A sprockets processor which rewrites the relative urls in a css when concatenating css files

urlrewriter.rb

require 'sprockets'
require 'pathname'
require 'uri'

module Sprockets
  class UrlRewriter < Processor
    def evaluate(context, locals)
      rel = Pathname.new(context.logical_path).parent
      data.gsub /url\(['"]?([^\s)]+\.[a-z]+)(\?\d+)?['"]?\)/ do |url|
        return url if URI.parse($1).absolute?
        new_path = rel.join Pathname.new($1)
        "url(#{new_path})"
      end
    end
  end
end