Hi Heiko,
What you have done should work in Windows. If you are in Unix, you would
need to use single quotes instead.
I copied your exact command and it works fine for me here.
You can test your condition with an extract command. Try these:
exiftool -keywords -if "not $Keywords=~/hugo/" *.jpg
exiftool -keywords -if "$Keywords=~/hugo/" *.jpg
The first command should print keywords for all files without "hugo" in the keywords,
and the second should print all the keywords with "hugo".
- Phil