Regex divisore e dividendo

[tampertools]

Non è con un'espressione regolare, ma questo funziona:

function parse_fraction ($fraction) {
    list($n, $d) = explode("/", $fraction);
    $n = strrev($n);
    foreach (array("n", "d") as $var) {
        $brackets = 0;
        $parsed = "";
        foreach (str_split($$var) as $token) {
            if ($token == "(") {
                if ($brackets != 0 && $brackets != -1) {
                    $parsed .= "(";
                }
                $brackets++;
            } else if ($token == ")") {
                if ($brackets != 0 && $brackets != 1) {
                    $parsed .= ")";
                }
                $brackets--;
            } else if (in_array($token, array("+", "*", "-")) && $brackets == 0) {
                break;
            } else {
                $parsed .= $token;
            }        }
        $$var = $parsed;
    }
    return array(strrev($n), $d);} 

print_r(parse_fraction("(5+2)/((3^4)+3)-5")); // stampa Array ( [0] => 5+2 [1] => (3^4)+3 )