boost::xpressive::blank — Matches a blank (horizonal white-space) character.
// In header: <boost/xpressive/regex_primitives.hpp> unspecified blank;
The regex traits are used to determine which characters are blank characters. To match any character that is not blank, use ~blank.
blank is equivalent to /[[:blank:]]/ in perl. ~blank is equivalent to /[[:^blank:]]/ in perl.