private function filter($text)
    {
        $text = preg_replace("/[^&a-z0-9-_\s']/i", '', $text);
        $text = str_replace(' ', $this->separator, $text);
        $text = trim(preg_replace("/{$this->separator}{2,}/", $this->separator, $text), $this->separator);

        return $text;
    }