rewrite_links.rb

require 'redcarpet'

class CustomRenderHTML < Redcarpet::Render::HTML
  def initialize(args)
    @link_prefix = args[:link_prefix] || ''
    super
  end

  def link(link, title, content)
    link = @link_prefix + link unless link.start_with? 'http'
    "<a href='#{link}'>#{content}</a>"
  end
end

render = CustomRenderHTML.new(link_prefix: 'http://google.com/search?q=')
markdown = Redcarpet::Markdown.new(render)
puts markdown.render(STDIN.read)

Just an example of extending redcarpets html renderer to add a prefix to hrefs in anchor tags.

I wanted to call super on line 11, but that's impossible in redcarpet because of vmg/redcarpet#51