Why does `std::match_results::size()` return 1 for a regex without capture groups?

Size of std::match_results

Question:

In the following C 11 code, why does matches.size() return 1 instead of the expected 3?

<code class="cpp">#include <iostream>
#include <string>
#include <regex>

int main() {
    std::string haystack("abcdefabcghiabc");
    std::regex needle("abc");
    std::smatch matches;
    std::regex_search(haystack, matches, needle);
    std::cout << matches.size() << std::endl;
}</code>

Answer:

The size() function of std::match_results returns the number of capture groups plus 1, indicating the complete match. In this case, there are no capture groups, so the size is 1.

Explanation:

The regex_search function finds the first occurrence of the regular expression in the input string. In this case, it finds "abc" at the beginning of the string. The matches object contains information about the match, including capture groups.

However, the provided regular expression contains no capture groups. Capture groups are parentheses in the regular expression that match specific parts of the input string. If capture groups were used, matches.size() would return the number of capture groups plus 1.

Finding Multiple Matches:

To find multiple matches, you can use an alternative approach that iterates over the matches:

<code class="cpp">int main() {
    std::regex r("abc");
    std::string s = "abcdefabcghiabc";
    int i = 0;
    std::sregex_iterator it(s.begin(), s.end(), r);
    std::sregex_iterator end;
    while (it != end) {
        std::smatch m = *it;
        std::cout << i++ << ": " << m.str() << std::endl;
        it++;
    }
    return 0;
}</code>

This code will print:

0: abc
1: abc
2: abc

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