5 Lexical conventions [lex]

Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an identifier, a literal, or an operator or 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, placeholder tokens produced by preprocessing import and module directives (import-keyword, module-keyword, and export-keyword), 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, or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both.

As described in [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:

• (3.1)
  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 (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
  encoding-prefix${}_{opt}$ R raw-string
• (3.2)
  Otherwise, if the next three characters are <​::​ and the subsequent character is neither : nor >, the < is treated as a preprocessing token by itself and not as the first character of the alternative token <:.
• (3.3)
  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, except that a header-name is only formed
  • (3.3.1)
    after the include or import preprocessing token in an #include ([cpp.include]) or import ([cpp.import]) directive, or
  • (3.3.2)
    within a has-include-expression.

#define R "x"
const char* s = R"y";           // ill-formed raw string, not "x" "y"

The import-keyword is produced by processing an import directive ([cpp.import]), the module-keyword is produced by preprocessing a module directive ([cpp.module]), and the export-keyword is produced by preprocessing either of the previous two directives.