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.

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

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