Challenge 3 — Solution

Task: Write a function maskCardNumber($number) that takes a 16-digit
card number string and uses preg_replace() with capturing groups to
keep only the last 4 digits visible, replacing the rest with asterisks
(e.g. "1234567812345678" becomes "************5678").

<?php
    function maskCardNumber(string $number): string {
        return preg_replace('/(\d{12})(\d{4})/', '************$2', $number);
    }

    echo maskCardNumber("1234567812345678");
?>


Output:
************5678

Notes:
- The pattern (\d{12})(\d{4}) splits a 16-digit number into two
  capturing groups: the first 12 digits, and the last 4 digits - both
  groups together must account for the entire 16-character string.
- The replacement string hardcodes twelve literal asterisks followed by
  $2, which refers back to the second capturing group (the last 4
  digits) - the first group ($1, the first 12 digits) is deliberately
  never referenced in the replacement, so its matched text is simply
  discarded and replaced by the asterisks instead.
- This pattern assumes exactly 16 digits with no spaces or dashes; a
  more production-ready version would first strip any non-digit
  characters from the input before applying this same masking logic,
  but that's outside what this specific exercise asked for.