The case body is executed upon a match, so is this not backwards? Also typical FreeBSD style puts the cases on a separate line, so something like:

case ${RELNUM} in
*-RELEASE | *-ALPHA* | *-BETA* | *-RC*)
        ;;
*)
        # error and exit
       ;;
esac

(also adding *-ALPHA* as noted on the svn mailing list)

Since we're going to add -ALPHA in here too I might drop the explicit -RELEASE form and make the text read more naturally with something along the lines of "is not a release (or alpha, beta or release candidate)." or "is not a release (including alpha, beta and release candidates)."