using System;
using System.Text.RegularExpressions;
 
class Demo {
    public static void Main()
    {
        Regex re = new Regex(@"^[-a-z0-9_@]{3,15}$");
        Console.WriteLine(re.IsMatch("a_b"));
        Console.WriteLine(re.IsMatch("abxyz1234567890"));
        Console.WriteLine(re.IsMatch("a!!"));
        Console.WriteLine(re.IsMatch("abxyz12345678901234"));
    }
}