From OpenNMS

In most cases, a service is considered down when the polled value exceeds certain value or falls below certain value. However, there are several cases where we have to check if the polled value lies within certain range. Unfortunatelly, OpenNMS service poller can only have one operand. The work around for this is to use regular expression operator (~) to evaluate the polled value.

For example, if the valid value of a monitored service should be in the range of 180 up to 240, we can use the following regular expression to evaluate it: "1[8-9][0-9]|2[0-3][0-9]|240"

Note that it's usually preferable, for a variety of reasons, to configure data collection (via SNMP, HTTP, JDBC, XMP, or another supported protocol) for the variables you're interested in and then to use a threshold to express and enforce the range of values within which those variables should fall.