markdown编辑和显示（语法高亮）

gem 'redcarpet'
gem 'pygments.rb'

show.html.erb

<%= markdown @post.content %>

helpers/application_helper.rb

module ApplicationHelper
  class HTMLwithPygments < Redcarpet::Render::HTML
    def block_code(code, language)
      Pygments.highlight(code, lexer: language)
    end
  end
  def markdown(text)
    renderer = HTMLwithPygments.new(filter_html: true, hard_wrap: true)
    options = {
      autolink: true,
      no_intra_emphasis: true,
      fenced_code_blocks: true,
      lax_html_blocks: true,
      strikethrough: true,
      superscript: true
    }
    Redcarpet::Markdown.new(renderer, options).render(text).html_safe
  end
end

assets/stylesheets/pygments.css.erb

<%= Pygments.css(:style => "default") %>

OK，这样就可以用markdown的语法来进入输入了，在显示的时候，会views会用helper中的markdown方法来调用Redcarpet::Markdown.new方法来渲染。
而pygments则是覆盖了Redcarpet::Markdown::HTML中的block_code方法，实现了语法高亮