Vim with Ack: Search for all #pragma not ending in once

Using Ack in vim I want to find all instance of #pragma that does not end in once within c++ related files. What is the regex I need to use?

#pragma\s+.*(?<!once)$ works in python and javascript but not sure how to get ack to accept it.

:Ack --cpp #pragma\s+.*(?<!once)$ produces the following error in vim:

E194: No alternate file name to substitute for '#'

Escaping the # with \\\ returns nothing. I have tried a few other modifications, but not getting what I want.

via Chebli Mohamed