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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 modern Go releases in module mode.
With Go installed:

go get github.com/russross/blackfriday/v2

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/v2"

and go get without parameters.

Legacy GOPATH mode is unsupported.

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

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/v2"
)

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

Custom options

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 AutoHeadingIDs extension 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/v2#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 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 –, 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

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