using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var x = HideNumber(16, "64537", "132");

        Console.WriteLine(x);

    }

    public static string HideNumber(int cardPanLength, string firstDigits = null, string lastDigits = null)

    {

        var cardPanHidden = new string('*', cardPanLength);

        if (cardPanLength >= firstDigits.Length + lastDigits.Length) 

        {

            if (!string.IsNullOrEmpty(lastDigits) && !string.IsNullOrEmpty(firstDigits))

            {

                cardPanHidden = cardPanHidden.Substring(0, cardPanHidden.Length - (firstDigits.Length + lastDigits.Length));

                var cardPan = String.Format("{0}{1}{2}", firstDigits.PadLeft(firstDigits.Length), cardPanHidden, lastDigits.PadRight(lastDigits.Length));

                return Regex.Replace(cardPan.ToString(), ".{4}", "$0 ");

            }

​

            if (!string.IsNullOrEmpty(lastDigits))

            {

                cardPanHidden = cardPanHidden.Substring(0, cardPanHidden.Length - lastDigits.Length);

                var cardPan = String.Format("{0}{1}", cardPanHidden, lastDigits.PadRight(lastDigits.Length));

                return Regex.Replace(cardPan.ToString(), ".{4}", "$0 ");

            }

​

            if (!string.IsNullOrEmpty(firstDigits))

            {

                cardPanHidden = cardPanHidden.Substring(0, cardPanHidden.Length - firstDigits.Length);

                var cardPan = String.Format("{0}{1}", firstDigits.PadRight(firstDigits.Length), cardPanHidden);

                return Regex.Replace(cardPan.ToString(), ".{4}", "$0 ");

            }

        }

        return null;

    }

}