Understanding c++ regex by a simple example -

i wrote following simple example:

#include <iostream> #include <string> #include <regex>  int main () {     std::string str("1231");     std::regex r("^(\\d)");     std::smatch m;     std::regex_search(str, m, r);     for(auto v: m) std::cout << v << std::endl; } 

demo

and got confused behavior. if understood purpose of match_result there correctly, 1 1 should have been printed. actually:

if successful, not empty , contains series of sub_match objects: first sub_match element corresponds entire match, and, if regex expression contained sub-expressions matched ([...])

the string passed function doesn't match regex, therefore should not have had the entire match.

what did miss?

you still entire match entire match not fit entire string fits entire regex.

for example consider this:

#include <iostream> #include <string> #include <regex>  int main() {     std::string str("1231");     std::regex r("^(\\d)\\d"); // entire match 2 numbers     std::smatch m;     std::regex_search(str, m, r);     for(auto v: m) std::cout << v << std::endl; } 

output:

12 1 

the entire match (first sub_match) entire regex matches against (part of string).

the second sub_match first (and only) capture group

looking @ original regex

std::regex r("^(\\d)");               |----| <- entire expression (sub_match #0)  std::regex r("^(\\d)");                |---| <- first capture group (sub_match #1) 

that 2 sub_matches come from.