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

int main()
{
    std::string s = "3.89C";

    // re = ^([-+]?[0-9]+(\.[0-9]*)?)([CF])$
    sregex re = bos                                 // ^
        >> (s1=                                     // (
                !(set= '-', '+')                    // [-+]?
                >> +range('0', '9')                 // [0-9]+
                >> !(s2= '.' >> *range('0', '9'))   // (\.[0-9]*)?
           )                                        // )
        >> (s3= (set= 'C', 'F'))                    // ([CF])
        >> eos;                                     // $

    smatch what;
    if (regex_match(s, what, re)) {
        std::cout << what[0] << std::endl;  // whole match
        std::cout << what[1] << std::endl;  // first capture
        std::cout << what[2] << std::endl;  // second capture
        std::cout << what[3] << std::endl;  // third capture
    }

    return 0;
}