Since all but one string indicates an "app is not OK" situation, why not write the monitor such that it simply checks for the one "app is OK" condition Receive String (whatever that string is), and let any other response naturally result in the health monitor check failing and the member eventually being marked down? Unless I'm missing some additional details that are not in your original question, it does not seem like you need a Receive Disable String on the monitor here. You do not need to describe all the false (not OK) conditions in a monitor to mark a resource as unavailable (down). You just need to define the one condition that produces the "success" result in the Receive String, and any other response is a "fail" result.
Your receive string should work, although the parenthesis are unnecessary in your regular expression. Try removing them to see if that makes a difference. If the monitor is still marking a pool member down, it most likely means the pool member is not returning the expected string. You should check the server's response payload to ensure it is producing what you expected. For additional information on troubleshooting LTM health monitors, you can refer to this article: K12531: Troubleshooting health monitors.
To test regular expressions against various strings, I often use regex101.com.
