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 modern Go releases in module mode.
With Go installed:

go get github.com/russross/blackfriday

will resolve and add the package to the current development module,
then build and install it. Alternatively, you can achieve the same
if you import it in a package:

import "github.com/russross/blackfriday"

and go get without parameters.

Old versions of Go and legacy GOPATH mode might work,
but no effort is made to keep them working.

Versions

Currently maintained and recommended version of Blackfriday is v2. It’s being
developed on its own branch: https://github.com/russross/blackfriday/tree/v2 and the
documentation is available at
https://pkg.go.dev/github.com/russross/blackfriday/v2.

It is go get-able in module mode at github.com/russross/blackfriday/v2.

Version 2 offers a number of improvements over v1:

• Cleaned up API
• A separate call to Parse, which produces an abstract syntax tree for
  the document
• Latest bug fixes
• Flexibility to easily add your own rendering extensions

Potential drawbacks:

• Our benchmarks show v2 to be slightly slower than v1. Currently in the
  ballpark of around 15%.
• API breakage. If you can’t afford modifying your code to adhere to the new API
  and don’t care too much about the new features, v2 is probably not for you.
• Several bug fixes are trailing behind and still need to be forward-ported to
  v2. See issue #348 for
  tracking.

If you are still interested in the legacy v1, you can import it from
github.com/russross/blackfriday. Documentation for the legacy v1 can be found
here: https://pkg.go.dev/github.com/russross/blackfriday.

Usage

v1

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)

v2

For the most sensible markdown processing, it is as simple as getting your input
into a byte slice and calling:

output := blackfriday.Run(input)

Your input will be parsed and the output rendered with a set of most popular
extensions enabled. If you want the most basic feature set, corresponding with
the bare Markdown specification, use:

output := blackfriday.Run(input, blackfriday.WithNoExtensions())

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.Run(input)
html := bluemonday.UGCPolicy().SanitizeBytes(unsafe)

Custom options, v1

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

Custom options, v2

If you want to customize the set of options, use blackfriday.WithExtensions,
blackfriday.WithRenderer and blackfriday.WithRefOverride.

blackfriday-tool

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:

• https://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://pkg.go.dev/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 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.

• bfchroma: provides convenience
  integration with the Chroma code
  highlighting library. bfchroma is only compatible with v2 of Blackfriday and
  provides a drop-in renderer ready to use with Blackfriday, as well as
  options and means for further customization.

• Blackfriday-Confluence: provides a Confluence Wiki Markup renderer.

• Blackfriday-Slack: converts markdown to slack message style

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