If the handler matches it correctly, it should be called. I’d think the issue is in your Regex. Can you pass your code this way?
For others who are curious how to write these handlers, there are a few threads around the forums and some info in the docs:
https://docs.thinkboxsoftware.com/products/deadline/10.0/1_User%20Manual/manual/application-plugins.html?highlight=stdouthandler