Railscast #272 with Redcarpet 2.0 helper code

application_helper.rb

def markdown(text)
  options = {hard_wrap: true, filter_html: true, autolink: true, no_intraemphasis: true, fenced_code: true, gh_blockcode: true}
  md = Redcarpet::Markdown.new(Redcarpet::Render::HTML, extensions = options)
  syntax_highlighter( md.render(text) ).html_safe
end

def syntax_highlighter(html)
  doc = Nokogiri::HTML(html)
  doc.search("//pre[@lang]").each do |pre|
    pre.replace Albino.colorize(pre.text.rstrip, pre[:lang])
  end
  doc.to_s
end

See comment by Braulio Carreno at end of comments for a better way of doing this with pygments.rb gem.