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