Why does `std::match_results::size` return 1 instead of 3 for multiple matches in a regex search?

std::match_results::size

In C , std::match_results::size is a function that returns the number of matched groups plus the overall match in a regex search. It's important to note that it does not return the total number of matches found.

In the example you provided:

<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>

You expected matches.size() to return 3, but instead, you get 1. This is because regex_search returns only one match, and size() returns the number of capture groups plus the whole match value. In this case, there is no capture group, so the size is 1 (the whole match itself).

To get multiple matches, you can use std::regex_iterator, which is demonstrated in the following alternative code:

<code class="cpp">std::regex rgx1("abc");
int i = 0;
smatch smtch;
while (regex_search(str, smtch, rgx1)) {
        std::cout << i << ": " << smtch[0] << std::endl;
        i += 1;
        str = smtch.suffix().str();
}</code>

This code destroys the input string, so here is another alternative using std::sregex_iterator:

<code class="cpp">std::regex r("ab(c)");
std::string s = "abcdefabcghiabc";
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
                             i != std::sregex_iterator();
                             ++i)
{
        std::smatch m = *i;
        std::cout << "Match value: " << m.str() << " at Position " << m.position() << '\n';
        std::cout << "    Capture: " << m[1].str() << " at Position " << m.position(1) << '\n';
}</code>

This iterator-based approach maintains the original string while allowing you to iterate through the matches.

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