Is there's any reason to NOT use "@contains" operator with "exclamation mark" ?

for example, I want to make a rule to identify if the browser is only using "CURL" and if not, don't complete the request for that file. so I made this rule :

SecRule SCRIPT_FILENAME "/srv/home/userdir/admin\.php$" \ "chain,log,deny,phase:2,t:removeNulls,t:lowercase,"

SecRule REQUEST_HEADERS:User-Agent "!@contains Curl"

but what happen is, if I use CURL it doesn't complete it ! although I'm using the  "exclamation mark". and also other browsers !

any suggestions?

Mostafa Badr