Because EBNF was designed prior to the existence of Unicode, it
is necessary to extend the syntax to be able to refer to Unicode
characters in grammar definitions. This specification makes use of
the standard unicode U+NNNN
syntax in grammar definitions, to refer to specific Unicode characters.
It also makes use of the syntax \p{t}
which should be understood to represent any Unicode character with
the property t. For example,
\p{Lowercase_Letter} describes
the set of characters that are both letters and are lowercase. The
syntax \P{t} should be understood
as the negation of \p{t};
it describes the set of characters without the property
t.