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. An experimental LaTeX output engine is also included.

It started as a translation from C of Sundown.

Installation

Blackfriday is compatible with Go 1. If you are using an older release of Go, consider using v1.1 of blackfriday, which was based on the last stable release of Go prior to Go 1. You can find it as a tagged commit on github.

With Go 1 and git installed:

go get github.com/russross/blackfriday

will download, compile, and install the package into your $GOPATH directory hierarchy. Alternatively, you can achieve the same if you import it into a project:

import "github.com/russross/blackfriday"

and go get without parameters.

Usage

For basic usage, it is as simple as getting your input into a byte slice and calling:

output := blackfriday.MarkdownBasic(input)

This renders it with no extensions enabled. To get a more useful feature set, use this instead:

output := blackfriday.MarkdownCommon(input)

Sanitize untrusted content

Blackfriday itself does nothing to protect against malicious content. If you are dealing with user-supplied markdown, we recommend running blackfriday's output through HTML sanitizer such as Bluemonday.

Here's an example of simple usage of blackfriday together with bluemonday:

import (
    "github.com/microcosm-cc/bluemonday"
    "github.com/russross/blackfriday"
)

// ...
unsafe := blackfriday.MarkdownCommon(input)
html := bluemonday.UGCPolicy().SanitizeBytes(unsafe)

Custom options

If you want to customize the set of options, first get a renderer (currently either the HTML or LaTeX output engines), then use it to call the more general Markdown function. For examples, see the implementations of MarkdownBasic and MarkdownCommon in markdown.go.

You can also check out blackfriday-tool for a more complete example of how to use it. Download and install it using:

go get github.com/russross/blackfriday-tool

This is a simple command-line tool that allows you to process a markdown file using a standalone program. You can also browse the source directly on github if you are just looking for some example code:

• http://github.com/russross/blackfriday-tool

Note that if you have not already done so, installing blackfriday-tool will be sufficient to download and install blackfriday in addition to the tool itself. The tool binary will be installed in $GOPATH/bin. This is a statically-linked binary that can be copied to wherever you need it without worrying about dependencies and library versions.

Sanitized anchor names

Blackfriday includes an algorithm for creating sanitized anchor names corresponding to a given input text. This algorithm is used to create anchors for headings when EXTENSION_AUTO_HEADER_IDS is enabled. The algorithm has a specification, so that other packages can create compatible anchor names and links to those anchors.

The specification is located at https://godoc.org/github.com/russross/blackfriday#hdr-Sanitized_Anchor_Names.

SanitizedAnchorName exposes this functionality, and can be used to create compatible links to the anchor names generated by blackfriday. This algorithm is also implemented in a small standalone package at github.com/shurcooL/sanitized_anchor_name. It can be useful for clients that want a small package and don't need full functionality of blackfriday.

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.

  To preserve classes of fenced code blocks while using the bluemonday HTML sanitizer, use the following policy:

  p := bluemonday.UGCPolicy()
  p.AllowAttrs("class").Matching(regexp.MustCompile("^language-[a-zA-Z0-9]+$")).OnElements("code")
  html := p.SanitizeBytes(unsafe)
  

• 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 (it is off by default in the MarkdownBasic and MarkdownCommon convenience functions), newlines in the input translate into line breaks in the output.

• 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 –, and --- is translated into —. 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 ⁴⁄₅.

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 header 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. This is currently part of the main Blackfriday repository, but may be split into its own project in the future. If you are interested in owning and maintaining the LaTeX output component, please be in touch.

  It renders some basic documents, but is only experimental at this point. In particular, it does not do any inline escaping, so input that happens to look like LaTeX code will be passed through without modification.

• Md2Vim: transforms markdown files into vimdoc format.

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.

License

Blackfriday is distributed under the Simplified BSD License