code: private String parseUncRoot(String path, String original) {
      Matcher uncMatcher = UNC_ROOT.matcher(path);
      if (uncMatcher.find()) {
        String host = uncMatcher.group(2);
        if (host == null) {
          throw new InvalidPathException(original, "UNC path is missing hostname");
        }
        String share = uncMatcher.group(3);
        if (share == null) {
          throw new InvalidPathException(original, "UNC path is missing sharename");
        }
  
        return path.substring(uncMatcher.start(), uncMatcher.end());
      } else {
        // probably shouldn't ever reach this
        throw new InvalidPathException(original, "Invalid UNC path");
      }
    }
docs: Parse the root of a UNC-style path, throwing an exception if the path does not start with a
  valid UNC root.