using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string pattern = "^(?=\\.{3,64}$)[a-z0-9\\.]+@[a-z0-9\\.]+$";

        string[] names = {

            "some.username@zen",

            "bob@zen", 

            "robert@zens",

            "soem.guy@some.really.quite.long.realm"

        };

        foreach (string name in names){

            Console.WriteLine("Match username: '" + name + "' Match success:" + Regex.Match(name, pattern).Success);

        }

    }

}