counted capture groups:

(?^msx: ((?^msx: \( ([^()]* | (??{ our $rx }))* \) )) [^()]* ((?^msx: \( ([^()]* | (??{ our $rx }))* \) )) )
        |           |                        |      |        |           |                        |      |
        1st begin   2nd begin                2-end  1st end  3rd begin   4th begin                4-end  3rd end