请帮我看看这个正则怎么用不了了

            string str = @"<li class=""ico_tel""><span class=""pri"" id=""t_phone"">
            18249375857
        </span>";
            Regex reg = new Regex(@"(?is)<li class=""ico_tel""><span class=""pri"" id=""t_phone"">\s*(\d+)\s*</span>");
            Console.WriteLine(reg.Match(str).Groups[1].Value);