6.4.8 Preprocessing numbers

pp-number:
                digit
                . digit
                pp-number digit
                pp-number identifier-nondigit
                pp-number e sign
                pp-number E sign
                pp-number p sign
                pp-number P sign
                pp-number .

922 A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed by valid identifier characters and the character sequences e+, e-, E+, E-, p+, p-, P+, or P-.

923 Preprocessing number tokens lexically include all floating and integer constant tokens.

924 A preprocessing number does not have type or a value;

925 it acquires both after a successful conversion (as part of translation phase 7) to a floating constant token or an integer constant token.

926 69) Thus, sequences of characters that resemble escape sequences cause undefined behavior.