how to mask credit card number (using regex?)

[eluser]esra[/eluser]
Something that I noted down in my journal last May for future reference as cc_help.php but have not used it myself. It was posted on a blog elsewhere but the blog no longer exists. According to my notes, this should result in a mask like xxxx-xxxx-xxxx-1234

Code:

<?php
function mask ( $str, $start = 0, $length = null ) {
    $mask = preg_replace ( "/\S/", "*", $str );
    if ( is_null ( $length )) {
        $mask = substr ( $mask, $start );
        $str = substr_replace ( $str, $mask, $start );
    } else {
        $mask = substr ( $mask, $start, $length );
        $str = substr_replace ( $str, $mask, $start, $length );
    }
    return $str;
}
?>