So (^/$|/[0-9a-zA-Z-]+.(html|css|js|txt)) should match the URLs you want to exclude - you can then put something like not http.request.uri.path matches "(^/$|/[0-9a-zA-Z-]+.(html|css|js|txt))"
So you would need to be on the Business or Enterprise plan to use regular expressions in your Firewall rules.
Without this, you would need to use equals but that will mean you’ll need to explicitly define each URL you want to trigger on (or not trigger on), or use the contains operators etc to do something without regular expressions.