Regex is like a language unto itself. Even a lot of seasoned programmers have trouble with it.
Using it requires not only understanding every possible variation you want to match, but also every possible variation you want to not match.
A lot of people do fairy well with the first, but fail with the second.
For example, using
(.)* matches everything, anything, and nothing.
I see it used way too often as a “short cut” to get things to match, but unfortunately it often results in matching what it shouldn’t.
I guess if it were under the “developers only” section it might be enough to scare off Admins that shouldn’t mess with it. But human nature being what it is, give out loaded guns and it’s only a matter of time before someone shoots themselves in their foot.
And as for a valid email regex, it is notoriously difficult to craft a fool-proof one. Many come close and are “good enough” but without additional processing there will likely be problems at some point.