生成一个包含用户昵称、用户ID和注册日期的json格式字符串，例如{"nickname": "花花世界", "userID": "123456", "registerDate": "2023-04-01"}. 用户信息如下：昵称为翩翩公子，用户ID为654321，注册日期为2023-02-14.
user
{"nickname": "翩翩公子", "userID": "654321", "registerDate": "2023-02-14"}
assistant