match_results operator[] in C++ STL

The match_results::operator[] is a built function in C++ which used to get the i-th match in the match_result object. It gives the reference to the match at the position given inside the operator.
Syntax:

smatch_name[N]

Note: smatch_name is an object of match_results class.

Parameters: It accepts a single parameter N which specifies the Match number. It is lower than match_results::size. The match number 0 represents the entire matched expression. Subsequent match numbers identify the sub-expressions if any. The passed integral is an unsigned integral type.
Return value: It returns the direct reference to the match at the N-th match. 
Note: First element always contains the whole regex match while the others contain the particular Capturing Group.
Below programs illustrates the above function. 
Program 1: 

CPP

// CPP program to illustrate
// match_results operator[] in C++
#include <bits/stdc++.h>
using namespace std;

int main()
{
    string s("Geeksforgeeks");
    regex re("(Geeks)(.*)");

    smatch match;

    regex_match(s, match, re);

    // use of operator[]--> returns the
    // reference to the match at i-th position
    cout << "Matches are:" << endl;
    for (int i = 0; i < match.size(); i++) {
        cout << "match " << i << " is " << match[i] << endl;
    }
}

Output:

Matches are:
match 0 is Geeksforgeeks
match 1 is Geeks
match 2 is forgeeks

Program 2: 

CPP

// CPP program to illustrate
// match_results operator[] in C++
// Find maximum length
#include <bits/stdc++.h>
using namespace std;

int main()
{
    string s("Geeksforgeeks");
    regex re("(Ge)(eks)(.*)");

    smatch match;

    regex_match(s, match, re);

    int max_length = 0;
    string str;

    // Since the first match is the whole string we do not consider it.
    for (int i = 1; i < match.size(); i++) {
        if (match.length(i) > max_length) {
            str = match[i];
            max_length = match.length(i);
        }
    }

    cout << "max-length sub match is " << str
         << " with a length of " << max_length << endl;
    return 0;
}

Output:

max-length sub match is forgeeks with a length of 8