how to mask credit card number (using regex?)

[eluser]coolfactor[/eluser]
I've managed to separate the two parts of the number. Here's my progress so far:

Code:

$cc = '1234-4344-2994-4394';

$pattern = '/^([0-9-]+)([0-9]{4})$/';

$matches = array();

preg_match($pattern, $cc, $matches);

The result is:

Code:

array(3)
{
    [0]=> string(19) "1234-4344-2994-4394"
    [1]=> string(15) "1234-4344-2994-"
    [2]=> string(4) "4394"
}

Now, how do I use preg_replace() instead to mask element [1]?