string-literal: encoding-prefix/opt " s-char-sequence/opt " encoding-prefix/opt R raw-string encoding-prefix/opt extraction-prefix " extraction-literal-contents " encoding-prefix/opt extraction-prefix R raw-extraction-string
extraction-prefix: f x F X
extraction-literal-contents: x-char-sequence/opt extraction-literal-contents x-char-sequence extraction-literal-contents { extraction-field } extraction-literal-contents {{ extraction-literal-contents }}
x-char-sequence: x-char x-char-sequence x-char
x-char: basic-x-char escape-sequence universal-character-name
basic-x-char any member of the translation character set except , ", {, }, \n
raw-extraction-string: " d-char-sequence/opt ( raw-extration-literal-contents } d-char-sequence/opt "
raw-extraction-literal-contents: y-char-sequence/opt raw-extraction-literal-contents y-char-sequence raw-extraction-literal-contents { extraction-field } raw-extraction-literal-contents {{ raw-extraction-literal-contents }}
y-char-sequence: y-char y-char-sequence y-char
y-char: any member of the translation character set except {, } and ) followed by the initial d-char-sequence followed by "
extraction-field: expression-field expression-field : format-spec
expression-field: simple-expression-field simple-expression-field ? expression-field : expression-field
simple-expression-field: e-token-sequence/opt simple-expression-field e-token-sequence simple-expression-field ( simple-expression-field ) simple-expression-field [ simple-expression-field ] simple-expression-field { simple-expression-field }
e-token-sequence: e-token e-char-sequence e-token
e-token: identifier pp-number character-literal character-literal user-defined-character-literal string-literal user-defined-string-literal preprocessing-op-or-punc except (. [, {, ), ], }, ?, :. each non-whitespace character that cannot be one of the above except (. [, {, ), ], }, ?, :.
format-spec: f-char-sequence/opt format-spec f-char-sequence format-spec { expression-field }
f-char-sequence: f-char f-char-sequence f-char
f-char: any member of the translation character set except {, }.
Note: As :: is not the same as two colons, due to maximum munch rule, a format-spec that starts with a colon can't be expressed. As format-spec fields can start with a space or a colon (both are used when container formatting proposal is accepted) the programmer can't reliably "fix" this by adding a space, and the format-spec defintion can't be changed to skip initial space either.