match_results length() in C++ STL

The match_results::length() is a inbuilt function in C++ which is used to return the length of a particular match in the match_results object.

Syntax:

smatch_name.length(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 length of the n-th match in the match_results object.

Note: First element always contains the whole regex match while the others conatain the particular Capturing Group.

Below programs illustrate the above function.

Program 1:

// CPP program to illustrate 
// match_results length() in C++ 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    string s("Geeksforgeeks"); 
    regex re("(Geeks)(.*)"); 
  
    smatch match; 
  
    regex_match(s, match, re); 
  
    for (int i = 0; i < match.size(); i++) { 
        cout << "match " << i << " has a length of "
             << match.length(i) << endl; 
    } 
    return 0; 
} 

Output:

match 0 has a length of 13
match 1 has a length of 5
match 2 has a length of 8

Program 2:

// CPP program to illustrate 
// match_results length() in C++ 
#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; 
    return 0; 
} 

Output:

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

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