code: private function getDefaultNormalizer()
      {
          $default = $this->getDefault();
          $trueRegex = $this->trueAnswerRegex;
          $falseRegex = $this->falseAnswerRegex;
  
          return function ($answer) use ($default, $trueRegex, $falseRegex) {
              if (is_bool($answer)) {
                  return $answer;
              }
              if (empty($answer) && !empty($default)) {
                  return $default;
              }
  
              if (preg_match($trueRegex, $answer)) {
                  return true;
              }
  
              if (preg_match($falseRegex, $answer)) {
                  return false;
              }
  
              return null;
          };
      }
docs: Returns the default answer normalizer.
  
  @return callable