ack: Exclude specific directories from search via regex

How do I ignore specific directories via RegEx with ack?

I can use the --ignore-dir option, but this does not let me specify a RegEx. I want to be able to ignore any directory, which has the words test or tests or more complicated patterns in its name.

I also tried a negative lookbehind via

ack -G '(?<!test)' pattern

but this does not work. It does not exclude the test directories.