Effeciency (in the case where there's no match).

Imagine trying to match against the string "(abc".

Without the (?>...)
-------------------
(abc<fail>
(ab<fail>
(a<fail>
(<fail>
<fail>
[download]

With the (?>...)
----------------
(abc<fail>
(<fail>
<fail>
[download]

It's safe to do so here since [^()] and (??{ $re }) are mutally exclusive.