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elisp: Syntax Flags

 
 34.2.2 Syntax Flags
 -------------------
 
 In addition to the classes, entries for characters in a syntax table can
 specify flags.  There are eight possible flags, represented by the
 characters ‘1’, ‘2’, ‘3’, ‘4’, ‘b’, ‘c’, ‘n’, and ‘p’.
 
    All the flags except ‘p’ are used to describe comment delimiters.
 The digit flags are used for comment delimiters made up of 2 characters.
 They 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 ‘b’, ‘c’, and ‘n’ are used to qualify the corresponding
 comment delimiter.
 
    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.  For a two-character comment starter, this flag
      is only significant on the second char, and for a 2-character
      comment ender it is only significant on the first char.
 
    • ‘c’ means that C as a comment delimiter belongs to the alternative
      “c” comment style.  For a two-character comment delimiter, ‘c’ on
      either character makes it of style “c”.
 
    • ‘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.
 
      Emacs supports several comment styles simultaneously in any one
      syntax table.  A comment style is a set of flags ‘b’, ‘c’, and ‘n’,
      so there can be up to 8 different comment styles.  Each comment
      delimiter has a style and only matches comment delimiters of the
      same style.  Thus if a comment starts with the comment-start
      sequence of style “bn”, it will extend until the next matching
      comment-end sequence of style “bn”.
 
      The appropriate comment syntax settings for C++ can be as follows:
 
      ‘/’
           ‘124’
      ‘*’
           ‘23b’
      newline
           ‘>’
 
      This defines four comment-delimiting sequences:
 
      ‘/*’
           This is a comment-start sequence for “b” style because the
           second character, ‘*’, has the ‘b’ flag.
 
      ‘//’
           This is a comment-start sequence for “a” style because the
           second character, ‘/’, does not have the ‘b’ flag.
 
      ‘*/’
           This is a comment-end sequence for “b” style because the first
           character, ‘*’, has the ‘b’ flag.
 
      newline
           This is a comment-end sequence for “a” style, because the
           newline character does not have the ‘b’ flag.
 
    • ‘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 (‘'’).  Motion and Syntax.
 

Info Catalog elisp: Syntax Class Table elisp: Syntax Descriptors