When substituting .for control variables (as ${:U...}) escape '$' and '\'
as well as ':' and '}' or ')'.
The parameters have just been expanded (and will be expanded again) later
so don't need expanding as part of the :U process.
Seems to fix pkgsrc breakage at line 34 of bsd.pkginstall.mk doing:
.for i in ${PKG_USERS}
_PKG_VARS.pkginstall+=  PKG_UID.${u} PKG_GECOS.${u} PKG_HOME.${u} PKG_SHELL.${u}
.endfor
when PKG_USERS = ${AVAHI_USER}:${AVAHI_GROUP}::Avahi\ user:/nonexistent
(set at line 41 of the ahavi Makefile)
I really dont think the above has the desired effect!

(dsl)