using System;
using System.Text.RegularExpressions;

public class Test
{

    public static void Main ()
    {

       // Define a regular expression for ISBN
       Regex rx = new Regex(@"ISBN\x20(?=.{13}$)\d{1,5}([- ])\d{1,7\1\d{1,6}1(\d|X)$");
       
       // Define test ISBN numbers
       string tests[] = {"ISBN 2-16678-888-1", "ISBN 1 84736 123 4"};
       
       // Check each test string against the regular expression.
       foreach (string test in tests)
       {
           if (rx.IsMatch(test))
           {
               Console.WriteLine("{0} is a valid ISBN.", test);
           }
           else
           {
               Console.WriteLine("{0} is not a valid ISBN.", test);
           }
       }
      
    } 
 
}