$re = '/(?:[^[]|^)?\((.*)\)(?!\])/m';
$str = '(hello world)
[oh (hello) there, (how) are you?]
(hey!!)
[(Uh, can you hear me?)]
(Yep, [fine])
[be sure (to be)( coming (tonight) for the (party)]
[(fasfj])';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php