Markdown.pl: allow documented single-quote ref titles

[markdown.git] / syntax.md

================
Markdown: Syntax
================

* [Markdown Basics]
* _[Syntax]( "Markdown Syntax Documentation")_
* [License]

- - - - -

*   [Overview]
    *   [Philosophy]
    *   [Inline HTML]
    *   [Automatic Escaping for Special Characters]
*   [Block Elements]
    *   [Paragraphs and Line Breaks]
    *   [Headers]
    *   [Blockquotes]
    *   [Lists]
    *   [Tables]
    *   [Style Sheet]
    *   [Code Blocks]
    *   [Horizontal Rules]
*   [Span Elements]
    *   [Links]
    *   [Emphasis]
    *   [Code]
    *   [Images]
*   [Miscellaneous]
    *   [Backslash Escapes]
    *   [Automatic Links]


**Note:** This document is itself written using Markdown; you
can [see the source for it by adding `.md` to the URL][src].

  [markdown basics]: basics.html  "Markdown Basics"
  [license]: license.html "License Information"
  [src]: syntax.md

- - - - -

--------
Overview
--------

~~~~~~~~~~
Philosophy
~~~~~~~~~~

Markdown is intended to be as easy-to-read and easy-to-write as is feasible.

Readability, however, is emphasized above all else. A Markdown-formatted
document should be publishable as-is, as plain text, without looking
like it's been marked up with tags or formatting instructions. While
Markdown's syntax has been influenced by several existing text-to-HTML
filters -- including [Setext] [1], [atx] [2], [Textile] [3], [reStructuredText] [4],
[Grutatext] [5], and [EtText] [6] -- the single biggest source of
inspiration for Markdown's syntax is the format of plain text email.

  [1]: http://docutils.sourceforge.net/mirror/setext.html
  [2]: http://www.aaronsw.com/2002/atx/
  [3]: http://textism.com/tools/textile/
  [4]: http://docutils.sourceforge.net/rst.html
  [5]: http://www.triptico.com/software/grutatxt.html
  [6]: http://ettext.taint.org/doc/

To this end, Markdown's syntax is comprised entirely of punctuation
characters, which punctuation characters have been carefully chosen so
as to look like what they mean. E.g., asterisks around a word actually
look like \*emphasis\*. Markdown lists look like, well, lists. Even
blockquotes look like quoted passages of text, assuming you've ever
used email.


~~~~~~~~~~~
Inline HTML
~~~~~~~~~~~

Markdown's syntax is intended for one purpose: to be used as a
format for *writing* for the web.

Markdown is not a replacement for HTML, or even close to it. Its
syntax is very small, corresponding only to a very small subset of
HTML tags. The idea is *not* to create a syntax that makes it easier
to insert HTML tags. In my opinion, HTML tags are already easy to
insert. The idea for Markdown is to make it easy to read, write, and
edit prose. HTML is a *publishing* format; Markdown is a *writing*
format. Thus, Markdown's formatting syntax only addresses issues that
can be conveyed in plain text.

For any markup that is not covered by Markdown's syntax, you simply
use HTML itself. There's no need to preface it or delimit it to
indicate that you're switching from Markdown to HTML; you just use
the tags.

The only restrictions are that block-level HTML elements -- e.g. `<div>`,
`<table>`, `<pre>`, `<p>`, etc. -- must be separated from surrounding
content by blank lines, and the start and end tags of the block should
not be indented with tabs or spaces. Markdown is smart enough not
to add extra (unwanted) `<p>` tags around HTML block-level tags.

For example, to add an HTML table to a Markdown article:

      This is a regular paragraph.

      <table>
          <tr>
              <td>Foo</td>
          </tr>
      </table>

      This is another regular paragraph.

Note that Markdown formatting syntax is not processed within block-level
HTML tags. E.g., you can't use Markdown-style `*emphasis*` inside an
HTML block.

Span-level HTML tags -- e.g. `<span>`, `<cite>`, or `<del>` -- can be
used anywhere in a Markdown paragraph, list item, or header. If you
want, you can even use HTML tags instead of Markdown formatting; e.g. if
you'd prefer to use HTML `<a>` or `<img>` tags instead of Markdown's
link or image syntax, go right ahead.

Unlike block-level HTML tags, Markdown syntax *is* processed within
span-level tags.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Automatic Escaping for Special Characters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In HTML, there are two characters that demand special treatment: `<`
and `&`. Left angle brackets are used to start tags; ampersands are
used to denote HTML entities. If you want to use them as literal
characters, you must escape them as entities, e.g. `&lt;`, and
`&amp;`.

Ampersands in particular are bedeviling for web writers. If you want to
write about 'AT&T', you need to write '`AT&amp;T`'. You even need to
escape ampersands within URLs. Thus, if you want to link to:

      http://images.google.com/images?num=30&q=larry+bird

you need to encode the URL as:

      http://images.google.com/images?num=30&amp;q=larry+bird

in your anchor tag `href` attribute. Needless to say, this is easy to
forget, and is probably the single most common source of HTML validation
errors in otherwise well-marked-up web sites.

Markdown allows you to use these characters naturally, taking care of
all the necessary escaping for you. If you use an ampersand as part of
an HTML entity, it remains unchanged; otherwise it will be translated
into `&amp;`.

So, if you want to include a copyright symbol in your article, you can write:

      &copy;

and Markdown will leave it alone. But if you write:

      AT&T

Markdown will translate it to:

      AT&amp;T

Similarly, because Markdown supports [inline HTML](#html), if you use
angle brackets as delimiters for HTML tags, Markdown will treat them as
such. But if you write:

      4 < 5

Markdown will translate it to:

      4 &lt; 5

However, inside Markdown code spans and blocks, angle brackets and
ampersands are *always* encoded automatically. This makes it easy to use
Markdown to write about HTML code. (As opposed to raw HTML, which is a
terrible format for writing about HTML syntax, because every single `<`
and `&` in your example code needs to be escaped.)


- - - - -

--------------
Block Elements
--------------

~~~~~~~~~~~~~~~~~~~~~~~~~~
Paragraphs and Line Breaks
~~~~~~~~~~~~~~~~~~~~~~~~~~

A paragraph is simply one or more consecutive lines of text, separated
by one or more blank lines.  (A blank line is any line that looks like a
blank line -- a line containing nothing but spaces or tabs is considered
blank.)  Normal paragraphs should not be indented with spaces or tabs.
Note that Markdown expands all tabs to spaces before doing anything else.

The implication of the "one or more consecutive lines of text" rule is
that Markdown supports "hard-wrapped" text paragraphs. This differs
significantly from most other text-to-HTML formatters (including Movable
Type's "Convert Line Breaks" option) which translate every line break
character in a paragraph into a `<br />` tag.

When you *do* want to insert a `<br />` break tag using Markdown, you
end a line with two or more spaces, then type return.

Yes, this takes a tad more effort to create a `<br />`, but a simplistic
"every line break is a `<br />`" rule wouldn't work for Markdown.
Markdown's email-style [blockquoting][bq] and multi-paragraph [list items][l]
work best -- and look better -- when you format them with hard breaks.

  [bq]: #blockquote
  [l]:  #list


~~~~~~~
Headers
~~~~~~~

Markdown supports two styles of headers, [Setext] [1] and [atx] [2].

Setext-style headers are "underlined" using equal signs (for first-level
headers), dashes (for second-level headers) and tildes (for third-level
headers). For example:

      This is an H1
      =============

      This is an H2
      -------------

      This is an H3
      ~~~~~~~~~~~~~

Any number of underlining `=`'s, `-`'s or `~`'s will work.  An optional
matching "overline" may precede the header like so:

      =============
      This is an H1
      =============

      -------------
      This is an H2
      -------------

      ~~~~~~~~~~~~~
      This is an H3
      ~~~~~~~~~~~~~

Atx-style headers use 1-6 hash characters at the start of the line,
corresponding to header levels 1-6. For example:

      # This is an H1

      ## This is an H2

      ###### This is an H6

Optionally, you may "close" atx-style headers. This is purely
cosmetic -- you can use this if you think it looks better. The
closing hashes don't even need to match the number of hashes
used to open the header. (The number of opening hashes
determines the header level.) :

      # This is an H1 #

      ## This is an H2 ##

      ### This is an H3 ######


~~~~~~~~~~~
Blockquotes
~~~~~~~~~~~

Markdown uses email-style `>` characters for blockquoting. If you're
familiar with quoting passages of text in an email message, then you
know how to create a blockquote in Markdown. It looks best if you hard
wrap the text and put a `>` before every line:

      > This is a blockquote with two paragraphs. Lorem ipsum dolor sit amet,
      > consectetuer adipiscing elit. Aliquam hendrerit mi posuere lectus.
      > Vestibulum enim wisi, viverra nec, fringilla in, laoreet vitae, risus.
      >
      > Donec sit amet nisl. Aliquam semper ipsum sit amet velit. Suspendisse
      > id sem consectetuer libero luctus adipiscing.

Markdown allows you to be lazy and only put the `>` before the first
line of a hard-wrapped paragraph:

      > This is a blockquote with two paragraphs. Lorem ipsum dolor sit amet,
      consectetuer adipiscing elit. Aliquam hendrerit mi posuere lectus.
      Vestibulum enim wisi, viverra nec, fringilla in, laoreet vitae, risus.

      > Donec sit amet nisl. Aliquam semper ipsum sit amet velit. Suspendisse
      id sem consectetuer libero luctus adipiscing.

Blockquotes can be nested (i.e. a blockquote-in-a-blockquote) by
adding additional levels of `>`:

      > This is the first level of quoting.
      >
      > > This is nested blockquote.
      >
      > Back to the first level.

Blockquotes can contain other Markdown elements, including headers, lists,
and code blocks:

      > ## This is a header.
      >
      > 1.  This is the first list item.
      > 2.  This is the second list item.
      >
      > Here's some example code:
      >
      >     return shell_exec("echo $input | $markdown_script");

Any decent text editor should make email-style quoting easy. For
example, with BBEdit, you can make a selection and choose Increase
Quote Level from the Text menu.


~~~~~
Lists
~~~~~

Markdown supports ordered (numbered, lettered or roman numeraled)
and unordered (bulleted) lists.

Unordered lists use asterisks, pluses, and hyphens -- interchangably
-- as list markers:

      *   Red
      *   Green
      *   Blue

is equivalent to:

      +   Red
      +   Green
      +   Blue

and:

      -   Red
      -   Green
      -   Blue

Ordered lists use numbers or letters (latin or greek) or roman numerals
followed by a period or right parenthesis `)`:

      1.  Bird
      2.  McHale
      3.  Parish

It's important to note that the actual numbers (or letters or roman
numerals) you use to mark the list *do* have an effect on the HTML
output Markdown produces, but only if you skip ahead and/or change
the list marker style.

The HTML Markdown produces from the above list is:

      <ol>
      <li>Bird</li>
      <li>McHale</li>
      <li>Parish</li>
      </ol>

If you instead wrote the list in Markdown like this:

      1.  Bird
      1.  McHale
      1.  Parish

or even:

      3. Bird
      1. McHale
      8. Parish

you'd get the exact same HTML output in the first case, but in the
second case the numbers would be in the sequence 3, 4 and 8 because
you are only allowed to skip ahead (and the first item in the list
must be numbered at least 0 [or `a`, `i`, etc.]).

The point is, if you want to, you can use ordinal numbers in your
ordered Markdown lists, so that the numbers in your source match the
numbers in your published HTML.  But if you want to be lazy, you don't
have to.

The style of the list marker is determined by the first list item.
If the first list item uses numbers the list style will be `decimal`.
If the first list item uses a roman numeral then the list style will
be either `lower-roman` or `upper-roman` depending on the case used.
Similarly for any non-roman letter you get `lower-alpha`, `upper-alpha`
or `lower-greek`.

However, if later list items change the style, an attempt is made to
modify the list numbering style for that item which should be effective
in just about any browser available today.

Similarly if a list item "skips ahead" an attempt is made to skip the
list number ahead which again should be effective in just about any
browser available today.

A right parenthesis ')' may be used in place of the `.` for any of the
numbering styles but it requires the [style sheet] to be included or
you will end up just seeing `.` instead.  For example this list:

      a)  Alpha
      b)  Beta
      c)  Gamma

will end up being displayed like this without the [style sheet]:

      a.  Alpha
      b.  Beta
      c.  Gamma

If you do use lazy list numbering, however, you should still start the
list with the number 1 (or letter A or a or roman numeral I or i) or even
a higher number if desired and then stick with that number (or letter) for
the rest of the items.  Since you may only skip forward in the numbering,
the items will end up numbered (or "lettered") starting with the value
used for the first item.

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.

Attempts to change an unordered list's style or switch from an ordered
list to an unordered list (or vice versa) in mid-list are ignored.

Lists end when the first non-blank, non-indented line (relative to the
current list nesting level) is encountered that does not begin with a
list marker.

To create two distinct lists when there are only blank lines between the
end of the first list and the start of the second, a separator line must
be inserted.  ([Horizontal rules] work just fine for this).

If desired, an HTML-style comment (e.g. `<!-- -->`) may be used for this
purpose provided it is preceded and followed by at least one blank line.

Any non-list-marker, non-blank, non-indented (relative to the current
list nesting level) line may be used for this purpose but the HTML-style
comment has the advantage of not causing anything extra to be shown when
the HTML output is displayed in a browser.

To make lists look nice, you can wrap items with hanging indents:

      *   Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
          Aliquam hendrerit mi posuere lectus. Vestibulum enim wisi,
          viverra nec, fringilla in, laoreet vitae, risus.
      *   Donec sit amet nisl. Aliquam semper ipsum sit amet velit.
          Suspendisse id sem consectetuer libero luctus adipiscing.

But if you want to be lazy, you don't have to:

      *   Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
      Aliquam hendrerit mi posuere lectus. Vestibulum enim wisi,
      viverra nec, fringilla in, laoreet vitae, risus.
      *   Donec sit amet nisl. Aliquam semper ipsum sit amet velit.
      Suspendisse id sem consectetuer libero luctus adipiscing.

If list items are separated by blank lines, Markdown will wrap the
items in `<p>` tags in the HTML output. For example, this input:

      *   Bird
      *   Magic

will turn into:

      <ul>
      <li>Bird</li>
      <li>Magic</li>
      </ul>

But this:

      *   Bird

      *   Magic

will turn into:

      <ul>
      <li><p>Bird</p></li>
      <li><p>Magic</p></li>
      </ul>

List items may consist of multiple paragraphs. Each subsequent
paragraph in a list item must be indented by 4 spaces:

      1.  This is a list item with two paragraphs. Lorem ipsum dolor
          sit amet, consectetuer adipiscing elit. Aliquam hendrerit
          mi posuere lectus.

          Vestibulum enim wisi, viverra nec, fringilla in, laoreet
          vitae, risus. Donec sit amet nisl. Aliquam semper ipsum
          sit amet velit.

      2.  Suspendisse id sem consectetuer libero luctus adipiscing.

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

      *   This is a list item with two paragraphs.

          This is the second paragraph in the list item. You're
      only required to indent the first line. Lorem ipsum dolor
      sit amet, consectetuer adipiscing elit.

      *   Another item in the same list.

To put a blockquote within a list item, the blockquote's `>`
delimiters need to be indented:

      *   A list item with a blockquote:

          > This is a blockquote
          > inside a list item.

To put a code block within a list item, the code block needs
to be indented *twice* (in other words 8 spaces):

      *   A list item with a code block:

              <code goes here>


It's worth noting that it's possible to trigger an ordered list by
accident, by writing something like this:

      1986. What a great season.

In other words, a *number-period-space* sequence at the beginning of a
line. To avoid this, you can backslash-escape the period:

      1986\. What a great season.

Markdown tries to be smart about this and requires either a blank line
before something that looks like a list item or requires that a list
definition is already active or requires that two lines in a row look
like list items in order for Markdown to recognize a list item.

So the above, by itself without the escaped ".", will not start a list
when it's outside of any list unless it's preceded by a blank line or
immediately followed by another line that looks like a list item (either
of the same kind or of a sublist).


~~~~~~
Tables
~~~~~~

Markdown supports simple tables like so:

    | Item | Price | Description |
    | ---- | -----:| ----------- |
    | Nut  | $1.29 | Delicious   |
    | Bean | $0.37 | Fiber       |

Output:

    <table>
      <tr><th>Item</th><th align="right">Price</th><th>Description</th></tr>
      <tr><td>Nut</td><td align="right">$1.29</td><td>Delicious</td></tr>
      <tr><td>Bean</td><td align="right">$0.37</td><td>Fiber</td></tr>
    </table>

The leading and trailing `|` on each line are optional unless there is only
a single column in which case at least one `|` is always required -- two if
the single column contains only whitespace.

Leading and trailing whitespace are always trimmed from each column's value
before using it.

To include a literal `|` (veritical bar) character in a column's value, precede
it with a `\` (backslash).  To include a literal `\` use `\\` (double them).

The number of columns in the separator row must match exactly the number of
columns in the header row in order for the table to be recognized.

Each separator in the separator line must be one or more `-` (dash) characters
optionally with a `:` (colon) on either or both ends.  With no colons the
column alignment will be the default.  With a colon only on the left the
alignment will be `left`.  With a colon only on the right the alignment will
be `right`.  And finally, with a colon on both ends the alignment will be
`center`.  The alignment will be applied to the column in both header and body
rows.

Body rows that contain fewer columns than the header row have empty columns
added.  Body rows that contain more columns than the header row have the
extra columns dropped.

The vertical bars do not need to be lined up, sloppy tables work just fine.
The above example could be rewritten like so:

    Item|Price|Description
    -|-:|-
    Nut|$1.29|Delicious
    Bean|$0.37|Fiber

Inline markup is recognized just fine within each column:

    |Example
    |:-
    |~~Strikeout~~ `code` _etc._


~~~~~~~~~~~
Style Sheet
~~~~~~~~~~~

If an unordered list item begins with `[ ]` or `[x]` then its bullet will
be suppressed and a nice checkbox shown instead.  In order for the fancy
checkboxes to show the markdown style sheet must be included.

It may be included in the output with the `--show-stylesheet` option.
To get just the style sheet, run `Markdown.pl` with no arguments with the
input redirected to `/dev/null`.  Without the style sheet these items
will show normally (i.e. with a bullet and as `[ ]` or `[x]`).

Ordered lists that make use of a `)` instead of a `.` to terminate the
marker also require the style sheet otherwise they will display with
the normal `.` marker termination.


~~~~~~~~~~~
Code Blocks
~~~~~~~~~~~

Pre-formatted code blocks are used for writing about programming or
markup source code. Rather than forming normal paragraphs, the lines
of a code block are interpreted literally. Markdown wraps a code block
in both `<pre>` and `<code>` tags.

To produce a code block in Markdown, simply indent every line of the
block by at least 4 spaces.  Alternatively precede the block with a
line consisting of 3 backtick quotes (or more) and follow it with a
line consisting of the same number of backtick quotes -- in this case the
code lines themselves do not require any additional indentation.
For example, given this input:

      This is a normal paragraph:

          This is a code block.

Or this equivalent input:

      This is a normal paragraph.

      ```
      This is a code block.
      ```

Markdown will generate:

      <p>This is a normal paragraph:</p>

      <pre><code>This is a code block.
      </code></pre>

Note that when using the 3 backtick quotes technique, the blank line
before the start of the code block is optional. One level of
indentation -- 4 spaces -- is removed from each line of the code block
unless the 3 backtick quotes are used.  For example, this:

      Here is an example of AppleScript:

          tell application "Foo"
              beep
          end tell

will turn into:

      <p>Here is an example of AppleScript:</p>

      <pre><code>tell application "Foo"
          beep
      end tell
      </code></pre>

A code block continues until it reaches a line that is not indented
(or the end of the article) when using the indentation technique or
until a line consisting of the same number of backtick quotes is found
when using the 3 backtick quotes technique.

Note that the 3 backtick quotes (or more) must appear at the beginning
of the line.  To include a code block within a list (or other indented
element), the indentation technique must be used.

Also note that within a backticks-delimited code block, tab characters
are always expanded with the tab stop locations 8 characters apart.

Within a code block, ampersands (`&`) and angle brackets (`<` and `>`)
are automatically converted into HTML entities. This makes it very
easy to include example HTML source code using Markdown -- just paste
it and indent it, and Markdown will handle the hassle of encoding the
ampersands and angle brackets. For example, this:

      <div class="footer">
          &copy; 2004 Foo Corporation
      </div>

will turn into:

      <pre><code>&lt;div class="footer"&gt;
          &amp;copy; 2004 Foo Corporation
      &lt;/div&gt;
      </code></pre>

Regular Markdown syntax is not processed within code blocks. E.g.,
asterisks are just literal asterisks within a code block. This means
it's also easy to use Markdown to write about Markdown's own syntax.


~~~~~~~~~~~~~~~~
Horizontal Rules
~~~~~~~~~~~~~~~~

You can produce a horizontal rule tag (`<hr />`) by placing three or
more hyphens, asterisks, or underscores on a line by themselves. If you
wish, you may use spaces between the hyphens or asterisks. Each of the
following lines will produce a horizontal rule:

      * * *

      ***

      *****

      - - -

      ---------------------------------------


- - - - -


-------------
Span Elements
-------------

~~~~~
Links
~~~~~

Markdown supports two style of links: *inline* and *reference*.

In both styles, the link text is delimited by [square brackets].

To create an inline link, use a set of regular parentheses immediately
after the link text's closing square bracket. Inside the parentheses,
put the URL where you want the link to point, along with an *optional*
title for the link, surrounded in quotes. For example:

      This is [an example](http://example.com/ "Title") inline link.

      [This link](http://example.net/) has no title attribute.

Will produce:

      <p>This is <a href="http://example.com/" title="Title">
      an example</a> inline link.</p>

      <p><a href="http://example.net/">This link</a> has no
      title attribute.</p>

If you're referring to a local resource on the same server, you can
use relative paths:

      See my [About](/about/) page for details.

Reference-style links use a second set of square brackets, inside
which you place a label of your choosing to identify the link:

      This is [an example][id] reference-style link.

You can optionally use a space to separate the sets of brackets:

      This is [an example] [id] reference-style link.

Then, anywhere in the document, you define your link label like this,
on a line by itself:

      [id]: http://example.com/  "Optional Title Here"

That is:

*   Square brackets containing the link identifier (optionally
    indented from the left margin using up to three spaces);
*   followed by a colon;
*   followed by one or more spaces (or tabs);
*   followed by the URL for the link;
*   optionally followed by a title attribute for the link, enclosed
    in double or single quotes, or enclosed in parentheses.

The following three link definitions are equivalent:

      [foo]: http://example.com/  "Optional Title Here"
      [foo]: http://example.com/  'Optional Title Here'
      [foo]: http://example.com/  (Optional Title Here)

**Note:** There is a known bug in Markdown.pl 1.0.3 which prevents
single quotes from being used to delimit link titles.

The link URL may, optionally, be surrounded by angle brackets:

      [id]: <http://example.com/>  "Optional Title Here"

You can put the title attribute on the next line and use extra spaces
or tabs for padding, which tends to look better with longer URLs:

      [id]: http://example.com/longish/path/to/resource/here
            "Optional Title Here"

Link definitions are only used for creating links during Markdown
processing, and are stripped from your document in the HTML output.

Link definition names may consist of letters, numbers, spaces, and
punctuation -- but they are *not* case sensitive. E.g. these two
links:

      [link text][a]
      [link text][A]

are equivalent.

The *implicit link name* shortcut allows you to omit the name of the
link, in which case the link text itself is used as the name.
Just use an empty set of square brackets (or none) -- e.g., to link the
word "Google" to the google.com web site, you could simply write:

      [Google][]

Or even just this:

      [Google]

And then define the link:

      [Google]: http://google.com/

Because link names may contain spaces, this shortcut even works for
multiple words in the link text:

      Visit [Daring Fireball] for more information.

And then define the link:

      [Daring Fireball]: http://daringfireball.net/

Text inside square brackets is left completely unchanged (including the
surrounding brackets) _unless_ it matches a link definition.  Furthermore,
the single pair of surrounding square brackets case is always checked
for last so you may only omit the trailing `[]` of an *implicit link name*
shortcut when the result would still be unambiguous.

Link definitions can be placed anywhere in your Markdown document. I
tend to put them immediately after each paragraph in which they're
used, but if you want, you can put them all at the end of your
document, sort of like footnotes.

All first, second and third level headers defined at the top-level
(in other words they are not in lists and start at the left margin)
using either the setext-style or atx-style automatically have an
anchor id and link definition added for them provided there is not
already a previous definition with the same id.  You can use this
to place a table-of-contents at the top of the document that links
to subsections later in the document.  Just like this document.

For example, all six of these links point to subsections later in
the same document:

      * Self Same
        * [Introduction]
        * [Part Two]
        * [Part Three]
      * Different
        * [Introduction](#Part-Two)
        * [Part Two](#Part_Three)
        * [Part Three](#introduction)

      ## Introduction

      ## Part Two

      ## Part Three

Here's an example of reference links in action:

      I get 10 times more traffic from [Google] [1] than from
      [Yahoo] [2] or [MSN] [3].

      [1]: http://google.com/        "Google"
      [2]: http://search.yahoo.com/  "Yahoo Search"
      [3]: http://search.msn.com/    "MSN Search"

Using the implicit link name shortcut, you could instead write:

      I get 10 times more traffic from [Google] than from
      [Yahoo] or [MSN].

      [google]: http://google.com/        "Google"
      [yahoo]:  http://search.yahoo.com/  "Yahoo Search"
      [msn]:    http://search.msn.com/    "MSN Search"

Both of the above examples will produce the following HTML output:

      <p>I get 10 times more traffic from <a href="http://google.com/"
      title="Google">Google</a> than from
      <a href="http://search.yahoo.com/" title="Yahoo Search">Yahoo</a>
      or <a href="http://search.msn.com/" title="MSN Search">MSN</a>.</p>

For comparison, here is the same paragraph written using
Markdown's inline link style:

      I get 10 times more traffic from [Google](http://google.com/ "Google")
      than from [Yahoo](http://search.yahoo.com/ "Yahoo Search") or
      [MSN](http://search.msn.com/ "MSN Search").

The point of reference-style links is not that they're easier to
write. The point is that with reference-style links, your document
source is vastly more readable. Compare the above examples: using
reference-style links, the paragraph itself is only 81 characters
long; with inline-style links, it's 176 characters; and as raw HTML,
it's 234 characters. In the raw HTML, there's more markup than there
is text.

With Markdown's reference-style links, a source document much more
closely resembles the final output, as rendered in a browser. By
allowing you to move the markup-related metadata out of the paragraph,
you can add links without interrupting the narrative flow of your
prose.


~~~~~~~~
Emphasis
~~~~~~~~

Markdown treats asterisks (`*`) and underscores (`_`) as indicators of
emphasis. Text wrapped with one `*` or `_` will be wrapped with an
HTML `<em>` tag; double `*`'s or `_`'s will be wrapped with an HTML
`<strong>` tag. Double `~`'s will be wrapped with an HTML `<strike>` tag.
E.g., this input:

      *single asterisks*

      _single underscores_

      **double asterisks**

      __double underscores__

      ~~double tildes~~

will produce:

      <em>single asterisks</em>

      <em>single underscores</em>

      <strong>double asterisks</strong>

      <strong>double underscores</strong>

      <strike>strike through</strike>

You can use whichever style you prefer; the lone restriction is that
the same character must be used to open and close an emphasis span.
Additionally `_` and double `_` are not recognized within words.

Emphasis using `*`'s or `~`'s can be used in the middle of a word:

      un*frigging*believable fan~~frigging~~tastic

But if you surround an `*`, `_` or `~` with spaces, it'll be treated as a
literal asterisk, underscore or tilde.

To produce a literal asterisk, underscore or tilde at a position where it
would otherwise be used as an emphasis delimiter, you can backslash
escape it:

      \*this text is surrounded by literal asterisks\*


~~~~
Code
~~~~

To indicate a span of code, wrap it with backtick quotes (`` ` ``).
Unlike a pre-formatted code block, a code span indicates code within a
normal paragraph. For example:

      Use the `printf()` function.

will produce:

      <p>Use the <code>printf()</code> function.</p>

To include a literal backtick character within a code span, you can use
multiple backticks as the opening and closing delimiters:

      ``There is a literal backtick (`) here.``

which will produce this:

      <p><code>There is a literal backtick (`) here.</code></p>

The backtick delimiters surrounding a code span may include spaces --
one after the opening, one before the closing. This allows you to place
literal backtick characters at the beginning or end of a code span:

      A single backtick in a code span: `` ` ``

      A backtick-delimited string in a code span: `` `foo` ``

will produce:

      <p>A single backtick in a code span: <code>`</code></p>

      <p>A backtick-delimited string in a code span: <code>`foo`</code></p>

With a code span, ampersands and angle brackets are encoded as HTML
entities automatically, which makes it easy to include example HTML
tags. Markdown will turn this:

      Please don't use any `<blink>` tags.

into:

      <p>Please don't use any <code>&lt;blink&gt;</code> tags.</p>

You can write this:

      `&#8212;` is the decimal-encoded equivalent of `&mdash;`.

to produce:

      <p><code>&amp;#8212;</code> is the decimal-encoded
      equivalent of <code>&amp;mdash;</code>.</p>


~~~~~~
Images
~~~~~~

Admittedly, it's fairly difficult to devise a "natural" syntax for
placing images into a plain text document format.

Markdown uses an image syntax that is intended to resemble the syntax
for links, allowing for two styles: *inline* and *reference*.

Inline image syntax looks like this:

      ![Alt text](/path/to/img.jpg)

      ![Alt text](/path/to/img.jpg "Optional title")

That is:

*   An exclamation mark: `!`;
*   followed by a set of square brackets, containing the `alt`
    attribute text for the image;
*   followed by a set of parentheses, containing the URL or path to
    the image, and an optional `title` attribute enclosed in double
    or single quotes.

Reference-style image syntax looks like this:

      ![Alt text][id]

Where "id" is the name of a defined image reference. Image references
are defined using syntax identical to link references:

      [id]: url/to/image  "Optional title attribute"

To specify one or both dimensions of an image, include the dimensions
in parentheses at the end of the title like so:

      [id]: url/to/image  "Optional title attribute (512x342)"

To resize in just one dimension, specify the other as a "?" like so:

      [id]: url/to/image  "Optional title attribute (?x342)"
      [id]: url/to/image  "Optional title attribute (512x?)"

The first dimension sets the "width" attribute and the second
dimension sets the "height" attribute.  The dimensions are then
removed from the "title" attribute.

It's possible to wrap the url when it's specified in a reference.
Both of these examples:

      [id]: url/\
            t\
            o/image
            "Optional title"

      [id]: url/to/image "Optional title"

Produce identical "img" tags.  Only the url can be wrapped and
only when it's in a reference.  The backslash ("\") must be the
last character on the line and the next line (after optional
ignored leading whitespace) must contain at least one additional
character that's part of the URL.

This can be useful for data: urls like so:

      ![image][1]

      [1]: data:image/gif;base64,R0lGODlhFwAXAPMAMf///+7u7t3d3czMzLu7u6qqqp\
           mZmYiIiHd3d2ZmZlVVVURERDMzMyIiIhEREQAAACwAAAAAFwAXAAAExxDISau9Mg\
           She8DURhhHWRLDB26FkSjKqxxFqlbBWOwF4fOGgsCycRkInI+ocEAQNBNWq0caCJ\
           i9aSqqGwwIL4MAsRATeMMMEykYHBLIt7DNHETrAPrBihVwDAh2ansBXygaAj5sa1\
           x7iTUAKomEBU53B0hGVoVMTleEg0hkCD0DJAhwAlVcQT6nLwgHR1liUQNaqgkMDT\
           NWXWkSbS6lZ0eKTUIWuTSbGzlNlkS3LSYksjtPK6YJCzEwNMAgbT9nKBwg6Onq6B\
           EAOw== "title (100x100)"

Thus allowing small amounts of image data to be embedded directly in the
source "text" file with minimal fuss.


- - - - -


-------------
Miscellaneous
-------------

~~~~~~~~~~~~~~~
Automatic Links
~~~~~~~~~~~~~~~

Markdown supports a shortcut style for creating "automatic" links for URLs
and email addresses: simply surround the URL or email address with angle
brackets or don't. What this means is that if you want to show the actual text
of a URL or email address, and also have it be a clickable link, you can do:

      <http://example.com/>

or this:

      http://example.com/

Markdown will turn that into:

      &lt;<a href="http://example.com/">http://example.com/</a>&gt;

or this:

      <a href="http://example.com/">http://example.com/</a>

If Markdown is not quite grabbing the right link when it's not surrounded
by angle brackets then just add the angle brackets to avoid the guessing.

Automatic links for email addresses work similarly, except that
Markdown will also perform a bit of randomized decimal and hex
entity-encoding to help obscure your address from address-harvesting
spambots. For example, Markdown will turn this:

      <address@example.com>

into something like this:

      <a href="&#x6D;&#x61;i&#x6C;&#x74;&#x6F;:&#x61;&#x64;&#x64;&#x72;&#x65;
      &#115;&#115;&#64;&#101;&#120;&#x61;&#109;&#x70;&#x6C;e&#x2E;&#99;&#111;
      &#109;">&#x61;&#x64;&#x64;&#x72;&#x65;&#115;&#115;&#64;&#101;&#120;&#x61;
      &#109;&#x70;&#x6C;e&#x2E;&#99;&#111;&#109;</a>

which will render in a browser as a clickable link to "address@example.com".

(This sort of entity-encoding trick will indeed fool many, if not
most, address-harvesting bots, but it definitely won't fool all of
them. It's better than nothing, but an address published in this way
will probably eventually start receiving spam.)


~~~~~~~~~~~~~~~~~
Backslash Escapes
~~~~~~~~~~~~~~~~~

Markdown allows you to use backslash escapes to generate literal
characters which would otherwise have special meaning in Markdown's
formatting syntax. For example, if you wanted to surround a word
with literal asterisks (instead of an HTML `<em>` tag), you can use
backslashes before the asterisks, like this:

      \*literal asterisks\*

Markdown provides backslash escapes for the following characters:

      \   backslash
      `   backtick
      *   asterisk
      _   underscore
      ~   tilde
      {}  curly braces
      []  square brackets
      ()  parentheses
      #   hash mark
      +   plus sign
      -   minus sign (hyphen)
      .   dot
      !   exclamation mark
      |   vertical bar (escape only needed/recognized in tables)