This will match every valid E-Mail address except
those belonging to top level domains (ex: root@localhost). However, in most cases on a web form you don't want to receive a top level E-Mail address anyway.
It will also match a lot of invalid E-Mail addresses, but that doesn't matter for the following reasons:
(1) This regex is sufficient to catch a vast majority of catch-able accidental E-Mail formatting errors.
(2) You won't find a regex that can validate all E-Mail address formats with 100% accuracy.
(3) Complex regex's of the type that can validate the address format of most
E-Mail addresses are expensive to parse and run.
(4) If a user intentionally
does not want to provide you with their E-Mail address, a regex will not force them to do so, no matter how complex it is. "firstname.lastname@example.org" is a syntactically valid E-Mail address that is trivial to type and is still totally useless to you.