c++ regex count number of matches

Here is an example of how you can count the number of matches using regular expressions in C++:

#include <iostream>
#include <regex>
#include <string>

int main() {
    std::string text = "Hello, this is a sample text with some numbers: 12345, 67890, and 54321.";
    std::regex pattern("\\d+"); // Match one or more digits

    std::sregex_iterator it(text.begin(), text.end(), pattern);
    std::sregex_iterator end;

    int count = 0;
    while (it != end) {
        ++count;
        ++it;
    }

    std::cout << "Number of matches: " << count << std::endl;

    return 0;
}

Let's go through the steps:

Include the necessary headers:
<iostream> for input/output operations
<regex> for regular expressions
<string> for working with strings.
Declare a main function, which is the entry point of your program.
Create a string variable text that contains the text you want to search for matches. In this example, we have the text "Hello, this is a sample text with some numbers: 12345, 67890, and 54321."
Create a regular expression pattern using the std::regex class. In this example, the pattern is "\d+", which matches one or more digits.
Create a std::sregex_iterator object it using the std::sregex_iterator constructor. The constructor takes three parameters: the beginning iterator of the text, the end iterator of the text, and the regular expression pattern.
Create another std::sregex_iterator object end to represent the end of the iteration.
Initialize a variable count to keep track of the number of matches found.
Use a while loop to iterate over the matches. In each iteration, increment the count variable and the it iterator.
After the loop ends, output the value of count using std::cout.
Finally, return 0 to indicate successful program execution.

This program will output "Number of matches: X" where X is the number of matches found in the text. In this example, it will output "Number of matches: 3" because there are three numbers (12345, 67890, and 54321) in the text that match the pattern "\d+".