> The sane thing would be to loop through building the output string, adding the replacement for each symbol as you go.

As follows:

    // SYMBOL_REF should be a class-level static final to avoid recompiling on each call.

    int pos = 0; // input[..pos] has been processed.
    StringBuilder out = new StringBuilder(); // could also guess at length here.
    Matcher m = SYMBOL_REF.matcher(input);
    while (m.find()) {
      String replacement = symbols.get(m.group(1));
      if (replacement == null) {
        continue; // no such symbol; keep literal `$foo`.
      }
      out.append(input, pos, m.start());
      out.append(replacement);
      pos = m.end();
    }
    out.append(input, pos, input.length());

Coding style must serve the purpose of aiding understanding. If you have strong opinions about the coding style of `replace` but those opinions don't lead to recognition that the implementation was incorrect and inefficient, your opinions are bad and you should feel bad. Stop writing garbage books and blog posts!

</rant>