using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ValidatePostcode("IP21 4AN").ToString());

    }

    private static bool ValidatePostcode(string postCode)

    {

        if (string.IsNullOrEmpty(postCode)) return false;

        var regexPostCode = @"^([Gg][Ii][Rr]0[Aa]{2})|((([A-Za-z][0-9]{1,2})|(([A-Za-z][A-Ha-hJ-Yj-y][0-9]{1,2})|(([AZa-z][0-9][A-Za-z])|([A-Za-z][A-Ha-hJ-Yj-y][0-9]?[A-Za-z]))))\\s?[0-9][A-Za-z]{2})$";

        var reg = new Regex(Regex.Unescape(regexPostCode), RegexOptions.IgnoreCase);

​

        if (!reg.IsMatch(postCode))

        {

            return false;

        }

​

        return true;

    }   

}