Regular Expression in Port Monitor

Regular Expression in Port Monitor

Hello I have set up a port monitor. the monitor executes a request and get a response in the format ":N " where N is a number indicating the length of a backlog. I want to monitor to make sure that backlog doesn't get too long. In the content match section of the port monitor I have entered the following regular expression: 

:[0-1][0-9]?$|:[2-9]$

which should match ":0" through ":19". Once the backlog reaches 20 it will change the status to trouble. However this does not work as intended. Instead I simply get an immediate trouble alert which reads:

The String ":[0-1][0-9]?$|:[2-9]$" is not present in the response

I've tried quoting the expression also with no luck. Are regular expressions accepted in the port monitor keyword check? If not is there another way to achieve this objective?