Extend the Redcarpet HTML Renderer to output language-XXX class

PreCodeLanguageMarkdown.rb

class PreCodeLanguageMarkdown < Redcarpet::Render::HTML
  def block_code(code, language)
    [
      '<pre>',
      "<code class='language-#{language}'>#{html_escape(code)}</code>",
      '</pre>'
    ].join()
  end

  private

  # Doesn't seem dangerous, keep an eye on it.
  def html_escape(string)
    string.gsub(/['&\"<>\/]/, {
        '&' => '&amp;',
        '<' => '&lt;',
        '>' => '&gt;',
        '"' => '&quot;',
        "'" => '&#x27;',
        "/" => '&#x2F;',
    })
  end
end

Put this file in Rail's ./lib

Require the file where you need to render:

 require_dependency('PreCodeLanguageMarkdown')

Then be sure to add the file to the watch list, so when you change it , your development env will reload it:

config.watchable_dirs['lib'] = [:rb]