Recursive regular expressions question

[Rekcuf]

Hi everyone:
Now I have a regular expressions with recursive pattern, here's my code:

$reg = '#
  \(
     (?P<outside>
       (?P<inside>[^()]+)
       |
       (?R)
     )*
  \)
#x';

$m = preg_match_all($reg, '(first_part)(second_part(third_part))', $matches);
var_dump($matches);

The result:

array(3) {
  [0]=>
  array(2) {
    [0]=>
    string(12) "(first_part)"
    [1]=>
    string(25) "(second_part(third_part))"
  }
  ["outside"]=>
  array(2) {
    [0]=>
    string(10) "first_part"
    [1]=>
    string(12) "(third_part)"  

  }
  ["inside"]=>
  array(2) {
    [0]=>
    string "first_part"
    [1]=>
    string "second_part"
  }
}

How does the parser group this pattern?
I don't know why group #outside is "(third_part)" while matching "(second_part(third_part))".