Boost Regex for formatting MAC Address string

Whats a good boost regex for formatting a MAC address string that looks like this - "xxxxxxxxxxxx" to "xx-xx-xx-xx-xx-xx". This is what I have so far

std::string s = "e8f5a4b3e8e4";
boost::regex expr("^ *([[:xdigit:]]{2})([[:xdigit:]]{2})([[:xdigit:]]{2})([[:xdigit:]]{2})([[:xdigit:]]{2})([[:xdigit:]]{2}) *$", boost::regex::extended | boost::regex::icase);
std::string fmt{"\\1-\\2-\\3-\\4-\\5-\\6"};
std::cout << boost::regex_replace(s, expr, fmt) << '\n';