HTTP Monitor

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.