01. Valid Usernames

using System;
using System.Linq;
using System.Collections.Generic;

namespace ValidUsername
{

    class Program
    {
        static void Main()
        {
            string[] usernames = Console.ReadLine().Split(", ");

            for (int i = 0; i < usernames.Length; i++)
            {
                string username = usernames[i];

                if (username.Length >= 3 && username.Length <= 16)
                {
                    bool isValid = true;
                    for (int j = 0; j < username.Length; j++)
                    {
                        if (!char.IsLetterOrDigit(username[j]) && username[j] != '-' && username[j] != '_')
                        {
                            isValid = false;
                            break;
                        }
                    }

                    if (isValid)
                    {
                        Console.WriteLine(username + " ");
                    }
                }
            }
        }
    }
}


REGEX SOLUTION:

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;

namespace ValidUsername
{

    class Program
    {
        static void Main()
        {
            string pattern = @"^[A-Za-z-_]{3,16}$";

            var usernames = Console.ReadLine().Split(", ");

            for (int i = 0; i < usernames.Length; i++)
            {
                if (Regex.IsMatch(usernames[i], pattern))
                {
                    Console.WriteLine(usernames[i]);
                }
            }
        }
    }
}