my $string = "A (B C D)";
get_groups($string);
$string = "A (B C D) (E F G)";
get_groups($string);

$string = "A B (C (D E) (F G)) (H I)";
$string = "(A B) M N ((C (D E)) (F G)) (Z(H I)) T R";
get_groups($string);