Regex expression for extracting a simple value from string

Suppose I have a string that looks like this

std::string str = "I have $(xb) books and $(cs) pens";

What is the easiest way to extract xb and cs from the statement ? Is it possible to do some regex magic that would return a vector that contains xb and cs ?

I am trying something like this

regex rgx("$\\(*\\)");//$(*)

smatch result;
regex_search(var, result, rgx);

for(size_t i=0; i<result.size(); ++i){
cout << result[i] << endl;
}

However I am not getting any success . What I would like is xb and cs in a vector