using System;

using System.Text.RegularExpressions;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var emailRegex = new Regex("^[_a-z0-9-]+(.[a-z0-9-]+)@[a-z0-9-]+(.[a-z0-9-]+)*(.[a-z]{2,4})$");

        var emailAddresses = new List<string>()

        {

            "user@domain.com",

            "user@domain.c",

            "@domain.com",

            "user@domain"

        };

        foreach (var emailAddress in emailAddresses)

        {

            Console.WriteLine(emailAddress + " - " + emailRegex.IsMatch(emailAddress));

        }

    }

}