Fix some thinkos in MSVC regex hack

Fix some thinkos in MSVC regex hack

The changed code is not used, but I tried to use a similar approach for
boost::regex, and found some problems:
- regex_replace and regex_search are implemented in the replacement, so they
  must not be used directly
- an smatch object must be given by reference (as in the called methods),
  otherwise an exception would be thrown at runtime
- the commented out regex_replace version is actually needed

This code is supposed to be deleted, but nevertheless I wanted to record here
how it had to be modified if it was actually needed.