If you're coming from Perl, or if you often use the command-line utility sed, you probably think of regexes primarily as a way to modify strings--for example, "remove all substrings matching this regex," or "replace all instances of this word with another word." The C++ standard library does provide a sort of replace-by-regex functionality, under the name std::regex_replace. It's based on the JavaScript String.prototype.replace method, which means that it comes with its own idiosyncratic formatting mini-language.
std::regex_replace(str, rx, "replacement") returns a std::string constructed by searching through str for every substring matching the regex rx and replacing each such substring with the literal string "replacement". For example:
std::string s = "apples and bananas...