Creating Functions to Register App Users

You are a junior developer working on a registration system for a mobile app at a small startup. The product managers have asked you to improve the current sign-up flow by integrating reusable Python functions that validate user input entered into the form.

Your task is to integrate these validation functions to approve or reject sign-up attempts before account creation. Together, these existing validation functions and the new registration functions you will build will significantly improve the robustness and quality of the sign-up process, enhancing onboarding for thousands of app users.