Regular expression return only certain values PHP

I cant remember what to use to return only a specific part of a string. I have a string like this:-

$str = "return(me or not?)";

I want to get the word which is after (. In this example me will be my result. How can I do this?

I dont think substr is what I am looking for. as substr returns value based on the index you provided. which in this case i dont know the index, it can vary. All I know is that I want to return whatever is after "(" and before the space " ". The index positions will always be different there for i cant use substr(..).