string s = "aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8";
        string[] parts = Regex.Split(s, @"(?:^|;)\w+,");
        string result = string.Join(",", parts).Trim(',');
        Response.Write(result);

------解决方案--------------------
探讨

你每一组之间的不是冒号啊，命名是分号，可以这样：

C# code

        string s = "aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8";
        string[] parts = Regex.Split(s, @"(?:^|;)\w+,");
        string result = string.Join(",",……


------解决方案--------------------
C# code

string s ="aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8;";
StringBuilder sb = new StringBuilder();
string[] ss = s.Split(';');
foreach(string s in ss)
{
   sb.Append(s.Split(',')[1]+",");
}
Response.Write(sb.ToString.TrimEnd(','));

------解决方案--------------------
或者这样也行：
C# code
        string s = "aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8";
        string result = Regex.Replace(s, @";?\w+,", ",").Trim(',');
        Response.Write(result);

------解决方案--------------------
探讨

你每一组之间的不是冒号啊，命名是分号，可以这样：

C# code

        string s = "aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8";
        string[] parts = Regex.Split(s, @"(?:^|;)\w+,");
        string result = string.Join(",",……


------解决方案--------------------
C# code

           string s = "aa,1;aa,2;aa,3;bb,4;bb,5;cc,6;dd,7;dd,8";
            MatchCollection mc = Regex.Matches(s, @"\w+,(\d+)");
            string s1 = "";
            foreach (Match m in mc)
            {
                s1 += m.Groups[1].Value + ",";
            }
            Response.Write(s1.TrimEnd(','));//1,2,3,4,5,6,7,8