function doBeginMatch(match) {
        const lexeme = match[0];
        const newMode = match.rule;

        const resp = new Response(newMode);
        // first internal before callbacks, then the public ones
        const beforeCallbacks = [newMode.__beforeBegin, newMode["on:begin"]];
        for (const cb of beforeCallbacks) {
          if (!cb) continue;
          cb(match, resp);
          if (resp.isMatchIgnored) return doIgnore(lexeme);
        }

        if (newMode.skip) {
          modeBuffer += lexeme;
        } else {
          if (newMode.excludeBegin) {
            modeBuffer += lexeme;
          }
          processBuffer();
          if (!newMode.returnBegin && !newMode.excludeBegin) {
            modeBuffer = lexeme;
          }
        }
        startNewMode(newMode, match);
        return newMode.returnBegin ? 0 : lexeme.length;
      }
Handle the start of a new potential mode match

@param {EnhancedMatch} match - the current match
@returns {number} how far to advance the parse cursor