Masking Credit Card Number using PHP regex Function

This php masking function will allow first and the last 4 digits for any credit card number. You can also provide the masking letter.

Credit card number has to be at least 13 character long.

function mask_cc($cc_num, $mask_ch='*') {
  
  if( strlen($cc_num) < 13) return $cc_num;
  
  else 
    return preg_replace('/(?!^.?)[0-9](?!(.){0,3}$)/', $mask_ch, $cc_num);

}

This is how you can call it.

mask_cc('1234123412341234', '#');
mask_cc('1234-1234-1234-1234', '*');