boost::xpressive::_w — Matches a word character.
// In header: <boost/xpressive/regex_primitives.hpp> unspecified _w;
'_w' matches a single word character. The regex traits are used to determine which characters are word characters. Use ~_w to match a character that is not a word character.
_w is like \w in perl. ~_w is like \W in perl.