[Exim] Disabling "me too" the hard way

As far as the documentation and posts here indicate, there's no
configuration option for turning off 'me too' posts resulting from
alias expansions.

I've attempted to fake this ability using the data line I've included
below, but it seems a nasty way of doing it. It would be a lot
simpler if I could guarantee that the expanded addresses were all
qualified. As it is, it removes the entire sender_address if the
sender's domain is not the one that unqualified usernames get given,
and removes the local_part with an optional domain if it is.

Can anyone suggest an improvement to this, or spot any mistakes,
below? Disabling me-too is a site policy, so leaving it enabled isn't
an option.

  data = ${if eq{${domain:$sender_address}}{$qualify_domain}\
                  {\
                    ${sg {\
                          ${lookup{$local_part}nis{mail.aliases}}\
                         }\

{^(.*)${local_part:$sender_address}(?:@${domain:$sender_address})?(.*)\$}\
                         {\N$1$2\N}\
                     }\
                  }\
                  {\
                    ${sg {\
                         ${lookup{$local_part}nis{mail.aliases}}\
                         }\
                         {^(.*)$sender_address(.*)\$}\
                         {\N$1$2\N}\
                     }\
                  }\
          }

Thanks,
Simon.