[exim] Pre-processing incoming mail

Hi list,

What would be the best way to pre-process incoming messages? By this I
mean that I'd like to single out certain messages and run one or more
search-and-replace routines over them before they're delivered.

For instance, I could write a perl script with some regular expressions
to do what I want, but then how would I get Exim to pipe these messages
through my script before delivery? Or would PCRE be a better choice?

Any suggestions would be much appreciated. Examples welcome too.

Jaap