sub captures {
    local $_ = shift;
    croak "$_ is not a compiled regexp" unless ref eq 'Regexp';

    my @p; # remember the kind of opening parentheses we've seen
    my ($n, $x) = (0, 1);
    /\G\(\?[ism]*(x?)[^:]*:/gc    # global /xism block
        && ( $x = $1 ? 1 : 0 );
    while( /\G(?=.)/gcs ) {
        $x > 0 ? /\G[^[\\()#]+/gc # ignore standard stuff (/x)
               : /\G[^[\\()]+/gc; # ignore standard stuff
        /\G(?:\\.)+/gcs;          # ignore backslashed stuff
        $x > 0 && /\G#.*/gc;      # ignore comments under /x
        /\G\[\^?\]?[^]]*\]/gc;    # character class
        /\G\(\?[ism]*(x?)[ism]*(?:-[ism]*(x?)[ism]*)?([:)])/gc && do {
            $x++, $3 eq ':' && push @p, '+x' if $1; # (?xism:...) and
            $x--, $3 eq ':' && push @p, '-x' if $2; # (?xism) blocks
        };
        /\G\(\?\(\d+\)/gc && push @p, 'sp';    # conditional regexp
        /\G\(\?/gc && push (@p, 'sp') && next; # other special regexp
        /\G\(/gc && ($n++, push @p, 'cp'); # a capturing parenthese
        /\G\)/gc && do {                   # a closing parenthese
            $x-- if $p[-1] eq '+x';        # compute /x state
            $x++ if $p[-1] eq '-x';
            pop @p;
        };
    }
    $n;
}