c++ - Difference between std::regex_match & std::regex_search? -

c++ - Difference between std::regex_match & std::regex_search? -

below programme has been written fetch "day" info using c++11 std::regex_match & std::regex_search. however, using first method returns false , sec method returns true(expected). read documentation , existing question related this, not understand difference between these 2 methods , when should utilize either of them? can both used interchangeably mutual problem?

difference between regex_match , regex_search?

#include<iostream> #include<string> #include<regex> int main() { std::string input{ "mon nov 25 20:54:36 2013" }; //day:: 2 number surrounded spaces in both side std::regex r{r"(\s\d{2}\s)"}; //std::regex r{"\\s\\d{2}\\s"}; std::smatch match; if (std::regex_match(input,match,r)) { std::cout << "found" << "\n"; } else { std::cout << "did not found" << "\n"; } if (std::regex_search(input, match,r)) { std::cout << "found" << "\n"; if (match.ready()){ std::string out = match[0]; std::cout << out << "\n"; } } else { std::cout << "did not found" << "\n"; } } output did not found found 25

why first regex method returns false in case?. regex seems right ideally both should have been returned true. ran above programme changing std::regex_match(input,match,r) std::regex_match(input,r) , found still returns false.

could explain above illustration and, in general, utilize cases of these methods?

regex_match returns true when entire input sequence has been matched, while regex_search succeed if sub-sequence matches regex.

quoting n3337,

§28.11.2/2 regex_match [re.alg.match] effects: determines whether there match between regular look e, , all of character sequence [first,last). ... returns true if such match exists, false otherwise.

the above description regex_match overload takes pair of iterators sequence matched. remaining overloads defined in terms of overload.

the corresponding regex_search overload described

§28.11.3/2 regex_search [re.alg.search] effects: determines whether there some sub-sequence within [first,last) matches regular look e. ... returns true if such sequence exists, false otherwise.

in example, if modify regex r{r"(.*?\s\d{2}\s.*)"}; both regex_match , regex_search succeed (but match result not day, entire date string).

live demo of modified version of illustration day beingness captured , displayed both regex_match , regex_search.

c++ regex c++11 visual-studio-2013 gcc4.9