<boost/url/rfc/pchars.hpp>
constexpr auto pchars = unreserved_chars + sub_delim_chars + ':' + '@';
Character sets are used with rules and the functions grammar::find_if and grammar::find_if_not.
system::result< decode_view > rv = grammar::parse( "Program%20Files", pchars );
pchar = unreserved / pct-encoded / sub-delims / ":" / "@"