Ack regex on filenames

According to this thread:

Q: My understanding is that ack uses Perl-derivative regular expressions, is that correct?

A: Yes. In fact, ack is written in Perl and just forwards the expressions to the internal engine.

Now, I have a file under the current directory called:

media_group_xml_producer.py

when I do:

> ack -n '.*group.*producer.*' 

or more directly:

> ack -n --python '.*group.*producer.*' 

I don't get any results. As far as I can tell, the regex should match this file. Why does it miss it?