[Exim] Rewriting based on subject line?

hi,
Just a quick one - I need to give a yes or no answer to this ASAP and
figured this might be the quickest way to find out :-)

Is there an easy way of rewriting a sender address depending on the
subject line. We have a brain dead piece of software that sends all email
from one account (say fred@???). We currently rewrite this with a
normal rewriting rule. However, we now need to rewrite it to one address
if the subject line starts with a certain magic word and to another in
all other cases. Ideally removing the magic word in the process but we
could live without that.

ie.
Email from fred@???
Subject: [magic] blah blah

   becomes
     From: eric@???
     Subject: blah blah

but email from fred@???
    Subject: blah blah

    becomes
      From: paul@???
      Subject: blah blah

does that make sense? Any ideas/clues/pointers? :-)

thanks,

Darren