tree
+document
+ block_quote
+ paragraph
+ str "Lorem ipsum dolor"
+ softbreak
+ str "sit amet."
+ list (type=bullet tight=true bullet_char=-)
+ list_item
+ paragraph
+ str "Qui "
+ emph
+ str "quodsi iracundia"
+ list_item
+ paragraph
+ str "aliquando id"
+```
+
+Notice how the [line ending] in the first paragraph has
+been parsed as a `softbreak`, and the asterisks in the first list item
+have become an `emph`.
+
+### An algorithm for parsing nested emphasis and links
+
+By far the trickiest part of inline parsing is handling emphasis,
+strong emphasis, links, and images. This is done using the following
+algorithm.
+
+When we're parsing inlines and we hit either
+
+- a run of `*` or `_` characters, or
+- a `[` or `.
+
+The [delimiter stack] is a doubly linked list. Each
+element contains a pointer to a text node, plus information about
+
+- the type of delimiter (`[`, `![`, `*`, `_`)
+- the number of delimiters,
+- whether the delimiter is "active" (all are active to start), and
+- whether the delimiter is a potential opener, a potential closer,
+ or both (which depends on what sort of characters precede
+ and follow the delimiters).
+
+When we hit a `]` character, we call the *look for link or image*
+procedure (see below).
+
+When we hit the end of the input, we call the *process emphasis*
+procedure (see below), with `stack_bottom` = NULL.
+
+#### *look for link or image*
+
+Starting at the top of the delimiter stack, we look backwards
+through the stack for an opening `[` or `![` delimiter.
+
+- If we don't find one, we return a literal text node `]`.
+
+- If we do find one, but it's not *active*, we remove the inactive
+ delimiter from the stack, and return a literal text node `]`.
+
+- If we find one and it's active, then we parse ahead to see if
+ we have an inline link/image, reference link/image, compact reference
+ link/image, or shortcut reference link/image.
+
+ + If we don't, then we remove the opening delimiter from the
+ delimiter stack and return a literal text node `]`.
+
+ + If we do, then
+
+ * We return a link or image node whose children are the inlines
+ after the text node pointed to by the opening delimiter.
+
+ * We run *process emphasis* on these inlines, with the `[` opener
+ as `stack_bottom`.
+
+ * We remove the opening delimiter.
+
+ * If we have a link (and not an image), we also set all
+ `[` delimiters before the opening delimiter to *inactive*. (This
+ will prevent us from getting links within links.)
+
+#### *process emphasis*
+
+Parameter `stack_bottom` sets a lower bound to how far we
+descend in the [delimiter stack]. If it is NULL, we can
+go all the way to the bottom. Otherwise, we stop before
+visiting `stack_bottom`.
+
+Let `current_position` point to the element on the [delimiter stack]
+just above `stack_bottom` (or the first element if `stack_bottom`
+is NULL).
+
+We keep track of the `openers_bottom` for each delimiter
+type (`*`, `_`). Initialize this to `stack_bottom`.
+
+Then we repeat the following until we run out of potential
+closers:
+
+- Move `current_position` forward in the delimiter stack (if needed)
+ until we find the first potential closer with delimiter `*` or `_`.
+ (This will be the potential closer closest
+ to the beginning of the input -- the first one in parse order.)
+
+- Now, look back in the stack (staying above `stack_bottom` and
+ the `openers_bottom` for this delimiter type) for the
+ first matching potential opener ("matching" means same delimiter).
+
+- If one is found:
+
+ + Figure out whether we have emphasis or strong emphasis:
+ if both closer and opener spans have length >= 2, we have
+ strong, otherwise regular.
+
+ + Insert an emph or strong emph node accordingly, after
+ the text node corresponding to the opener.
+
+ + Remove any delimiters between the opener and closer from
+ the delimiter stack.
+
+ + Remove 1 (for regular emph) or 2 (for strong emph) delimiters
+ from the opening and closing text nodes. If they become empty
+ as a result, remove them and remove the corresponding element
+ of the delimiter stack. If the closing node is removed, reset
+ `current_position` to the next element in the stack.
+
+- If none in found:
+
+ + Set `openers_bottom` to the element before `current_position`.
+ (We know that there are no openers for this kind of closer up to and
+ including this point, so this puts a lower bound on future searches.)
+
+ + If the closer at `current_position` is not a potential opener,
+ remove it from the delimiter stack (since we know it can't
+ be a closer either).
+
+ + Advance `current_position` to the next element in the stack.
+
+After we're done, we remove all delimiters above `stack_bottom` from the
+delimiter stack.
+
Code Review / simantics / platform.git / blobdiff