class MyFilter
  def self.filter(controller)
    insert_text controller, :before, /<\/body>/i, 'bar'
  end

  def self.insert_text(controller, position, pattern, new_text)
    index = if match = controller.response.body.match(pattern)
      match.offset(0)[position == :before ? 0 : 1]
    else
      controller.response.body.size
    end
    controller.response.body = controller.response.body.insert index, new_text
  end
end

class ActionController::Base
  after_filter MyFilter
end