// #include <boost/format.hpp>
// or
#include <boost/optional.hpp>

#include <boost/xpressive/xpressive.hpp>
#include <boost/xpressive/regex_actions.hpp>
using namespace boost::xpressive;

int main(int,char**)
{
   int nec=0;

   std::map<std::string, int> nec_map;
   nec_map["O"] = 1;
   nec_map["W"] = 2;

   sregex rx_res =
         *_s
            >> optional( ((a1 = nec_map ) >> ':')[ ref(nec) = a1 | 0 ] );

   return 0;
}