I did some searching to confirm this, but it appears the Filter in the Syslog Monitor (NetXMS Management Console) is very basic in that it does not accept regular expressions or boolean things like "hostname AND deny"?
I know I can do this and a lot more in the actual syslog View, but sometimes just being able to filter things out on the fly as they come through is all that's required (regex would be a winner), as opposed to having to re-execute a search query over and over while looking at the logs as they come in.

Did I miss something in the documentation or is this currently not possible?

Cheers