protected function validatePart($part)
    {
        if (strpos($part, '=>') !== false) {
            $subs = explode('=>', $part);
            $subs = array_filter($subs);

            foreach ($subs as $sub) {
                if (!$this->validateNoSpaces($sub)) {
                    return false;
                }
            }

            return true;
        }

        return $this->validateNoSpaces($part);
    }