[Exim] Possibility of rewriting the subject...

Hi,

how can I rewrite the suject of a message?
I'm thinking of headers_remove and headers_add in conjunction with
the variable h_subject .
I think it would be easy to add characters to the h_subject, but how
can I remove i.e. "FW: " ??
Is it possible to use regular expressions within headers_add??

Thanx so far,

Michael