#include <iostream>
#include <string>
#include <regex>

int main() 
{
  std::string expr = "", s = "";
  std::smatch matches;
  if (std::regex_match(s, matches, std::regex(expr)))
  {
    // matches[0] contains the original string.
    // matches[n] contains a sub_match object for each matching subexpression
    for (auto i = 0; i < matches.size(); ++i)
    {
      // sub_match is pair of iterators to the first
      // and one past the last chars of the matching subexpression
      // (matches[i].first, matches[i].second); 
      std::string match = matches[i];
      std::cout << "\tmatches[" << i << "] = " << match << '\n';
    }
  }
  else std::cout << "The regexp \"" << expr 
                 << "\" does not match \"" << s << "\"\n";
}