function join(regexps, separator = "|") {
  // backreferenceRe matches an open parenthesis or backreference. To avoid
  // an incorrect parse, it additionally matches the following:
  // - [...] elements, where the meaning of parentheses and escapes change
  // - other escape sequences, so we do not misparse escape sequences as
  //   interesting elements
  // - non-matching or lookahead parentheses, which do not capture. These
  //   follow the '(' with a '?'.
  const backreferenceRe = /\[(?:[^\\\]]|\\.)*\]|\(\??|\\([1-9][0-9]*)|\\./;
  let numCaptures = 0;
  let ret = '';
  for (let i = 0; i < regexps.length; i++) {
    numCaptures += 1;
    const offset = numCaptures;
    let re = source(regexps[i]);
    if (i > 0) {
      ret += separator;
    }
    ret += "(";
    while (re.length > 0) {
      const match = backreferenceRe.exec(re);
      if (match == null) {
        ret += re;
        break;
      }
      ret += re.substring(0, match.index);
      re = re.substring(match.index + match[0].length);
      if (match[0][0] === '\\' && match[1]) {
        // Adjust the backreference.
        ret += '\\' + String(Number(match[1]) + offset);
      } else {
        ret += match[0];
        if (match[0] === '(') {
          numCaptures++;
        }
      }
    }
    ret += ")";
  }
  return ret;
}
@param {(string | RegExp)[]} regexps
@param {string} separator
@returns {string}