Regular expressions in an Objective-C Cocoa application

I make it easy. I add a new C++ file to my Objective C project, rename it as .mm, and then create a standard C++ class inside. Then, I make a static class method in the "public:" section for a C++ function that takes an NSString and returns an NSString (or NSArray, if that's what you want). I then convert NSString to C++ std::string like so:

// If anyone knows a more efficient way, let me know in the comments.
// The "if" condition below is because ObjC crashes if converting to
// std::string if the string is nil or empty.
// assume #include <string>
std::string s = "";
if (([sInput != nil]) && (!([sInput isEqualTo:@""]))) {
std::string sTemp([sInput UTF8String]);
s = sTemp;
}

From there, I can use regex_replace like so:

// assume #include <regex>
std::string sResult = std::regex_replace(sSource,sRegExp,sReplaceWith);

Then, I can convert that std::string back into an NSString with:

NSString *sResponse2 = @(sResult.c_str());

If you're only using this C++ just for this function, then you may find it suitable to call this file extra.mm (class name Extra) and put this static class method in, and then add other static class methods when the situation arrives where it just makes sense to do it in C++ because it's less hassle in some cases. (There are cases where ObjC does something with less lines of code, and some cases where C++ does it with less lines of code.)

P.S. Still yet another way with this is to use a .mm file but make an Objective C wrapper around the use of std::string and std::regex_replace() (or regex_match()).