Comment by c0nstantine

[a-z] is equivalent to 'a|b|...|z' in the normal regex language.

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).