[Serusers] setting prefix according to $SIP_HF_FROM?

hi, is there some way, to prefix a given uri (=~[1-9]\d*, for example) based on the From-header field? suppose i want to extract a specific part (the area-code) from the From-header (like in bla(a)04711.sip...), i want to add prefix(echo $SIP_HF_FROM | sed 's/.*\@\([^.]*\)\..*/\1/') or something like that in my SER config, but (even with the exec- module, which isn't at all that flexible in my opinion) there seems to be no way to accomplish this. background is my previous mail regarding aliases based on SIP_DOMAIN. if i could just extract that area-code from the From-header, then i could easily prefix it to the To-header, so i would have no problems with aliases at all. any clues? brgds, g. feiner