Procmail Regex (revised question)

[Linux Geeks]

Hi All,

Right now I have a global account which reside in my ISP's server, and
I twice a day get all my emails using fetchmail + cron and delivered
them to each mailbox using procmail.

Right now I'm using an abundant rules to check agains 'To' and
'Received' header from incoming mails as shown below

====================
:0
* ^To:.+a at domain\.com
! a at domain.com

:0
* ^Received:.+a at domain\.com
! a at domain.com

:0
* ^To:.+b at domain\.com
! a at domain.com

:0
* ^Received:.+b at domain\.com
! a at domain.com

:0
* ^To:.+c at domain\.com
! a at domain.com

:0
* ^Received:.+c at domain\.com
! a at domain.com
 ====================

This rule is very inefficient. As I create new user, I have to add two
additional rule entry. Is there any regex alternative to my problem  ?

Thanks,

Dino