How to build a Raw string for regex from string variable

How build a regex from a string variable, and interpret that as Raw format.

std::regex re{R"pattern"};

For the above code, is there a way to replace the fixed string "pattern" with a std::string pattern; variable that is either built from compile time or run time.

I tried this but didn't work:

std::string pattern = "key";
std::string pattern = std::string("R(\"") + pattern + ")\"";
std::regex re( pattern ); // does not work as expected