code: function regexpToRegexp(regex, keys, options) {
    if (!Array.isArray(keys)) return regex.source;
  
    let groups = regex.source.match(/\((?!\?)/g);
    if (!groups) return regex.source;
  
    let i = 0;
    let group = () => ({
      name: i++,
      prefix: null,
      delimiter: null,
      optional: false,
      repeat: false,
      partial: false,
      pattern: null
    });
  
    groups.forEach(() => keys.push(group()));
    return regex.source;
  }
docs: Create keys from match groups in the given regex
  @param  {!RegExp} path
  @param  {Array=}  keys
  @return {!RegExp}