Convert lowercase to uppercase

I need to assure that the output from a PHS system is only in uppercase.  It appears to be a McKesson bug that even though CAPS lock is on the interface still sends information in lowercase.  It sounded like it could be simply handled by converting only the lowercase letters to uppercase with something like regsub….  I was looking at something like

regsub -all {[a-z]} $msg {[A-Z]} newmsg  (This doesn’t do the desired)

Any ideas?