[exim] Advice on a Regexp requested

Folks,

Given a subordinate 'Received' header containing the likes of:

Received: by giky28.corgiky.com (PowerMTA(TM) v3.0c2) id <the rest redacted>.

With a target of matching the 'PowerMTA' substring

AND an unpredictable number of characters preceeding it...

What might we have in Exim's toolbox AND NOT an external call, that is as
predictable / reliable as an SQL 'LIKE' comparison?

But more efficient of resources...

?

Thanks,

Bill Hacker