Description:
------------
I can't tell if this is universal, but with this particular regex it certainly is a bug. I think it is to do with using parentheses directly next to each other.

Reproduce code:
---------------
<?
    $str = 'arr[sub][sub_sub]';
    die(preg_replace('/([^\[\]]*)(.*)/','[\1]\2',$str));
?>

just for the record, this works as expected if i use the pattern:

preg_replace('/([^\[\]]*)\[(.*)\]/','[\1][\2]',$el);

so it appears that the problem is when parenthesised (is that correct?) expression appear next to each other.

Expected result:
----------------
[arr][sub][sub_sub]

Actual result:
--------------
[arr][sub][sub_sub][]