When you go to Audiences you will find many prebuilt browser selection options. If you need something not already built out (i.e., Safari 11) you can use a Profile Script to evaluate anything in the useragent string. Something like this for your profile script will return a 'true' or 'false':
I don't exactly know what you should be looking for to identify Safari 11, but this should work as a template for you. Then you can create an audience rule that keys off of the true/false value of this profile script. All profile scripts once activated can be selected in an audience rule under Visitor Profile with the name you gave the profile script prefaced with a "user."
