// f : Matcher -> S
static S regexReplace(S s, S pat, O f) {
  Matcher m = Pattern.compile(pat).matcher(s);
  new StringBuffer buf;
  while (m.find())
    m.appendReplacement(buf, m.quoteReplacement(str(callF(f, m))));
  m.appendTail(buf);
  ret str(buf);
}