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.

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