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Blackfriday: a markdown processor for Go
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# Blackfriday

Blackfriday is a Markdown processor implemented in Go. It is paranoid about its input (so you can safely feed it user-supplied data), it is fast, it supports common extensions (tables, smart punctuation substitutions, etc.), and it is safe for all utf-8 (unicode) input.

HTML output is currently supported, along with Smartypants extensions.

It started as a translation from C of Sundown.

## Installation

Blackfriday is compatible with any modern Go release. With Go 1.7 and git installed:

go get gopkg.in/russross/blackfriday.v2


## Features

All features of Sundown are supported, including:

• Compatibility. The Markdown v1.0.3 test suite passes with the --tidy option. Without --tidy, the differences are mostly in whitespace and entity escaping, where blackfriday is more consistent and cleaner.

• Common extensions, including table support, fenced code blocks, autolinks, strikethroughs, non-strict emphasis, etc.

• Safety. Blackfriday is paranoid when parsing, making it safe to feed untrusted user input without fear of bad things happening. The test suite stress tests this and there are no known inputs that make it crash. If you find one, please let me know and send me the input that does it.

NOTE: "safety" in this context means runtime safety only. In order to protect yourself against JavaScript injection in untrusted content, see this example.

• Fast processing. It is fast enough to render on-demand in most web applications without having to cache the output.

• Thread safety. You can run multiple parsers in different goroutines without ill effect. There is no dependence on global shared state.

• Minimal dependencies. Blackfriday only depends on standard library packages in Go. The source code is pretty self-contained, so it is easy to add to any project, including Google App Engine projects.

• Standards compliant. Output successfully validates using the W3C validation tool for HTML 4.01 and XHTML 1.0 Transitional.

## Extensions

In addition to the standard markdown syntax, this package implements the following extensions:

• Intra-word emphasis supression. The _ character is commonly used inside words when discussing code, so having markdown interpret it as an emphasis command is usually the wrong thing. Blackfriday lets you treat all emphasis markers as normal characters when they occur inside a word.

• Tables. Tables can be created by drawing them in the input using a simple syntax:

Name    | Age
--------|------
Bob     | 27
Alice   | 23

• Fenced code blocks. In addition to the normal 4-space indentation to mark code blocks, you can explicitly mark them and supply a language (to make syntax highlighting simple). Just mark it like this:

​go
func getTrue() bool {
return true
}
​


You can use 3 or more backticks to mark the beginning of the block, and the same number to mark the end of the block.

• Definition lists. A simple definition list is made of a single-line term followed by a colon and the definition for that term.

Cat
: Fluffy animal everyone likes

Internet
: Vector of transmission for pictures of cats


Terms must be separated from the previous definition by a blank line.

• Footnotes. A marker in the text that will become a superscript number; a footnote definition that will be placed in a list of footnotes at the end of the document. A footnote looks like this:

This is a footnote.[^1]

[^1]: the footnote text.

• Autolinking. Blackfriday can find URLs that have not been explicitly marked as links and turn them into links.

• Strikethrough. Use two tildes (~~) to mark text that should be crossed out.

• Hard line breaks. With this extension enabled newlines in the input translate into line breaks in the output. This extension is off by default.

• Smart quotes. Smartypants-style punctuation substitution is supported, turning normal double- and single-quote marks into curly quotes, etc.

• LaTeX-style dash parsing is an additional option, where -- is translated into &ndash;, and --- is translated into &mdash;. This differs from most smartypants processors, which turn a single hyphen into an ndash and a double hyphen into an mdash.

• Smart fractions, where anything that looks like a fraction is translated into suitable HTML (instead of just a few special cases like most smartypant processors). For example, 4/5 becomes <sup>4</sup>&frasl;<sub>5</sub>, which renders as 45.

• MathJaX Support is an additional feature which is supported by many markdown editor. It translate inline math equation quoted by $ and display math block quoted by $$ into MathJax compatible format. hyphen _ won't break LaTeX render within a math element any more. $$ \left[ \begin{array}{a} a^l_1 \\ ⋮ \\ a^l_{d_l} \end{array}\right] = \sigma( \left[ \begin{matrix} w^l_{1,1} & ⋯ & w^l_{1,d_{l-1}} \\ ⋮ & ⋱ & ⋮ \\ w^l_{d_l,1} & ⋯ & w^l_{d_l,d_{l-1}} \\ \end{matrix}\right] · \left[ \begin{array}{x} a^{l-1}_1 \\ ⋮ \\ ⋮ \\ a^{l-1}_{d_{l-1}} \end{array}\right] + \left[ \begin{array}{b} b^l_1 \\ ⋮ \\ b^l_{d_l} \end{array}\right])$\$


## Other renderers

Blackfriday is structured to allow alternative rendering engines. Here are a few of note:

• github_flavored_markdown: provides a GitHub Flavored Markdown renderer with fenced code block highlighting, clickable heading anchor links.

It's not customizable, and its goal is to produce HTML output equivalent to the GitHub Markdown API endpoint, except the rendering is performed locally.

• markdownfmt: like gofmt, but for markdown.

• LaTeX output: renders output as LaTeX.

## Todo

• More unit testing
• Improve unicode support. It does not understand all unicode rules (about what constitutes a letter, a punctuation symbol, etc.), so it may fail to detect word boundaries correctly in some instances. It is safe on all utf-8 input.