Interesting question. RFC1123 states that TLDs should be alphabetic, and that the first (or only) character must be a letter or a digit, which should exclude a TLD of '@'. local-parts can include most printable characters other than ()<>[];:@\,.

/^+@+$/ is easy and the false positives it allows are weird enough to accept.

People who have weird email addresses are used to them not working.

There is nothing before the first @, and nothing after the last @.