Validations for User class:

- username must be present and unique, length between 3 and 25 characters

- email must be present and unique, length max of 105 characters

- validate email format using regex (regular expression)

You can find the code added to the user model during the video here: https://github.com/udemyrailscourse/alpha-blog-6/commit/1834571a5a219fb3347f0a3d658a7a8e89607a78

The ruby regular expression used to match the format of valid email addresses is listed below.

/\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i