using System;

using System.Web;

using System.Collections.Specialized;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

         var cardNumber = "3456123434561234";

​

var firstDigits = cardNumber.Substring(0, 12);

var lastDigits = cardNumber.Substring(cardNumber.Length - 4, 4);

​

var requiredMask = new String('X', cardNumber.Length - firstDigits.Length - lastDigits.Length);

​

var maskedString = string.Concat(firstDigits, requiredMask, lastDigits);

var maskedCardNumberWithSpaces = Regex.Replace(maskedString, ".{4}", "$0 ");

​

        Console.WriteLine(maskedCardNumberWithSpaces);

    }

}