line endings in body tests

Hi,

SpamAssassin version 4.0.0
  running on Perl version 5.36.0

Debian 12 (bookworm)

I’m pulling my hair out.
A simple mail containing

------[snip]-----
teststring
some random text
------[snap]-----

and a rule in local.cf:
body TESTRULE /teststring$/
score TESTRULE 5.0

does NOT match when testing with spamassassin -t
BUT when the mail body looks like

------[snip]-----
teststring

sometext
------[snap]-----

the rule matches - why, why, why? There are no additional or hidden control characters.

body TESTRULE /^teststring/
score TESTRULE 5.0

matches in any case. spamassassin --lint does not complain.
I’m going slightly mad 🙁

Background: I want to tag all mails that have lines ending with " ." (space period), if this pattern occures at least 3 times.

I thought

------[snip]-----
body __2025030701 /\s\.$/
tflags __2025030701 multiple
meta 2025030701 __2025030701 >= 3
score 2025030701 5.0
describe 2025030701 Mindestens 3 Zeilen enden auf " ."
------[snap]-----

would do this, but I cannot get spamassassin to recognize the line endings.

Any hint is appreciated,

Rainer