code: private static int parseOctalMode(CharSequenceScanner parse) throws IllegalArgumentException {
  
      String octals = parse.readWhile(CharFilter.LATIN_DIGIT_FILTER);
      if (octals.length() == 0) {
        return -1;
      }
      if (octals.length() > 4) {
        throw new IllegalArgumentException();
      }
      return Integer.parseInt(octals, 8);
    }
docs: This method parses the current state of the {@link #chmod(String) chmod} argument given by {@code parse} in octal
  mode. If no digits are detected, {@code parse} remains unchanged and {@code -1} is returned, else all digits are
  consumed and the parsed octal-mode is returned.
  
  @param parse is the current parser state of the the {@link #chmod(String) chmod} argument.
  @return the parsed octal-mode or {@code -1} if {@code parse} does NOT point to a digit.
  @throws IllegalArgumentException if there are more the 4 digits or non-octal digits (8 or 9).