$re = '/(?<=\))(?!\()[^()]+|\((?!\()[^)]+\)/m';
$str = '((Gripper|Open==true OR RIT|Turning==false) AND Robot|PosX >=3 OR (Test|Close==false OR (Gripper|Open==false AND RIT|Turning==false)))';
preg_match_all($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