Regular Expression C# Tutorial

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.RegularExpressions;
using System.Reflection;
public class MainClass{
   public static void Main(){
        Regex r = new Regex(@"(?(^\d)^\d+$|^\D+$)");
        Console.WriteLine(r.Match("cat too"));
        Console.WriteLine(r.Match("dag"));
        Console.WriteLine(r.Match("2else"));
   }
}
cat too
dag