public function getKeys($pattern = null)
    {
        $keys = $this->get('__stored_keys__', []);
        if (is_null($pattern)) {
            return $keys;
        }

        $matches = [];
        $pattern = $this->_normalizePattern($pattern);
        foreach ($keys as $key) {
            if (preg_match($pattern, $key)) {
                $matches[] = $key;
            }
        }

        return $matches;
    }