docs: private static void checkRegexp(String regexp) {
          String checking = regexp.replaceAll("\\\\\\(", "X").replaceAll("\\(\\?", "X");
          int checked = checking.indexOf('(');
          if (checked > -1) {
              StringBuilder msg = new StringBuilder("Only non-capturing groups allowed:\r\n" +
                      regexp + "\r\n");
              for (int i = 0; i < checked; i++) {
                  msg.append(" ");
              }
              msg.append("^");
              throw new IllegalArgumentException(msg.toString());
          }
      }