Given a credit card number as a string, mask all characters except the last 4 by replacing them with the '#' character. Return null for null input and an empty string for empty input. If the string has 4 or fewer characters, return it unchanged.
Examples:
Input: cardNumber = '4111111111111111'
Output: '############1111'
Explanation: All characters except the last 4 are replaced with '#'
Input: cardNumber = '12345'
Output: '#2345'
Explanation: Only the first character is masked, last 4 are preserved
Input: cardNumber = '1234'
Output: '1234'
Explanation: String has 4 or fewer characters, return unchanged
Input: cardNumber = null
Output: null
Explanation: Null input returns null
Input: cardNumber = ''
Output: ''
Explanation: Empty string returns empty string
Apex Code Editor
Welcome to Lightning Challenge!
How It Works
- • Write your solution in the code editor
- • Connect your Salesforce org to test
- • Submit to check if your solution passes
- • Use hints if you get stuck
Contest Alert
🏆 #CodeEveryDay October 2026 (Oct 4–15)Sponsored by FLXBL
Contest runs October 4 - 15. Complete challenges to climb the leaderboard!
Prizes 1 & 2: Salesforce certification exam voucher — won by random drawing, where every point is a ticket. see all prizes
Only the 12 daily challenges shown during this contest count toward points. Earlier dailies don't carry over.