QueryPath\CSS\QueryPathEventHandler::parseAnB PHP Method

    protected function parseAnB($rule)
    {
        if ($rule == 'even') {
            return array(2, 0);
        } elseif ($rule == 'odd') {
            return array(2, 1);
        } elseif ($rule == 'n') {
            return array(1, 0);
        } elseif (is_numeric($rule)) {
            return array(0, (int) $rule);
        }
        $rule = explode('n', $rule);
        if (count($rule) == 0) {
            throw new ParseException('nth-child value is invalid.');
        }
        // Each of these is legal: 1, -1, and -. '-' is shorthand for -1.
        $aVal = trim($rule[0]);
        $aVal = $aVal == '-' ? -1 : (int) $aVal;
        $bVal = !empty($rule[1]) ? (int) trim($rule[1]) : 0;
        return array($aVal, $bVal);
    }