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vmg / redcarpet

The safe Markdown parser, reloaded.
MIT License
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Redcarpet is written with sugar, spice and everything nice

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Redcarpet is a Ruby library for Markdown processing that smells like butterflies and popcorn.

This library is written by people

Redcarpet was written by Vicent Martí. It is maintained by Robin Dupret and Matt Rogers.

Redcarpet would not be possible without the Sundown library and its authors (Natacha Porté, Vicent Martí, and its many awesome contributors).

You can totally install it as a Gem

Redcarpet is readily available as a Ruby gem. It will build some native extensions, but the parser is standalone and requires no installed libraries. Starting with Redcarpet 3.0, the minimum required Ruby version is 1.9.2 (or Rubinius in 1.9 mode).

$ [sudo] gem install redcarpet

If you need to use it with Ruby 1.8.7, you will have to stick with 2.3.0:

$ [sudo] gem install redcarpet -v 2.3.0

The Redcarpet source is available at GitHub:

$ git clone git://github.com/vmg/redcarpet.git

And it's like really simple to use

The core of the Redcarpet library is the Redcarpet::Markdown class. Each instance of the class is attached to a Renderer object; the Markdown class performs parsing of a document and uses the attached renderer to generate output.

The Redcarpet::Markdown object is encouraged to be instantiated once with the required settings, and reused between parses.

# Initializes a Markdown parser
markdown = Redcarpet::Markdown.new(renderer, extensions = {})

Here, the renderer variable refers to a renderer object, inheriting from Redcarpet::Render::Base. If the given object has not been instantiated, the library will do it with default arguments.

Rendering with the Markdown object is done through Markdown#render. Unlike in the RedCloth API, the text to render is passed as an argument and not stored inside the Markdown instance, to encourage reusability. Example:

markdown.render("This is *bongos*, indeed.")
# => "<p>This is <em>bongos</em>, indeed.</p>"

You can also specify a hash containing the Markdown extensions which the parser will identify. The following extensions are accepted:

Example:

markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, autolink: true, tables: true)

Darling, I packed you a couple renderers for lunch

Redcarpet comes with two built-in renderers, Redcarpet::Render::HTML and Redcarpet::Render::XHTML, which output HTML and XHTML, respectively. These renderers are actually implemented in C and hence offer brilliant performance — several degrees of magnitude faster than other Ruby Markdown solutions.

All the rendering flags that previously applied only to HTML output have now been moved to the Redcarpet::Render::HTML class, and may be enabled when instantiating the renderer:

Redcarpet::Render::HTML.new(render_options = {})

Initializes an HTML renderer. The following flags are available:

Example:

renderer = Redcarpet::Render::HTML.new(no_links: true, hard_wrap: true)

The HTML renderer has an alternate version, Redcarpet::Render::HTML_TOC, which will output a table of contents in HTML based on the headers of the Markdown document.

When instantiating this render object, you can optionally pass a nesting_level option which takes an integer or a range and allows you to make it render only headers at certain levels.

Redcarpet also includes a plaintext renderer, Redcarpet::Render::StripDown, that strips out all the formatting:

require 'redcarpet'
require 'redcarpet/render_strip'

markdown = Redcarpet::Markdown.new(Redcarpet::Render::StripDown)

markdown.render("**This** _is_ an [example](http://example.org/).")
# => "This is an example (http://example.org/)."

And you can even cook your own

Custom renderers are created by inheriting from an existing renderer. The built-in renderers, HTML and XHTML may be extended as such:

# Create a custom renderer that sets a custom class for block-quotes.
class CustomRender < Redcarpet::Render::HTML
  def block_quote(quote)
    %(<blockquote class="my-custom-class">#{quote}</blockquote>)
  end
end

markdown = Redcarpet::Markdown.new(CustomRender, fenced_code_blocks: true)

But new renderers can also be created from scratch by extending the abstract base class Redcarpet::Render::Base (see lib/redcarpet/render_man.rb for an example implementation of a Manpage renderer):

class ManPage < Redcarpet::Render::Base
  # you get the drill -- keep going from here
end

The following instance methods may be implemented by the renderer:

Block-level calls

If the return value of the method is nil, the block will be skipped. Therefore, make sure that your renderer has at least a paragraph method implemented. If the method for a document element is not implemented, the block will be skipped.

Example:

class RenderWithoutCode < Redcarpet::Render::HTML
  def block_code(code, language)
    nil
  end
end

Span-level calls

A return value of nil will not output any data. If the method for a document element is not implemented, the contents of the span will be copied verbatim:

Note: When overriding a renderer's method, be sure to return a HTML element with a level that matches the level of that method (e.g. return a block element when overriding a block-level callback). Otherwise, the output may be unexpected.

Low level rendering

Header of the document

Rendered before any another elements:

Footer of the document

Rendered after all the other elements:

Pre/post-process

Special callback: preprocess or postprocess the whole document before or after the rendering process begins:

You can look at "How to extend the Redcarpet 2 Markdown library?" for some more explanations.

Also, now our Pants are much smarter

Redcarpet 2 comes with a standalone SmartyPants implementation. It is fully compliant with the original implementation. It is the fastest SmartyPants parser there is, with a difference of several orders of magnitude.

The SmartyPants parser can be found in Redcarpet::Render::SmartyPants. It has been implemented as a module, so it can be used standalone or as a mixin.

When mixed with a Renderer class, it will override the postprocess method to perform SmartyPants replacements once the rendering is complete.

# Mixin
class HTMLWithPants < Redcarpet::Render::HTML
  include Redcarpet::Render::SmartyPants
end

# Standalone
Redcarpet::Render::SmartyPants.render("<p>Oh SmartyPants, you're so crazy...</p>")

SmartyPants works on top of already-rendered HTML, and will ignore replacements inside the content of HTML tags and inside specific HTML blocks (pre, code, var, samp, kbd, math, script, style).

What? You really want to mix Markdown renderers?

Redcarpet used to be a drop-in replacement for Redcloth. This is no longer the case since version 2 -- it now has its own API, but retains the old name. Yes, that does mean that Redcarpet is not backwards-compatible with the 1.X versions.

Each renderer has its own API and its own set of extensions: you should choose one (it doesn't have to be Redcarpet, though that would be great!), write your software accordingly, and force your users to install it. That's the only way to have reliable and predictable Markdown output on your program.

Markdown is already ill-specified enough; if you create software that is renderer-independent, the results will be completely unreliable!

Still, if major forces (let's say, tornadoes or other natural disasters) force you to keep a Markdown-compatibility layer, Redcarpet also supports this:

require 'redcarpet/compat'

Requiring the compatibility library will declare a Markdown class with the classical RedCloth API, e.g.

Markdown.new('this is my text').to_html

This class renders 100% standards compliant Markdown with 0 extensions. Nada. Don't even try to enable extensions with a compatibility layer, because that's a maintenance nightmare and won't work.

On a related topic: if your Markdown gem has a lib/markdown.rb file that monkeypatches the Markdown class, you're a terrible human being. Just saying.

Boring legal stuff

Copyright (c) 2011-2016, Vicent Martí

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.