smatch | Regex (Regular Expressions) in C++
Last Updated :
18 Jan, 2023
smatch is an instantiation of the match_results class template for matches on string objects. Functions that can be called using smatch: str(), position(), and length() member functions of the match_results object can be called to get the text that was matched, or the starting position and its length of the match relative to the subject string.
- Call these member functions without a parameter or with 0 as the parameter to get the overall regex match.
- Call them passing 1 or greater to get the match of a particular capturing group.
- The size() member function indicates the number of capturing groups plus one for the overall match.
- Thus you can pass a value up to size()-1 to the other three member functions(str(), position(), length() ).
What is capturing group ? Examples:
Example-1:
Suppose you create a regex object like : regex re("(geeks)(.*)")
Here no of capturing group is = 2
[ one is "geeks" and second is any character after "geeks" ].
Example-2:
regex re("a(b)c")
Here no of capturing group is = 1[ 'b' is the capturing group].
whatever within '(' and ')' braces is treated as capturing group.
Below is the program to show the working of smatch:
CPP
#include <bits/stdc++.h>
using namespace std;
int main()
{
string sp("geeksforgeeks");
regex re("(geeks)(.*)");
smatch match;
if (regex_search(sp, match, re) == true ) {
cout << "Match size = " << match.size() << endl;
cout << "Whole match : " << match.str(0) << endl;
cout << "First capturing group is '" << match.str(1)
<< "' which is captured at index " << match.position(1)
<< endl;
cout << "Second capturing group is '" << match.str(2)
<< "' which is captured at index " << match.position(2)
<< endl;
}
else {
cout << "No match is found" << endl;
}
return 0;
}
|
Output:
Match size = 3
Whole match : geeksforgeeks
First capturing group is 'geeks' which is captured at index 0
Second capturing group is 'forgeeks' which is captured at index 5
Time Complexity: O(n)
Auxiliary Space: O(1)
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