code: func (sh *Helper) GetBytes(key string) ([]byte, error) {
  	if i, ok := sh.IndexMap[key]; ok {
  		if i <= len(sh.submatch) {
  			return sh.submatch[i], nil
  		}
  		return []byte{}, ErrIndexOutOfRange
  	}
  	return []byte{}, ErrNoSuchKey
  }
docs: // GetBytes : Get the matching []byte (if any) extracted from
  // the data matched by the Helper's regexp. Helper needs to be
  // prepared using the Prepare function to work properly.