[Exim] acl_check_content regex problem

Hello,

I got a problem with the acl_check_content ACL.
I had installed exim-4.20-4_06 rpms, and just upgraded to 4.20-6_07
to no avail. (on RedHat 7)

The config file reads :

acl_check_content:
   warn message = X-ACL: This message matches a blacklisted regex
($regex_match_string)
        regex = \[ADV\]

As far as I understand, this should point out messages containing
[ADV] somewhere in the header or the body.

I have put the 'warn' instead of 'deny' because every single email
got rejected. They all seem to match the regex ?!?!?

The incoming message got the following line inserted in the headers:
X-ACL: This message matches a blacklisted regex ([ADV])

Even if they do not contain [ADV] at all.

What do I do wrong, other rules in the same ACL do work fine
(demime, spam)

Thanks,
Yann.