RegexrReplace w/o C++20

RegexReplace.cpp

#include <iostream>
#include <string>
#include <vector>
#include <regex>
#include <functional>

/// @brief Replaces occurrences of a regular expression pattern in a string with a replacement string.
///
/// @param input The string to search in.
/// @param pattern The regular expression pattern to match.
/// @param replacement The string to replace the matched occurrences with.
/// @return A new string with the replacements made.
std::string RegexReplace(const std::string& input, const std::string& pattern, const std::string& replacement) {
    std::string result = "";
    std::regex re(pattern);
    std::smatch match;
    std::string::const_iterator searchStart(input.cbegin());

    while (std::regex_search(searchStart, input.cend(), match, re)) {
        result.append(searchStart, match[0].first);
        result.append(replacement);
        searchStart = match[0].second;
    }

    result.append(searchStart, input.cend());
    return result;
}

/// @brief Replaces occurrences of a regular expression pattern in a string with the result of a lambda function.
///
/// @tparam ReplacementFunc The type of the replacement function (must be callable with a std::smatch).
/// @param input The string to search in.
/// @param pattern The regular expression pattern to match.
/// @param replacementFunc A function (or lambda) that takes a std::smatch object and returns the replacement string.
/// @return A new string with the replacements made.
template<typename ReplacementFunc>
std::string RegexReplace(const std::string& input, const std::string& pattern,
                                 ReplacementFunc replacementFunc) {
    // Check if the ReplacementFunc is a string.  If it is, call the string overload.
    if constexpr (std::is_same_v<ReplacementFunc, std::string>) {
        return RegexReplace(input, pattern, "");
    }
    std::string result = "";
    std::regex re(pattern);
    std::smatch match;
    std::string::const_iterator searchStart(input.cbegin());

    while (std::regex_search(searchStart, input.cend(), match, re)) {
        result.append(searchStart, match[0].first);
        result.append(replacementFunc(match)); // Call the lambda
        searchStart = match[0].second;
    }

    result.append(searchStart, input.cend());
    return result;
}

/// @brief Main function to run the Google Tests.
int main(int argc, char** argv) {
std::string text = "apple, banana, cherry";
std::string pattern = "(\\w+)";
std::string expected = "APPLE, BANANA, CHERRY";
std::string result = RegexReplace(text, pattern,
	[](const std::smatch& m) -> std::string
	{
		std::string word = m[1].str();
		std::string upperCaseWord = "";
		for (char c : word)
		{
			upperCaseWord += std::toupper(c);
		}
		return upperCaseWord;
	});

    std::cout << "Original:    " << text << std::endl;
    std::cout << "Replaced:    " << result << std::endl;

    return 0;
}