Is this to find a file extension?

Yes, I'm afraid. (Still trying to understand the code 😛 )

https://github.com/babashka/fs/blob/master/API.md#babashka.fs/extension :)

Or just https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/nio/file/Path.html#endsWith(java.lang.String)

The grouping seems unnecessary?

I'd go with (.endsWith f ".jpg") 🤷

Or, if you need to support multiple endings, just (re-matches #"\.jpe?g$" f)

Was about to say. 😃

Though it still may make more sense to use babashka.fs or similar to extract the extension, lowercase it and test it against some list.

(re-matches #"(?i)\.jpe?g$" f) should be fast and precise. Anyways, I was just surprised be the initial code snippet I had found and it took me a while to understand what it really wanted to achieve. 😉

I like the term code perl, btw. TIL.

Presumably "code perl" is code that is write-only and named at PERL code?