code: public function isValid($value)
      {
          if (($this->allowEmpty && empty($value)) || $this->validator->isValid($value)) {
              $this->errorMessage = '';
              return true;
          }
          $this->errorMessage = (empty($this->label) ? '' : "{$this->label}: ") . join(' / ', $this->validator->getMessages());
          return false;
      }
docs: {@inheritdoc}