protected function splitValues($value)
    {
        $delimiter = preg_quote($this->delimiter, '/');

        if ($this->trim_whitespace) {
            $expression = '/\s*' . $delimiter . '\s*/u';
        } else {
            $expression = '/' . $delimiter . '/u';
        }

        return preg_split($expression, $value, -1, PREG_SPLIT_NO_EMPTY);
    }