Because there's non-programmers in this community, if you aren't sure what this means but are too afraid to ask, it's a Regular Expression that better represents the terms "Linux" and "Unix."

Though if we're going to be that pedantic, it would be [nN][uiI][xX]$. That extra pipe wouldn't actually do anything in the last example, because regexp picks one character from the set by default.

And if we want to be really pedantic,

(?!nix)[nN][uI][xX]$

Would be the most accurate.

Edit: based on comments, I think...

(nux|NIX)$

...would be the best. Then you don't wind up with weird matches with things like UNiX.

Isn't that the globbing operator?

There's a good idea and some real potential here, but it didn't quite land for me.