sS regexpQuote_useBackslashes(S s) {
  if (s == null) null;
  StringBuilder buf = null;
  int n = l(s);
  for i to n: {
    char c = s.charAt(i);
    if (regexpSpecialCharacters().indexOf(c) < 0) {
      if (buf != null) buf.append(c);
    } else {
      if (buf == null) buf = new StringBuilder(takeFirst(i, s));
      buf.append('\\');
      buf.append(c);
    }
  }
  ret buf == null ? s : buf.toString();
}