Fix C++11 std::regex incompatibility

Fix C++11 std::regex incompatibility

boost::regex supports escape sequences starting with a backslash in format
strings of regex_replace, but std::regex does not. Therefore format strings
involving literal backslashes have to be written differently for both flavours.

The special MSVC handling in regex.h is removed, since it is not needed
anymore, and using grep syntax would definitely be wrong.