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Latest commit e2b825e Aug 10, 2018



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Ruby wrapper for libcmark-gfm, GitHub's fork of the reference parser for CommonMark. It passes all of the C tests, and is therefore spec-complete. It also includes extensions to the CommonMark spec as documented in the GitHub Flavored Markdown spec, such as support for tables, strikethroughs, and autolinking.

For more information on available extensions, see the documentation below.


Add this line to your application's Gemfile:

gem 'commonmarker'

And then execute:

$ bundle

Or install it yourself as:

$ gem install commonmarker


Converting to HTML

Call render_html on a string to convert it to HTML:

require 'commonmarker'
CommonMarker.render_html('Hi *there*', :DEFAULT)
# <p>Hi <em>there</em></p>\n

The second argument is optional--see below for more information.

Generating a document

You can also parse a string to receive a Document node. You can then print that node to HTML, iterate over the children, and other fun node stuff. For example:

require 'commonmarker'

doc = CommonMarker.render_doc('*Hello* world', :DEFAULT)
puts(doc.to_html) # <p>Hi <em>there</em></p>\n

doc.walk do |node|
  puts node.type # [:document, :paragraph, :text, :emph, :text]

The second argument is optional--see below for more information.

Example: walking the AST

You can use walk or each to iterate over nodes:

  • walk will iterate on a node and recursively iterate on a node's children.
  • each will iterate on a node and its children, but no further.
require 'commonmarker'

# parse the files specified on the command line
doc = CommonMarker.render_doc("# The site\n\n [GitHub](https://www.github.com)")

# Walk tree and print out URLs for links
doc.walk do |node|
  if node.type == :link
    printf("URL = %s\n", node.url)

# Capitalize all regular text in headers
doc.walk do |node|
  if node.type == :header
    node.each do |subnode|
      if subnode.type == :text
        subnode.string_content = subnode.string_content.upcase

# Transform links to regular text
doc.walk do |node|
  if node.type == :link

Creating a custom renderer

You can also derive a class from CommonMarker's HtmlRenderer class. This produces slower output, but is far more customizable. For example:

class MyHtmlRenderer < CommonMarker::HtmlRenderer
  def initialize
    @headerid = 1

  def header(node)
    block do
      out("<h", node.header_level, " id=\"", @headerid, "\">",
               :children, "</h", node.header_level, ">")
      @headerid += 1

# this renderer prints directly to STDOUT, instead
# of returning a string
myrenderer = MyHtmlRenderer.new

# Print any warnings to STDERR
renderer.warnings.each do |w|


CommonMarker accepts the same options that CMark does, as symbols. Note that there is a distinction in CMark for "parse" options and "render" options, which are represented in the tables below.

Parse options

Name Description
:DEFAULT The default parsing system.
:FOOTNOTES Parse footnotes.
:LIBERAL_HTML_TAG Support liberal parsing of inline HTML tags.
:SMART Use smart punctuation (curly quotes, etc.).
:STRIKETHROUGH_DOUBLE_TILDE Parse strikethroughs by double tildes (compatibility with redcarpet)
:VALIDATE_UTF8 Replace illegal sequences with the replacement character U+FFFD.

Render options

Name Description
:DEFAULT The default rendering system.
:GITHUB_PRE_LANG Use GitHub-style <pre lang> for fenced code blocks.
:HARDBREAKS Treat \n as hardbreaks (by adding <br/>).
:NOBREAKS Translate \n in the source to a single whitespace.
:SAFE Suppress raw HTML and unsafe links.
:SOURCEPOS Include source position in rendered HTML.
:TABLE_PREFER_STYLE_ATTRIBUTES Use style insted of align for table cells
:FULL_INFO_STRING Include full info strings of code blocks in separate attribute

Passing options

To apply a single option, pass it in as a symbol argument:

CommonMarker.render_doc("\"Hello,\" said the spider.", :SMART)
# <p>“Hello,” said the spider.</p>\n

To have multiple options applied, pass in an array of symbols:

CommonMarker.render_html("\"'Shelob' is my name.\"", [:HARDBREAKS, :SOURCEPOS])

For more information on these options, see the CMark documentation.


Both render_html and render_doc take an optional third argument defining the extensions you want enabled as your CommonMark document is being processed. The documentation for these extensions are defined in this spec, and the rationale is provided in this blog post.

The available extensions are:

  • :table - This provides support for tables.
  • :strikethrough - This provides support for strikethroughs.
  • :autolink - This provides support for automatically converting URLs to anchor tags.
  • :tagfilter - This strips out several "unsafe" HTML tags from being used.

Developing locally

After cloning the repo:

bundle exec rake compile

If there were no errors, you're done! Otherwise, make sure to follow the CMark dependency instructions.


Some rough benchmarks:

$ bundle exec rake benchmark

input size = 11063727 bytes

  0.070000   0.020000   0.090000 (  0.079641)
  0.070000   0.010000   0.080000 (  0.083535)
commonmarker with to_html
  0.100000   0.010000   0.110000 (  0.111947)
commonmarker with ruby HtmlRenderer
  1.830000   0.030000   1.860000 (  1.866203)
  4.610000   0.070000   4.680000 (  4.678398)