C++ Academy · Lesson

Capturing Groups

Extract matched parts.

What Is a Capturing Group?

Parentheses in a pattern create a capturing group, remembering the part of the text they matched so you can extract it afterward.

(\\d+) captures a run of digits.
Results land in a std::smatch.

#include <iostream>
#include <regex>

int main() {
    std::regex p("(\\d+)");
    std::smatch m;
    std::string s = "order 42";
    if (std::regex_search(s, m, p)) {
        std::cout << "Captured: " << m[1] << '\n';
    }
    return 0;
}

The smatch Result

std::smatch stores the match results. Index [0] is the whole match, and [1], [2], ... are the capturing groups in order.

#include <iostream>
#include <regex>

int main() {
    std::regex p("(\\w+)@(\\w+)");
    std::smatch m;
    std::string s = "user@host";
    if (std::regex_search(s, m, p)) {
        std::cout << "Whole: " << m[0] << '\n';
        std::cout << "User: " << m[1] << ", Host: " << m[2] << '\n';
    }
    return 0;
}

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