Hello Arash,
If you have a scenario where you have to put the email address which contain only one letter before @symbol then you have to customize create user method with the new regex pattern. Kindly find a link from
here, which has the method of register user, which you can use to make a rest call for registering your user with App42 and set your email format at your end.
Also, find a below code snippet to call that method & let us know if it solve your issue:
App42UserService app42UserService = new App42UserService("api_key", "secret_key");
User userObj = app42UserService.registerUser("Your_username","Your_password","Your_emailId");
Thanks,
Himanshu Sharma