new Regex(?(^\d)^\d+$|^\D+$) : Regular Expression « Regular Expression « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. Regular Expression
3. Regular Expression

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