问奇怪的bug - std::regex只匹配前两个字符串

EN

#include <iostream>
#include <regex>
#include <vector>

int main()
{
        std::regex reg("rmvb|avi|rm|mp4|256");

        std::vector<std::string> ext{"rmvb", "avi", "rm", "mp4", "256", "null"};

        for (int i = 0; i < 6; i++)
        {
                std::cout << ext[i] << "\t" << std::boolalpha << std::regex_match(ext[i], reg) << std::endl;
        }

        return 0;
}

rmvb    true
avi true
rm  false
mp4 false
256 false
null    false