No need to use regular expressions for things like that - a simple [man]str_replace/man will do.
If you've got a long list of them to do, you could make an array like so for easier management:
$array (
':)' => 'http://site.com/img.gif',
// etc.
);
$data = str_replace(array_keys($array), array_values($array), $data);
