how to issue a `start of string` pattern for ag

I want to find every file in from the current dir and downwards whose filename starts with foo using __[ag](https://github.com/ggreer/the_silver_searcher)__(the silver searcher). I have tried: ag -g '^foo' ag -g '\^foo' ag -g '\Afoo' no luck. But it should work as ag implements PCRE syntax, right? What am I missing here?