Are you sure this is the best regex for validation?

There is this regex: https://emailregex.com/

Seeing the amount of code let's me think of left-pad. Minimal code that should just be part of your own project instead of using a third party dependency.

Also obligatory: don't validate emails, just send the user an email and make them confirm by clicking a link in the mail.

No, this is not the best regex. This is just to set up the initial phase of the project, in the future it will be improved.

I don't want to be the old fart killjoy, but why not use net/mail.Parse()?

Using regexp for email parsing is really error prone and gives email validators that don't accept characters like + that are often used for aliases, etc

This is just the basic initial setup. You will see improvements!