out.cs

using System;
using System.Text.RegularExpressions;

namespace TestNamespace
{
    internal class Program
    {

        public static string validaralfanum()
        {
            bool bandera = false;
            Regex regex = new Regex(@"^[a-zA-Z0-9\s,]{4}$");
            string unNum;
            do
            {
                unNum = Console.ReadLine();

                if (string.IsNullOrEmpty(unNum))
                {
                    Console.WriteLine("Ingrese un ID");
                }
                else if (!regex.IsMatch(unNum))
                {
                    Console.WriteLine("El ID debe ser alfanumérico");
                }
                else
                {
                    bandera = true;
                }
            }
            while (!bandera);
            return unNum;
        }

        public static void Main(string[] args)
        {
            Console.WriteLine("Plese enter an alphanumeric 4-digit string");
            String answer = validaralfanum();
        }
    }
}