implement Input Validation for User Registration, Login and Train Addition

[vivi2004]

Users can create an account by providing necessary information. Input fields include username, email, password, and confirm password. Validation checks: Username must be unique and follow a specific format (e.g., alphanumeric, minimum and maximum length). Email must be in a valid format. Password must meet complexity requirements (e.g., minimum length, include uppercase, lowercase, numbers, special characters). Password and confirm password must match

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