MarkDown之一：溯源与基础学习

Posted on Edited on In , Changyan: Symbols count in article: 32k Reading time ≈ 29 mins.

如果只是想用 Markdown 记录一些东西，还有不错的排版，也许只需要了解发明人 JOHN GRUBER 对 Markdown 的定义和坚持就 OK 了：语法简单，书写自由，基本够用。

[目录]

一、资料

  1. JOHN GRUBER：发明人的MarkDown官方站点
  2. Markdown维基：维基百科（中文）
  3. Markdown指南：关于MarkDown的一切
  4. Markdown Community：GitHub上的MarkDown社区
  5. Markdown 简介：简书上一篇介绍，稍显啰嗦

二、溯源

2004 年，苹果工程师 John Gruber 因为编写冗长、费力的 HTML 标签来正确格式化自己的内容感到厌倦，所以他有了一个想法，在 Aaron Swartz 协作下，他设计了简单的书写系统，让基于 Web 文档的内容在原始状态下就易于书写和易于阅读。

定义

by JOHN GRUBER

Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML). Thus, “Markdown” is two things: (1) a plain text formatting syntax; and (2) a software tool, written in Perl, that converts the plain text formatting to HTML.

The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that 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, the single biggest source of inspiration for Markdown’s syntax is the format of plain text email.

by 维基百科

Markdown是一种轻量级标记语言，创始人为约翰·格鲁伯。它允许人们使用易读易写的纯文本格式编写文档，然后转换成有效的XHTML（或者HTML）文档。 由于Markdown的轻量化、易读易写特性，并且对于图片，图表、数学式都有支持，目前许多网站都广泛使用Markdown来撰写帮助文档或是用于论坛上发表消息。如GitHub、Reddit、Diaspora、Stack Exchange、OpenStreetMap 、SourceForge、简书等，甚至还能被用来撰写电子书。

[注] XHTML（可扩展超文本标记语言, eXtensible HyperText Markup Language）：

  • 一种标记语言，表现方式与超文本标记语言（HTML）类似，不过语法上更加严格。
  • 从继承关系上讲，HTML是一种基于标准通用标记语言（SGML）的应用，是一种非常灵活的置标语言，而XHTML则基于可扩展标记语言（XML），XML是SGML的一个子集。XHTML is part of the family of XML markup languages.
  • XHTML1.1为XHTML最后的独立标准，2.0止于草案阶段。
  • XHTML5则是属于HTML5标准的一部分，且名称已改为“以XML序列化的HTML5”（an XML adaptation of the HTML5 specification），而非“可扩展的HTML”。

版权声明

Markdown 1.0.1 (18 KB) — 17 Dec 2004

Copyright (c) 2004, John Gruber
http://daringfireball.net/
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name "Markdown" nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

This software is provided by the copyright holders and contributors "as is" and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. In no event shall the copyright owner or contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.

++中文++：

如果满足以下条件，则允许以源代码和二进制形式重新分发和使用，无论是否经过修改：

  • 重新分发源代码必须保留上述版权声明、此条件列表和以下免责声明。
  • 以二进制形式重新分发必须在随分发提供的文档和/或其他材料中复制上述版权声明、此条件列表和以下免责声明。
  • 未经事先书面许可，不得使用“Markdown”名称或其贡献者的名称来认可或推广源自本软件的产品。

本软件由版权所有者和贡献者“按原样”提供，并且不提供任何明示或暗示的保证，包括但不限于对适销性和特定用途适用性的暗示保证。在任何情况下，版权所有者或贡献者均不对任何直接、间接、偶然、特殊、惩戒性或后果性损害（包括但不限于采购替代商品或服务；使用、数据或利润损失；或业务中断）以任何方式引起的以及任何责任理论，无论是合同、严格责任或因使用本软件而以任何方式引起的侵权（包括疏忽或其他），即使已被告知此类损害的可能性。

注：以上基于BSD许可协议。

BSD许可协议（Berkeley Software Distribution license） 是自由软件中使用最广泛的许可协议之一。BSD许可证被认为是copycenter（中间著作权），介乎标准的copyright与GPL的copyleft之间。"Take it down to the copy center and make as many copies as you want"。可以说，GPL强迫后续版本必须一样是自由软件，BSD的后续版本可以选择要继续是BSD或其他自由软件条款或封闭软件等等。

三、语法要点

（基于 JOHN GRUBER Markdown: Basics）

The examples on this page are written in a before/after style, showing example syntax and the HTML output produced by Markdown.

段落、标题、引用 | Paragraphs, Headers, Blockquotes

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.【正常段落无需用空格或制表符进行缩进】。

Markdown offers two styles of headers: Setext and atx. Setext-style headers for <h1> and <h2> are created by "underlining" with equal signs (=) and hyphens (-), respectively. To create an atx-style header, you put 1-6 hash marks (#) at the beginning of the line -- the number of hashes equals the resulting HTML header level.

注1：Setext - Structure Enhanced text, is a lightweight markup language used to format plain text documents, was first introduced in 1991 for use in electronic newsletter. 注2：atx - In 2002 Aaron Swartz created atx, "the true structured text format". Swartz was a major contributor to Markdown, and author of its html2text translator.

Blockquotes are indicated using email-style '>' angle brackets.

Markdown:

A First Level Header
====================

A Second Level Header
---------------------

Now is the time for all good men to come to the aid of their country. This is just a regular paragraph.

The quick brown fox jumped over the lazy dog's back.

### Header 3

> This is a blockquote.
> 
> This is the second paragraph in the blockquote.
>
> ## This is an H2 in a blockquote

Output:

<h1>A First Level Header</h1>

<h2>A Second Level Header</h2>

<p>Now is the time for all good men to come to the aid of their country. This is just a regular paragraph.</p>

<p>The quick brown fox jumped over the lazy dog's back.</p>

<h3>Header 3</h3>

<blockquote>
    <p>This is a blockquote.</p>
    
    <p>This is the second paragraph in the blockquote.</p>
    
    <h2>This is an H2 in a blockquote</h2>
</blockquote>

强调 | Phrase Emphasis

Markdown uses asterisks and underscores to indicate spans of emphasis.

Markdown:

Some of these words *are emphasized*.
Some of these words _are emphasized also_.

Use two asterisks for **strong emphasis**.
Or, if you prefer, __use two underscores instead__.

Output:

<p>Some of these words <em>are emphasized</em>.
Some of these words <em>are emphasized also</em>.</p>

<p>Use two asterisks for <strong>strong emphasis</strong>.
Or, if you prefer, <strong>use two underscores instead</strong>.</p>

列表 | Lists

Unordered (bulleted) lists use asterisks, pluses, and hyphens (*, +, and -) as list markers. These three markers are interchangable; this:

*   Candy.
*   Gum.
*   Booze.

this:

+   Candy.
+   Gum.
+   Booze.

and this:

-   Candy.
-   Gum.
-   Booze.

all produce the same output:

<ul>
<li>Candy.</li>
<li>Gum.</li>
<li>Booze.</li>
</ul>

Ordered (numbered) lists use regular numbers, followed by periods, as list markers:

1.  Red
2.  Green
3.  Blue

Output:

<ol>
<li>Red</li>
<li>Green</li>
<li>Blue</li>
</ol>

If you put blank lines between items, you'll get <p> tags for the list item text. You can create multi-paragraph list items by ==indenting the paragraphs by 4 spaces or 1 tab==: 【缩进4个空格或1个制表符】

*   A list item.

    With multiple paragraphs.

*   Another item in the list.

Output:

<ul>
<li><p>A list item.</p>
<p>With multiple paragraphs.</p></li>
<li><p>Another item in the list.</p></li>
</ul>

Markdown supports two styles for creating links: inline and reference. With both styles, you use square brackets to delimit the text you want to turn into a link.

Inline-style links use parentheses immediately after the link text. For example:

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

Output:

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

Optionally, you may include a title attribute in the parentheses:

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

Output:

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

Reference-style links allow you to refer to your links by names, which you define elsewhere in your document:

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"

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>

The title attribute is optional. Link names may contain letters, numbers and spaces, but are ==not case sensitive==:

I start my morning with a cup of coffee and [The New York Times][NY Times].

[ny times]: http://www.nytimes.com/

Output:

<p>I start my morning with a cup of coffee and <a href="http://www.nytimes.com/">The New York Times</a>.</p>

图像 | Images

Image syntax is very much like link syntax.

Inline (titles are optional):

![alt text](/path/to/img.jpg "Title")

Reference-style:

![alt text][id]

[id]: /path/to/img.jpg "Title"

Both of the above examples produce the same output:

<img src="/path/to/img.jpg" alt="alt text" title="Title" />

代码 | Code

In a regular paragraph, you can create code span by wrapping text in backtick quotes. Any ampersands (&) and angle brackets (< or >) will automatically be translated into HTML entities. This makes it easy to use Markdown to write about HTML example code:

I strongly recommend against using any `<blink>` tags.

I wish SmartyPants used named entities like `&mdash;`
instead of decimal-encoded entites like `&#8212;`.

Output:

<p>I strongly recommend against using any
<code>&lt;blink&gt;</code> tags.</p>

<p>I wish SmartyPants used named entities like
<code>&amp;mdash;</code> instead of decimal-encoded
entites like <code>&amp;#8212;</code>.</p>

To specify an entire block of pre-formatted code, indent every line of the block by 4 spaces or 1 tab. Just like with code spans, &, <, and > characters will be escaped automatically.

Markdown:

If you want your page to validate under XHTML 1.0 Strict,
you've got to put paragraph tags in your blockquotes:

    <blockquote>
        <p>For example.</p>
    </blockquote>

Output:

<p>If you want your page to validate under XHTML 1.0 Strict,
you've got to put paragraph tags in your blockquotes:</p>

<pre><code>&lt;blockquote&gt;
    &lt;p&gt;For example.&lt;/p&gt;
&lt;/blockquote&gt;
</code></pre>

四、语法详解

（基于JOHN GRUBER Markdown: Syntax）

Overview

哲学 | Philosophy

Markdown is intended to be as easy-to-read and easy-to-write as is feasible.【易读易写】

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. The single biggest source of inspiration for Markdown's syntax is the format of plain text email.

To this end, Markdown's syntax is comprised entirely of punctuation characters,【其语法完全由标点符号组成】 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.

内联HTML | Inline HTML

Markdown's syntax is intended for one purpose: to be used as a format for writing for the web.【一个目的：WEB写作】

The idea is not to create a syntax that makes it easier to insert HTML tags. 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. 【HTML-出版格式，MarkDown-写作格式】 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. 【可直接使用HTML标签（tags）作为MarkDown语法之外的补充】

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, 【块级HTML元素需用空行独立出来】 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 A1</td> <td>Foo B1</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.

自动转义特殊字符 | 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 【折磨人的】 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, 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.)

块元素 | Block Elements

段落与换行符 | 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.

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 and multi-paragraph list items work best -- and look better -- when you format them with hard breaks.

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

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

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

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

Any number of underlining ='s or -'s will work. 【任何个数的=或-都同样OK】

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.) : 【闭合的#仅为装饰】

# This is an H1 #

## This is an H2 ##

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

块引用 | Blockquotes

Markdown uses email-style > characters for blockquoting. 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) 【嵌套】 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");

列表 | Lists

Markdown supports ordered (numbered) and unordered (bulleted【项目符号】) 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 followed by periods:

1.  Bird
2.  McHale
3.  Parish

It's important to note that the actual numbers you use to mark the list have no effect on the HTML output Markdown produces. 【实际序号数字对HTML输出并无影响】 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. 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.

If you do use lazy list numbering, however, you should still start the list with the number 1. At some point in the future, Markdown may support starting ordered lists at an arbitrary number.

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. 【列表标记后面必须跟一个或多个空格或制表符。】

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 either 4 spaces or one tab:

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: 【块引用分隔符(>)需要缩进，以在列表内部使用引用】

*   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 -- 8 spaces or two tabs: 【为在列表内部使用代码块，缩进量需要翻倍（8空格或2制表符）】

*   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: 【使用反斜杠转义句号（.），以避免输出非预期的有序列表】

1986\. What a great season.

代码块 | 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 or 1 tab. For example, given this 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>

One level of indentation -- 4 spaces or 1 tab -- is removed from each line of the code block. 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).

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.

水平分割线 | 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:

* * *

***

*****

- - -

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

跨度元素 | Span Elements

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 【冒号:】;
  • 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. 【title可用双引号、单引号或圆括号】

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 1.0.1 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 【删除】 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 -- e.g., to link the word "Google" to the google.com web site, you could simply write:

[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/

Link definitions can be placed anywhere in your Markdown document. If you want, you can put them all at the end of your document, sort of like footnotes.

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.

With Markdown's reference-style links, a source document much more closely resembles 【接近】 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.

强调 | 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. E.g., this input:

*single asterisks*

_single underscores_

**double asterisks**

__double underscores__

will produce:

<em>single asterisks</em>

<em>single underscores</em>

<strong>double asterisks</strong>

<strong>double underscores</strong>

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.

Emphasis can be used in the middle of a word:

un*frigging*believable

But if you surround an * or _ with spaces, it'll be treated as a literal asterisk or underscore. 【表示强调的定界符（*、_）与被强调部分之间不能有空格】

To produce a literal asterisk or underscore 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\*

代码 | Code

To indicate a span of code, wrap it with backtick quotes (`) 【反引号】. 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: 【要在代码跨度内包含一个反引号字符（`），可用多个反引号作为该代码跨度的定界符】

``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>

图像 | 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: !; 【感叹号】
  • 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"

As of this writing, Markdown has no syntax for specifying the dimensions of an image; if this is important to you, you can simply use regular HTML <img> tags.

其他 | Miscellaneous

Markdown supports a shortcut style for creating "automatic" links for URLs and email addresses: simply surround the URL or email address with angle brackets. 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 this:

<http://example.com/>

Markdown will turn this into:

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

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.)

反斜杠转义 | 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 【反斜杠转义】 for the following characters:

\   backslash
`   backtick
*   asterisk
_   underscore
{}  curly braces
[]  square brackets
()  parentheses
#   hash mark
+   plus sign
-   minus sign (hyphen)
.   dot
!   exclamation mark