获得字符串中开始和结束字符串中间得值

/// <summary>
        /// 获得字符串中开始和结束字符串中间得值
        /// </summary>
        /// <param name="str">字符串</param>
        /// <param name="s">开始</param>
        /// <param name="e">结束</param>
        /// <returns></returns> 
        public static string GetMidValue(string str, string s, string e)
        {
            Regex rg = new Regex("(?<=(" + s + "))[.\s\S]*?(?=(" + e + "))", RegexOptions.Multiline | RegexOptions.Singleline);
            return s.Trim('\') + rg.Match(str).Value + e.Trim('\');
        }

List<string> listdetail=new List<string>();
            string strhtml = "testaaa{aaa}testbbb{bbb}testccc{ccc}";
            for (int i = 0; i < 30; i++)
            {
                string find = GetMidValue(strhtml, "{", "}");
                if (find != "{}")
                {
                    listdetail.Add(find);
                    if (i == 0)
                    {
                        strhtml = strhtml.Replace(find, "<start>");
                    }
                    else
                    {
                        strhtml = strhtml.Replace(find, "<end>");
                    }
                }
                else
                {
                    break;
                }
            }
            string end = strhtml;