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Syntax Flags

In addition to the classes, entries for characters in a syntax table can include flags. There are six possible flags, represented by the characters `1', `2', `3', `4', `b' and `p'.

All the flags except `p' are used to describe multi-character comment delimiters. The digit flags indicate that a character can also be part of a comment sequence, in addition to the syntactic properties associated with its character class. The flags are independent of the class and each other for the sake of characters such as `*' in C mode, which is a punctuation character, and the second character of a start-of-comment sequence (`/*'), and the first character of an end-of-comment sequence (`*/').

The flags for a character c are:

• `1' means c is the start of a two-character comment start sequence.

• `2' means c is the second character of such a sequence.

• `3' means c is the start of a two-character comment end sequence.

• `4' means c is the second character of such a sequence.

• `b' means that c as a comment delimiter belongs to the alternative "b" comment style.

  Emacs can now supports two comment styles simultaneously. (This is for the sake of C++.) More specifically, it can recognize two different comment-start sequences. Both must share the same first character; only the second character may differ. Mark the second character of the "b"-style comment start sequence with the `b' flag.

  The two styles of comment can have different comment-end sequences. A comment-end sequence (one or two characters) applies to the "b" style if its first character has the `b' flag set; otherwise, it applies to the "a" style.

  The appropriate comment syntax settings for C++ are as follows:

  `/'

  `124b'

  `*'

  `23'

  newline

  `>b'

  Thus `/*' is a comment-start sequence for "a" style, `//' is a comment-start sequence for "b" style, `*/' is a comment-end sequence for "a" style, and newline is a comment-end sequence for "b" style.

• `p' identifies an additional "prefix character" for Lisp syntax. These characters are treated as whitespace when they appear between expressions. When they appear within an expression, they are handled according to their usual syntax codes.

  The function backward-prefix-chars moves back over these characters, as well as over characters whose primary syntax class is prefix (`'').

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