using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var phoneNumber = "00123456789";

          var regex = new Regex(Regex.Escape("0"));

           phoneNumber = regex.Replace(phoneNumber, "81", 1);

        Console.WriteLine(phoneNumber);

​

        !Regex.IsMatch(PhoneNumber, @"^\d{10,11}$", RegexOptions.IgnoreCase))

    }

}