code: public function getRegions($type = null)
      {
          if (null === $type) {
              return array_keys($this->matrix);
          }
  
          $regions = [];
          foreach ($this->matrix as $region => $types) {
              if (in_array($type, $types)) {
                  $regions[] = $region;
              }
          }
  
          return $regions;
      }
docs: Returns the available regions for a specified type.
  If not specified, all regions are returned.
  
  @param null $type
  
  @return array