using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(CheckVatNumber("NL 8123 048 83B 01"));

    }

    private static bool CheckVatNumber(string vat)

    {

        if (string.IsNullOrEmpty(vat))

        {

            return false;

        }

​

        vat = vat.ToUpper();

        vat = Regex.Replace(vat, "(\\s|-|\\.)+", string.Empty);

        Console.WriteLine("After applying regex replace: " + vat);

​

        var match = Regex.Match(

            vat,

            "^(NL)(\\d{9})B\\d{2}$",

            RegexOptions.None,

            TimeSpan.FromMilliseconds(100));

​

        if (match.Success)

        {

            var total = 0;

            var vatNumber = match.Groups[2].Value;

            Console.WriteLine("After extracting vat number: " + vatNumber);

            var multipliers = new int[] { 9, 8, 7, 6, 5, 4, 3, 2, 1 };

​

            // Extract the next digit and multiply by the counter.

            for (var i = 0; i < 8; i++)

            {

                total += int.Parse(vatNumber[i].ToString()) *

                    multipliers[i];

            }

            Console.WriteLine("Total before modulus 11: " + total);

​

            total %= 11;

            Console.WriteLine("Total after modulus 11: " + total);

            if (total > 9)

            {

                total = 0;

            }

            Console.WriteLine("Comparing total " + total + " with last digit " + vatNumber.Substring(8));

​

            // Compare it with the last character of the VAT number.

            // If it's the same, then it's valid.

            if (total == int.Parse(vatNumber.Substring(8)))

            {

                return true;

            }

        }

​

        return false;

    }

}