#include <iostream>
#include <regex>
int main() {
std::string bstr = "111111111111111111111111111111111111111110";
std::regex re(".{6}");
bstr = std::regex_replace(bstr, re, "|$00");
std::cout << "bstr: " << bstr << std::endl;
return 0;
}
Why when I compile the same code using visual studio 2022 i get a different value stored on bstr after running the regex_replace function?
Short answer: Use $& as a reference to whole match. In this case the correct format string is:
bstr = std::regex_replace(bstr, re, "|$&");
Long answer: Well, this is a rare case where MSVC is right and gcc and clang are (technically) buggy.
C++ default regex flavour is based on ECMAScript standard. This standard defines $n and $nn as backreference to a capture group number n or nn, but neither of them allows 0 as valid group number. An invalid substitution should be left as is (i.e. treated like any other plain text).
This is what MSVC is doing. It recognizes that $00 substitution is invalid per standard and treats it as plain text. gcc and clang on the other hand made it work like std::regex_match and treat group number 0 as the whole match. A reasonable assumption, but technically incorrect by standard.
This can be confirmed also with regex101: |$00 is replaced with just that text, |$& is replaced with the match.
