The HTML <input type="email"> regex is a good starting point for "what's a valid email address": [1]

    /^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/

If you want to be a little more precise, the '.' characters in the local part cannot be consecutive nor occur at the start or end of the string. Also, the syntax does not account for EAI (add any non-C1 control non-ASCII Unicode character to the list of allowable characters), nor does it account for IDN (complicated, but it's IDN, there's libraries for it).

Unfortunately, email is in a weird space where there's a lot of clearly incorrect validators out there that fail on common stuff (e.g., dashes in domain names, weird TLDs, etc.), but trying to find a good one is hard because many people want to "help" by flexing how good they are at covering really, really weird cases that you honestly shouldn't want to support, like relay routing, or RFC 822 comments in email addresses (which aren't actually part of the email address!).

[1] https://html.spec.whatwg.org/multipage/input.html#email-stat...