#include <iostream>
#include <regex>
 
int main()
{
    std::regex const pattern { "a?" };
    std::cout << "'a?' match with '': " << std::boolalpha << 
        std::regex_match("", pattern) << std::endl;
        
    std::cout << "'a?' match with 'a': " << std::boolalpha << 
        std::regex_match("a", pattern) << std::endl;
        
    std::cout << "'a?' match with 'aaaa': " << std::boolalpha << 
        std::regex_match("aaaa", pattern) << std::endl;
}