upper/lower- case conversion of $MATCH

This is working:

        :0w
        * ^(Sender|To|Cc):.*\/majordomo-(users|docs|workers)
        | $APPNMAIL $LISTS/$MATCH

except when some person decides to send to Majordomo-Users or  
MajorDomo-Users or Majordomo-USERS

what's the best way to get this to lowercase?

{{ There was a mention of this some time ago, why can't I find it? }}

Efficiency is the goal.

TjL