using System;

using System.Text.RegularExpressions;

public class Program {

    public static void Main() {

        var cardNumber = "476173**********10";

        cardNumber = cardNumber.Replace(" ", "");                   // remove spaces

        var firstMaskNumbers = 6;

        var lastMaskNumbers = 4;

        var cardNumberSize = 16;

        var cardInit = "000000";

        int myInt;                                                  // needed for tryparse

        // Parsing card initial numbers

        if (int.TryParse(cardNumber.Substring(0 , firstMaskNumbers), out myInt)){

            cardInit = cardNumber.Substring(0 , firstMaskNumbers);

        }

        // Parsing card end numbers

        var cardEnd = "9999";

        if (int.TryParse(cardNumber.Substring(cardNumber.Length - lastMaskNumbers, lastMaskNumbers), out myInt)){

            cardEnd = cardNumber.Substring(cardNumber.Length - lastMaskNumbers, lastMaskNumbers);

        }

        // Adding Xs to mask

        var cardMiddleSize = cardNumberSize - cardInit.Length - cardEnd.Length;

        var cardMiddle = "";

        while (cardMiddle.Length < cardMiddleSize){ // add Xs till required size

            cardMiddle += "X";

        }

        Console.WriteLine(cardInit + cardMiddle + cardEnd);

    }

}

​

​