// e.g. s = "hello $1" and m = Matches("world")
static S expandDollarRefsToMatches(S s, Matches m) {
  ret expandDollarRefsToMatches(s, m, false);
}

static S expandDollarRefsToMatches(S s, Matches m, bool alwaysQuote) {
  L<S> tok = javaTok(s);
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (!t.startsWith("$")) continue;
    t = dropFirst(t);
    int j = parseIntAtEnd(t);
    if (j < 1) continue;
    S x = get(m.m, j-1);
    if (x == null) continue;
    if (alwaysQuote || startsWith(t, "q"))
      x = quote(x);
    tok.set(i, x);
  }
  ret join(tok);
}