protected function callSelector($key)
    {
        foreach ($this->selectors as $selector => $callback) {
            $regex = '/' . $selector . '/i';
            if (preg_match($regex, $key, $matches)) {
                return call_user_func_array($callback, array($this, $key, $matches));
            }
        }

        throw new Exception\Index('Unknown index: ' . $key);
    }