Regex empty string or email

Question

I found a lot of Regex email validation in SO but I did not find any that will accept an empty string. Is this possible through Regex only? Accepting either empty string or

Answer 1

The answers above work ($ for empty), but I just tried this and it also works to just leave empty like so:

/\A(INTENSE_EMAIL_REGEX|)\z/i

Same thing in reverse order

/\A(|INTENSE_EMAIL_REGEX)\z/i