using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string pattern = @"^(\d *){13}$";

        string input = @"33       33          333                 333              333";

        foreach (Match m in Regex.Matches(input, pattern))

        {

            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);

        }

    }

}