using System.Text.RegularExpressions;
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))