code: public function setOwner(?string $owner)
      {
          if (null !== $owner) {
              if (!preg_match('/^[a-z]+$/i', $owner)) {
                  throw new DomainException('Owner is not valid.');
              }
          }
  
          $this->owner = $owner;
  
          return $this;
      }
docs: Set owner
  
  @param string $owner
  @return object
  @throws \DomainException