boost::xpressive::xdigit — Matches a hexadecimal digit character.
// In header: <boost/xpressive/regex_primitives.hpp> unspecified xdigit;
The regex traits are used to determine which characters are hex digits. To match any character that is not a hex digit, use ~xdigit.
xdigit is equivalent to /[[:xdigit:]]/ in perl. ~xdigit is equivalent to /[[:^xdigit:]]/ in perl.
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