docs: public static String getURLProtocol(String urlString) {
          String protocol = null;
          int firstColon = urlString.indexOf(':');
          if (firstColon >= 0) {
              String specifiedProtocol = urlString.substring(0, firstColon);
              if (FindBugs.knownURLProtocolSet.contains(specifiedProtocol)) {
                  protocol = specifiedProtocol;
              }
          }
          return protocol;
      }