const string src = "abcdefghijklmnopqrstuvwxyz0123456789";
        private void button1_Click(object sender, EventArgs e)
        {
            Regex re = new Regex(@"([a-zA-Z]+)(\d+)");
            Match result = re.Match(src);

            string alphaPart = result.Groups[1].Value;
            string numberPart = result.Groups[2].Value;

            int firstlength = 5;

            var sb = new StringBuilder();
            Random RNG = new Random();
            for (var i = 0; i < firstlength; i++)
            {
                var c = numberPart[RNG.Next(0, numberPart.Length)];
                sb.Append(c);
            }
            sb.Insert(2, ".");
            int secondlength = 3;
            sb.Append(' ');
            for (var i = 0; i < secondlength; i++)
            {
                var c = alphaPart[RNG.Next(0, alphaPart.Length)];
                sb.Append(c.ToString().ToUpper());
            }
            MessageBox.Show(sb.ToString());
        }