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
.