code: function createRegexString(prefix) {
    var key = 'createRegexString:' + prefix;
    if (cache.hasOwnProperty(key)) {
      return cache[key];
    }
    var str = (prefix + '(\\d+)$}').replace(/\\?([${}])/g, '\\$1');
    cache[key] = str;
    return str;
  }
docs: Create a string to pass into `RegExp` for checking for and finding async ids.
  @param  {String} `prefix` prefix to use for the first part of the regex
  @return {String} string to pass into `RegExp`