I believe the subtext is that "proper" validation of an email is deceptively complex when you would expect a single regex to do. I can't really vouch for if that assumption holds or not.

The code itself is very helpful and interesting, https://github.com/dominicsayers/isemail/blob/master/is_emai....

Point is that checking whether something is a valid email or not is 1179 lines long (I know there are a lot of comments). Particularly interesting is to look at all of the possible error conditions, and the fact that "whether a string is an email address" is not so much a binary yes-no, but the library provides a "likeliness threshold" option.

Essentially just pointing out that checking if a string is/can be an email address is surprisingly complex.

Yes. And not much point in all that careful checking against the RFCs - many email servers use, accept, and correctly process email addressed to technically invalid addresses.

This library runs the risk of blocking actual, working email addresses.