code: func parseContentRangeFirstByte(s string) int64 {
  	matches := regexp.MustCompile(`(\d+)-`).FindStringSubmatch(s)
  	if matches == nil {
  		return 0
  	}
  	ret, _ := strconv.ParseInt(matches[1], 0, 64)
  	return ret
  }
docs: // Parses out the first byte from a Content-Range header. Returns 0 if it
  // isn't found, which is what is implied if there is no header.