Why does "[\\s\\S]" not working in C++11's regex

I have the code below this:

#include <string>
#include <regex>

int main(int argc, char const *argv[]) {
  std::string s = "_apple_";

  std::regex r1("_(\\s|\\S)+_");
  std::regex r2("_[\\s\\S]+_");
  std::regex r3("_.+_");
  std::regex r4("_[pale]+_");

  std::smatch sm;
  printf("r1:%d r2:%d r3:%d r4:%d\n", 
        std::regex_match(s, sm, r1), 
        std::regex_match(s, sm, r2), 
        std::regex_match(s, sm, r3), 
        std::regex_match(s, sm, r4));

  return 0;
}

output:r1:1 r2:0 r3:1 r4:1

I can not understand why r2 is not match?