I have implemented the following code to try and count the number of matches in the given string; in this case it should be 1.

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

unsigned countMatches(std::string text, std::regex expr);
    
int main()
{
    std::string phrase = "Hello world";

    std::regex pattern = std::regex("world");

    std::cout << countMatches(phrase, pattern) << std::endl;
    
    return 0;
}

unsigned countMatches(std::string text, std::regex expr)
{
    std::smatch matches;
    
    while(std::regex_search(text, matches, expr))
    text = matches.suffix().str();
   
    return matches.size();
}

However it always prints 0 and I can't see why.

CodePudding user response：

If you just want to iterate over the input string, and count a number of matches, you can use std::sregex_iterator.

[Demo]

unsigned countMatches(const std::string& text, std::regex expr) {
    unsigned ret{};
    for (auto it = std::sregex_iterator(text.begin(), text.end(), expr);
        it != std::sregex_iterator{};
          it,   ret);
    return ret;
}

// Outputs: 1

CodePudding user response：

However it always prints 0 and I can't see why.

You call regex_search in a while loop. The body of the loop (eventhough wrongly indented) is updating text. The first iteration does find 1 match. But then you update text to be an empty string (the suffix of the match) and in the next iteration there are 0 matches. This is the value your function returns.

Instead you should accumulate the number of matches:

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

size_t countMatches(std::string text, std::regex const & expr)
{
    std::smatch matches;
    size_t result{ 0 };

    while (std::regex_search(text, matches, expr))
    {
        result  = matches.size();  // <--- accumulate number of matches
        text = matches.suffix().str();
    }

    return result;
}

int main()
{
    std::string phrase = "Hello world";
    std::cout << countMatches(phrase, std::regex("world")) << std::endl;
    std::cout << countMatches(phrase, std::regex("o")) << std::endl;
    return 0;
}

Output:

1
2

Note that I changed the return value to size_t as this is the type of matches.size(). I also added const & to the expr parameter to avoid a copy.