Regex works very well for what it was originally designed: describing/validating regular languages. It can work ok if your language is simple and almost regular

Regex works very well for what it was originally designed: describing/validating regular languages. It can work ok if your language is simple and almost regular. They work very badly for validating non-regulars languages, even when extensions are added Perl-style to support that. And, unfortunately, most structured formats you might care to valdiate are in fact not regular languages at all. Email addresses in particular are surprisingly complicated and far from being regular languages. I don't know how commonly real servers support the full feature set, but even if they just support non-ascii names they quickly become a pain.