Creating new modes for CodeMirror

I want to highlight only keywords that look like this: {KEYWORD} (basically UPPERCASE words wrapped between single {} parentheses)

I tried this by copying the code from the Mustache Overlay demo, and by replacing the double brackets with single ones:

CodeMirror.defineMode('mymode', function(config, parserConfig) {
  var mymodeOverlay = {
    token: function(stream, state) {
      if (stream.match("{")) {
        while ((ch = stream.next()) != null)
          if (ch == "}" && stream.next() == "}") break;
        return 'mymode';
      }
      while (stream.next() != null && !stream.match("{", false)) {}
      return null;
    }
  };
  return CodeMirror.overlayParser(CodeMirror.getMode(config, parserConfig.backdrop || "text/html"), mymodeOverlay);
});

but it doesn't work very good :)

Any ideas?