public function filter($value)
    {
        if (empty($value) && isset($this->filterData[$this->fieldToSlugFrom])) {
            $value = $this->filterData[$this->fieldToSlugFrom];
        }

        if (is_null($value)) {
            return $this->setEmpty();
        }

        $value = transliterator_transliterate($this->transliterator, $value);
        $value = iconv("UTF-8", "ASCII//TRANSLIT//IGNORE", $value);
        $value = preg_replace('/[^\w\d]+/', '-', $value);
        $value = trim($value, '-');
        return strtolower($value);
    }