using System;

using System.Text.RegularExpressions;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string[] tests =

        {

            "CC# 123456789012345",

            "Credit Card 123456 7890 123456",

            "Text 1234 5678 9012 3456 Text",

            "Something 123 456 789 012 3456",

            "Not a card (too long): 12345689123456789123",

            "Just a value 123"

        };

​

        var report = string.Join(Environment.NewLine, tests

            .Select(test => $"{test,45} => {MaskCreditCardNumber(test)}"));

        Console.WriteLine(report);

        Console.WriteLine("123456789012345".MaskCreditCardNumber());

    }

    private static string MaskCreditCardNumber(string text)

    {

        if (string.IsNullOrEmpty(text))

        {

            return text;

        }

​

        return Regex.Replace(text, "[0-9][0-9 ]{13,}[0-9]", match =>

        {

            string digits = string.Concat(match.Value.Where(char.IsDigit));

​

            return digits.Length == 16 || digits.Length == 15

                ? new string('X', digits.Length - 4) + digits[^4..]

                : match.Value;

        });

    }

}

​

public static class Extensions

{

    public static string MaskCreditCardNumber(this string sender, char maskCharacter = 'X')

    {

        if (string.IsNullOrEmpty(sender))

        {

            return sender;

        }

​

        return Regex.Replace(sender, "[0-9][0-9 ]{13,}[0-9]", match =>

        {

            string digits = string.Concat(match.Value.Where(char.IsDigit));

​

            return digits.Length is 16 or 15

                ? new string(maskCharacter, digits.Length - 4) + digits[^4..]

                : match.Value;

        });

    }

}