regexp for SMTP AUTH

[Rainer Fügenstein]

hi,

mails delivered via sendmail SMTP AUTH contain a Received: header like this:

Received: from [192.168.5.238] (xyz.example.com [90.217.201.80])
     (authenticated bits=0)
     by myserver.mydomain.at (8.13.8/8.13.8) with ESMTP id s0VE3Iwj027715
     (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
     for <m...@home.org>; Fri, 31 Jan 2014 15:03:19 +0100

could anyone here help with a rule that matches this header, please? I
tried for quite some time now but can't get it done :-(

important parts to match are "(authenticated bits=\d+)" and
"myserver.mydomain.at"

thank you.