my $str = "((A,B,(C,D,(E,F),G),H),(I,J,K))";
our @nodes;
our $re;
$re = qr{
    \(
    (?:
        (?:
            ((?> [^()]+ ))    # Non-parens without backtracking
            (?{  /./ })    # <--- any regex here segfaults
        )+
    |
        (??{ $re })     # Group with matching parens
    )*
    \)
}x;