[Exim] Rewriting incoming message subject line

Hi,

I'm on the Ruby mailing list and they send subject lines in the
following format:

Subject: [ruby-talk:03200] Re: Ruby Tutorial? URLS please.

I want to set up a filter to remove the [...] part from the subject.
(Come to think of it, I also might want this for the exim mailing
list's [exim]). Can anyone say how to do this?

regards,
Richard