Indeed: technically, it's impossible. Email addresses, per RFC 822, are not a regular language -- though it's doable in Perl because Perl "regular expressions" are unapologetically more powerful than real regular expressions.
RFC 822, section 3.4.3, defines comments in email addresses to be surrounded by matching parentheses, and which nest -- a classic example of a language which is not regular.
Chalk this up to "stupid technicalities I learned one day when I decided to write a regex to match RFC 822 perfectly". It turned out to be a fun but useless exercise, because most email programs don't accept half the crap that's in RFC 822 anyway.
RFC 822, section 3.4.3, defines comments in email addresses to be surrounded by matching parentheses, and which nest -- a classic example of a language which is not regular.
Chalk this up to "stupid technicalities I learned one day when I decided to write a regex to match RFC 822 perfectly". It turned out to be a fun but useless exercise, because most email programs don't accept half the crap that's in RFC 822 anyway.