Case insensitive search

Hi,

I have a server with an rfc2307bis schema and would like to search for users
being member of a group, but the uid  might be accidentally put in upper or
lower case or a combination.  An ldapsearch of uid is case insensitive, but
when I look for memberuid it is case sensitive. How can I created an
ldapsearch which is case insensitive ? I tried:

# filter: (&(memberuid:caseIgnoreMatch:=user)(cn=groupa))
# requesting: ALL
#
# search result

search: 2
result: 12 Critical extension is unavailable
text: Bad search filter

but it fails.

What can I do ?

Thank you
Markus