#include <iostream> /* C++ iostream C++98/11 */
#include <string> /* C++ strings C++98/11 */ /* RegEx Boost */
#include <boost/algorithm/string.hpp>
int main() {
std::string str = "\\dir\\dir2\\dir4\\dir4\\dir6";
auto itr = boost::find_nth(str, "\\", 4);
if(itr.empty()) std::cout << "empty";
else std::cout << itr.begin() - str.begin();
}
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