I just want to make sure I understood this correctly.
(rx (one-or-more (any upper lower)))
is equal to
(rx (one-or-more (any "A-Z" "a-z")))
Correct?
The macro rx returns regexp strings that can be passed to other Emacs functions.
ELISP> (rx (one-or-more (any upper lower)))
"[[:lower:][:upper:]]+"
ELISP> (rx (one-or-more (any "A-Z" "a-z")))
"[A-Za-z]+"
That doesn't answer your question directly; it pushes the question to "are these two regexes identical?" So, let's look for an uppercase or lowercase letter that is not between A and Z, or a and z. Let's try á.
ELISP> (string-match-p (rx (one-or-more (any upper lower))) "á")
0 (#o0, #x0, ?\C-@)
ELISP> (string-match-p (rx (one-or-more (any "A-Z" "a-z"))) "á")
nil
So the regexes are not identical. Presumably you want to use (rx (one-or-more (any upper lower))) most of the time; it not only includes characters most people think of as letters, but I'd argue is also more readable.
[A-Z]matches only an ASCII uppercase letter, that is, a letter fromAthroughZ. There are other, non-ASCII uppercase letters (e.g., in languages other than English).