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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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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.
*nix is more likely to be a glob, therefore an accurate version would be *n?x
Edit: global -> glob dang autocorrect
Actually *nix isn't a Regular Expression, because the star operator * requires a preceding character or object to apply to. This is a wildcard for the shell style globbing, where a single star doesn't require a second object.
Isn't (I|U) equivalent to ([IU])?
Yes, but you can really only do that with single characters, since your first example is an ordered group and the second is an unordered set in a capturing group. The equivalency drops off when you include more characters.
Plus, you can do things like [a-zA-Z], and you can't do that with the former example.
I would imagine there's a difference in computing overhead, too, but I have no idea which is more performant.
We're talking about Unix so being as pedantic as possible is actually required.
at that point we could just flip the switch for the case insensitive mode
But then you'd match terms like "liNuX" and "UniX," and that's just silly. 😆
b{2}|[^b]{2}
(two B's or no two B's)