Question

How to mask a number with stars except the last 4 digits in C#

Answer 1

using System;

public class Program
{
    public static string MaskNumber(string input, char maskChar = '*')
    {
        if (string.IsNullOrEmpty(input) || input.Length <= 4)
            return input;

        // Take last 4 characters and pad left with maskChar up to full length
        return input.Substring(input.Length - 4).PadLeft(input.Length, maskChar);

        // Alternatively, using modern C# range syntax:
        // return input[^4..].PadLeft(input.Length, maskChar);
    }

    public static void Main()
    {
        Console.WriteLine(MaskNumber("1234567890123456")); // ************3456
        Console.WriteLine(MaskNumber("9876543210"));       // ******3210
        Console.WriteLine(MaskNumber("123"));              // 123
    }
}



/*
run:

************3456
******3210
123

*/

 

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Answer 2

using System;

public class Program
{
    public static string MaskNumberExplicit(string input, char maskChar = '*')
    {
        if (string.IsNullOrEmpty(input) || input.Length <= 4)
            return input;
    
        int maskLength = input.Length - 4;
        
        return new string(maskChar, maskLength) + input.Substring(maskLength);
    }

    public static void Main()
    {
        Console.WriteLine(MaskNumberExplicit("1234567890123456")); // ************3456
    }
}



/*
run:

************3456

*/

 

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Answer 3

using System;
using System.Text.RegularExpressions;

public class Program
{
    public static string MaskDigitsKeepFormat(string input, int visibleDigits = 4, 
                                              char maskChar = '*')
    {
        if (string.IsNullOrEmpty(input))
            return input;

        // Count total digits in the string
        int totalDigits = Regex.Matches(input, @"\d").Count;
        int digitsToMask = Math.Max(0, totalDigits - visibleDigits);
        int maskedCount = 0;

        // Replace digits left-to-right until digitsToMask threshold is reached
        return Regex.Replace(input, @"\d", match =>
        {
            if (maskedCount < digitsToMask)
            {
                maskedCount++;
                return maskChar.ToString();
            }
            return match.Value;
        });
    }

    public static void Main()
    {
        Console.WriteLine(MaskDigitsKeepFormat("1234-5678-9012-3456")); // ****-****-****-3456
        Console.WriteLine(MaskDigitsKeepFormat("+1 (800) 555-0199"));    // +* (***) ***-0199
    }
}



/*
run:

****-****-****-3456
+* (***) ***-0199

*/

 

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