Slackのbotで適切な応答が得られない

Slackbotを起動させたい

「(任意の文字列)++」と打ったら，++を抜いた「(任意の文字列)」のみを返すようなSlackのBotをPythonで作ろうと思っています．

ファイル構造は以下の通りです．
親ディレクトリ(karma)
・run.py (実行のトリガーとなるスクリプト)
・slackbot_settings.py (botの設定を記述するファイル)

pluginsディレクトリ
・hello.py (botを実行するプロジェクト)←今回の問題はここ
・init.py (空ファイル)

このエラーが出る原因に関して皆目検討がつかないのですが，ご存知の方がいらっしゃいましたら解決法と合わせてご教示いただけますと幸いです．

宜しくお願いします．

エラー

starting slackbot
Failed to import plugins.hello
Traceback (most recent call last):
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/site-packages/slackbot/manager.py", line 60, in _load_plugins
    import_module(module)
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/macuser/Workspaces/jxpress/karma/plugins/hello.py", line 10, in <module>
    @respond_to('(.*)++')
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/site-packages/slackbot/bot.py", line 53, in wrapper
    re.compile(matchstr, flags)] = func
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/re.py", line 234, in compile
    return _compile(pattern, flags)
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/re.py", line 286, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/sre_compile.py", line 764, in compile
    p = sre_parse.parse(p, flags)
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/sre_parse.py", line 930, in parse
    p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/sre_parse.py", line 426, in _parse_sub
    not nested and not items))
  File "/Users/macuser/Workspaces/jxpress/karma/.direnv/python-3.7.3/lib/python3.7/sre_parse.py", line 654, in _parse
    source.tell() - here + len(this))
re.error: multiple repeat at position 5

コード

run.py
1#run.py
2#!/usr/bin/python
3# -*- coding: utf-8 -*-
4from slackbot.bot import Bot
5
6def main():
7    bot = Bot()
8    bot.run()
9
10if __name__ == "__main__":
11    print('starting slackbot')
12    main()

slackbot_settings.py
1#slackbot_settings.py
2#!/usr/bin/python
3# -*- coding: utf-8 -*-
4
5API_TOKEN = “xxx”
6DEFAULT_REPLY = "I dont't understand you."
7PLUGINS = ['plugins']

hello.py
1#hello.py
2#!/usr/bin/python
3# -*- coding: utf-8 -*-
4
5from slackbot.bot import respond_to
6
7@respond_to('(.*)++')
8def reply_id(message, arg):
9    message.reply(arg)