The unreserved character set
<boost/url/rfc/unreserved_chars.hpp>
constexpr grammar::lut_chars unreserved_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "abcdefghijklmnopqrstuvwxyz"
    "0123456789"
    "-._~";
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", pct_encoded_rule( unreserved_chars ) );
unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"