public static function removeStopWords($string, $asString = false)
    {
        $delimiters = preg_quote(static::$delimiters, '/');
        $stopWords = explode(',', static::$stopWords);

        $result = array_map(function ($token) {
            return $token;
        }, array_filter(
            array_map(function ($t) {
                return mb_strtolower($t, 'UTF-8');
            }, preg_split("/[\\s$delimiters]+/", $string, -1, PREG_SPLIT_NO_EMPTY)),
            function ($word) use ($stopWords) {
                return !in_array($word, $stopWords);
            }
        ));
        if ($asString) {
            return implode(' ', $result);
        }

        return $result;
    }