foreach (string str in Regex.Split(textBox1.Text, @"^\d\.$", RegexOptions.Multiline | RegexOptions.IgnoreCase))
            {
                Console.WriteLine("@" + str);
            }

foreach (Match m in Regex.Matches(yourStr, @"(?m)(?<=^\d\.).+"))
{
   richTextBox2.Text += "@" + m.Value + "\n";
}
/*-----输出-----
@aaa
@bbb
@ccc 
*/