[exim] cannot test regex condition in MIME ACL

I did some initial trials of exiscan on a test system using exim-4.43 with the
exiscan patch. This was before exim-4.50 was released. On the basis of that I have
started setting up exim-4.50 built with WITH_CONTENT_SCAN but not with
WITH_OLD_DEMIME.

In my MIME ACL I have a regex test which worked fine in the trial system fine, but
gives "cannot test regex condition in MIME ACL" on the 4.50 system. There is no
problem using mime_regex, but I want to test the raw data.

Sectio 40.4 specifically says that regex is available in a MIME ACL and makes no
reference to any build-time requirements. I had a quick search of the source for
the error message, but can't find where it is.

What have I got wrong?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter