Yeah. Similarly, for the range transformations, instead of `[a:A-z:Z]`, I would suggest `[a-z:A-Z]`; and instead of `[a:b-y:zz:a]`, something like `[a-y:b-z;z:a]`, perhaps.
Then there is no syntactic special case. This is just EXPR:EXPR; the special case is that both EXPR are character class syntax, and so the tr-like range mapping applies.
So if we do [a-z]:[A-Z] it should be expanded to:
(a|b|...|z):(A|B|...|Z)
which is pretty legal in trre but has different meaning of mapping any a-z to ALL the A-Z (generating A-Z on each occurrence of lowercase letter).
Distinct syntactic forms can be given distinct semantics, as long as there is rhyme and reason.
Moreover, the right side of the colon is not the normal regex language, it only borrows its syntax. So there, we may be justified in denying that the usual equivalence holds between character class syntax and a disjunction of the symbols denoted by the class.
But I got your point. Maybe there could be some ways to do it in consistent way. Just straight tr-like syntax won't work, e.g I really want it something like this to be valid:
[a-b]:(x|y) (pairs a:x, b:x, a:y, b:y)
and I prefer not handle these in some ad-hoc way.