#include <iostream>
#include <regex>
 
int main()
{
    std::regex const pattern { R"((.).*\1)" };
    std::cout << "'(.).*\\1' match with '': " << std::boolalpha << 
        std::regex_match("", pattern) << std::endl;
 
    std::cout << "'(.).*\\1' match with 'abc': " << std::boolalpha << 
        std::regex_match("abc", pattern) << std::endl;
 
    std::cout << "'(.).*\\1' match with 'aba': " << std::boolalpha << 
        std::regex_match("aba", pattern) << std::endl;
 
    std::cout << "'(.).*\\1' match with 'abcdefa': " << std::boolalpha << 
        std::regex_match("abcdefa", pattern) << std::endl;
}