Rolling out the Redcarpet

Here at GitHub, we love Markdown. We use it everywhere: to render the wikis, issues, pull requests, and all user-generated comments. We even encourage developers to write their READMEs in this awesome markup language. In fact, we use it so much that we’ve learnt a few lessons on Markdown parsing the hard way.

Every day, GitHub renders thousands of Markdown documents with all kinds of user-submitted content, ranging from poorly formatted to downright malicious. Your average Markdown parser is not prepared to deal with potentially pathological inputs, and hence is vulnerable to DOS attacks. That’s why we’ve decided to take Natacha Porté‘s awesome library, Upskirt, and pimped it with everything you’d expect in a Markdown library for the web – both in features and in security.

Our fork of the library also comes with a Ruby wrapper, aptly named Redcarpet. Redcarpet works as a drop-in replacement for BlueCloth and RDiscount; we’ve been slowly deploying it through all our frontend machines, and so far none of them has caught 🔥. We consider this a tremendous success, but since we strive for perfection, please report any rendering errors you may encounter in your Markdown documents to help us improve the library.

Finally, to celebrate the release of the new library we’re enabling syntax highlighted code blocks in GitHub Flavored Markdown.

Four space indentation is now no longer required when including code, backtraces and other text in a comment, issue, Gist or any other Markdown-enabled text. Instead, simply create a fenced block with ```. An optional language identifier after the backticks will syntax highlight the code in that language.

``` ruby
require 'redcarpet'
markdown = Redcarpet.new("Hello World!")
puts markdown.to_html

``` ruby
require 'redcarpet'
markdown = Redcarpet.new("Hello World!")
puts markdown.to_html