sed in makefile is not working as expected when using regex

Got a makefile whith this command which convert folder names on ./cmd/ from snake_case to PascalCase

test:
	@for f in $(shell ls ./cmd/); do \
		echo $${f}; \
 		echo $${f} | sed -r 's/(^|_)([a-z])/\U\2/g'; \
	 done

What I get when I run it is, with a prefixed uppercase U:

api_get_manual
UapiUgetUmanual

And what I expect to get:

ApiGetManual