public static void Main()

        {

            string str = @"http://item.taobao.com/item.htm?id=21456688754&ali_refid=a3_420763_1007:1102249745:7:4043f4c103d9bc6af0a150eaaa84293d:809ab74692c8b2532811f1c84fbbe72b&ali_trackid=1_809ab74692c8b2532811f1c84fbbe72b";

            Regex reg = new Regex(@"(?<=id=)(\d+)");

            if (reg.IsMatch(str))

            {

                Match m = reg.Match(str);

                Console.WriteLine(m.Value);

            }

            Console.Read();

        }

		string s = "http://item.taobao.com/item.htm?id=21456688754&ali_refid=a3_420763_1007:1102249745:7:4043f4c103d9bc6af0a150eaaa84293d:809ab74692c8b2532811f1c84fbbe72b&ali_trackid=1_809ab74692c8b2532811f1c84fbbe72b";

		string id = HttpUtility.ParseQueryString(new Uri(s).Query)["id"];

		Response.Write(id);