X-Git-Url: https://gerrit.simantics.org/r/gitweb?p=simantics%2Fplatform.git;a=blobdiff_plain;f=tests%2Forg.simantics.scl.compiler.tests%2Fsrc%2Forg%2Fsimantics%2Fscl%2Fcompiler%2Ftests%2Fmarkdown%2Fspec.txt;fp=tests%2Forg.simantics.scl.compiler.tests%2Fsrc%2Forg%2Fsimantics%2Fscl%2Fcompiler%2Ftests%2Fmarkdown%2Fspec.txt;h=bdaed436dd9e20ae200ea1e49be4b10c679c730f;hp=0000000000000000000000000000000000000000;hb=0364f8f54b009e9e5de482d5c9d1cb7efb023141;hpb=7ecf07ff9aacab300f1fb900f1f0f97beb1be139 diff --git a/tests/org.simantics.scl.compiler.tests/src/org/simantics/scl/compiler/tests/markdown/spec.txt b/tests/org.simantics.scl.compiler.tests/src/org/simantics/scl/compiler/tests/markdown/spec.txt new file mode 100644 index 000000000..bdaed436d --- /dev/null +++ b/tests/org.simantics.scl.compiler.tests/src/org/simantics/scl/compiler/tests/markdown/spec.txt @@ -0,0 +1,9252 @@ +--- +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 `

` 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 +. +

foo→baz→→bim
+
+```````````````````````````````` + + +```````````````````````````````` example + →foo→baz→→bim +. +
foo→baz→→bim
+
+```````````````````````````````` + + +```````````````````````````````` example + a→a + ὐ→a +. +
a→a
+ὐ→a
+
+```````````````````````````````` + + +```````````````````````````````` example + - foo + +→bar +. + +```````````````````````````````` + +```````````````````````````````` example +- foo + +→→bar +. + +```````````````````````````````` + +```````````````````````````````` example +>→→foo +. +
+
  foo
+
+
+```````````````````````````````` + +```````````````````````````````` example +-→→foo +. + +```````````````````````````````` + + +```````````````````````````````` example + foo +→bar +. +
foo
+bar
+
+```````````````````````````````` + +```````````````````````````````` example + - foo + - bar +→ - baz +. + +```````````````````````````````` + + + +## 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` +. + +```````````````````````````````` + + +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 +*** +--- +___ +. +
+
+
+```````````````````````````````` + + +Wrong characters: + +```````````````````````````````` example ++++ +. +

+++

+```````````````````````````````` + + +```````````````````````````````` example +=== +. +

===

+```````````````````````````````` + + +Not enough characters: + +```````````````````````````````` example +-- +** +__ +. +

-- +** +__

+```````````````````````````````` + + +One to three spaces indent are allowed: + +```````````````````````````````` example + *** + *** + *** +. +
+
+
+```````````````````````````````` + + +Four spaces is too many: + +```````````````````````````````` example + *** +. +
***
+
+```````````````````````````````` + + +```````````````````````````````` example +Foo + *** +. +

Foo +***

+```````````````````````````````` + + +More than three characters may be used: + +```````````````````````````````` example +_____________________________________ +. +
+```````````````````````````````` + + +Spaces are allowed between the characters: + +```````````````````````````````` example + - - - +. +
+```````````````````````````````` + + +```````````````````````````````` example + ** * ** * ** * ** +. +
+```````````````````````````````` + + +```````````````````````````````` example +- - - - +. +
+```````````````````````````````` + + +Spaces are allowed at the end: + +```````````````````````````````` example +- - - - +. +
+```````````````````````````````` + + +However, no other characters may occur in the line: + +```````````````````````````````` example +_ _ _ _ a + +a------ + +---a--- +. +

_ _ _ _ a

+

a------

+

---a---

+```````````````````````````````` + + +It is required that all of the [non-whitespace characters] be the same. +So, this is not a thematic break: + +```````````````````````````````` example + *-* +. +

-

+```````````````````````````````` + + +Thematic breaks do not need blank lines before or after: + +```````````````````````````````` example +- foo +*** +- bar +. + +
+ +```````````````````````````````` + + +Thematic breaks can interrupt a paragraph: + +```````````````````````````````` example +Foo +*** +bar +. +

Foo

+
+

bar

+```````````````````````````````` + + +If a line of dashes that meets the above conditions for being a +thematic break could also be interpreted as the underline of a [setext +heading], the interpretation as a +[setext heading] takes precedence. Thus, for example, +this is a setext heading, not a paragraph followed by a thematic break: + +```````````````````````````````` example +Foo +--- +bar +. +

Foo

+

bar

+```````````````````````````````` + + +When both a thematic break and a list item are possible +interpretations of a line, the thematic break takes precedence: + +```````````````````````````````` example +* Foo +* * * +* Bar +. + +
+ +```````````````````````````````` + + +If you want a thematic break in a list item, use a different bullet: + +```````````````````````````````` example +- Foo +- * * * +. + +```````````````````````````````` + + +## 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 +. +

foo

+

foo

+

foo

+

foo

+
foo
+
foo
+```````````````````````````````` + + +More than six `#` characters is not a heading: + +```````````````````````````````` example +####### foo +. +

####### foo

+```````````````````````````````` + + +At least one space is required between the `#` characters and the +heading's contents, unless the heading is empty. Note that many +implementations currently do not require the space. However, the +space was required by the +[original ATX implementation](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 +. +

#5 bolt

+

#hashtag

+```````````````````````````````` + + +A tab will not work: + +```````````````````````````````` example +#→foo +. +

#→foo

+```````````````````````````````` + + +This is not a heading, because the first `#` is escaped: + +```````````````````````````````` example +\## foo +. +

## foo

+```````````````````````````````` + + +Contents are parsed as inlines: + +```````````````````````````````` example +# foo *bar* \*baz\* +. +

foo bar *baz*

+```````````````````````````````` + + +Leading and trailing blanks are ignored in parsing inline content: + +```````````````````````````````` example +# foo +. +

foo

+```````````````````````````````` + + +One to three spaces indentation are allowed: + +```````````````````````````````` example + ### foo + ## foo + # foo +. +

foo

+

foo

+

foo

+```````````````````````````````` + + +Four spaces are too much: + +```````````````````````````````` example + # foo +. +
# foo
+
+```````````````````````````````` + + +```````````````````````````````` example +foo + # bar +. +

foo +# bar

+```````````````````````````````` + + +A closing sequence of `#` characters is optional: + +```````````````````````````````` example +## foo ## + ### bar ### +. +

foo

+

bar

+```````````````````````````````` + + +It need not be the same length as the opening sequence: + +```````````````````````````````` example +# foo ################################## +##### foo ## +. +

foo

+
foo
+```````````````````````````````` + + +Spaces are allowed after the closing sequence: + +```````````````````````````````` example +### foo ### +. +

foo

+```````````````````````````````` + + +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 +. +

foo ### b

+```````````````````````````````` + + +The closing sequence must be preceded by a space: + +```````````````````````````````` example +# foo# +. +

foo#

+```````````````````````````````` + + +Backslash-escaped `#` characters do not count as part +of the closing sequence: + +```````````````````````````````` example +### foo \### +## foo #\## +# foo \# +. +

foo ###

+

foo ###

+

foo #

+```````````````````````````````` + + +ATX headings need not be separated from surrounding content by blank +lines, and they can interrupt paragraphs: + +```````````````````````````````` example +**** +## foo +**** +. +
+

foo

+
+```````````````````````````````` + + +```````````````````````````````` example +Foo bar +# baz +Bar foo +. +

Foo bar

+

baz

+

Bar foo

+```````````````````````````````` + + +ATX headings can be empty: + +```````````````````````````````` example +## +# +### ### +. +

+

+

+```````````````````````````````` + + +## 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* +--------- +. +

Foo bar

+

Foo bar

+```````````````````````````````` + + +The content of the header may span more than one line: + +```````````````````````````````` example +Foo *bar +baz* +==== +. +

Foo bar +baz

+```````````````````````````````` + + +The underlining can be any length: + +```````````````````````````````` example +Foo +------------------------- + +Foo += +. +

Foo

+

Foo

+```````````````````````````````` + + +The heading content can be indented up to three spaces, and need +not line up with the underlining: + +```````````````````````````````` example + Foo +--- + + Foo +----- + + Foo + === +. +

Foo

+

Foo

+

Foo

+```````````````````````````````` + + +Four spaces indent is too much: + +```````````````````````````````` example + Foo + --- + + Foo +--- +. +
Foo
+---
+
+Foo
+
+
+```````````````````````````````` + + +The setext heading underline can be indented up to three spaces, and +may have trailing spaces: + +```````````````````````````````` example +Foo + ---- +. +

Foo

+```````````````````````````````` + + +Four spaces is too much: + +```````````````````````````````` example +Foo + --- +. +

Foo +---

+```````````````````````````````` + + +The setext heading underline cannot contain internal spaces: + +```````````````````````````````` example +Foo += = + +Foo +--- - +. +

Foo += =

+

Foo

+
+```````````````````````````````` + + +Trailing spaces in the content line do not cause a line break: + +```````````````````````````````` example +Foo +----- +. +

Foo

+```````````````````````````````` + + +Nor does a backslash at the end: + +```````````````````````````````` example +Foo\ +---- +. +

Foo\

+```````````````````````````````` + + +Since indicators of block structure take precedence over +indicators of inline structure, the following are setext headings: + +```````````````````````````````` example +`Foo +---- +` + + +. +

`Foo

+

`

+

<a title="a lot

+

of dashes"/>

+```````````````````````````````` + + +The setext heading underline cannot be a [lazy continuation +line] in a list item or block quote: + +```````````````````````````````` example +> Foo +--- +. +
+

Foo

+
+
+```````````````````````````````` + + +```````````````````````````````` example +> foo +bar +=== +. +
+

foo +bar +===

+
+```````````````````````````````` + + +```````````````````````````````` example +- Foo +--- +. + +
+```````````````````````````````` + + +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 +--- +. +

Foo +Bar

+```````````````````````````````` + + +But in general a blank line is not required before or after +setext headings: + +```````````````````````````````` example +--- +Foo +--- +Bar +--- +Baz +. +
+

Foo

+

Bar

+

Baz

+```````````````````````````````` + + +Setext headings cannot be empty: + +```````````````````````````````` example + +==== +. +

====

+```````````````````````````````` + + +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 +--- +--- +. +
+
+```````````````````````````````` + + +```````````````````````````````` example +- foo +----- +. + +
+```````````````````````````````` + + +```````````````````````````````` example + foo +--- +. +
foo
+
+
+```````````````````````````````` + + +```````````````````````````````` example +> foo +----- +. +
+

foo

+
+
+```````````````````````````````` + + +If you want a heading with `> foo` as its literal text, you can +use backslash escapes: + +```````````````````````````````` example +\> foo +------ +. +

> foo

+```````````````````````````````` + + +**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 +. +

Foo

+

bar

+

baz

+```````````````````````````````` + + +Authors who want interpretation 2 can put blank lines around +the thematic break, + +```````````````````````````````` example +Foo +bar + +--- + +baz +. +

Foo +bar

+
+

baz

+```````````````````````````````` + + +or use a thematic break that cannot count as a [setext heading +underline], such as + +```````````````````````````````` example +Foo +bar +* * * +baz +. +

Foo +bar

+
+

baz

+```````````````````````````````` + + +Authors who want interpretation 3 can use backslash escapes: + +```````````````````````````````` example +Foo +bar +\--- +baz +. +

Foo +bar +--- +baz

+```````````````````````````````` + + +## 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 +. +
a simple
+  indented code block
+
+```````````````````````````````` + + +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 +. + +```````````````````````````````` + + +```````````````````````````````` example +1. foo + + - bar +. +
    +
  1. +

    foo

    +
      +
    • bar
    • +
    +
  2. +
+```````````````````````````````` + + + +The contents of a code block are literal text, and do not get parsed +as Markdown: + +```````````````````````````````` example +
+ *hi* + + - one +. +
<a/>
+*hi*
+
+- one
+
+```````````````````````````````` + + +Here we have three chunks separated by blank lines: + +```````````````````````````````` example + chunk1 + + chunk2 + + + + chunk3 +. +
chunk1
+
+chunk2
+
+
+
+chunk3
+
+```````````````````````````````` + + +Any initial spaces beyond four will be included in the content, even +in interior blank lines: + +```````````````````````````````` example + chunk1 + + chunk2 +. +
chunk1
+  
+  chunk2
+
+```````````````````````````````` + + +An indented code block cannot interrupt a paragraph. (This +allows hanging indents and the like.) + +```````````````````````````````` example +Foo + bar + +. +

Foo +bar

+```````````````````````````````` + + +However, any non-blank line with fewer than four leading spaces ends +the code block immediately. So a paragraph may occur immediately +after indented code: + +```````````````````````````````` example + foo +bar +. +
foo
+
+

bar

+```````````````````````````````` + + +And indented code can occur immediately before and after other kinds of +blocks: + +```````````````````````````````` example +# Heading + foo +Heading +------ + foo +---- +. +

Heading

+
foo
+
+

Heading

+
foo
+
+
+```````````````````````````````` + + +The first line can be indented more than four spaces: + +```````````````````````````````` example + foo + bar +. +
    foo
+bar
+
+```````````````````````````````` + + +Blank lines preceding or following an indented code block +are not included in it: + +```````````````````````````````` example + + + foo + + +. +
foo
+
+```````````````````````````````` + + +Trailing spaces are included in the code block's content: + +```````````````````````````````` example + foo +. +
foo  
+
+```````````````````````````````` + + + +## 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 +``` +< + > +``` +. +
<
+ >
+
+```````````````````````````````` + + +With tildes: + +```````````````````````````````` example +~~~ +< + > +~~~ +. +
<
+ >
+
+```````````````````````````````` + + +The closing code fence must use the same character as the opening +fence: + +```````````````````````````````` example +``` +aaa +~~~ +``` +. +
aaa
+~~~
+
+```````````````````````````````` + + +```````````````````````````````` example +~~~ +aaa +``` +~~~ +. +
aaa
+```
+
+```````````````````````````````` + + +The closing code fence must be at least as long as the opening fence: + +```````````````````````````````` example +```` +aaa +``` +`````` +. +
aaa
+```
+
+```````````````````````````````` + + +```````````````````````````````` example +~~~~ +aaa +~~~ +~~~~ +. +
aaa
+~~~
+
+```````````````````````````````` + + +Unclosed code blocks are closed by the end of the document +(or the enclosing [block quote][block quotes] or [list item][list items]): + +```````````````````````````````` example +``` +. +
+```````````````````````````````` + + +```````````````````````````````` example +````` + +``` +aaa +. +

+```
+aaa
+
+```````````````````````````````` + + +```````````````````````````````` example +> ``` +> aaa + +bbb +. +
+
aaa
+
+
+

bbb

+```````````````````````````````` + + +A code block can have all empty lines as its content: + +```````````````````````````````` example +``` + + +``` +. +

+  
+
+```````````````````````````````` + + +A code block can be empty: + +```````````````````````````````` example +``` +``` +. +
+```````````````````````````````` + + +Fences can be indented. If the opening fence is indented, +content lines will have equivalent opening indentation removed, +if present: + +```````````````````````````````` example + ``` + aaa +aaa +``` +. +
aaa
+aaa
+
+```````````````````````````````` + + +```````````````````````````````` example + ``` +aaa + aaa +aaa + ``` +. +
aaa
+aaa
+aaa
+
+```````````````````````````````` + + +```````````````````````````````` example + ``` + aaa + aaa + aaa + ``` +. +
aaa
+ aaa
+aaa
+
+```````````````````````````````` + + +Four spaces indentation produces an indented code block: + +```````````````````````````````` example + ``` + aaa + ``` +. +
```
+aaa
+```
+
+```````````````````````````````` + + +Closing fences may be indented by 0-3 spaces, and their indentation +need not match that of the opening fence: + +```````````````````````````````` example +``` +aaa + ``` +. +
aaa
+
+```````````````````````````````` + + +```````````````````````````````` example + ``` +aaa + ``` +. +
aaa
+
+```````````````````````````````` + + +This is not a closing fence, because it is indented 4 spaces: + +```````````````````````````````` example +``` +aaa + ``` +. +
aaa
+    ```
+
+```````````````````````````````` + + + +Code fences (opening and closing) cannot contain internal spaces: + +```````````````````````````````` example +``` ``` +aaa +. +

+aaa

+```````````````````````````````` + + +```````````````````````````````` example +~~~~~~ +aaa +~~~ ~~ +. +
aaa
+~~~ ~~
+
+```````````````````````````````` + + +Fenced code blocks can interrupt paragraphs, and can be followed +directly by paragraphs, without a blank line between: + +```````````````````````````````` example +foo +``` +bar +``` +baz +. +

foo

+
bar
+
+

baz

+```````````````````````````````` + + +Other blocks can also occur before and after fenced code blocks +without an intervening blank line: + +```````````````````````````````` example +foo +--- +~~~ +bar +~~~ +# baz +. +

foo

+
bar
+
+

baz

+```````````````````````````````` + + +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 +``` +. +
def foo(x)
+  return 3
+end
+
+```````````````````````````````` + + +```````````````````````````````` example +~~~~ ruby startline=3 $%@#$ +def foo(x) + return 3 +end +~~~~~~~ +. +
def foo(x)
+  return 3
+end
+
+```````````````````````````````` + + +```````````````````````````````` example +````; +```` +. +
+```````````````````````````````` + + +[Info strings] for backtick code blocks cannot contain backticks: + +```````````````````````````````` example +``` aa ``` +foo +. +

aa +foo

+```````````````````````````````` + + +Closing code fences cannot have [info strings]: + +```````````````````````````````` example +``` +``` aaa +``` +. +
``` aaa
+
+```````````````````````````````` + + + +## 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 ``, or the end of the line.\ +**End condition:** line contains an end tag +``, ``, or `` (case-insensitive; it +need not match the start tag). + +2. **Start condition:** line begins with the string ``. + +3. **Start condition:** line begins with the string ``. + +4. **Start condition:** line begins with the string ``. + +5. **Start condition:** line begins with the string +``. + +6. **Start condition:** line begins the string `<` or ``, 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 + + + + +
+ hi +
+ +okay. +. + + + + +
+ hi +
+

okay.

+```````````````````````````````` + + +```````````````````````````````` example +
+ *hello* + +. +
+ *hello* + +```````````````````````````````` + + +A block can also start with a closing tag: + +```````````````````````````````` example +
+*foo* +. +
+*foo* +```````````````````````````````` + + +Here we have two HTML blocks with a Markdown paragraph between them: + +```````````````````````````````` example +
+ +*Markdown* + +
+. +
+

Markdown

+
+```````````````````````````````` + + +The tag on the first line can be partial, as long +as it is split where there would be whitespace: + +```````````````````````````````` example +
+
+. +
+
+```````````````````````````````` + + +```````````````````````````````` example +
+
+. +
+
+```````````````````````````````` + + +An open tag need not be closed: +```````````````````````````````` example +
+*foo* + +*bar* +. +
+*foo* +

bar

+```````````````````````````````` + + + +A partial tag need not even be completed (garbage +in, garbage out): + +```````````````````````````````` example +
+. +
*foo*
+```````````````````````````````` + + +```````````````````````````````` example +
+foo +
+. +
+foo +
+```````````````````````````````` + + +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 +
+``` c +int x = 33; +``` +. +
+``` 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 + +*bar* + +. + +*bar* + +```````````````````````````````` + + +In type 7 blocks, the [tag name] can be anything: + +```````````````````````````````` example + +*bar* + +. + +*bar* + +```````````````````````````````` + + +```````````````````````````````` example + +*bar* + +. + +*bar* + +```````````````````````````````` + + +```````````````````````````````` example + +*bar* +. + +*bar* +```````````````````````````````` + + +These rules are designed to allow us to work with tags that +can function as either block-level or inline-level tags. +The `` tag is a nice example. We can surround content with +`` tags in three different ways. In this case, we get a raw +HTML block, because the `` tag is on a line by itself: + +```````````````````````````````` example + +*foo* + +. + +*foo* + +```````````````````````````````` + + +In this case, we get a raw HTML block that just includes +the `` tag (because it ends with the following blank +line). So the contents get interpreted as CommonMark: + +```````````````````````````````` example + + +*foo* + + +. + +

foo

+
+```````````````````````````````` + + +Finally, in this case, the `` 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 +*foo* +. +

foo

+```````````````````````````````` + + +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 +

+import Text.HTML.TagSoup
+
+main :: IO ()
+main = print $ parseTags tags
+
+. +

+import Text.HTML.TagSoup
+
+main :: IO ()
+main = print $ parseTags tags
+
+```````````````````````````````` + + +A script tag (type 1): + +```````````````````````````````` example + +. + +```````````````````````````````` + + +A style tag (type 1): + +```````````````````````````````` example + +. + +```````````````````````````````` + + +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 + +*foo* +. + +

foo

+```````````````````````````````` + + +```````````````````````````````` example +*bar* +*baz* +. +*bar* +

baz

+```````````````````````````````` + + +Note that anything on the last line after the +end tag will be included in the [HTML block]: + +```````````````````````````````` example +1. *bar* +. +1. *bar* +```````````````````````````````` + + +A comment (type 2): + +```````````````````````````````` example + +. + +```````````````````````````````` + + + +A processing instruction (type 3): + +```````````````````````````````` example +'; + +?> +. +'; + +?> +```````````````````````````````` + + +A declaration (type 4): + +```````````````````````````````` example + +. + +```````````````````````````````` + + +CDATA (type 5): + +```````````````````````````````` example + +. + +```````````````````````````````` + + +The opening tag can be indented 1-3 spaces, but not 4: + +```````````````````````````````` example + + + +. + +
<!-- foo -->
+
+```````````````````````````````` + + +```````````````````````````````` example +
+ +
+. +
+
<div>
+
+```````````````````````````````` + + +An HTML block of types 1--6 can interrupt a paragraph, and need not be +preceded by a blank line. + +```````````````````````````````` example +Foo +
+bar +
+. +

Foo

+
+bar +
+```````````````````````````````` + + +However, a following blank line is needed, except at the end of +a document, and except for blocks of types 1--5, above: + +```````````````````````````````` example +
+bar +
+*foo* +. +
+bar +
+*foo* +```````````````````````````````` + + +HTML blocks of type 7 cannot interrupt a paragraph: + +```````````````````````````````` example +Foo + +baz +. +

Foo + +baz

+```````````````````````````````` + + +This rule differs from John Gruber's original Markdown syntax +specification, which says: + +> The only restrictions are that block-level HTML elements — +> e.g. `
`, ``, `
`, `

`, etc. — must be separated from +> surrounding content by blank lines, and the start and end tags of the +> block should not be indented with tabs or spaces. + +In some ways Gruber's rule is more restrictive than the one given +here: + +- 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 +

+ +*Emphasized* text. + +
+. +
+

Emphasized text.

+
+```````````````````````````````` + + +```````````````````````````````` example +
+*Emphasized* text. +
+. +
+*Emphasized* text. +
+```````````````````````````````` + + +Some Markdown implementations have adopted a convention of +interpreting content inside tags as text if the open tag has +the attribute `markdown=1`. The rule given above seems a simpler and +more elegant way of achieving the same expressive power, which is also +much simpler to parse. + +The main potential drawback is that one can no longer paste HTML +blocks into Markdown documents with 100% reliability. However, +*in most cases* this will work fine, because the blank lines in +HTML are usually followed by HTML block tags. For example: + +```````````````````````````````` example +
+ + + + + + + +
+Hi +
+. + + + + +
+Hi +
+```````````````````````````````` + + +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 + + + + + + + + +
+ Hi +
+. + + +
<td>
+  Hi
+</td>
+
+ +
+```````````````````````````````` + + +Fortunately, blank lines are usually not necessary and can be +deleted. The exception is inside `
` tags, but as described
+above, raw HTML blocks starting with `
` *can* contain blank
+lines.
+
+## 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]
+.
+

foo

+```````````````````````````````` + + +```````````````````````````````` example + [foo]: + /url + 'the title' + +[foo] +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +[Foo*bar\]]:my_(url) 'title (with parens)' + +[Foo*bar\]] +. +

Foo*bar]

+```````````````````````````````` + + +```````````````````````````````` example +[Foo bar]: + +'title' + +[Foo bar] +. +

Foo bar

+```````````````````````````````` + + +The title may extend over multiple lines: + +```````````````````````````````` example +[foo]: /url ' +title +line1 +line2 +' + +[foo] +. +

foo

+```````````````````````````````` + + +However, it may not contain a [blank line]: + +```````````````````````````````` example +[foo]: /url 'title + +with blank line' + +[foo] +. +

[foo]: /url 'title

+

with blank line'

+

[foo]

+```````````````````````````````` + + +The title may be omitted: + +```````````````````````````````` example +[foo]: +/url + +[foo] +. +

foo

+```````````````````````````````` + + +The link destination may not be omitted: + +```````````````````````````````` example +[foo]: + +[foo] +. +

[foo]:

+

[foo]

+```````````````````````````````` + + +Both title and destination can contain backslash escapes +and literal backslashes: + +```````````````````````````````` example +[foo]: /url\bar\*baz "foo\"bar\baz" + +[foo] +. +

foo

+```````````````````````````````` + + +A link can come before its corresponding definition: + +```````````````````````````````` example +[foo] + +[foo]: url +. +

foo

+```````````````````````````````` + + +If there are several matching definitions, the first one takes +precedence: + +```````````````````````````````` example +[foo] + +[foo]: first +[foo]: second +. +

foo

+```````````````````````````````` + + +As noted in the section on [Links], matching of labels is +case-insensitive (see [matches]). + +```````````````````````````````` example +[FOO]: /url + +[Foo] +. +

Foo

+```````````````````````````````` + + +```````````````````````````````` example +[ΑΓΩ]: /φου + +[αγω] +. +

αγω

+```````````````````````````````` + + +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 +. +

bar

+```````````````````````````````` + + +This is not a link reference definition, because there are +[non-whitespace characters] after the title: + +```````````````````````````````` example +[foo]: /url "title" ok +. +

[foo]: /url "title" ok

+```````````````````````````````` + + +This is a link reference definition, but it has no title: + +```````````````````````````````` example +[foo]: /url +"title" ok +. +

"title" ok

+```````````````````````````````` + + +This is not a link reference definition, because it is indented +four spaces: + +```````````````````````````````` example + [foo]: /url "title" + +[foo] +. +
[foo]: /url "title"
+
+

[foo]

+```````````````````````````````` + + +This is not a link reference definition, because it occurs inside +a code block: + +```````````````````````````````` example +``` +[foo]: /url +``` + +[foo] +. +
[foo]: /url
+
+

[foo]

+```````````````````````````````` + + +A [link reference definition] cannot interrupt a paragraph. + +```````````````````````````````` example +Foo +[bar]: /baz + +[bar] +. +

Foo +[bar]: /baz

+

[bar]

+```````````````````````````````` + + +However, it can directly follow other block elements, such as headings +and thematic breaks, and it need not be followed by a blank line. + +```````````````````````````````` example +# [Foo] +[foo]: /url +> bar +. +

Foo

+
+

bar

+
+```````````````````````````````` + + +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] +. +

foo, +bar, +baz

+```````````````````````````````` + + +[Link reference definitions] can occur +inside block containers, like lists and block quotations. They +affect the entire document, not just the container in which they +are defined: + +```````````````````````````````` example +[foo] + +> [foo]: /url +. +

foo

+
+
+```````````````````````````````` + + + +## 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 +. +

aaa

+

bbb

+```````````````````````````````` + + +Paragraphs can contain multiple lines, but no blank lines: + +```````````````````````````````` example +aaa +bbb + +ccc +ddd +. +

aaa +bbb

+

ccc +ddd

+```````````````````````````````` + + +Multiple blank lines between paragraph have no effect: + +```````````````````````````````` example +aaa + + +bbb +. +

aaa

+

bbb

+```````````````````````````````` + + +Leading spaces are skipped: + +```````````````````````````````` example + aaa + bbb +. +

aaa +bbb

+```````````````````````````````` + + +Lines after the first may be indented any amount, since indented +code blocks cannot interrupt paragraphs. + +```````````````````````````````` example +aaa + bbb + ccc +. +

aaa +bbb +ccc

+```````````````````````````````` + + +However, the first line may be indented at most three spaces, +or an indented code block will be triggered: + +```````````````````````````````` example + aaa +bbb +. +

aaa +bbb

+```````````````````````````````` + + +```````````````````````````````` example + aaa +bbb +. +
aaa
+
+

bbb

+```````````````````````````````` + + +Final spaces are stripped before inline parsing, so a paragraph +that ends with two or more spaces will not end with a [hard line +break]: + +```````````````````````````````` example +aaa +bbb +. +

aaa
+bbb

+```````````````````````````````` + + +## 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 + + +. +

aaa

+

aaa

+```````````````````````````````` + + + +# 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 +. +
+

Foo

+

bar +baz

+
+```````````````````````````````` + + +The spaces after the `>` characters can be omitted: + +```````````````````````````````` example +># Foo +>bar +> baz +. +
+

Foo

+

bar +baz

+
+```````````````````````````````` + + +The `>` characters can be indented 1-3 spaces: + +```````````````````````````````` example + > # Foo + > bar + > baz +. +
+

Foo

+

bar +baz

+
+```````````````````````````````` + + +Four spaces gives us a code block: + +```````````````````````````````` example + > # Foo + > bar + > baz +. +
> # Foo
+> bar
+> baz
+
+```````````````````````````````` + + +The Laziness clause allows us to omit the `>` before a +paragraph continuation line: + +```````````````````````````````` example +> # Foo +> bar +baz +. +
+

Foo

+

bar +baz

+
+```````````````````````````````` + + +A block quote can contain some lazy and some non-lazy +continuation lines: + +```````````````````````````````` example +> bar +baz +> foo +. +
+

bar +baz +foo

+
+```````````````````````````````` + + +Laziness only applies to lines that would have been continuations of +paragraphs had they been prepended with [block quote markers]. +For example, the `> ` cannot be omitted in the second line of + +``` markdown +> foo +> --- +``` + +without changing the meaning: + +```````````````````````````````` example +> foo +--- +. +
+

foo

+
+
+```````````````````````````````` + + +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 +. +
+
    +
  • foo
  • +
+
+
    +
  • bar
  • +
+```````````````````````````````` + + +For the same reason, we can't omit the `> ` in front of +subsequent lines of an indented or fenced code block: + +```````````````````````````````` example +> foo + bar +. +
+
foo
+
+
+
bar
+
+```````````````````````````````` + + +```````````````````````````````` example +> ``` +foo +``` +. +
+
+
+

foo

+
+```````````````````````````````` + + +Note that in the following case, we have a paragraph +continuation line: + +```````````````````````````````` example +> foo + - bar +. +
+

foo +- bar

+
+```````````````````````````````` + + +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 +> +. +
+
+```````````````````````````````` + + +```````````````````````````````` example +> +> +> +. +
+
+```````````````````````````````` + + +A block quote can have initial or final blank lines: + +```````````````````````````````` example +> +> foo +> +. +
+

foo

+
+```````````````````````````````` + + +A blank line always separates block quotes: + +```````````````````````````````` example +> foo + +> bar +. +
+

foo

+
+
+

bar

+
+```````````````````````````````` + + +(Most current Markdown implementations, including John Gruber's +original `Markdown.pl`, will parse this example as a single block quote +with two paragraphs. But it seems better to allow the author to decide +whether two block quotes or one are wanted.) + +Consecutiveness means that if we put these block quotes together, +we get a single block quote: + +```````````````````````````````` example +> foo +> bar +. +
+

foo +bar

+
+```````````````````````````````` + + +To get a block quote with two paragraphs, use: + +```````````````````````````````` example +> foo +> +> bar +. +
+

foo

+

bar

+
+```````````````````````````````` + + +Block quotes can interrupt paragraphs: + +```````````````````````````````` example +foo +> bar +. +

foo

+
+

bar

+
+```````````````````````````````` + + +In general, blank lines are not needed before or after block +quotes: + +```````````````````````````````` example +> aaa +*** +> bbb +. +
+

aaa

+
+
+
+

bbb

+
+```````````````````````````````` + + +However, because of laziness, a blank line is needed between +a block quote and a following paragraph: + +```````````````````````````````` example +> bar +baz +. +
+

bar +baz

+
+```````````````````````````````` + + +```````````````````````````````` example +> bar + +baz +. +
+

bar

+
+

baz

+```````````````````````````````` + + +```````````````````````````````` example +> bar +> +baz +. +
+

bar

+
+

baz

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

foo +bar

+
+
+
+```````````````````````````````` + + +```````````````````````````````` example +>>> foo +> bar +>>baz +. +
+
+
+

foo +bar +baz

+
+
+
+```````````````````````````````` + + +When including an indented code block in a block quote, +remember that the [block quote marker] includes +both the `>` and a following space. So *five spaces* are needed after +the `>`: + +```````````````````````````````` example +> code + +> not code +. +
+
code
+
+
+
+

not code

+
+```````````````````````````````` + + + +## 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. +. +

A paragraph +with two lines.

+
indented code
+
+
+

A block quote.

+
+```````````````````````````````` + + +And let *M* be the marker `1.`, and *N* = 2. Then rule #1 says +that the following is an ordered list item with start number 1, +and the same contents as *Ls*: + +```````````````````````````````` example +1. A paragraph + with two lines. + + indented code + + > A block quote. +. +
    +
  1. +

    A paragraph +with two lines.

    +
    indented code
    +
    +
    +

    A block quote.

    +
    +
  2. +
+```````````````````````````````` + + +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 +. +
    +
  • one
  • +
+

two

+```````````````````````````````` + + +```````````````````````````````` example +- one + + two +. +
    +
  • +

    one

    +

    two

    +
  • +
+```````````````````````````````` + + +```````````````````````````````` example + - one + + two +. +
    +
  • one
  • +
+
 two
+
+```````````````````````````````` + + +```````````````````````````````` example + - one + + two +. +
    +
  • +

    one

    +

    two

    +
  • +
+```````````````````````````````` + + +It is tempting to think of this in terms of columns: the continuation +blocks must be indented at least to the column of the first +[non-whitespace character] after the list marker. However, that is not quite right. +The spaces after the list marker determine how much relative indentation +is needed. Which column this indentation reaches will depend on +how the list item is embedded in other constructions, as shown by +this example: + +```````````````````````````````` example + > > 1. one +>> +>> two +. +
+
+
    +
  1. +

    one

    +

    two

    +
  2. +
+
+
+```````````````````````````````` + + +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 +. +
+
+
    +
  • one
  • +
+

two

+
+
+```````````````````````````````` + + +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 +. +

-one

+

2.two

+```````````````````````````````` + + +A list item may not contain blocks that are separated by more than +one blank line. Thus, two blank lines will end a list, unless the +two blanks are contained in a [fenced code block]. + +```````````````````````````````` example +- foo + + bar + +- foo + + + bar + +- ``` + foo + + + bar + ``` + +- baz + + + ``` + foo + + + bar + ``` +. +
    +
  • +

    foo

    +

    bar

    +
  • +
  • +

    foo

    +
  • +
+

bar

+
    +
  • +
    foo
    +
    +
    +bar
    +
    +
  • +
  • +

    baz

    +
      +
    • +
      foo
      +
      +
      +bar
      +
      +
    • +
    +
  • +
+```````````````````````````````` + + +A list item may contain any kind of block: + +```````````````````````````````` example +1. foo + + ``` + bar + ``` + + baz + + > bam +. +
    +
  1. +

    foo

    +
    bar
    +
    +

    baz

    +
    +

    bam

    +
    +
  2. +
+```````````````````````````````` + + +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 +. +
    +
  • +

    Foo

    +
    bar
    +
    +baz
    +
    +
  • +
+```````````````````````````````` + + +```````````````````````````````` example +- Foo + + bar + + + baz +. +
    +
  • +

    Foo

    +
    bar
    +
    +
  • +
+
  baz
+
+```````````````````````````````` + + +Note that ordered list start numbers must be nine digits or less: + +```````````````````````````````` example +123456789. ok +. +
    +
  1. ok
  2. +
+```````````````````````````````` + + +```````````````````````````````` example +1234567890. not ok +. +

1234567890. not ok

+```````````````````````````````` + + +A start number may begin with 0s: + +```````````````````````````````` example +0. ok +. +
    +
  1. ok
  2. +
+```````````````````````````````` + + +```````````````````````````````` example +003. ok +. +
    +
  1. ok
  2. +
+```````````````````````````````` + + +A start number may not be negative: + +```````````````````````````````` example +-1. not ok +. +

-1. not ok

+```````````````````````````````` + + + +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 +. +
    +
  • +

    foo

    +
    bar
    +
    +
  • +
+```````````````````````````````` + + +And in this case it is 11 spaces: + +```````````````````````````````` example + 10. foo + + bar +. +
    +
  1. +

    foo

    +
    bar
    +
    +
  2. +
+```````````````````````````````` + + +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 +. +
indented code
+
+

paragraph

+
more code
+
+```````````````````````````````` + + +```````````````````````````````` example +1. indented code + + paragraph + + more code +. +
    +
  1. +
    indented code
    +
    +

    paragraph

    +
    more code
    +
    +
  2. +
+```````````````````````````````` + + +Note that an additional space indent is interpreted as space +inside the code block: + +```````````````````````````````` example +1. indented code + + paragraph + + more code +. +
    +
  1. +
     indented code
    +
    +

    paragraph

    +
    more code
    +
    +
  2. +
+```````````````````````````````` + + +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 +. +

foo

+

bar

+```````````````````````````````` + + +```````````````````````````````` example +- foo + + bar +. +
    +
  • foo
  • +
+

bar

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

    foo

    +

    bar

    +
  • +
+```````````````````````````````` + + +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 +. +
    +
  • foo
  • +
  • +
    bar
    +
    +
  • +
  • +
    baz
    +
    +
  • +
+```````````````````````````````` + +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 +. +
    +
  • foo
  • +
+```````````````````````````````` + + +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 +. +
    +
  • +
+

foo

+```````````````````````````````` + + +Here is an empty bullet list item: + +```````````````````````````````` example +- foo +- +- bar +. +
    +
  • foo
  • +
  • +
  • bar
  • +
+```````````````````````````````` + + +It does not matter whether there are spaces following the [list marker]: + +```````````````````````````````` example +- foo +- +- bar +. +
    +
  • foo
  • +
  • +
  • bar
  • +
+```````````````````````````````` + + +Here is an empty ordered list item: + +```````````````````````````````` example +1. foo +2. +3. bar +. +
    +
  1. foo
  2. +
  3. +
  4. bar
  5. +
+```````````````````````````````` + + +A list may start or end with an empty list item: + +```````````````````````````````` example +* +. +
    +
  • +
+```````````````````````````````` + + + +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. +. +
    +
  1. +

    A paragraph +with two lines.

    +
    indented code
    +
    +
    +

    A block quote.

    +
    +
  2. +
+```````````````````````````````` + + +Indented two spaces: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +
    +
  1. +

    A paragraph +with two lines.

    +
    indented code
    +
    +
    +

    A block quote.

    +
    +
  2. +
+```````````````````````````````` + + +Indented three spaces: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +
    +
  1. +

    A paragraph +with two lines.

    +
    indented code
    +
    +
    +

    A block quote.

    +
    +
  2. +
+```````````````````````````````` + + +Four spaces indent gives a code block: + +```````````````````````````````` example + 1. A paragraph + with two lines. + + indented code + + > A block quote. +. +
1.  A paragraph
+    with two lines.
+
+        indented code
+
+    > A block quote.
+
+```````````````````````````````` + + + +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. +. +
    +
  1. +

    A paragraph +with two lines.

    +
    indented code
    +
    +
    +

    A block quote.

    +
    +
  2. +
+```````````````````````````````` + + +Indentation can be partially deleted: + +```````````````````````````````` example + 1. A paragraph + with two lines. +. +
    +
  1. A paragraph +with two lines.
  2. +
+```````````````````````````````` + + +These examples show how laziness can work in nested structures: + +```````````````````````````````` example +> 1. > Blockquote +continued here. +. +
+
    +
  1. +
    +

    Blockquote +continued here.

    +
    +
  2. +
+
+```````````````````````````````` + + +```````````````````````````````` example +> 1. > Blockquote +> continued here. +. +
+
    +
  1. +
    +

    Blockquote +continued here.

    +
    +
  2. +
+
+```````````````````````````````` + + + +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 +. +
    +
  • foo +
      +
    • bar +
        +
      • baz
      • +
      +
    • +
    +
  • +
+```````````````````````````````` + + +One is not enough: + +```````````````````````````````` example +- foo + - bar + - baz +. +
    +
  • foo
  • +
  • bar
  • +
  • baz
  • +
+```````````````````````````````` + + +Here we need four, because the list marker is wider: + +```````````````````````````````` example +10) foo + - bar +. +
    +
  1. foo +
      +
    • bar
    • +
    +
  2. +
+```````````````````````````````` + + +Three is not enough: + +```````````````````````````````` example +10) foo + - bar +. +
    +
  1. foo
  2. +
+
    +
  • bar
  • +
+```````````````````````````````` + + +A list may be the first block in a list item: + +```````````````````````````````` example +- - foo +. +
    +
  • +
      +
    • foo
    • +
    +
  • +
+```````````````````````````````` + + +```````````````````````````````` example +1. - 2. foo +. +
    +
  1. +
      +
    • +
        +
      1. foo
      2. +
      +
    • +
    +
  2. +
+```````````````````````````````` + + +A list item can contain a heading: + +```````````````````````````````` example +- # Foo +- Bar + --- + baz +. +
    +
  • +

    Foo

    +
  • +
  • +

    Bar

    +baz
  • +
+```````````````````````````````` + + +### Motivation + +John Gruber's Markdown spec says the following about list items: + +1. "List markers typically start at the left margin, but may be indented + by up to three spaces. List markers must be followed by one or more + spaces or a tab." + +2. "To make lists look nice, you can wrap items with hanging indents.... + But if you don't want to, you don't have to." + +3. "List items may consist of multiple paragraphs. Each subsequent + paragraph in a list item must be indented by either 4 spaces or one + tab." + +4. "It looks nice if you indent every line of the subsequent paragraphs, + but here again, Markdown will allow you to be lazy." + +5. "To put a blockquote within a list item, the blockquote's `>` + delimiters need to be indented." + +6. "To put a code block within a list item, the code block needs to be + indented twice — 8 spaces or two tabs." + +These rules specify that a paragraph under a list item must be indented +four spaces (presumably, from the left margin, rather than the start of +the list marker, but this is not said), and that code under a list item +must be indented eight spaces instead of the usual four. They also say +that a block quote must be indented, but not by how much; however, the +example given has four spaces indentation. Although nothing is said +about other kinds of block-level content, it is certainly reasonable to +infer that *all* block elements under a list item, including other +lists, must be indented four spaces. This principle has been called the +*four-space rule*. + +The four-space rule is clear and principled, and if the reference +implementation `Markdown.pl` had followed it, it probably would have +become the standard. However, `Markdown.pl` allowed paragraphs and +sublists to start with only two spaces indentation, at least on the +outer level. Worse, its behavior was inconsistent: a sublist of an +outer-level list needed two spaces indentation, but a sublist of this +sublist needed three spaces. It is not surprising, then, that different +implementations of Markdown have developed very different rules for +determining what comes under a list item. (Pandoc and python-Markdown, +for example, stuck with Gruber's syntax description and the four-space +rule, while discount, redcarpet, marked, PHP Markdown, and others +followed `Markdown.pl`'s behavior more closely.) + +Unfortunately, given the divergences between implementations, there +is no way to give a spec for list items that will be guaranteed not +to break any existing documents. However, the spec given here should +correctly handle lists formatted with either the four-space rule or +the more forgiving `Markdown.pl` behavior, provided they are laid out +in a way that is natural for a human to read. + +The strategy here is to let the width and indentation of the list marker +determine the indentation necessary for blocks to fall under the list +item, rather than having a fixed and arbitrary number. The writer can +think of the body of the list item as a unit which gets indented to the +right enough to fit the list marker (and any indentation on the list +marker). (The laziness rule, #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 +
    +
  • foo
  • +
+

bar

+
    +
  • baz
  • +
+``` + +as the four-space rule demands, rather than a single list, + +``` html +
    +
  • +

    foo

    +

    bar

    +
      +
    • baz
    • +
    +
  • +
+``` + +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 +
    +
  • +

    one

    +

    two

    +
  • +
+``` + +and similarly + +``` markdown +> - one +> +> two +``` + +as + +``` html +
+
    +
  • +

    one

    +

    two

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

` tags, while paragraphs in a tight list are not.) + +Changing the bullet or ordered list delimiter starts a new list: + +```````````````````````````````` example +- foo +- bar ++ baz +. +

    +
  • foo
  • +
  • bar
  • +
+
    +
  • baz
  • +
+```````````````````````````````` + + +```````````````````````````````` example +1. foo +2. bar +3) baz +. +
    +
  1. foo
  2. +
  3. bar
  4. +
+
    +
  1. baz
  2. +
+```````````````````````````````` + + +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 +. +

Foo

+
    +
  • bar
  • +
  • baz
  • +
+```````````````````````````````` + + +`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. +. +

The number of windows in my house is

+
    +
  1. The number of doors is 6.
  2. +
+```````````````````````````````` + + +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 `

` 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 +. +

    +
  • +

    foo

    +
  • +
  • +

    bar

    +
  • +
+
    +
  • baz
  • +
+```````````````````````````````` + + +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 +. +
    +
  • foo
  • +
+

bar

+
    +
  • baz
  • +
+```````````````````````````````` + + +Indeed, two blank lines will end *all* containing lists: + +```````````````````````````````` example +- foo + - bar + - baz + + + bim +. +
    +
  • foo +
      +
    • bar +
        +
      • baz
      • +
      +
    • +
    +
  • +
+
  bim
+
+```````````````````````````````` + + +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 +. +
    +
  • foo
  • +
  • bar
  • +
+
    +
  • baz
  • +
  • bim
  • +
+```````````````````````````````` + + +```````````````````````````````` example +- foo + + notcode + +- foo + + + code +. +
    +
  • +

    foo

    +

    notcode

    +
  • +
  • +

    foo

    +
  • +
+
code
+
+```````````````````````````````` + + +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 +. +
    +
  • a
  • +
  • b
  • +
  • c
  • +
  • d
  • +
  • e
  • +
  • f
  • +
  • g
  • +
  • h
  • +
  • i
  • +
+```````````````````````````````` + + +```````````````````````````````` example +1. a + + 2. b + + 3. c +. +
    +
  1. +

    a

    +
  2. +
  3. +

    b

    +
  4. +
  5. +

    c

    +
  6. +
+```````````````````````````````` + + +This is a loose list, because there is a blank line between +two of the list items: + +```````````````````````````````` example +- a +- b + +- c +. +
    +
  • +

    a

    +
  • +
  • +

    b

    +
  • +
  • +

    c

    +
  • +
+```````````````````````````````` + + +So is this, with a empty second item: + +```````````````````````````````` example +* a +* + +* c +. +
    +
  • +

    a

    +
  • +
  • +
  • +

    c

    +
  • +
+```````````````````````````````` + + +These are loose lists, even though there is no space between the items, +because one of the items directly contains two block-level elements +with a blank line between them: + +```````````````````````````````` example +- a +- b + + c +- d +. +
    +
  • +

    a

    +
  • +
  • +

    b

    +

    c

    +
  • +
  • +

    d

    +
  • +
+```````````````````````````````` + + +```````````````````````````````` example +- a +- b + + [ref]: /url +- d +. +
    +
  • +

    a

    +
  • +
  • +

    b

    +
  • +
  • +

    d

    +
  • +
+```````````````````````````````` + + +This is a tight list, because the blank lines are in a code block: + +```````````````````````````````` example +- a +- ``` + b + + + ``` +- c +. +
    +
  • a
  • +
  • +
    b
    +
    +
    +
    +
  • +
  • c
  • +
+```````````````````````````````` + + +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 +. +
    +
  • a +
      +
    • +

      b

      +

      c

      +
    • +
    +
  • +
  • d
  • +
+```````````````````````````````` + + +This is a tight list, because the blank line is inside the +block quote: + +```````````````````````````````` example +* a + > b + > +* c +. +
    +
  • a +
    +

    b

    +
    +
  • +
  • c
  • +
+```````````````````````````````` + + +This list is tight, because the consecutive block elements +are not separated by blank lines: + +```````````````````````````````` example +- a + > b + ``` + c + ``` +- d +. +
    +
  • a +
    +

    b

    +
    +
    c
    +
    +
  • +
  • d
  • +
+```````````````````````````````` + + +A single-paragraph list is tight: + +```````````````````````````````` example +- a +. +
    +
  • a
  • +
+```````````````````````````````` + + +```````````````````````````````` example +- a + - b +. +
    +
  • a +
      +
    • b
    • +
    +
  • +
+```````````````````````````````` + + +This list is loose, because of the blank line between the +two block elements in the list item: + +```````````````````````````````` example +1. ``` + foo + ``` + + bar +. +
    +
  1. +
    foo
    +
    +

    bar

    +
  2. +
+```````````````````````````````` + + +Here the outer list is loose, the inner list tight: + +```````````````````````````````` example +* foo + * bar + + baz +. +
    +
  • +

    foo

    +
      +
    • bar
    • +
    +

    baz

    +
  • +
+```````````````````````````````` + + +```````````````````````````````` example +- a + - b + - c + +- d + - e + - f +. +
    +
  • +

    a

    +
      +
    • b
    • +
    • c
    • +
    +
  • +
  • +

    d

    +
      +
    • e
    • +
    • f
    • +
    +
  • +
+```````````````````````````````` + + +# 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` +. +

hilo`

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

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

+```````````````````````````````` + + +Backslashes before other characters are treated as literal +backslashes: + +```````````````````````````````` example +\→\A\a\ \3\φ\« +. +

\→\A\a\ \3\φ\«

+```````````````````````````````` + + +Escaped characters are treated as regular characters and do +not have their usual Markdown meanings: + +```````````````````````````````` example +\*not emphasized* +\
not a tag +\[not a link](/foo) +\`not code` +1\. not a list +\* not a list +\# not a heading +\[foo]: /url "not a reference" +. +

*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"

+```````````````````````````````` + + +If a backslash is itself escaped, the following character is not: + +```````````````````````````````` example +\\*emphasis* +. +

\emphasis

+```````````````````````````````` + + +A backslash at the end of the line is a [hard line break]: + +```````````````````````````````` example +foo\ +bar +. +

foo
+bar

+```````````````````````````````` + + +Backslash escapes do not work in code blocks, code spans, autolinks, or +raw HTML: + +```````````````````````````````` example +`` \[\` `` +. +

\[\`

+```````````````````````````````` + + +```````````````````````````````` example + \[\] +. +
\[\]
+
+```````````````````````````````` + + +```````````````````````````````` example +~~~ +\[\] +~~~ +. +
\[\]
+
+```````````````````````````````` + + +```````````````````````````````` example + +. +

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

+```````````````````````````````` + + +```````````````````````````````` example + +. + +```````````````````````````````` + + +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") +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +[foo] + +[foo]: /bar\* "ti\*tle" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +``` foo\+bar +foo +``` +. +
foo
+
+```````````````````````````````` + + + +## 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 +is used as an authoritative source for the valid entity +references and their corresponding code points. + +```````````````````````````````` example +  & © Æ Ď +¾ ℋ ⅆ +∲ ≧̸ +. +

  & © Æ Ď +¾ ℋ ⅆ +∲ ≧̸

+```````````````````````````````` + + +[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 � � +. +

# Ӓ Ϡ � �

+```````````````````````````````` + + +[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 +. +

" ആ ಫ

+```````````````````````````````` + + +Here are some nonentities: + +```````````````````````````````` example +  &x; &#; &#x; +&ThisIsNotDefined; &hi?; +. +

&nbsp &x; &#; &#x; +&ThisIsNotDefined; &hi?;

+```````````````````````````````` + + +Although HTML5 does accept some entity references +without a trailing semicolon (such as `©`), these are not +recognized here, because it makes the grammar too ambiguous: + +```````````````````````````````` example +© +. +

&copy

+```````````````````````````````` + + +Strings that are not on the list of HTML5 named entities are not +recognized as entity references either: + +```````````````````````````````` example +&MadeUpEntity; +. +

&MadeUpEntity;

+```````````````````````````````` + + +Entity and numeric character references are recognized in any +context besides code spans or code blocks, including +URLs, [link titles], and [fenced code block][] [info strings]: + +```````````````````````````````` example + +. + +```````````````````````````````` + + +```````````````````````````````` example +[foo](/föö "föö") +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +[foo] + +[foo]: /föö "föö" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +``` föö +foo +``` +. +
foo
+
+```````````````````````````````` + + +Entity and numeric character references are treated as literal +text in code spans and code blocks: + +```````````````````````````````` example +`föö` +. +

f&ouml;&ouml;

+```````````````````````````````` + + +```````````````````````````````` example + föfö +. +
f&ouml;f&ouml;
+
+```````````````````````````````` + + +## 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` +. +

foo

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

foo ` bar

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

``

+```````````````````````````````` + + +[Line endings] are treated like spaces: + +```````````````````````````````` example +`` +foo +`` +. +

foo

+```````````````````````````````` + + +Interior spaces and [line endings] are collapsed into +single spaces, just as they would be by a browser: + +```````````````````````````````` example +`foo bar + baz` +. +

foo bar baz

+```````````````````````````````` + + +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 +`
` 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` +. +

foo `` bar

+```````````````````````````````` + + +Note that backslash escapes do not work in code spans. All backslashes +are treated literally: + +```````````````````````````````` example +`foo\`bar` +. +

foo\bar`

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

*foo*

+```````````````````````````````` + + +And this is not parsed as a link: + +```````````````````````````````` example +[not a `link](/foo`) +. +

[not a link](/foo)

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

<a href="">`

+```````````````````````````````` + + +But this is an HTML tag: + +```````````````````````````````` example +
` +. +

`

+```````````````````````````````` + + +And this is code: + +```````````````````````````````` example +`` +. +

<http://foo.bar.baz>`

+```````````````````````````````` + + +But this is an autolink: + +```````````````````````````````` example +` +. +

http://foo.bar.`baz`

+```````````````````````````````` + + +When a backtick string is not closed by a matching backtick string, +we just have literal backticks: + +```````````````````````````````` example +```foo`` +. +

```foo``

+```````````````````````````````` + + +```````````````````````````````` example +`foo +. +

`foo

+```````````````````````````````` + + +## 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 +> `` tag; double `*`'s or `_`'s will be wrapped with an HTML `` +> tag. + +This is enough for most users, but these rules leave much undecided, +especially when it comes to nested emphasis. The original +`Markdown.pl` test suite makes it clear that triple `***` and +`___` delimiters can be used for strong emphasis, and most +implementations have also allowed the following patterns: + +``` 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 `...` is always preferred to + `...`. + +14. An interpretation `...` is always + preferred to `..`. + +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 `foo _bar baz_` rather + than `*foo bar* baz`. For the same reason, + `**foo*bar**` is parsed as `foobar*` + rather than `foo*bar`. + +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 bar baz` + rather than `foo **bar baz`. + +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 `*foo*` rather than as + `[foo](bar)`. + +These rules can be illustrated through a series of examples. + +Rule 1: + +```````````````````````````````` example +*foo bar* +. +

foo bar

+```````````````````````````````` + + +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* +. +

a * foo bar*

+```````````````````````````````` + + +This is not emphasis, because the opening `*` is preceded +by an alphanumeric and followed by punctuation, and hence +not part of a [left-flanking delimiter run]: + +```````````````````````````````` example +a*"foo"* +. +

a*"foo"*

+```````````````````````````````` + + +Unicode nonbreaking spaces count as whitespace, too: + +```````````````````````````````` example +* a * +. +

* a *

+```````````````````````````````` + + +Intraword emphasis with `*` is permitted: + +```````````````````````````````` example +foo*bar* +. +

foobar

+```````````````````````````````` + + +```````````````````````````````` example +5*6*78 +. +

5678

+```````````````````````````````` + + +Rule 2: + +```````````````````````````````` example +_foo bar_ +. +

foo bar

+```````````````````````````````` + + +This is not emphasis, because the opening `_` is followed by +whitespace: + +```````````````````````````````` example +_ foo bar_ +. +

_ foo bar_

+```````````````````````````````` + + +This is not emphasis, because the opening `_` is preceded +by an alphanumeric and followed by punctuation: + +```````````````````````````````` example +a_"foo"_ +. +

a_"foo"_

+```````````````````````````````` + + +Emphasis with `_` is not allowed inside words: + +```````````````````````````````` example +foo_bar_ +. +

foo_bar_

+```````````````````````````````` + + +```````````````````````````````` example +5_6_78 +. +

5_6_78

+```````````````````````````````` + + +```````````````````````````````` example +пристаням_стремятся_ +. +

пристаням_стремятся_

+```````````````````````````````` + + +Here `_` does not generate emphasis, because the first delimiter run +is right-flanking and the second left-flanking: + +```````````````````````````````` example +aa_"bb"_cc +. +

aa_"bb"_cc

+```````````````````````````````` + + +This is emphasis, even though the opening delimiter is +both left- and right-flanking, because it is preceded by +punctuation: + +```````````````````````````````` example +foo-_(bar)_ +. +

foo-(bar)

+```````````````````````````````` + + +Rule 3: + +This is not emphasis, because the closing delimiter does +not match the opening delimiter: + +```````````````````````````````` example +_foo* +. +

_foo*

+```````````````````````````````` + + +This is not emphasis, because the closing `*` is preceded by +whitespace: + +```````````````````````````````` example +*foo bar * +. +

*foo bar *

+```````````````````````````````` + + +A newline also counts as whitespace: + +```````````````````````````````` example +*foo bar +* +. +

*foo bar

+
    +
  • +
+```````````````````````````````` + + +This is not emphasis, because the second `*` is +preceded by punctuation and followed by an alphanumeric +(hence it is not part of a [right-flanking delimiter run]: + +```````````````````````````````` example +*(*foo) +. +

*(*foo)

+```````````````````````````````` + + +The point of this restriction is more easily appreciated +with this example: + +```````````````````````````````` example +*(*foo*)* +. +

(foo)

+```````````````````````````````` + + +Intraword emphasis with `*` is allowed: + +```````````````````````````````` example +*foo*bar +. +

foobar

+```````````````````````````````` + + + +Rule 4: + +This is not emphasis, because the closing `_` is preceded by +whitespace: + +```````````````````````````````` example +_foo bar _ +. +

_foo bar _

+```````````````````````````````` + + +This is not emphasis, because the second `_` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +_(_foo) +. +

_(_foo)

+```````````````````````````````` + + +This is emphasis within emphasis: + +```````````````````````````````` example +_(_foo_)_ +. +

(foo)

+```````````````````````````````` + + +Intraword emphasis is disallowed for `_`: + +```````````````````````````````` example +_foo_bar +. +

_foo_bar

+```````````````````````````````` + + +```````````````````````````````` example +_пристаням_стремятся +. +

_пристаням_стремятся

+```````````````````````````````` + + +```````````````````````````````` example +_foo_bar_baz_ +. +

foo_bar_baz

+```````````````````````````````` + + +This is emphasis, even though the closing delimiter is +both left- and right-flanking, because it is followed by +punctuation: + +```````````````````````````````` example +_(bar)_. +. +

(bar).

+```````````````````````````````` + + +Rule 5: + +```````````````````````````````` example +**foo bar** +. +

foo bar

+```````````````````````````````` + + +This is not strong emphasis, because the opening delimiter is +followed by whitespace: + +```````````````````````````````` example +** foo bar** +. +

** foo bar**

+```````````````````````````````` + + +This is not strong emphasis, because the opening `**` is preceded +by an alphanumeric and followed by punctuation, and hence +not part of a [left-flanking delimiter run]: + +```````````````````````````````` example +a**"foo"** +. +

a**"foo"**

+```````````````````````````````` + + +Intraword strong emphasis with `**` is permitted: + +```````````````````````````````` example +foo**bar** +. +

foobar

+```````````````````````````````` + + +Rule 6: + +```````````````````````````````` example +__foo bar__ +. +

foo bar

+```````````````````````````````` + + +This is not strong emphasis, because the opening delimiter is +followed by whitespace: + +```````````````````````````````` example +__ foo bar__ +. +

__ foo bar__

+```````````````````````````````` + + +A newline counts as whitespace: +```````````````````````````````` example +__ +foo bar__ +. +

__ +foo bar__

+```````````````````````````````` + + +This is not strong emphasis, because the opening `__` is preceded +by an alphanumeric and followed by punctuation: + +```````````````````````````````` example +a__"foo"__ +. +

a__"foo"__

+```````````````````````````````` + + +Intraword strong emphasis is forbidden with `__`: + +```````````````````````````````` example +foo__bar__ +. +

foo__bar__

+```````````````````````````````` + + +```````````````````````````````` example +5__6__78 +. +

5__6__78

+```````````````````````````````` + + +```````````````````````````````` example +пристаням__стремятся__ +. +

пристаням__стремятся__

+```````````````````````````````` + + +```````````````````````````````` example +__foo, __bar__, baz__ +. +

foo, bar, baz

+```````````````````````````````` + + +This is strong emphasis, even though the opening delimiter is +both left- and right-flanking, because it is preceded by +punctuation: + +```````````````````````````````` example +foo-__(bar)__ +. +

foo-(bar)

+```````````````````````````````` + + + +Rule 7: + +This is not strong emphasis, because the closing delimiter is preceded +by whitespace: + +```````````````````````````````` example +**foo bar ** +. +

**foo bar **

+```````````````````````````````` + + +(Nor can it be interpreted as an emphasized `*foo bar *`, because of +Rule 11.) + +This is not strong emphasis, because the second `**` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +**(**foo) +. +

**(**foo)

+```````````````````````````````` + + +The point of this restriction is more easily appreciated +with these examples: + +```````````````````````````````` example +*(**foo**)* +. +

(foo)

+```````````````````````````````` + + +```````````````````````````````` example +**Gomphocarpus (*Gomphocarpus physocarpus*, syn. +*Asclepias physocarpa*)** +. +

Gomphocarpus (Gomphocarpus physocarpus, syn. +Asclepias physocarpa)

+```````````````````````````````` + + +```````````````````````````````` example +**foo "*bar*" foo** +. +

foo "bar" foo

+```````````````````````````````` + + +Intraword emphasis: + +```````````````````````````````` example +**foo**bar +. +

foobar

+```````````````````````````````` + + +Rule 8: + +This is not strong emphasis, because the closing delimiter is +preceded by whitespace: + +```````````````````````````````` example +__foo bar __ +. +

__foo bar __

+```````````````````````````````` + + +This is not strong emphasis, because the second `__` is +preceded by punctuation and followed by an alphanumeric: + +```````````````````````````````` example +__(__foo) +. +

__(__foo)

+```````````````````````````````` + + +The point of this restriction is more easily appreciated +with this example: + +```````````````````````````````` example +_(__foo__)_ +. +

(foo)

+```````````````````````````````` + + +Intraword strong emphasis is forbidden with `__`: + +```````````````````````````````` example +__foo__bar +. +

__foo__bar

+```````````````````````````````` + + +```````````````````````````````` example +__пристаням__стремятся +. +

__пристаням__стремятся

+```````````````````````````````` + + +```````````````````````````````` example +__foo__bar__baz__ +. +

foo__bar__baz

+```````````````````````````````` + + +This is strong emphasis, even though the closing delimiter is +both left- and right-flanking, because it is followed by +punctuation: + +```````````````````````````````` example +__(bar)__. +. +

(bar).

+```````````````````````````````` + + +Rule 9: + +Any nonempty sequence of inline elements can be the contents of an +emphasized span. + +```````````````````````````````` example +*foo [bar](/url)* +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +*foo +bar* +. +

foo +bar

+```````````````````````````````` + + +In particular, emphasis and strong emphasis can be nested +inside emphasis: + +```````````````````````````````` example +_foo __bar__ baz_ +. +

foo bar baz

+```````````````````````````````` + + +```````````````````````````````` example +_foo _bar_ baz_ +. +

foo bar baz

+```````````````````````````````` + + +```````````````````````````````` example +__foo_ bar_ +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +*foo *bar** +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +*foo **bar** baz* +. +

foo bar baz

+```````````````````````````````` + + +But note: + +```````````````````````````````` example +*foo**bar**baz* +. +

foobarbaz

+```````````````````````````````` + + +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* +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +*foo **bar*** +. +

foo bar

+```````````````````````````````` + + +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*** +. +

foobar**

+```````````````````````````````` + + + +Indefinite levels of nesting are possible: + +```````````````````````````````` example +*foo **bar *baz* bim** bop* +. +

foo bar baz bim bop

+```````````````````````````````` + + +```````````````````````````````` example +*foo [*bar*](/url)* +. +

foo bar

+```````````````````````````````` + + +There can be no empty emphasis or strong emphasis: + +```````````````````````````````` example +** is not an empty emphasis +. +

** is not an empty emphasis

+```````````````````````````````` + + +```````````````````````````````` example +**** is not an empty strong emphasis +. +

**** is not an empty strong emphasis

+```````````````````````````````` + + + +Rule 10: + +Any nonempty sequence of inline elements can be the contents of an +strongly emphasized span. + +```````````````````````````````` example +**foo [bar](/url)** +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +**foo +bar** +. +

foo +bar

+```````````````````````````````` + + +In particular, emphasis and strong emphasis can be nested +inside strong emphasis: + +```````````````````````````````` example +__foo _bar_ baz__ +. +

foo bar baz

+```````````````````````````````` + + +```````````````````````````````` example +__foo __bar__ baz__ +. +

foo bar baz

+```````````````````````````````` + + +```````````````````````````````` example +____foo__ bar__ +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +**foo **bar**** +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +**foo *bar* baz** +. +

foo bar baz

+```````````````````````````````` + + +But note: + +```````````````````````````````` example +**foo*bar*baz** +. +

foobarbaz**

+```````````````````````````````` + + +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** +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +**foo *bar*** +. +

foo bar

+```````````````````````````````` + + +Indefinite levels of nesting are possible: + +```````````````````````````````` example +**foo *bar **baz** +bim* bop** +. +

foo bar baz +bim bop

+```````````````````````````````` + + +```````````````````````````````` example +**foo [*bar*](/url)** +. +

foo bar

+```````````````````````````````` + + +There can be no empty emphasis or strong emphasis: + +```````````````````````````````` example +__ is not an empty emphasis +. +

__ is not an empty emphasis

+```````````````````````````````` + + +```````````````````````````````` example +____ is not an empty strong emphasis +. +

____ is not an empty strong emphasis

+```````````````````````````````` + + + +Rule 11: + +```````````````````````````````` example +foo *** +. +

foo ***

+```````````````````````````````` + + +```````````````````````````````` example +foo *\** +. +

foo *

+```````````````````````````````` + + +```````````````````````````````` example +foo *_* +. +

foo _

+```````````````````````````````` + + +```````````````````````````````` example +foo ***** +. +

foo *****

+```````````````````````````````` + + +```````````````````````````````` example +foo **\*** +. +

foo *

+```````````````````````````````` + + +```````````````````````````````` example +foo **_** +. +

foo _

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

*foo

+```````````````````````````````` + + +```````````````````````````````` example +*foo** +. +

foo*

+```````````````````````````````` + + +```````````````````````````````` example +***foo** +. +

*foo

+```````````````````````````````` + + +```````````````````````````````` example +****foo* +. +

***foo

+```````````````````````````````` + + +```````````````````````````````` example +**foo*** +. +

foo*

+```````````````````````````````` + + +```````````````````````````````` example +*foo**** +. +

foo***

+```````````````````````````````` + + + +Rule 12: + +```````````````````````````````` example +foo ___ +. +

foo ___

+```````````````````````````````` + + +```````````````````````````````` example +foo _\__ +. +

foo _

+```````````````````````````````` + + +```````````````````````````````` example +foo _*_ +. +

foo *

+```````````````````````````````` + + +```````````````````````````````` example +foo _____ +. +

foo _____

+```````````````````````````````` + + +```````````````````````````````` example +foo __\___ +. +

foo _

+```````````````````````````````` + + +```````````````````````````````` example +foo __*__ +. +

foo *

+```````````````````````````````` + + +```````````````````````````````` example +__foo_ +. +

_foo

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

foo_

+```````````````````````````````` + + +```````````````````````````````` example +___foo__ +. +

_foo

+```````````````````````````````` + + +```````````````````````````````` example +____foo_ +. +

___foo

+```````````````````````````````` + + +```````````````````````````````` example +__foo___ +. +

foo_

+```````````````````````````````` + + +```````````````````````````````` example +_foo____ +. +

foo___

+```````````````````````````````` + + +Rule 13 implies that if you want emphasis nested directly inside +emphasis, you must use different delimiters: + +```````````````````````````````` example +**foo** +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +*_foo_* +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +__foo__ +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +_*foo*_ +. +

foo

+```````````````````````````````` + + +However, strong emphasis within strong emphasis is possible without +switching delimiters: + +```````````````````````````````` example +****foo**** +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +____foo____ +. +

foo

+```````````````````````````````` + + + +Rule 13 can be applied to arbitrarily long sequences of +delimiters: + +```````````````````````````````` example +******foo****** +. +

foo

+```````````````````````````````` + + +Rule 14: + +```````````````````````````````` example +***foo*** +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +_____foo_____ +. +

foo

+```````````````````````````````` + + +Rule 15: + +```````````````````````````````` example +*foo _bar* baz_ +. +

foo _bar baz_

+```````````````````````````````` + + +```````````````````````````````` example +**foo*bar** +. +

foobar*

+```````````````````````````````` + + +```````````````````````````````` example +*foo __bar *baz bim__ bam* +. +

foo bar *baz bim bam

+```````````````````````````````` + + +Rule 16: + +```````````````````````````````` example +**foo **bar baz** +. +

**foo bar baz

+```````````````````````````````` + + +```````````````````````````````` example +*foo *bar baz* +. +

*foo bar baz

+```````````````````````````````` + + +Rule 17: + +```````````````````````````````` example +*[bar*](/url) +. +

*bar*

+```````````````````````````````` + + +```````````````````````````````` example +_foo [bar_](/url) +. +

_foo bar_

+```````````````````````````````` + + +```````````````````````````````` example +* +. +

*

+```````````````````````````````` + + +```````````````````````````````` example +** +. +

**

+```````````````````````````````` + + +```````````````````````````````` example +__ +. +

__

+```````````````````````````````` + + +```````````````````````````````` example +*a `*`* +. +

a *

+```````````````````````````````` + + +```````````````````````````````` example +_a `_`_ +. +

a _

+```````````````````````````````` + + +```````````````````````````````` example +**a +. +

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

+```````````````````````````````` + + +```````````````````````````````` example +__a +. +

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

+```````````````````````````````` + + + +## 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") +. +

link

+```````````````````````````````` + + +The title may be omitted: + +```````````````````````````````` example +[link](/uri) +. +

link

+```````````````````````````````` + + +Both the title and the destination may be omitted: + +```````````````````````````````` example +[link]() +. +

link

+```````````````````````````````` + + +```````````````````````````````` example +[link](<>) +. +

link

+```````````````````````````````` + + +The destination cannot contain spaces or line breaks, +even if enclosed in pointy brackets: + +```````````````````````````````` example +[link](/my uri) +. +

[link](/my uri)

+```````````````````````````````` + + +```````````````````````````````` example +[link]() +. +

[link](</my uri>)

+```````````````````````````````` + + +```````````````````````````````` example +[link](foo +bar) +. +

[link](foo +bar)

+```````````````````````````````` + + +```````````````````````````````` example +[link]() +. +

[link]()

+```````````````````````````````` + +Parentheses inside the link destination may be escaped: + +```````````````````````````````` example +[link](\(foo\)) +. +

link

+```````````````````````````````` + +One level of balanced parentheses is allowed without escaping: + +```````````````````````````````` example +[link]((foo)and(bar)) +. +

link

+```````````````````````````````` + +However, if you have parentheses within parentheses, you need to escape +or use the `<...>` form: + +```````````````````````````````` example +[link](foo(and(bar))) +. +

[link](foo(and(bar)))

+```````````````````````````````` + + +```````````````````````````````` example +[link](foo(and\(bar\))) +. +

link

+```````````````````````````````` + + +```````````````````````````````` example +[link]() +. +

link

+```````````````````````````````` + + +Parentheses and other symbols can also be escaped, as usual +in Markdown: + +```````````````````````````````` example +[link](foo\)\:) +. +

link

+```````````````````````````````` + + +A link can contain fragment identifiers and queries: + +```````````````````````````````` example +[link](#fragment) + +[link](http://example.com#fragment) + +[link](http://example.com?foo=3#frag) +. +

link

+

link

+

link

+```````````````````````````````` + + +Note that a backslash before a non-escapable character is +just a backslash: + +```````````````````````````````` example +[link](foo\bar) +. +

link

+```````````````````````````````` + + +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) +. +

link

+```````````````````````````````` + + +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") +. +

link

+```````````````````````````````` + + +Titles may be in single quotes, double quotes, or parentheses: + +```````````````````````````````` example +[link](/url "title") +[link](/url 'title') +[link](/url (title)) +. +

link +link +link

+```````````````````````````````` + + +Backslash escapes and entity and numeric character references +may be used in titles: + +```````````````````````````````` example +[link](/url "title \""") +. +

link

+```````````````````````````````` + + +Nested balanced quotes are not allowed without escaping: + +```````````````````````````````` example +[link](/url "title "and" title") +. +

[link](/url "title "and" title")

+```````````````````````````````` + + +But it is easy to work around this by using a different quote type: + +```````````````````````````````` example +[link](/url 'title "and" title') +. +

link

+```````````````````````````````` + + +(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" ) +. +

link

+```````````````````````````````` + + +But it is not allowed between the link text and the +following parenthesis: + +```````````````````````````````` example +[link] (/uri) +. +

[link] (/uri)

+```````````````````````````````` + + +The link text may contain balanced brackets, but not unbalanced ones, +unless they are escaped: + +```````````````````````````````` example +[link [foo [bar]]](/uri) +. +

link [foo [bar]]

+```````````````````````````````` + + +```````````````````````````````` example +[link] bar](/uri) +. +

[link] bar](/uri)

+```````````````````````````````` + + +```````````````````````````````` example +[link [bar](/uri) +. +

[link bar

+```````````````````````````````` + + +```````````````````````````````` example +[link \[bar](/uri) +. +

link [bar

+```````````````````````````````` + + +The link text may contain inline content: + +```````````````````````````````` example +[link *foo **bar** `#`*](/uri) +. +

link foo bar #

+```````````````````````````````` + + +```````````````````````````````` example +[![moon](moon.jpg)](/uri) +. +

moon

+```````````````````````````````` + + +However, links may not contain other links, at any level of nesting. + +```````````````````````````````` example +[foo [bar](/uri)](/uri) +. +

[foo bar](/uri)

+```````````````````````````````` + + +```````````````````````````````` example +[foo *[bar [baz](/uri)](/uri)*](/uri) +. +

[foo [bar baz](/uri)](/uri)

+```````````````````````````````` + + +```````````````````````````````` example +![[[foo](uri1)](uri2)](uri3) +. +

[foo](uri2)

+```````````````````````````````` + + +These cases illustrate the precedence of link text grouping over +emphasis grouping: + +```````````````````````````````` example +*[foo*](/uri) +. +

*foo*

+```````````````````````````````` + + +```````````````````````````````` example +[foo *bar](baz*) +. +

foo *bar

+```````````````````````````````` + + +Note that brackets that *aren't* part of links do not take +precedence: + +```````````````````````````````` example +*foo [bar* baz] +. +

foo [bar baz]

+```````````````````````````````` + + +These cases illustrate the precedence of HTML tags, code spans, +and autolinks over link grouping: + +```````````````````````````````` example +[foo +. +

[foo

+```````````````````````````````` + + +```````````````````````````````` example +[foo`](/uri)` +. +

[foo](/uri)

+```````````````````````````````` + + +```````````````````````````````` example +[foo +. +

[foohttp://example.com/?search=](uri)

+```````````````````````````````` + + +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" +. +

foo

+```````````````````````````````` + + +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 +. +

link [foo [bar]]

+```````````````````````````````` + + +```````````````````````````````` example +[link \[bar][ref] + +[ref]: /uri +. +

link [bar

+```````````````````````````````` + + +The link text may contain inline content: + +```````````````````````````````` example +[link *foo **bar** `#`*][ref] + +[ref]: /uri +. +

link foo bar #

+```````````````````````````````` + + +```````````````````````````````` example +[![moon](moon.jpg)][ref] + +[ref]: /uri +. +

moon

+```````````````````````````````` + + +However, links may not contain other links, at any level of nesting. + +```````````````````````````````` example +[foo [bar](/uri)][ref] + +[ref]: /uri +. +

[foo bar]ref

+```````````````````````````````` + + +```````````````````````````````` example +[foo *bar [baz][ref]*][ref] + +[ref]: /uri +. +

[foo bar baz]ref

+```````````````````````````````` + + +(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 +. +

*foo*

+```````````````````````````````` + + +```````````````````````````````` example +[foo *bar][ref] + +[ref]: /uri +. +

foo *bar

+```````````````````````````````` + + +These cases illustrate the precedence of HTML tags, code spans, +and autolinks over link grouping: + +```````````````````````````````` example +[foo + +[ref]: /uri +. +

[foo

+```````````````````````````````` + + +```````````````````````````````` example +[foo`][ref]` + +[ref]: /uri +. +

[foo][ref]

+```````````````````````````````` + + +```````````````````````````````` example +[foo + +[ref]: /uri +. +

[foohttp://example.com/?search=][ref]

+```````````````````````````````` + + +Matching is case-insensitive: + +```````````````````````````````` example +[foo][BaR] + +[bar]: /url "title" +. +

foo

+```````````````````````````````` + + +Unicode case fold is used: + +```````````````````````````````` example +[Толпой][Толпой] is a Russian word. + +[ТОЛПОЙ]: /url +. +

Толпой is a Russian word.

+```````````````````````````````` + + +Consecutive internal [whitespace] is treated as one space for +purposes of determining matching: + +```````````````````````````````` example +[Foo + bar]: /url + +[Baz][Foo bar] +. +

Baz

+```````````````````````````````` + + +No [whitespace] is allowed between the [link text] and the +[link label]: + +```````````````````````````````` example +[foo] [bar] + +[bar]: /url "title" +. +

[foo] bar

+```````````````````````````````` + + +```````````````````````````````` example +[foo] +[bar] + +[bar]: /url "title" +. +

[foo] +bar

+```````````````````````````````` + + +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] +. +

bar

+```````````````````````````````` + + +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 +. +

[bar][foo!]

+```````````````````````````````` + + +[Link labels] cannot contain brackets, unless they are +backslash-escaped: + +```````````````````````````````` example +[foo][ref[] + +[ref[]: /uri +. +

[foo][ref[]

+

[ref[]: /uri

+```````````````````````````````` + + +```````````````````````````````` example +[foo][ref[bar]] + +[ref[bar]]: /uri +. +

[foo][ref[bar]]

+

[ref[bar]]: /uri

+```````````````````````````````` + + +```````````````````````````````` example +[[[foo]]] + +[[[foo]]]: /url +. +

[[[foo]]]

+

[[[foo]]]: /url

+```````````````````````````````` + + +```````````````````````````````` example +[foo][ref\[] + +[ref\[]: /uri +. +

foo

+```````````````````````````````` + + +Note that in this example `]` is not backslash-escaped: + +```````````````````````````````` example +[bar\\]: /uri + +[bar\\] +. +

bar\

+```````````````````````````````` + + +A [link label] must contain at least one [non-whitespace character]: + +```````````````````````````````` example +[] + +[]: /uri +. +

[]

+

[]: /uri

+```````````````````````````````` + + +```````````````````````````````` example +[ + ] + +[ + ]: /uri +. +

[ +]

+

[ +]: /uri

+```````````````````````````````` + + +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" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +[*foo* bar][] + +[*foo* bar]: /url "title" +. +

foo bar

+```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +[Foo][] + +[foo]: /url "title" +. +

Foo

+```````````````````````````````` + + + +As with full reference links, [whitespace] is not +allowed between the two sets of brackets: + +```````````````````````````````` example +[foo] +[] + +[foo]: /url "title" +. +

foo +[]

+```````````````````````````````` + + +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" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +[*foo* bar] + +[*foo* bar]: /url "title" +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +[[*foo* bar]] + +[*foo* bar]: /url "title" +. +

[foo bar]

+```````````````````````````````` + + +```````````````````````````````` example +[[bar [foo] + +[foo]: /url +. +

[[bar foo

+```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +[Foo] + +[foo]: /url "title" +. +

Foo

+```````````````````````````````` + + +A space after the link text should be preserved: + +```````````````````````````````` example +[foo] bar + +[foo]: /url +. +

foo bar

+```````````````````````````````` + + +If you just want bracketed text, you can backslash-escape the +opening bracket to avoid links: + +```````````````````````````````` example +\[foo] + +[foo]: /url "title" +. +

[foo]

+```````````````````````````````` + + +Note that this is a link, because a link label ends with the first +following closing bracket: + +```````````````````````````````` example +[foo*]: /url + +*[foo*] +. +

*foo*

+```````````````````````````````` + + +Full references take precedence over shortcut references: + +```````````````````````````````` example +[foo][bar] + +[foo]: /url1 +[bar]: /url2 +. +

foo

+```````````````````````````````` + + +In the following case `[bar][baz]` is parsed as a reference, +`[foo]` as normal text: + +```````````````````````````````` example +[foo][bar][baz] + +[baz]: /url +. +

[foo]bar

+```````````````````````````````` + + +Here, though, `[foo][bar]` is parsed as a reference, since +`[bar]` is defined: + +```````````````````````````````` example +[foo][bar][baz] + +[baz]: /url1 +[bar]: /url2 +. +

foobaz

+```````````````````````````````` + + +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 +. +

[foo]bar

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

foo

+```````````````````````````````` + + +```````````````````````````````` example +![foo *bar*] + +[foo *bar*]: train.jpg "train & tracks" +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +![foo ![bar](/url)](/url2) +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +![foo [bar](/url)](/url2) +. +

foo bar

+```````````````````````````````` + + +Though this spec is concerned with parsing, not rendering, it is +recommended that in rendering to HTML, only the plain string content +of the [image description] be used. Note that in +the above example, the alt attribute's value is `foo bar`, not `foo +[bar](/url)` or `foo bar`. Only the plain string +content is rendered, without formatting. + +```````````````````````````````` example +![foo *bar*][] + +[foo *bar*]: train.jpg "train & tracks" +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +![foo *bar*][foobar] + +[FOOBAR]: train.jpg "train & tracks" +. +

foo bar

+```````````````````````````````` + + +```````````````````````````````` example +![foo](train.jpg) +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +My ![foo bar](/path/to/train.jpg "title" ) +. +

My foo bar

+```````````````````````````````` + + +```````````````````````````````` example +![foo]() +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +![](/url) +. +

+```````````````````````````````` + + +Reference-style: + +```````````````````````````````` example +![foo][bar] + +[bar]: /url +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +![foo][bar] + +[BAR]: /url +. +

foo

+```````````````````````````````` + + +Collapsed: + +```````````````````````````````` example +![foo][] + +[foo]: /url "title" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +![*foo* bar][] + +[*foo* bar]: /url "title" +. +

foo bar

+```````````````````````````````` + + +The labels are case-insensitive: + +```````````````````````````````` example +![Foo][] + +[foo]: /url "title" +. +

Foo

+```````````````````````````````` + + +As with reference links, [whitespace] is not allowed +between the two sets of brackets: + +```````````````````````````````` example +![foo] +[] + +[foo]: /url "title" +. +

foo +[]

+```````````````````````````````` + + +Shortcut: + +```````````````````````````````` example +![foo] + +[foo]: /url "title" +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +![*foo* bar] + +[*foo* bar]: /url "title" +. +

foo bar

+```````````````````````````````` + + +Note that link labels cannot contain unescaped brackets: + +```````````````````````````````` example +![[foo]] + +[[foo]]: /url "title" +. +

![[foo]]

+

[[foo]]: /url "title"

+```````````````````````````````` + + +The link labels are case-insensitive: + +```````````````````````````````` example +![Foo] + +[foo]: /url "title" +. +

Foo

+```````````````````````````````` + + +If you just want bracketed text, you can backslash-escape the +opening `!` and `[`: + +```````````````````````````````` example +\!\[foo] + +[foo]: /url "title" +. +

![foo]

+```````````````````````````````` + + +If you want a link after a literal `!`, backslash-escape the +`!`: + +```````````````````````````````` example +\![foo] + +[foo]: /url "title" +. +

!foo

+```````````````````````````````` + + +## 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

+```````````````````````````````` + + +```````````````````````````````` example + +. +

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

+```````````````````````````````` + + +```````````````````````````````` example + +. +

irc://foo.bar:2233/baz

+```````````````````````````````` + + +Uppercase is also fine: + +```````````````````````````````` example + +. +

MAILTO:FOO@BAR.BAZ

+```````````````````````````````` + + +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

+```````````````````````````````` + + +```````````````````````````````` example + +. +

made-up-scheme://foo,bar

+```````````````````````````````` + + +```````````````````````````````` example + +. +

http://../

+```````````````````````````````` + + +```````````````````````````````` example + +. +

localhost:5001/foo

+```````````````````````````````` + + +Spaces are not allowed in autolinks: + +```````````````````````````````` example + +. +

<http://foo.bar/baz bim>

+```````````````````````````````` + + +Backslash-escapes do not work inside autolinks: + +```````````````````````````````` example + +. +

http://example.com/\[\

+```````````````````````````````` + + +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

+```````````````````````````````` + + +```````````````````````````````` example + +. +

foo+special@Bar.baz-bar0.com

+```````````````````````````````` + + +Backslash-escapes do not work inside email autolinks: + +```````````````````````````````` example + +. +

<foo+@bar.example.com>

+```````````````````````````````` + + +These are not autolinks: + +```````````````````````````````` example +<> +. +

<>

+```````````````````````````````` + + +```````````````````````````````` example +< http://foo.bar > +. +

< http://foo.bar >

+```````````````````````````````` + + +```````````````````````````````` example + +. +

<m:abc>

+```````````````````````````````` + + +```````````````````````````````` example + +. +

<foo.bar.baz>

+```````````````````````````````` + + +```````````````````````````````` example +http://example.com +. +

http://example.com

+```````````````````````````````` + + +```````````````````````````````` example +foo@bar.example.com +. +

foo@bar.example.com

+```````````````````````````````` + + +## 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 ``. + +An [HTML comment](@) consists of ``, +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 ``, and the string +`?>`. + +A [declaration](@) consists of the +string ``, and the character `>`. + +A [CDATA section](@) consists of +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 + +. +

+```````````````````````````````` + + +Empty elements: + +```````````````````````````````` example + +. +

+```````````````````````````````` + + +[Whitespace] is allowed: + +```````````````````````````````` example + +. +

+```````````````````````````````` + + +With attributes: + +```````````````````````````````` example + +. +

+```````````````````````````````` + + +Custom tag names can be used: + +```````````````````````````````` example +Foo +. +

Foo

+```````````````````````````````` + + +Illegal tag names, not parsed as HTML: + +```````````````````````````````` example +<33> <__> +. +

<33> <__>

+```````````````````````````````` + + +Illegal attribute names: + +```````````````````````````````` example +
+. +

<a h*#ref="hi">

+```````````````````````````````` + + +Illegal attribute values: + +```````````````````````````````` example +
+. +

</a href="foo">

+```````````````````````````````` + + +Comments: + +```````````````````````````````` example +foo +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +foo +. +

foo <!-- not a comment -- two hyphens -->

+```````````````````````````````` + + +Not comments: + +```````````````````````````````` example +foo foo --> + +foo +. +

foo <!--> foo -->

+

foo <!-- foo--->

+```````````````````````````````` + + +Processing instructions: + +```````````````````````````````` example +foo +. +

foo

+```````````````````````````````` + + +Declarations: + +```````````````````````````````` example +foo +. +

foo

+```````````````````````````````` + + +CDATA sections: + +```````````````````````````````` example +foo &<]]> +. +

foo &<]]>

+```````````````````````````````` + + +Entity and numeric character references are preserved in HTML +attributes: + +```````````````````````````````` example +foo
+. +

foo

+```````````````````````````````` + + +Backslash escapes do not work in HTML attributes: + +```````````````````````````````` example +foo +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example + +. +

<a href=""">

+```````````````````````````````` + + +## 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 `
` tag): + +```````````````````````````````` example +foo +baz +. +

foo
+baz

+```````````````````````````````` + + +For a more visible alternative, a backslash before the +[line ending] may be used instead of two spaces: + +```````````````````````````````` example +foo\ +baz +. +

foo
+baz

+```````````````````````````````` + + +More than two spaces can be used: + +```````````````````````````````` example +foo +baz +. +

foo
+baz

+```````````````````````````````` + + +Leading spaces at the beginning of the next line are ignored: + +```````````````````````````````` example +foo + bar +. +

foo
+bar

+```````````````````````````````` + + +```````````````````````````````` example +foo\ + bar +. +

foo
+bar

+```````````````````````````````` + + +Line breaks can occur inside emphasis, links, and other constructs +that allow inline content: + +```````````````````````````````` example +*foo +bar* +. +

foo
+bar

+```````````````````````````````` + + +```````````````````````````````` example +*foo\ +bar* +. +

foo
+bar

+```````````````````````````````` + + +Line breaks do not occur inside code spans + +```````````````````````````````` example +`code +span` +. +

code span

+```````````````````````````````` + + +```````````````````````````````` example +`code\ +span` +. +

code\ span

+```````````````````````````````` + + +or HTML tags: + +```````````````````````````````` example +
+. +

+```````````````````````````````` + + +```````````````````````````````` example + +. +

+```````````````````````````````` + + +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\ +. +

foo\

+```````````````````````````````` + + +```````````````````````````````` example +foo +. +

foo

+```````````````````````````````` + + +```````````````````````````````` example +### foo\ +. +

foo\

+```````````````````````````````` + + +```````````````````````````````` example +### foo +. +

foo

+```````````````````````````````` + + +## 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 +. +

foo +baz

+```````````````````````````````` + + +Spaces at the end of the line and beginning of the next line are +removed: + +```````````````````````````````` example +foo + baz +. +

foo +baz

+```````````````````````````````` + + +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 +. +

hello $.;'there

+```````````````````````````````` + + +```````````````````````````````` example +Foo χρῆν +. +

Foo χρῆν

+```````````````````````````````` + + +Internal spaces are preserved verbatim: + +```````````````````````````````` example +Multiple spaces +. +

Multiple spaces

+```````````````````````````````` + + + + +# 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 `![` + +we insert a text node with these symbols as its literal content, and we +add a pointer to this text node to the [delimiter stack](@). + +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. +