Emacs query-replace with textual transformation

Question

I want to find any text in a file that matches a regexp of the form t[A-Z]u (i.e., a match t followed by a capital letter and another match u, and transform the matched text so that the capital letter is lowercase. For example, for the regexp x[A-Z]y

xAy

becomes

xay

and

xZy

becomes

xzy

Emacs' query-replace function allows back-references, but AFAIK not the transformation of the matched text. Is there a built-in function that does this? Does anybody have a short Elisp function I could use?

UPDATE

@Marcel Levy has it: \, in a replacement expression introduces an (arbitrary?) Elisp expression. E.g., the solution to the above is

M-x replace-regexp <RET> x\([A-Z]\)z <RET> x\,(downcase \1)z