preprocessing-token: header-name identifier pp-number character-literal user-defined-character-literal string-literal user-defined-string-literal preprocessing-op-or-punc each non-white-space character that cannot be one of the above
Each preprocessing token that is converted to a token ([lex.token]) shall have the lexical form of a keyword, an identifier, a literal, an operator, or a punctuator.
A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing token are: header names, identifiers, preprocessing numbers, character literals (including user-defined character literals), string literals (including user-defined string literals), preprocessing operators and punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories. If a ' or a " character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments ([lex.comment]), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in Clause [cpp], in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space can appear within a preprocessing token only as part of a header name or between the quotation characters in a character literal or string literal.
If the input stream has been parsed into preprocessing tokens up to a given character:
If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as R", the next preprocessing token shall be a raw string literal. Between the initial and final double quote characters of the raw string, any transformations performed in phases 1 and 2 (trigraphs, universal-character-names, and line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified. The raw string literal is defined as the shortest sequence of characters that matches the raw-string pattern
Otherwise, if the next three characters are <:: and the subsequent character is neither : nor >, the < is treated as a preprocessor token by itself and not as the first character of the alternative token <:.
Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail.
#define R "x" const char* s = R"y"; // ill-formed raw string, not "x" "y"
— end example ]
[ Example: The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer literal token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating literal token), whether or not E is a macro name. — end example ]
[ Example: The program fragment x+++++y is parsed as x ++ ++ + y, which, if x and y have integral types, violates a constraint on increment operators, even though the parse x ++ + ++ y might yield a correct expression. — end example ]