How to act every time a file is (re)-created with some words

I have a file /usr/local/var/log/locate/upd_locatedbs_core.log which is created every night. Normally it does not contain the string "Excluding ALL external volumes". When it does, I want to show a notification (via a shell script).
I created the following rule watching the folder /usr/local/var/log/locate/:

This works -- but only once: When I delete the file and recreate it (either by writing to it or by copying it), nothing happens again. However that's exactly what I want to test for: Whether the file was (re)created with a certain content....
Any idea how I can fix this? Many thanks in advance!
I created the following rule watching the folder /usr/local/var/log/locate/:

This works -- but only once: When I delete the file and recreate it (either by writing to it or by copying it), nothing happens again. However that's exactly what I want to test for: Whether the file was (re)created with a certain content....
Any idea how I can fix this? Many thanks in advance!