IOS XR route policy : BGP help (reg exp)

I'm trying to do something which I know should be simply but I'm failing :/

I want a simple route-policy which will allow me to advertise an as if it originates from a certain AS number but ONLY if it originates from that AS and doesn't pass through any other AS's.

I've got:

route-policy CUSTOMER-AS
if as-path originates-from '12345' then pass
endif
end-policy

But this will allow my router to advertise the prefixes even if they pass through multiple AS's before reaching me.

Thanks