static S expandDollarRefsToMatches(S s, Matches m) {
  L<S> tok = javaTok(s);
  for (int i = 1; i < l(tok); i += 2) {
    if (tok.get(i).startsWith("$") && isInteger(tok.get(i).substring(1))) {
      S x = get(m.m, parseInt(tok.get(i).substring(1))-1);
      tok.set(i, unnull(x));
    }
  }
  ret join(tok);
}