The right side is a normal regex language syntactically. Semantically it is a generator instead of a parser (consumer).

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.