I’m on enterprise. Are you sure this would not be treated as a folder, looking to purge everything under foo/ rather than anything that begins with foo?
It does seem to work as you want. I created https://cf.sjr.dev/foo-1.jpg (and 2 and 3), loaded them a few times to get cache HIT, then purging the prefix cf.sjr.dev/foo and all 3 showed a cache MISS on first load.