code: function regExpPrefix(re) {
      var prefix = /^\^((?:\\[^a-zA-Z0-9]|[^\\\[\]\^$*+?.()|{}]+)*)/.exec(re.source);
      return (prefix != null) ? prefix[1].replace(/\\(.)/g, "$1") : '';
    }
docs: Returns a string that is a prefix of all strings matching the RegExp