sS handleHashtags(S text, IF1<S> handler) {
  LS tok = regexpICMatchesAsCNC(regexpNegativeLookbehind("\\w") + "#\\w+\\b", text);
  if (l(tok) == 1) ret text; // no hashtags found
  
  for (int i = 1; i < l(tok); i += 2)
    tok.set(i, unnull(handler.get(tok.get(i))));

  ret join(tok);
}