Difference between std::regex_match & std::regex_search?

Question

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?

Difference between regex_match and regex_search?

#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"(\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 this case?. The regex seems to be correct so ideally both should have been returned true. I ran the above program by changing the std::regex_match(input,match,r) to std::regex_match(input,r) and found that it still returns false.

Could somebody explain the above example and, in general, use cases of these methods?

Answer 1

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

Quoting from N3337,

§28.11.2/2 regex_match [re.alg.match]

Effects: Determines whether there is a match between the regular expression e, and all of the character sequence [first,last). ... Returns true if such a match exists, false otherwise.

The above description is for the regex_match overload that takes a pair of iterators to the sequence to be matched. The remaining overloads are defined in terms of this overload.

The corresponding regex_search overload is described as

§28.11.3/2 regex_search [re.alg.search]

Effects: Determines whether there is some sub-sequence within [first,last) that matches the regular expression e. ... Returns true if such a sequence exists, false otherwise.

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In your example, if you modify the regex to r{R"(.*?\s\d{2}\s.*)"}; both regex_match and regex_search will succeed (but the match result is not just the day, but the entire date string).

Live demo of a modified version of your example where the day is being captured and displayed by both regex_match and regex_search.

Answer 2

It's very simple. regex_search looks through the string to find if any portion of the string matches the regex. regex_match checks if the whole string is a match for the regex. As a simple example, given the following string:

"one two three four" 

If I use regex_search on that string with the expression "three", it will succeed, because "three" can be found in "one two three four"

However, if I use regex_match instead, it will fail, because "three" is not the whole string, but only a part of it.