[Exim] ACL / string expansion question

in my RCPT ACL, I'm trying to accept mail bounces for lists I have,
regardless of how broken the sending mail server is, so I'm trying to add a
an accept ACL like this one:

  accept  domains       = +local_domains
          condition     = ${extract{1}{-bounces+}{$local_part}{false}}

The RCPT I'm trying to match is:
Envelope-to: <svlug-bounces+lsann=hotmail.com@???>

Ideally, I'd like to get 'svlug' out of it if the match is successful so
that I can then check that svlug is a valid list before I accept the RCPT.

Unfortunately, my condition doesn't seem to work:

rcpt to: <svlug-bounces+lsann=hotmail.com@???>
>>> using ACL "acl_check_rcpt"
(...)

>>> check condition = ${extract{1}{-bounces+}{$local_part}{false}}
>>>                 = false

Suggestions?

Thanks
Marc
--
Microsoft is to operating systems & security ....
                                      .... what McDonalds is to gourmet cooking

Home page: http://marc.merlins.org/ | Finger marc_f@??? for PGP key