36. Syntax Tables

• Syntax Table Concepts
• Syntax Descriptors
• Syntax Table Functions
• Syntax Properties
• Motion and Syntax
• Parsing Balanced Expressions
• Some Standard Syntax Tables
• Syntax Table Internals
• Categories

A syntax table specifies the syntactic textual function of each character. This information is used by the parsing functions, 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.

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36.1. Syntax Table Concepts

A syntax table is a char-table (see section Char-Tables). The element at index c describes the character with code c. The element's value should be a list 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. (Some Lisp systems provide ways to redefine the read syntax, but we decided to leave this feature out of Emacs Lisp for simplicity.)

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 that 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." Just changing the standard syntax for a characters affects all syntax tables which inherit from it.

^topFunction: syntax-table-p object

This function returns t if object is a syntax table.

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36.2. Syntax Descriptors

• Table of Syntax Classes
• Syntax Flags

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 syntax you want.

The syntax table specifies a syntax class for each character. 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 that is frequently 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 that 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 syntax 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).

^top 36.2.1. Table of Syntax Classes

Here is a table of syntax classes, the characters that stand for them, their meanings, and examples of their use.

^topSyntax class: whitespace character

Whitespace characters (designated by ` ' or `-') separate symbols and words from each other. Typically, whitespace characters have no other syntactic significance, and multiple whitespace characters are syntactically equivalent to a single one. Space, tab, newline and formfeed are classified as whitespace in almost all major modes.

^topSyntax class: word constituent

Word constituents (designated by `w') are parts of normal English words and are typically used in variable and command names in programs. All upper- and lower-case letters, and the digits, are typically word constituents.

^topSyntax class: symbol constituent

Symbol constituents (designated by `_') are the extra characters that are used in variable and command names along with word constituents. For example, the symbol constituents class is used in Lisp mode to indicate that certain characters may be part of symbol names even though they are not part of English words. These characters are `$&*+-_<>'. In standard C, the only non-word-constituent character that is valid in symbols is underscore (`_').

^topSyntax class: punctuation character

Punctuation characters (designated by `.') are those characters that are used as punctuation in English, or are used in some way in a programming language to separate symbols from one another. Most programming language modes, including Emacs Lisp mode, have no characters in this class since the few characters that are not symbol or word constituents all have other uses.

^topSyntax class: open parenthesis character

^topSyntax class: close parenthesis character

Open and close parenthesis characters are characters used in dissimilar pairs to surround sentences or expressions. Such a grouping is begun with an open parenthesis character and terminated with a close. Each open parenthesis character matches a particular close parenthesis character, and vice versa. Normally, Emacs indicates momentarily the matching open parenthesis when you insert a close parenthesis. See section Blinking Parentheses.

The class of open parentheses is designated by `(', and that of close parentheses by `)'.

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.

^topSyntax class: string quote

String quote characters (designated by `"') are used in many languages, including Lisp and C, to delimit string constants. The same string quote character appears at the beginning and the end of a string. Such quoted strings do not nest.

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.

^topSyntax class: escape

An escape character (designated by `\') starts an escape sequence such as is used in C string and character constants. The character `\' belongs to this class in both C and Lisp. (In C, it is used thus only inside strings, but it turns out to cause no trouble to treat it this way throughout C code.)

Characters in this class count as part of words if words-include-escapes is non-nil. See section Motion by Words.

^topSyntax class: character quote

A character quote character (designated by `/') quotes the following character so that it loses its normal syntactic meaning. This differs from an escape character in that only the character immediately following is ever affected.

Characters in this class count as part of words if words-include-escapes is non-nil. See section Motion by Words.

This class is used for backslash in TeX mode.

^topSyntax class: paired delimiter

Paired delimiter characters (designated by `$') are like string quote characters except that the syntactic properties of the characters between the delimiters are not suppressed. Only TeX mode uses a paired delimiter presently--the `$' that both enters and leaves math mode.

^topSyntax class: expression prefix

An expression prefix operator (designated by `'') is used for syntactic operators that are considered as part of an expression if they appear next to one. In Lisp modes, these characters include the apostrophe, `'' (used for quoting), the comma, `,' (used in macros), and `#' (used in the read syntax for certain data types).

^topSyntax class: comment starter

^topSyntax class: comment ender

The comment starter and comment ender characters are used in various languages to delimit comments. These classes are designated by `<' and `>', respectively.

English text has no comment characters. In Lisp, the semicolon (`;') starts a comment and a newline or formfeed ends one.

^topSyntax class: inherit

This syntax class does not specify a particular syntax. It says to look in the standard syntax table to find the syntax of this character. The designator for this syntax code is `@'.

^topSyntax class: generic comment delimiter

A generic comment delimiter character starts or ends a special kind of comment. Any generic comment delimiter matches any generic comment delimiter, but they cannot match a comment starter or comment ender; generic comment delimiters can only match each other.

This syntax class is primarily meant for use with the syntax-table text property (see section Syntax Properties). You can mark any range of characters as forming a comment, by giving the first and last characters of the range syntax-table properties identifying them as generic comment delimiters.

^topSyntax class: generic string delimiter

A generic string delimiter character starts or ends a string. This class differs from the string quote class in that any generic string delimiter can match any other generic string delimiter; but they do not match ordinary string quote characters.

This syntax class is primarily meant for use with the syntax-table text property (see section Syntax Properties). You can mark any range of characters as forming a string constant, by giving the first and last characters of the range syntax-table properties identifying them as generic string delimiters.

^top 36.2.2. Syntax Flags

In addition to the classes, entries for characters in a syntax table can specify 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 (`*/').

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:

  ^top`/'

  `124b'

  ^top`*'

  `23'

  ^topnewline

  `>b'

  This defines four comment-delimiting sequences:

  ^top`/*'

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

  ^top`//'

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

  ^top`*/'

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

  ^topnewline

  This is a comment-end sequence for "b" style, because the newline character has 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 codes. The function backward-prefix-chars moves back over these characters, as well as over characters whose primary syntax class is prefix (`''). See section Motion and Syntax.

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36.3. Syntax Table Functions

In this section we describe functions for creating, accessing and altering syntax tables.

^topFunction: make-syntax-table

This function creates a new syntax table. It inherits the syntax for letters and control characters from the standard syntax table. For other characters, the syntax is copied from the standard syntax table.

Most major mode syntax tables are created in this way.

^topFunction: copy-syntax-table &optional table

This function constructs a copy of table and returns it. If table is not supplied (or is nil), it returns a copy of the current syntax table. Otherwise, an error is signaled if table is not a syntax table.

^topCommand: modify-syntax-entry char syntax-descriptor &optional table

This function sets the syntax entry for char according to syntax-descriptor. The syntax is changed only for table, which defaults to the current buffer's syntax table, and not in any other syntax table. The argument syntax-descriptor specifies the desired syntax; this is a string beginning with a class designator character, and optionally containing a matching character and flags as well. See section Syntax Descriptors.

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 ?/ ". 14")
     => nil

^topFunction: char-syntax character

This function returns the syntax class of character, represented by its mnemonic designator character. This returns only the class, not any matching parenthesis or flags.

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 sequences. 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, `)'.

(string (char-syntax ?\ ))
     => " "

(string (char-syntax ?/))
     => "."

(string (char-syntax ?\())
     => "("

We use string to make it easier to see the character returned by char-syntax.

^topFunction: set-syntax-table table

This function makes table the syntax table for the current buffer. It returns table.

^topFunction: syntax-table

This function returns the current syntax table, which is the table for the current buffer.

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36.4. Syntax Properties

When the syntax table is not flexible enough to specify the syntax of a language, you can use syntax-table text properties to override the syntax table for specific character occurrences in the buffer. See section Text Properties.

The valid values of syntax-table text property are:

^topsyntax-table

If the property value is a syntax table, that table is used instead of the current buffer's syntax table to determine the syntax for this occurrence of the character.

^top(syntax-code . matching-char)

A cons cell of this format specifies the syntax for this occurrence of the character.

^topnil

If the property is nil, the character's syntax is determined from the current syntax table in the usual way.

^topVariable: parse-sexp-lookup-properties

If this is non-nil, the syntax scanning functions pay attention to syntax text properties. Otherwise they use only the current syntax table.

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36.5. Motion and Syntax

This section describes functions for moving across characters that have certain syntax classes.

^topFunction: skip-syntax-forward syntaxes &optional limit

This function moves point forward across characters having syntax classes mentioned in syntaxes. It stops when it encounters the end of the buffer, or position limit (if specified), or a character it is not supposed to skip. The return value is the distance traveled, which is a nonnegative integer.

^topFunction: skip-syntax-backward syntaxes &optional limit

This function moves point backward across characters whose syntax classes are mentioned in syntaxes. It stops when it encounters the beginning of the buffer, or position limit (if specified), or a character it is not supposed to skip.

The return value indicates the distance traveled. It is an integer that is zero or less.

^topFunction: backward-prefix-chars

This function moves point backward over any number of characters with expression prefix syntax. This includes both characters in the expression prefix syntax class, and characters with the `p' flag.

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36.6. Parsing Balanced Expressions

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.

^topFunction: parse-partial-sexp start limit &optional target-depth stop-before state stop-comment

This function parses a sexp in the current buffer starting at start, not scanning past limit. It stops at position limit or when certain criteria described below are met, and sets point to the location where parsing stops. It returns a value describing the status of the parse at the point where it stops.

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. If stop-comment is the symbol syntax-table, parsing stops after the start of a comment or a string, or the end of a comment or a string, whichever comes first.

The fifth argument state is a nine-element list of the same form as the value of this function, described below. (It is OK to omit the last element of the nine.) 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 nine elements describing the final state of the parse:

1. The depth in parentheses, counting from 0.
2. The character position of the start of the innermost parenthetical grouping containing the stopping point; nil if none.
3. The character position of the start of the last complete subexpression terminated; nil if none.
4. Non-nil if inside a string. More precisely, this is the character that will terminate the string, or t if a generic string delimiter character should terminate it.
5. t if inside a comment (of either style).
6. t if point is just after a quote character.
7. The minimum parenthesis depth encountered during this scan.
8. What kind of comment is active: nil for a comment of style "a", t for a comment of style "b", and syntax-table for a comment that should be ended by a generic comment delimiter character.
9. The string or comment start position. While inside a comment, this is the position where the comment began; while inside a string, this is the position where the string began. When outside of strings and comments, this element is nil.

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.

^topFunction: scan-lists from count depth

This function scans forward count balanced parenthetical groupings from position from. It returns the position where the scan stops. If count is negative, the scan moves backwards.

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 depth levels of parenthesis.

Scanning ignores comments if parse-sexp-ignore-comments is non-nil.

If the 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.

^topFunction: scan-sexps from count

This function scans forward count sexps from position from. It returns the position where the scan stops. If count is negative, the scan moves backwards.

Scanning ignores comments if parse-sexp-ignore-comments is non-nil.

If the scan reaches the beginning or end of (the accessible part of) the buffer while 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.

^topVariable: parse-sexp-ignore-comments

If the value is non-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.

^topFunction: forward-comment count

This function moves point forward across count comments (backward, if count is negative). If it finds anything other than a comment or whitespace, it stops, leaving point at the place where it stopped. It also stops after satisfying count.

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 in the buffer cannot exceed that many.

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36.7. Some Standard Syntax Tables

Most of the major modes in Emacs have their own syntax tables. Here are several of them:

^topFunction: standard-syntax-table

This function returns the standard syntax table, which is the syntax table used in Fundamental mode.

^topVariable: text-mode-syntax-table

The value of this variable is the syntax table used in Text mode.

^topVariable: c-mode-syntax-table

The value of this variable is the syntax table for C-mode buffers.

^topVariable: emacs-lisp-mode-syntax-table

The value of this variable is the syntax table used in Emacs Lisp mode by editing commands. (It has no effect on the Lisp read function.)

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36.8. Syntax Table Internals

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). Nonetheless, here we document the internal format.

Each element of a syntax table is a cons cell of the form (syntax-code . matching-char). The CAR, syntax-code, is an integer that encodes the syntax class, and any flags. The CDR, matching-char, is non-nil if a character to match was specified.

This table gives the value of syntax-code which corresponds to each syntactic type.

For example, the usual syntax value for `(' is (4 . 41). (41 is the character code for `)'.)

The flags are encoded in higher order bits, starting 16 bits from the least significant bit. This table gives the power of two which corresponds to each syntax flag.

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36.9. Categories

Categories provide an alternate way of classifying characters syntactically. You can define several categories as needed, then independently assign each character to one or more categories. Unlike syntax classes, categories are not mutually exclusive; it is normal for one character to belong to several categories.

Each buffer has a category table which records which categories are defined and also which characters belong to each category. Each category table defines its own categories, but normally these are initialized by copying from the standard categories table, so that the standard categories are available in all modes.

Each category has a name, which is an ASCII printing character in the range ` ' to `~'. You specify the name of a category when you define it with define-category.

The category table is actually a char-table (see section Char-Tables). The element of the category table at index c is a category set---a bool-vector--that indicates which categories character c belongs to. In this category set, if the element at index cat is t, that means category cat is a member of the set, and that character c belongs to category cat.

^topFunction: define-category char docstring &optional table

This function defines a new category, with name char and documentation docstring.

The new category is defined for category table table, which defaults to the current buffer's category table.

^topFunction: category-docstring category &optional table

This function returns the documentation string of category category in category table table.

(category-docstring ?a)
     => "ASCII"
(category-docstring ?l)
     => "Latin"

^topFunction: get-unused-category table

This function returns a category name (a character) which is not currently defined in table. If all possible categories are in use in table, it returns nil.

^topFunction: category-table

This function returns the current buffer's category table.

^topFunction: category-table-p object

This function returns t if object is a category table, otherwise nil.

^topFunction: standard-category-table

This function returns the standard category table.

^topFunction: copy-category-table &optional table

This function constructs a copy of table and returns it. If table is not supplied (or is nil), it returns a copy of the current category table. Otherwise, an error is signaled if table is not a category table.

^topFunction: set-category-table table

This function makes table the category table for the current buffer. It returns table.

^topFunction: make-category-set categories

This function returns a new category set--a bool-vector--whose initial contents are the categories listed in the string categories. The elements of categories should be category names; the new category set has t for each of those categories, and nil for all other categories.

(make-category-set "al")
     => #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"

^topFunction: char-category-set char

This function returns the category set for character char. This is the bool-vector which records which categories the character char belongs to. The function char-category-set does not allocate storage, because it returns the same bool-vector that exists in the category table.

(char-category-set ?a)
     => #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"

^topFunction: category-set-mnemonics category-set

This function converts the category set category-set into a string containing the names of all the categories that are members of the set.

(category-set-mnemonics (char-category-set ?a))
     => "al"

^topFunction: modify-category-entry character category &optional table reset

This function modifies the category set of character in category table table (which defaults to the current buffer's category table).

Normally, it modifies the category set by adding category to it. But if reset is non-nil, then it deletes category instead.