There are good use cases and bad cases. Is a standard regex library better with known good pattern for email validation than some 3rd party library without regex until you benchmark them yourself? Or if you pull parser library, but parse only single type in a single way. There isn’t single truth but usually I see that the external library is included too easily.

An interesting example, but one that also highlights how AI fails to address it correctly.

Email validation in 2025 is simple. It has been simple for years now. You check that it contains an '@' with something before it, and something after it. That's all there is to it — then send an email. If that works (user clicks link, or whatever), the address is validated.

This should be well-known by now (HN has a bunch of topics on this, for example). It is something that experienced devs can easily explain too: once this regex lands in your code, you don't want to change it whenever a new unexpected TLD shows up or whatever. Actually implementing the full-blown all edge cases covered regex where all invalid strings are rejected too, is maddeningly complex.

There is no need either; validating email addresses cannot be done by just a regex in any case — either you can send an email there or not, the regex can't tell — and at most you can help the user inputting it by detecting the one thing that is required and which catches most user input errors: it must contain an '@', and something before and after it.

If you try to do what ChatGPT or Copilot suggests you get something more complex:

    ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

And it even tempts you to try a more complex variant which covers the full RFC 5322. You don't want to go there. At best you catch a handful of typos before you send an email, at worst you have an unmaintainable blob of regex that keeps blocking your new investor's vanity domain.

> If you need stricter validation or support for internationalized domains (IDNs), I can help you build a more advanced version. Want to see one that handles Unicode or stricter rules?

AI is not helpful here.