[exim] ACL for headers check

Hello,
we would like to add an ACL rule to check a specific header
against a mysql database.

For example, we would like to check "From" for a regexp match.

Let's assume that our database is like this:

+----------------------------+
+ header | string            +
+----------------------------+
+ From   | /.*<mytest@.*>$/  +
+----------------------------+

Our lookup query should be something like this:

SELECT string FROM table WHERE header = 'From' \
AND $eximVariable REGEXP string


But we are unable to create the acl condition.
Can someone help us?

Thank you.