[Exim] Checking for sender_host_address

I need to check whether a message has been sent from one of our
own networks which are of form

a.b.c.d/19
e.f.g.h/20
j.k.l.m/21

Is there an easier way than a long-winded check on $sender_host_address
with net19-lsearch;/my/networks 
     net20-lsearch;/my/networks   and then
     net21-lsearch;/my/networks

Dave
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Dave Lewney  
Unix System Manager             Email:  D.M.Lewney@??? 
University of Brighton          phone:  01273 642669
UK                              fax:    01273 642666