Below program has been written to fetch the "Day" information using the C++11 std::regex_match & std::regex_search. However, using the first method returns false and second method returns true(expected). I read the documentation and already existing SO question related to this, but I do not understand the difference between these two methods and when we should use either of them? Can they both be used interchangeably for any common problem?

regex_match 和 regex_search 的区别?

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

int main()
{
    std::string input{ "Mon Nov 25 20:54:36 2013" };
    //Day:: Exactly Two Number surrounded by spaces in both side
    std::regex  r{R"(sd{2}s)"};
    //std::regex  r{"\s\d{2}\s"};
    std::smatch match;

if (std::regex_match(input,match,r)) {
        std::cout << "Found" << "
";
    } else {
        std::cout << "Did Not Found" << "
";
    }

    if (std::regex_search(input, match,r)) {
        std::cout << "Found" << "
";
        if (match.ready()){
            std::string out = match[0];
            std::cout << out << "
";
        }
    }
    else {
        std::cout << "Did Not Found" << "
";
    }
}

输出

Did Not Found

Found

 25 

在这种情况下，为什么第一个正则表达式方法返回 false?.regex 似乎是正确的，所以理想情况下两者都应该返回 true.我通过将 std::regex_match(input,match,r) 更改为 std::regex_match(input,r) 来运行上面的程序，发现它仍然返回 <代码>假.