C++ has had regular expressions (in some degree of support) in-language since C++11, which is now ten or eleven years old. Unless you have to use Boost for this, you may just want to fall back to the language-native implementations. As I was looking to remember how to retrieve matches I found the std::regex_iterator entity. And the example in the C++ reference online is close enough to what you're looking for: regex_iterator

#include <regex>
#include <iterator>
#include <iostream>
#include <string>
 
int main()
{
    const std::string s = "Quick brown fox.";
 
    std::regex words_regex("[^\\s]+");
    auto words_begin = 
        std::sregex_iterator(s.begin(), s.end(), words_regex);
    auto words_end = std::sregex_iterator();
 
    std::cout << "Found " 
              << std::distance(words_begin, words_end) 
              << " words:\n";
 
    for (std::sregex_iterator i = words_begin; i != words_end; ++i) {
        std::smatch match = *i;                                       
+          
        std::string match_str = match.str(); 
        std::cout << match_str << '\n';
    }   
}
[download]

As you can see in the example, they're counting as a word anything that doesn't contain whitespace. You probably want to also exclude punctuation. So you would want to enumerate that in the character class. It gets harder when you want to deal with apostrophes, allowing them in words, while excluding single quoted constructs. That gets complicated fast.