link_converter.rb

module RPH
  module LinkConverter
    def self.included(klass)
      klass.extend ClassMethods
    end

    module ClassMethods
      def convert_links_for(*columns)
        methods = []

        columns.each do |column|
          define_method "converted_link_#{column}" do
            html = self[column].to_s
            html.gsub!(/\swww\./, ' http://www.').to_s
            html.gsub!(/((https?:\/\/|www\.)([-\w\.]+)+(:\d+)?(\/([\w\/_\.]*(\?\S+)?)?)?)/, '<a href="\1">\1</a>')
            self[column] = html
          end

          methods << "converted_link_#{column}".to_sym
        end

        before_save *methods
      end
    end
  end
end

using_link_converter.rb

class Comment < ActiveRecord::Base
  include RPH::LinkConverter
  convert_links_for :body
end