A syntax table specifies the syntactic textual function of each character. This information is used by the parsing commands, the complex movement commands, and others to determine where words, symbols, and other syntactic constructs begin and end. The current syntax table controls the meaning of the word motion functions (see section Motion by Words) and the list motion functions (see section Moving over Balanced Expressions) as well as the functions in this chapter.
A syntax table is a vector of 256 elements; it contains one entry for each of the 256 ASCII characters of an 8-bit byte. Each element is an integer that encodes the syntax of the character in question.
Syntax tables are used only for moving across text, not for the Emacs Lisp reader. Emacs Lisp uses built-in syntactic rules when reading Lisp expressions, and these rules cannot be changed.
Each buffer has its own major mode, and each major mode has its own idea of the syntactic class of various characters. For example, in Lisp mode, the character `;' begins a comment, but in C mode, it terminates a statement. To support these variations, Emacs makes the choice of syntax table local to each buffer. Typically, each major mode has its own syntax table and installs that table in each buffer which uses that mode. Changing this table alters the syntax in all those buffers as well as in any buffers subsequently put in that mode. Occasionally several similar modes share one syntax table. See section Major Mode Examples, for an example of how to set up a syntax table.
A syntax table can inherit the data for some characters from the standard syntax table, while specifying other characters itself. The "inherit" syntax class means "inherit this character's syntax from the standard syntax table." Most major modes' syntax tables inherit the syntax of character codes 0 through 31 and 128 through 255. This is useful with character sets such as ISO Latin-1 that have additional alphabetic characters in the range 128 to 255. Just changing the standard syntax for these characters affects all major modes.
t if object is a vector of length 256
elements. This means that the vector may be a syntax table. However,
according to this test, any vector of length 256 is considered to be a
syntax table, no matter what its contents.
This section describes the syntax classes and flags that denote the
syntax of a character, and how they are represented as a syntax
descriptor, which is a Lisp string that you pass to
modify-syntax-entry to specify the desired syntax.
Emacs defines a number of syntax classes. Each syntax table puts each character into one class. There is no necessary relationship between the class of a character in one syntax table and its class in any other table.
Each class is designated by a mnemonic character which serves as the name of the class when you need to specify a class. Usually the designator character is one which is frequently put in that class; however, its meaning as a designator is unvarying and independent of what syntax that character currently has.
A syntax descriptor is a Lisp string which specifies a syntax class, a matching character (used only for the parenthesis classes) and flags. The first character is the designator for a syntax class. The second character is the character to match; if it is unused, put a space there. Then come the characters for any desired flags. If no matching character or flags are needed, one character is sufficient.
For example, the descriptor for the character `*' in C mode is `. 23' (i.e., punctuation, matching character slot unused, second character of a comment-starter, first character of an comment-ender), and the entry for `/' is `. 14' (i.e., punctuation, matching character slot unused, first character of a comment-starter, second character of a comment-ender).
Here is a table syntax classes, the characters that stand for them, their meanings, and examples of their use.
The class of open parentheses is designated with `(', and that of close parentheses with `)'.
In English text, and in C code, the parenthesis pairs are `()', `[]', and `{}'. In Emacs Lisp, the delimiters for lists and vectors (`()' and `[]') are classified as parenthesis characters.
The parsing facilities of Emacs consider a string as a single token. The usual syntactic meanings of the characters in the string are suppressed.
The Lisp modes have two string quote characters: double-quote (`"') and vertical bar (`|'). `|' is not used in Emacs Lisp, but it is used in Common Lisp. C also has two string quote characters: double-quote for strings, and single-quote (`'') for character constants.
English text has no string quote characters because English is not a programming language. Although quotation marks are used in English, we do not want them to turn off the usual syntactic properties of other characters in the quotation.
Characters in this class count as part of words if
words-include-escapes is non-nil. See section Motion by Words.
Characters in this class count as part of words if
words-include-escapes is non-nil. See section Motion by Words.
This class is not currently used in any standard Emacs modes.
English text has no comment characters. In Lisp, the semicolon (`;') starts a comment and a newline or formfeed ends one.
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:
backward-prefix-chars moves back over these
characters, as well as over characters whose primary syntax class is
prefix (`''). See section Motion and Syntax.
In this section we describe functions for creating, accessing and altering syntax tables.
Most major mode syntax tables are created in this way.
nil), it returns a copy of the
current syntax table. Otherwise, an error is signaled if table is
not a syntax table.
This function always returns nil. The old syntax information in
the table for this character is discarded.
An error is signaled if the first character of the syntax descriptor is not one of the twelve syntax class designator characters. An error is also signaled if char is not a character.
Examples:
;; Put the space character in class whitespace.
(modify-syntax-entry ?\ " ")
=> nil
;; Make `$' an open parenthesis character,
;; with `^' as its matching close.
(modify-syntax-entry ?$ "(^")
=> nil
;; Make `^' a close parenthesis character,
;; with `$' as its matching open.
(modify-syntax-entry ?^ ")$")
=> nil
;; Make `/' a punctuation character,
;; the first character of a start-comment sequence,
;; and the second character of an end-comment sequence.
;; This is used in C mode.
(modify-syntax-entry ?/ ".13")
=> nil
An error is signaled if char is not a character.
The following examples apply to C mode. The first example shows that the syntax class of space is whitespace (represented by a space). The second example shows that the syntax of `/' is punctuation. This does not show the fact that it is also part of comment start and end sequence. The third example shows that open parenthesis is in the class of open parentheses. This does not show the fact that it has a matching character, `)'.
(char-to-string (char-syntax ?\ ))
=> " "
(char-to-string (char-syntax ?/))
=> "."
(char-to-string (char-syntax ?\())
=> "("
This section describes functions for moving across characters in certain syntax classes. None of these functions exists in Emacs version 18 or earlier.
Here are several functions for parsing and scanning balanced expressions, also known as sexps, in which parentheses match in pairs. The syntax table controls the interpretation of characters, so these functions can be used for Lisp expressions when in Lisp mode and for C expressions when in C mode. See section Moving over Balanced Expressions, for convenient higher-level functions for moving over balanced expressions.
If state is nil, start is assumed to be at the top
level of parenthesis structure, such as the beginning of a function
definition. Alternatively, you might wish to resume parsing in the
middle of the structure. To do this, you must provide a state
argument that describes the initial status of parsing.
If the third argument target-depth is non-nil, parsing
stops if the depth in parentheses becomes equal to target-depth.
The depth starts at 0, or at whatever is given in state.
If the fourth argument stop-before is non-nil, parsing
stops when it comes to any character that starts a sexp. If
stop-comment is non-nil, parsing stops when it comes to the
start of a comment.
The fifth argument state is an eight-element list of the same
form as the value of this function, described below. The return value
of one call may be used to initialize the state of the parse on another
call to parse-partial-sexp.
The result is a list of eight elements describing the final state of the parse:
nil if none.
nil if none.
nil if inside a string. More precisely, this is the
character that will terminate the string.
t if inside a comment.
t if point is just after a quote character.
t if inside a comment of style "b".
Elements 0, 3, 4, 5 and 7 are significant in the argument state.
This function is most often used to compute indentation for languages that have nested parentheses.
If depth is nonzero, parenthesis depth counting begins from that
value. The only candidates for stopping are places where the depth in
parentheses becomes zero; scan-lists counts count such
places and then stops. Thus, a positive value for depth means go
out levels of parenthesis.
Scanning ignores comments if parse-sexp-ignore-comments is
non-nil.
If scan reaches the beginning or end of the buffer (or its accessible
portion), and the depth is not zero, an error is signaled. If the depth
is zero but the count is not used up, nil is returned.
Scanning ignores comments if parse-sexp-ignore-comments is
non-nil.
If scan reaches the beginning or end of (the accessible part of) the
buffer in the middle of a parenthetical grouping, an error is signaled.
If it reaches the beginning or end between groupings but before count is
used up, nil is returned.
nil, then comments are treated as
whitespace by the functions in this section and by forward-sexp.
In older Emacs versions, this feature worked only when the comment
terminator is something like `*/', and appears only to end a
comment. In languages where newlines terminate comments, it was
necessary make this variable nil, since not every newline is the
end of a comment. This limitation no longer exists.
You can use forward-comment to move forward or backward over
one comment or several comments.
To move forward over all comments and whitespace following point, use
(forward-comment (buffer-size)). (buffer-size) is a good
argument to use, because the number of comments to in the buffer cannot
exceed that many.
Each of the major modes in Emacs has its own syntax table. Here are several of them:
read
function.)
Each element of a syntax table is an integer that encodes the syntax of one character: the syntax class, possible matching character, and flags. Lisp programs don't usually work with the elements directly; the Lisp-level syntax table functions usually work with syntax descriptors (see section Syntax Descriptors).
The low 8 bits of each element of a syntax table indicate the syntax class.
The next 8 bits are the matching opposite parenthesis (if the character has parenthesis syntax); otherwise, they are not meaningful. The next 6 bits are the flags.