string str = "1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD ";
            string[] arrstr=str.Split('1', '2', '3', '4', '5', '6', '7', '8', '9', '0');
            foreach (string s in arrstr)
            {
                Console.WriteLine(s.Trim('.'));
            }

Regex regex = new Regex("([1-9]\\.([A-Z]{4})\\s{0,1})+?");
MatchCollection matchs = regex.Matches("1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD");
foreach (Match match in matchs)
{
    Response.Write(match.Groups[2].Value);//<------ match.Groups[2].Value 就是你想要的值
    Response.Write("<Br>");
}

string str = "1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD";
            string[] s = str.Split(new char[] { ' ', '.','1','2','3','4','5','6','7','8','9' });

            foreach (string ss in s)
            {
                Console.WriteLine(ss);
            }

string src = "HK5";
string dst = @"1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD 
6.DFKJNJBU 
7.DFPTNF  DFVBGG";
int num = int.Parse(src.Substring(src.IndexOf("HK") + 2, 1));
string[] arr = dst.Split(' ');
for (int i = 0; i < num; i++)
{
    richTextBox2.Text += arr[i].Substring(2) + "\n";
}

Regex regex = new Regex("([1-9]\\.([A-Z]+)\\s{0,1})+?");
MatchCollection matchs = regex.Matches("1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD");
foreach (Match match in matchs)
{
Response.Write(match.Groups[2].Value);// <------ match.Groups[2].Value 就是你想要的值
Response.Write(" <Br>");
} 

              string str = "1.DDCF 2.DDER 3.DDCV 4.DERD 5.DEDD ";
            str = str.Replace(" ","");
            string[] arrstr=str.Split('1', '2', '3', '4', '5', '6', '7', '8', '9', '0');
            foreach (string s in arrstr)
            {
                Console.WriteLine(s.Trim('.'));
            }

            string sol = string.Empty;
            MatchCollection m = Regex.Matches(textBox2.Text, @"(?<=HK)\d+(?=\s)|(?<=\s*\d+\.)[^\s]+(?=\s)", RegexOptions.IgnoreCase);

            for (int  i = 1; i <=Convert.ToInt32(m[0].Value); i++)
            {
                sol +=  m[i].Value + Environment.NewLine;
            }
            this.textBox1.Text = sol;