Full email validation regexp

At over 23k, the current email validation regexp is getting pretty big. That doesn’t include the nested comment
stripping, which adds just over 1k, or the domain literal (IPv4 & IPv6) rules, which add another 1k.

It’s a testament to PCRE that it even runs.

And for nested comments:

And domain-literal IP addresses:

The full regexp for RFC2822 addresses was a mere 3.7k.