35.2.2 Syntax Flags

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

All the flags except “n” and “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 (`*/').

Here is a table of the possible flags for a character c, and what they mean:

• `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 supports two comment styles simultaneously in any one syntax table. This is for the sake of C++. Each style of comment syntax has its own comment-start sequence and its own comment-end sequence. Each comment must stick to one style or the other; thus, if it starts with the comment-start sequence of style “b,” it must also end with the comment-end sequence of style “b.”

  The two comment-start sequences must begin with the same character; only the second character may differ. Mark the second character of the “b”-style comment-start sequence with the “b” flag.

  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'

  This defines four comment-delimiting sequences:

  `/*'

  This is a comment-start sequence for “a” style because the second character, `*', does not have the “b” flag.
  

  `//'

  This is a comment-start sequence for “b” style because the second character, `/', does have the “b” flag.
  

  `*/'

  This is a comment-end sequence for “a” style because the first character, `*', does not have the “b” flag.
  

  newline

  This is a comment-end sequence for “b” style, because the newline character has the “b” flag.

• “n” on a comment delimiter character specifies that this kind of comment can be nested. For a two-character comment delimiter, “n” on either character makes it nestable.
• “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 classes.

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