$re = '/(?:(\(((?>[^()]+|(?1))*)\))|\G(?!^)(?:\s*(\w+)\s*)(?=\())/m';
$str = '((weight gt 10) OR (weight lt 100)) AND (length lt 50)
(length lt 50) AND ((weight gt 10) OR (weight lt 100))
((weight gt 10) OR (weight lt 100)) AND (length lt 50) AND (length lt 50)';
preg_match_all($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