* NEWS: Document installing packages from directories.

[emacs.git] / doc / lispref / syntax.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2015 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Syntax Tables
@chapter Syntax Tables
@cindex parsing buffer text
@cindex syntax table
@cindex text parsing

  A @dfn{syntax table} specifies the syntactic role of each character
in a buffer.  It can be used to determine where words, symbols, and
other syntactic constructs begin and end.  This information is used by
many Emacs facilities, including Font Lock mode (@pxref{Font Lock
Mode}) and the various complex movement commands (@pxref{Motion}).

@menu
* Basics: Syntax Basics.     Basic concepts of syntax tables.
* Syntax Descriptors::       How characters are classified.
* Syntax Table Functions::   How to create, examine and alter syntax tables.
* Syntax Properties::        Overriding syntax with text properties.
* Motion and Syntax::        Moving over characters with certain syntaxes.
* Parsing Expressions::      Parsing balanced expressions
                                using the syntax table.
* Syntax Table Internals::   How syntax table information is stored.
* Categories::               Another way of classifying character syntax.
@end menu

@node Syntax Basics
@section Syntax Table Concepts

  A syntax table is a data structure which can be used to look up the
@dfn{syntax class} and other syntactic properties of each character.
Syntax tables are used by Lisp programs for scanning and moving across
text.

  Internally, a syntax table is a char-table (@pxref{Char-Tables}).
The element at index @var{c} describes the character with code
@var{c}; its value is a cons cell which specifies the syntax of the
character in question.  @xref{Syntax Table Internals}, for details.
However, instead of using @code{aset} and @code{aref} to modify and
inspect syntax table contents, you should usually use the higher-level
functions @code{char-syntax} and @code{modify-syntax-entry}, which are
described in @ref{Syntax Table Functions}.

@defun syntax-table-p object
This function returns @code{t} if @var{object} is a syntax table.
@end defun

  Each buffer has its own major mode, and each major mode has its own
idea of the syntax class of various characters.  For example, in Lisp
mode, the character @samp{;} begins a comment, but in C mode, it
terminates a statement.  To support these variations, the syntax table
is local to each buffer.  Typically, each major mode has its own
syntax table, which it installs in all buffers that use that mode.
For example, the variable @code{emacs-lisp-mode-syntax-table} holds
the syntax table used by Emacs Lisp mode, and
@code{c-mode-syntax-table} holds the syntax table used by C mode.
Changing a major mode's syntax table alters the syntax in all of that
mode's buffers, as well as in any buffers subsequently put in that
mode.  Occasionally, several similar modes share one syntax table.
@xref{Example Major Modes}, for an example of how to set up a syntax
table.

@cindex standard syntax table
@cindex inheritance, syntax table
  A syntax table can @dfn{inherit} from another syntax table, which is
called its @dfn{parent syntax table}.  A syntax table can leave the
syntax class of some characters unspecified, by giving them the
``inherit'' syntax class; such a character then acquires the syntax
class specified by the parent syntax table (@pxref{Syntax Class
Table}).  Emacs defines a @dfn{standard syntax table}, which is the
default parent syntax table, and is also the syntax table used by
Fundamental mode.

@defun standard-syntax-table
This function returns the standard syntax table, which is the syntax
table used in Fundamental mode.
@end defun

  Syntax tables are not used by the Emacs Lisp reader, which has its
own built-in syntactic rules which 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.)

@node Syntax Descriptors
@section Syntax Descriptors
@cindex syntax class

  The @dfn{syntax class} of a character describes its syntactic role.
Each syntax table specifies the syntax class of 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 syntax class is designated by a mnemonic character, which
serves as the name of the class when you need to specify a class.
Usually, this designator character is one that is often assigned that
class; however, its meaning as a designator is unvarying and
independent of what syntax that character currently has.  Thus,
@samp{\} as a designator character always means ``escape character''
syntax, regardless of whether the @samp{\} character actually has that
syntax in the current syntax table.
@ifnottex
@xref{Syntax Class Table}, for a list of syntax classes and their
designator characters.
@end ifnottex

@cindex syntax descriptor
  A @dfn{syntax descriptor} is a Lisp string that describes the syntax
class and other syntactic properties of a character.  When you want to
modify the syntax of a character, that is done by calling the function
@code{modify-syntax-entry} and passing a syntax descriptor as one of
its arguments (@pxref{Syntax Table Functions}).

  The first character in a syntax descriptor must be a syntax class
designator character.  The second character, if present, specifies a
matching character (e.g., in Lisp, the matching character for
@samp{(} is @samp{)}); a space specifies that there is no matching
character.  Then come characters specifying additional syntax
properties (@pxref{Syntax Flags}).

  If no matching character or flags are needed, only one character
(specifying the syntax class) is sufficient.

  For example, the syntax descriptor for the character @samp{*} in C
mode is @code{". 23"} (i.e., punctuation, matching character slot
unused, second character of a comment-starter, first character of a
comment-ender), and the entry for @samp{/} is @samp{@w{. 14}} (i.e.,
punctuation, matching character slot unused, first character of a
comment-starter, second character of a comment-ender).

  Emacs also defines @dfn{raw syntax descriptors}, which are used to
describe syntax classes at a lower level.  @xref{Syntax Table
Internals}.

@menu
* Syntax Class Table::      Table of syntax classes.
* Syntax Flags::            Additional flags each character can have.
@end menu

@node Syntax Class Table
@subsection Table of Syntax Classes
@cindex syntax class table

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

@table @asis
@item Whitespace characters: @samp{@ } or @samp{-}
Characters that 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, and formfeed are classified as whitespace in
almost all major modes.

This syntax class can be designated by either @w{@samp{@ }} or
@samp{-}.  Both designators are equivalent.

@item Word constituents: @samp{w}
Parts of words in human languages.  These are typically used in
variable and command names in programs.  All upper- and lower-case
letters, and the digits, are typically word constituents.

@item Symbol constituents: @samp{_}
Extra characters used in variable and command names along with word
constituents.  Examples include the characters @samp{$&*+-_<>} in Lisp
mode, which may be part of a symbol name even though they are not part
of English words.  In standard C, the only non-word-constituent
character that is valid in symbols is underscore (@samp{_}).

@item Punctuation characters: @samp{.}
Characters used as punctuation in a human language, or used in a
programming language to separate symbols from one another.  Some
programming language modes, such as Emacs Lisp mode, have no
characters in this class since the few characters that are not symbol
or word constituents all have other uses.  Other programming language
modes, such as C mode, use punctuation syntax for operators.

@item Open parenthesis characters: @samp{(}
@itemx Close parenthesis characters: @samp{)}
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.  @xref{Blinking}.

In human languages, and in C code, the parenthesis pairs are
@samp{()}, @samp{[]}, and @samp{@{@}}.  In Emacs Lisp, the delimiters
for lists and vectors (@samp{()} and @samp{[]}) are classified as
parenthesis characters.

@item String quotes: @samp{"}
Characters used 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 (@samp{"})
and vertical bar (@samp{|}).  @samp{|} 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 (@samp{'}) for character
constants.

Human text has no string quote characters.  We do not want quotation
marks to turn off the usual syntactic properties of other characters
in the quotation.

@item Escape-syntax characters: @samp{\}
Characters that start an escape sequence, such as is used in string
and character constants.  The character @samp{\} 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
@code{words-include-escapes} is non-@code{nil}.  @xref{Word Motion}.

@item Character quotes: @samp{/}
Characters used to quote 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
@code{words-include-escapes} is non-@code{nil}.  @xref{Word Motion}.

This class is used for backslash in @TeX{} mode.

@item Paired delimiters: @samp{$}
Similar to string quote characters, except that the syntactic
properties of the characters between the delimiters are not
suppressed.  Only @TeX{} mode uses a paired delimiter presently---the
@samp{$} that both enters and leaves math mode.

@item Expression prefixes: @samp{'}
Characters 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, @samp{'} (used for quoting), the
comma, @samp{,} (used in macros), and @samp{#} (used in the read
syntax for certain data types).

@item Comment starters: @samp{<}
@itemx Comment enders: @samp{>}
@cindex comment syntax
Characters used in various languages to delimit comments.  Human text
has no comment characters.  In Lisp, the semicolon (@samp{;}) starts a
comment and a newline or formfeed ends one.

@item Inherit standard syntax: @samp{@@}
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.

@item Generic comment delimiters: @samp{!}
Characters that start or end a special kind of comment.  @emph{Any}
generic comment delimiter matches @emph{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
@code{syntax-table} text property (@pxref{Syntax Properties}).  You
can mark any range of characters as forming a comment, by giving the
first and last characters of the range @code{syntax-table} properties
identifying them as generic comment delimiters.

@item Generic string delimiters: @samp{|}
Characters that start or end a string.  This class differs from the
string quote class in that @emph{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
@code{syntax-table} text property (@pxref{Syntax Properties}).  You
can mark any range of characters as forming a string constant, by
giving the first and last characters of the range @code{syntax-table}
properties identifying them as generic string delimiters.
@end table

@node Syntax Flags
@subsection Syntax Flags
@cindex 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 @samp{1}, @samp{2}, @samp{3}, @samp{4}, @samp{b}, @samp{c},
@samp{n}, and @samp{p}.

  All the flags except @samp{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 @emph{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 @samp{*} in
C mode, which is a punctuation character, @emph{and} the second
character of a start-of-comment sequence (@samp{/*}), @emph{and} the
first character of an end-of-comment sequence (@samp{*/}).  The flags
@samp{b}, @samp{c}, and @samp{n} are used to qualify the corresponding
comment delimiter.

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

@itemize @bullet
@item
@samp{1} means @var{c} is the start of a two-character comment-start
sequence.

@item
@samp{2} means @var{c} is the second character of such a sequence.

@item
@samp{3} means @var{c} is the start of a two-character comment-end
sequence.

@item
@samp{4} means @var{c} is the second character of such a sequence.

@item
@samp{b} means that @var{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.

@item
@samp{c} means that @var{c} as a comment delimiter belongs to the
alternative ``c'' comment style.  For a two-character comment
delimiter, @samp{c} on either character makes it of style ``c''.

@item
@samp{n} on a comment delimiter character specifies
that this kind of comment can be nested.  For a two-character
comment delimiter, @samp{n} on either character makes it
nestable.

@cindex comment style
Emacs supports several comment styles simultaneously in any one syntax
table.  A comment style is a set of flags @samp{b}, @samp{c}, and
@samp{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:

@table @asis
@item @samp{/}
@samp{124}
@item @samp{*}
@samp{23b}
@item newline
@samp{>}
@end table

This defines four comment-delimiting sequences:

@table @asis
@item @samp{/*}
This is a comment-start sequence for ``b'' style because the
second character, @samp{*}, has the @samp{b} flag.

@item @samp{//}
This is a comment-start sequence for ``a'' style because the second
character, @samp{/}, does not have the @samp{b} flag.

@item @samp{*/}
This is a comment-end sequence for ``b'' style because the first
character, @samp{*}, has the @samp{b} flag.

@item newline
This is a comment-end sequence for ``a'' style, because the newline
character does not have the @samp{b} flag.
@end table

@item
@samp{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 @code{backward-prefix-chars} moves back over these
characters, as well as over characters whose primary syntax class is
prefix (@samp{'}).  @xref{Motion and Syntax}.
@end itemize

@node Syntax Table Functions
@section Syntax Table Functions

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

@defun make-syntax-table &optional table
This function creates a new syntax table.  If @var{table} is
non-@code{nil}, the parent of the new syntax table is @var{table};
otherwise, the parent is the standard syntax table.

In the new syntax table, all characters are initially given the
``inherit'' (@samp{@@}) syntax class, i.e., their syntax is inherited
from the parent table (@pxref{Syntax Class Table}).
@end defun

@defun copy-syntax-table &optional table
This function constructs a copy of @var{table} and returns it.  If
@var{table} is omitted or @code{nil}, it returns a copy of the
standard syntax table.  Otherwise, an error is signaled if @var{table}
is not a syntax table.
@end defun

@deffn Command modify-syntax-entry char syntax-descriptor  &optional table
@cindex syntax entry, setting
This function sets the syntax entry for @var{char} according to
@var{syntax-descriptor}.  @var{char} must be a character, or a cons
cell of the form @code{(@var{min} . @var{max})}; in the latter case,
the function sets the syntax entries for all characters in the range
between @var{min} and @var{max}, inclusive.

The syntax is changed only for @var{table}, which defaults to the
current buffer's syntax table, and not in any other syntax table.

The argument @var{syntax-descriptor} is a syntax descriptor, i.e., a
string whose first character is a syntax class designator and whose
second and subsequent characters optionally specify a matching
character and syntax flags.  @xref{Syntax Descriptors}.  An error is
signaled if @var{syntax-descriptor} is not a valid syntax descriptor.

This function always returns @code{nil}.  The old syntax information in
the table for this character is discarded.

@example
@group
@exdent @r{Examples:}

;; @r{Put the space character in class whitespace.}
(modify-syntax-entry ?\s " ")
     @result{} nil
@end group

@group
;; @r{Make @samp{$} an open parenthesis character,}
;;   @r{with @samp{^} as its matching close.}
(modify-syntax-entry ?$ "(^")
     @result{} nil
@end group

@group
;; @r{Make @samp{^} a close parenthesis character,}
;;   @r{with @samp{$} as its matching open.}
(modify-syntax-entry ?^ ")$")
     @result{} nil
@end group

@group
;; @r{Make @samp{/} a punctuation character,}
;;   @r{the first character of a start-comment sequence,}
;;   @r{and the second character of an end-comment sequence.}
;;   @r{This is used in C mode.}
(modify-syntax-entry ?/ ". 14")
     @result{} nil
@end group
@end example
@end deffn

@defun char-syntax character
This function returns the syntax class of @var{character}, represented
by its designator character (@pxref{Syntax Class Table}).  This
returns @emph{only} the class, not its matching character or syntax
flags.

The following examples apply to C mode.  (We use @code{string} to make
it easier to see the character returned by @code{char-syntax}.)

@example
@group
;; Space characters have whitespace syntax class.
(string (char-syntax ?\s))
     @result{} " "
@end group

@group
;; Forward slash characters have punctuation syntax.
;; Note that this @code{char-syntax} call does not reveal
;; that it is also part of comment-start and -end sequences.
(string (char-syntax ?/))
     @result{} "."
@end group

@group
;; Open parenthesis characters have open parenthesis syntax.
;; Note that this @code{char-syntax} call does not reveal that
;; it has a matching character, @samp{)}.
(string (char-syntax ?\())
     @result{} "("
@end group
@end example

@end defun

@defun set-syntax-table table
This function makes @var{table} the syntax table for the current buffer.
It returns @var{table}.
@end defun

@defun syntax-table
This function returns the current syntax table, which is the table for
the current buffer.
@end defun

@deffn Command describe-syntax &optional buffer
This command displays the contents of the syntax table of
@var{buffer} (by default, the current buffer) in a help buffer.
@end deffn

@defmac with-syntax-table table body@dots{}
This macro executes @var{body} using @var{table} as the current syntax
table.  It returns the value of the last form in @var{body}, after
restoring the old current syntax table.

Since each buffer has its own current syntax table, we should make that
more precise: @code{with-syntax-table} temporarily alters the current
syntax table of whichever buffer is current at the time the macro
execution starts.  Other buffers are not affected.
@end defmac

@node Syntax Properties
@section Syntax Properties
@kindex syntax-table @r{(text property)}

When the syntax table is not flexible enough to specify the syntax of
a language, you can override the syntax table for specific character
occurrences in the buffer, by applying a @code{syntax-table} text
property.  @xref{Text Properties}, for how to apply text properties.

  The valid values of @code{syntax-table} text property are:

@table @asis
@item @var{syntax-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 the
underlying text character.

@item @code{(@var{syntax-code} . @var{matching-char})}
A cons cell of this format is a raw syntax descriptor (@pxref{Syntax
Table Internals}), which directly specifies a syntax class for the
underlying text character.

@item @code{nil}
If the property is @code{nil}, the character's syntax is determined from
the current syntax table in the usual way.
@end table

@defvar parse-sexp-lookup-properties
If this is non-@code{nil}, the syntax scanning functions, like
@code{forward-sexp}, pay attention to syntax text properties.
Otherwise they use only the current syntax table.
@end defvar

@defvar syntax-propertize-function
This variable, if non-@code{nil}, should store a function for applying
@code{syntax-table} properties to a specified stretch of text.  It is
intended to be used by major modes to install a function which applies
@code{syntax-table} properties in some mode-appropriate way.

The function is called by @code{syntax-ppss} (@pxref{Position Parse}),
and by Font Lock mode during syntactic fontification (@pxref{Syntactic
Font Lock}).  It is called with two arguments, @var{start} and
@var{end}, which are the starting and ending positions of the text on
which it should act.  It is allowed to call @code{syntax-ppss} on any
position before @var{end}.  However, it should not call
@code{syntax-ppss-flush-cache}; so, it is not allowed to call
@code{syntax-ppss} on some position and later modify the buffer at an
earlier position.
@end defvar

@defvar syntax-propertize-extend-region-functions
This abnormal hook is run by the syntax parsing code prior to calling
@code{syntax-propertize-function}.  Its role is to help locate safe
starting and ending buffer positions for passing to
@code{syntax-propertize-function}.  For example, a major mode can add
a function to this hook to identify multi-line syntactic constructs,
and ensure that the boundaries do not fall in the middle of one.

Each function in this hook should accept two arguments, @var{start}
and @var{end}.  It should return either a cons cell of two adjusted
buffer positions, @code{(@var{new-start} . @var{new-end})}, or
@code{nil} if no adjustment is necessary.  The hook functions are run
in turn, repeatedly, until they all return @code{nil}.
@end defvar

@node Motion and Syntax
@section Motion and Syntax
@cindex moving across syntax classes
@cindex skipping characters of certain syntax

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

@defun skip-syntax-forward syntaxes &optional limit
This function moves point forward across characters having syntax
classes mentioned in @var{syntaxes} (a string of syntax class
characters).  It stops when it encounters the end of the buffer, or
position @var{limit} (if specified), or a character it is not supposed
to skip.

If @var{syntaxes} starts with @samp{^}, then the function skips
characters whose syntax is @emph{not} in @var{syntaxes}.

The return value is the distance traveled, which is a nonnegative
integer.
@end defun

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

If @var{syntaxes} starts with @samp{^}, then the function skips
characters whose syntax is @emph{not} in @var{syntaxes}.

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

@defun 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 @samp{p} flag.
@end defun

@node Parsing Expressions
@section Parsing Expressions
@cindex parsing expressions
@cindex scanning expressions

  This section describes functions for parsing and scanning balanced
expressions.  We will refer to such expressions as @dfn{sexps},
following the terminology of Lisp, even though these functions can act
on languages other than Lisp.  Basically, a sexp is either a balanced
parenthetical grouping, a string, or a ``symbol'' (i.e., a sequence
of characters whose syntax is either word constituent or symbol
constituent).  However, characters in the expression prefix syntax
class (@pxref{Syntax Class Table}) are treated as part of the sexp if
they appear next to it.

  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.  @xref{List Motion}, for convenient
higher-level functions for moving over balanced expressions.

  A character's syntax controls how it changes the state of the
parser, rather than describing the state itself.  For example, a
string delimiter character toggles the parser state between
``in-string'' and ``in-code'', but the syntax of characters does not
directly say whether they are inside a string.  For example (note that
15 is the syntax code for generic string delimiters),

@example
(put-text-property 1 9 'syntax-table '(15 . nil))
@end example

@noindent
does not tell Emacs that the first eight chars of the current buffer
are a string, but rather that they are all string delimiters.  As a
result, Emacs treats them as four consecutive empty string constants.

@menu
* Motion via Parsing::       Motion functions that work by parsing.
* Position Parse::           Determining the syntactic state of a position.
* Parser State::             How Emacs represents a syntactic state.
* Low-Level Parsing::        Parsing across a specified region.
* Control Parsing::          Parameters that affect parsing.
@end menu

@node Motion via Parsing
@subsection Motion Commands Based on Parsing
@cindex motion based on parsing

  This section describes simple point-motion functions that operate
based on parsing expressions.

@defun scan-lists from count depth
This function scans forward @var{count} balanced parenthetical
groupings from position @var{from}.  It returns the position where the
scan stops.  If @var{count} is negative, the scan moves backwards.

If @var{depth} is nonzero, treat the starting position as being
@var{depth} parentheses deep.  The scanner moves forward or backward
through the buffer until the depth changes to zero @var{count} times.
Hence, a positive value for @var{depth} has the effect of moving out
@var{depth} levels of parenthesis from the starting position, while a
negative @var{depth} has the effect of moving deeper by @var{-depth}
levels of parenthesis.

Scanning ignores comments if @code{parse-sexp-ignore-comments} is
non-@code{nil}.

If the scan reaches the beginning or end of the accessible part of the
buffer before it has scanned over @var{count} parenthetical groupings,
the return value is @code{nil} if the depth at that point is zero; if
the depth is non-zero, a @code{scan-error} error is signaled.
@end defun

@defun scan-sexps from count
This function scans forward @var{count} sexps from position @var{from}.
It returns the position where the scan stops.  If @var{count} is
negative, the scan moves backwards.

Scanning ignores comments if @code{parse-sexp-ignore-comments} is
non-@code{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, @code{nil} is returned.
@end defun

@defun forward-comment count
This function moves point forward across @var{count} complete comments
     (that is, including the starting delimiter and the terminating
delimiter if any), plus any whitespace encountered on the way.  It
moves backward if @var{count} is negative.  If it encounters anything
other than a comment or whitespace, it stops, leaving point at the
place where it stopped.  This includes (for instance) finding the end
of a comment when moving forward and expecting the beginning of one.
The function also stops immediately after moving over the specified
number of complete comments.  If @var{count} comments are found as
expected, with nothing except whitespace between them, it returns
@code{t}; otherwise it returns @code{nil}.

This function cannot tell whether the ``comments'' it traverses are
embedded within a string.  If they look like comments, it treats them
as comments.

To move forward over all comments and whitespace following point, use
@code{(forward-comment (buffer-size))}.  @code{(buffer-size)} is a
good argument to use, because the number of comments in the buffer
cannot exceed that many.
@end defun

@node Position Parse
@subsection Finding the Parse State for a Position
@cindex parse state for a position

  For syntactic analysis, such as in indentation, often the useful
thing is to compute the syntactic state corresponding to a given buffer
position.  This function does that conveniently.

@defun syntax-ppss &optional pos
This function returns the parser state that the parser would reach at
position @var{pos} starting from the beginning of the buffer.
@iftex
See the next section for
@end iftex
@ifnottex
@xref{Parser State},
@end ifnottex
for a description of the parser state.

The return value is the same as if you call the low-level parsing
function @code{parse-partial-sexp} to parse from the beginning of the
buffer to @var{pos} (@pxref{Low-Level Parsing}).  However,
@code{syntax-ppss} uses a cache to speed up the computation.  Due to
this optimization, the second value (previous complete subexpression)
and sixth value (minimum parenthesis depth) in the returned parser
state are not meaningful.

This function has a side effect: it adds a buffer-local entry to
@code{before-change-functions} (@pxref{Change Hooks}) for
@code{syntax-ppss-flush-cache} (see below).  This entry keeps the
cache consistent as the buffer is modified.  However, the cache might
not be updated if @code{syntax-ppss} is called while
@code{before-change-functions} is temporarily let-bound, or if the
buffer is modified without running the hook, such as when using
@code{inhibit-modification-hooks}.  In those cases, it is necessary to
call @code{syntax-ppss-flush-cache} explicitly.
@end defun

@defun syntax-ppss-flush-cache beg &rest ignored-args
This function flushes the cache used by @code{syntax-ppss}, starting
at position @var{beg}.  The remaining arguments, @var{ignored-args},
are ignored; this function accepts them so that it can be directly
used on hooks such as @code{before-change-functions} (@pxref{Change
Hooks}).
@end defun

  Major modes can make @code{syntax-ppss} run faster by specifying
where it needs to start parsing.

@defvar syntax-begin-function
If this is non-@code{nil}, it should be a function that moves to an
earlier buffer position where the parser state is equivalent to
@code{nil}---in other words, a position outside of any comment,
string, or parenthesis.  @code{syntax-ppss} uses it to further
optimize its computations, when the cache gives no help.
@end defvar

@node Parser State
@subsection Parser State
@cindex parser state

  A @dfn{parser state} is a list of ten elements describing the state
of the syntactic parser, after it parses the text between a specified
starting point and a specified end point in the buffer.  Parsing
functions such as @code{syntax-ppss}
@ifnottex
(@pxref{Position Parse})
@end ifnottex
return a parser state as the value.  Some parsing functions accept a
parser state as an argument, for resuming parsing.

  Here are the meanings of the elements of the parser state:

@enumerate 0
@item
The depth in parentheses, counting from 0.  @strong{Warning:} this can
be negative if there are more close parens than open parens between
the parser's starting point and end point.

@item
@cindex innermost containing parentheses
The character position of the start of the innermost parenthetical
grouping containing the stopping point; @code{nil} if none.

@item
@cindex previous complete subexpression
The character position of the start of the last complete subexpression
terminated; @code{nil} if none.

@item
@cindex inside string
Non-@code{nil} if inside a string.  More precisely, this is the
character that will terminate the string, or @code{t} if a generic
string delimiter character should terminate it.

@item
@cindex inside comment
@code{t} if inside a non-nestable comment (of any comment style;
@pxref{Syntax Flags}); or the comment nesting level if inside a
comment that can be nested.

@item
@cindex quote character
@code{t} if the end point is just after a quote character.

@item
The minimum parenthesis depth encountered during this scan.

@item
What kind of comment is active: @code{nil} if not in a comment or in a
comment of style @samp{a}; 1 for a comment of style @samp{b}; 2 for a
comment of style @samp{c}; and @code{syntax-table} for a comment that
should be ended by a generic comment delimiter character.

@item
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 @code{nil}.

@item
Internal data for continuing the parsing.  The meaning of this
data is subject to change; it is used if you pass this list
as the @var{state} argument to another call.
@end enumerate

  Elements 1, 2, and 6 are ignored in a state which you pass as an
argument to continue parsing, and elements 8 and 9 are used only in
trivial cases.  Those elements are mainly used internally by the
parser code.

  One additional piece of useful information is available from a
parser state using this function:

@defun syntax-ppss-toplevel-pos state
This function extracts, from parser state @var{state}, the last
position scanned in the parse which was at top level in grammatical
structure.  ``At top level'' means outside of any parentheses,
comments, or strings.

The value is @code{nil} if @var{state} represents a parse which has
arrived at a top level position.
@end defun

@node Low-Level Parsing
@subsection Low-Level Parsing

  The most basic way to use the expression parser is to tell it
to start at a given position with a certain state, and parse up to
a specified end position.

@defun parse-partial-sexp start limit &optional target-depth stop-before state stop-comment
This function parses a sexp in the current buffer starting at
@var{start}, not scanning past @var{limit}.  It stops at position
@var{limit} or when certain criteria described below are met, and sets
point to the location where parsing stops.  It returns a parser state
@ifinfo
(@pxref{Parser State})
@end ifinfo
describing the status of the parse at the point where it stops.

@cindex parenthesis depth
If the third argument @var{target-depth} is non-@code{nil}, parsing
stops if the depth in parentheses becomes equal to @var{target-depth}.
The depth starts at 0, or at whatever is given in @var{state}.

If the fourth argument @var{stop-before} is non-@code{nil}, parsing
stops when it comes to any character that starts a sexp.  If
@var{stop-comment} is non-@code{nil}, parsing stops when it comes to the
start of a comment.  If @var{stop-comment} is the symbol
@code{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.

If @var{state} is @code{nil}, @var{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 @var{state}
argument that describes the initial status of parsing.  The value
returned by a previous call to @code{parse-partial-sexp} will do
nicely.
@end defun

@node Control Parsing
@subsection Parameters to Control Parsing
@cindex parsing, control parameters

@defvar multibyte-syntax-as-symbol
If this variable is non-@code{nil}, @code{scan-sexps} treats all
non-@acronym{ASCII} characters as symbol constituents regardless
of what the syntax table says about them.  (However, text properties
can still override the syntax.)
@end defvar

@defopt parse-sexp-ignore-comments
@cindex skipping comments
If the value is non-@code{nil}, then comments are treated as
whitespace by the functions in this section and by @code{forward-sexp},
@code{scan-lists} and @code{scan-sexps}.
@end defopt

@vindex parse-sexp-lookup-properties
The behavior of @code{parse-partial-sexp} is also affected by
@code{parse-sexp-lookup-properties} (@pxref{Syntax Properties}).

You can use @code{forward-comment} to move forward or backward over
one comment or several comments.

@node Syntax Table Internals
@section Syntax Table Internals
@cindex syntax table internals

  Syntax tables are implemented as char-tables (@pxref{Char-Tables}),
but most Lisp programs don't work directly with their elements.
Syntax tables do not store syntax data as syntax descriptors
(@pxref{Syntax Descriptors}); they use an internal format, which is
documented in this section.  This internal format can also be assigned
as syntax properties (@pxref{Syntax Properties}).

@cindex syntax code
@cindex raw syntax descriptor
  Each entry in a syntax table is a @dfn{raw syntax descriptor}: a
cons cell of the form @code{(@var{syntax-code}
. @var{matching-char})}.  @var{syntax-code} is an integer which
encodes the syntax class and syntax flags, according to the table
below.  @var{matching-char}, if non-@code{nil}, specifies a matching
character (similar to the second character in a syntax descriptor).

  Here are the syntax codes corresponding to the various syntax
classes:

@multitable @columnfractions .2 .3 .2 .3
@item
@i{Code} @tab @i{Class} @tab @i{Code} @tab @i{Class}
@item
0 @tab whitespace @tab 8 @tab paired delimiter
@item
1 @tab punctuation @tab 9 @tab escape
@item
2 @tab word @tab 10 @tab character quote
@item
3 @tab symbol @tab 11 @tab comment-start
@item
4 @tab open parenthesis @tab 12 @tab comment-end
@item
5 @tab close parenthesis @tab 13 @tab inherit
@item
6 @tab expression prefix @tab 14 @tab generic comment
@item
7 @tab string quote @tab 15 @tab generic string
@end multitable

@noindent
For example, in the standard syntax table, the entry for @samp{(} is
@code{(4 . 41)}.  41 is the character code for @samp{)}.

  Syntax 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.

@multitable @columnfractions .15 .3 .15 .3
@item
@i{Prefix} @tab @i{Flag} @tab @i{Prefix} @tab @i{Flag}
@item
@samp{1} @tab @code{(lsh 1 16)} @tab @samp{p} @tab @code{(lsh 1 20)}
@item
@samp{2} @tab @code{(lsh 1 17)} @tab @samp{b} @tab @code{(lsh 1 21)}
@item
@samp{3} @tab @code{(lsh 1 18)} @tab @samp{n} @tab @code{(lsh 1 22)}
@item
@samp{4} @tab @code{(lsh 1 19)}
@end multitable

@defun string-to-syntax desc
Given a syntax descriptor @var{desc} (a string), this function returns
the corresponding raw syntax descriptor.
@end defun

@defun syntax-after pos
This function returns the raw syntax descriptor for the character in
the buffer after position @var{pos}, taking account of syntax
properties as well as the syntax table.  If @var{pos} is outside the
buffer's accessible portion (@pxref{Narrowing, accessible portion}),
the return value is @code{nil}.
@end defun

@defun syntax-class syntax
This function returns the syntax code for the raw syntax descriptor
@var{syntax}.  More precisely, it takes the raw syntax descriptor's
@var{syntax-code} component, masks off the high 16 bits which record
the syntax flags, and returns the resulting integer.

If @var{syntax} is @code{nil}, the return value is returns @code{nil}.
This is so that the expression

@example
(syntax-class (syntax-after pos))
@end example

@noindent
evaluates to @code{nil} if @code{pos} is outside the buffer's
accessible portion, without throwing errors or returning an incorrect
code.
@end defun

@node Categories
@section Categories
@cindex categories of characters
@cindex character categories

  @dfn{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.

@cindex category table
  Each buffer has a @dfn{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 @acronym{ASCII} printing character in
the range @w{@samp{ }} to @samp{~}.  You specify the name of a category
when you define it with @code{define-category}.

@cindex category set
  The category table is actually a char-table (@pxref{Char-Tables}).
The element of the category table at index @var{c} is a @dfn{category
set}---a bool-vector---that indicates which categories character @var{c}
belongs to.  In this category set, if the element at index @var{cat} is
@code{t}, that means category @var{cat} is a member of the set, and that
character @var{c} belongs to category @var{cat}.

For the next three functions, the optional argument @var{table}
defaults to the current buffer's category table.

@defun define-category char docstring &optional table
This function defines a new category, with name @var{char} and
documentation @var{docstring}, for the category table @var{table}.

Here's an example of defining a new category for characters that have
strong right-to-left directionality (@pxref{Bidirectional Display})
and using it in a special category table:

@example
(defvar special-category-table-for-bidi
  (let ((category-table (make-category-table))
        (uniprop-table (unicode-property-table-internal 'bidi-class)))
    (define-category ?R "Characters of bidi-class R, AL, or RLO"
                     category-table)
    (map-char-table
     #'(lambda (key val)
         (if (memq val '(R AL RLO))
             (modify-category-entry key ?R category-table)))
     uniprop-table)
    category-table))
@end example
@end defun

@defun category-docstring category &optional table
This function returns the documentation string of category @var{category}
in category table @var{table}.

@example
(category-docstring ?a)
     @result{} "ASCII"
(category-docstring ?l)
     @result{} "Latin"
@end example
@end defun

@defun get-unused-category &optional table
This function returns a category name (a character) which is not
currently defined in @var{table}.  If all possible categories are in use
in @var{table}, it returns @code{nil}.
@end defun

@defun category-table
This function returns the current buffer's category table.
@end defun

@defun category-table-p object
This function returns @code{t} if @var{object} is a category table,
otherwise @code{nil}.
@end defun

@defun standard-category-table
This function returns the standard category table.
@end defun

@defun copy-category-table &optional table
This function constructs a copy of @var{table} and returns it.  If
@var{table} is not supplied (or is @code{nil}), it returns a copy of the
standard category table.  Otherwise, an error is signaled if @var{table}
is not a category table.
@end defun

@defun set-category-table table
This function makes @var{table} the category table for the current
buffer.  It returns @var{table}.
@end defun

@defun make-category-table
This creates and returns an empty category table.  In an empty category
table, no categories have been allocated, and no characters belong to
any categories.
@end defun

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

@example
(make-category-set "al")
     @result{} #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"
@end example
@end defun

@defun char-category-set char
This function returns the category set for character @var{char} in the
current buffer's category table.  This is the bool-vector which
records which categories the character @var{char} belongs to.  The
function @code{char-category-set} does not allocate storage, because
it returns the same bool-vector that exists in the category table.

@example
(char-category-set ?a)
     @result{} #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"
@end example
@end defun

@defun category-set-mnemonics category-set
This function converts the category set @var{category-set} into a string
containing the characters that designate the categories that are members
of the set.

@example
(category-set-mnemonics (char-category-set ?a))
     @result{} "al"
@end example
@end defun

@defun modify-category-entry char category &optional table reset
This function modifies the category set of @var{char} in category
table @var{table} (which defaults to the current buffer's category
table).  @var{char} can be a character, or a cons cell of the form
@code{(@var{min} . @var{max})}; in the latter case, the function
modifies the category sets of all characters in the range between
@var{min} and @var{max}, inclusive.

Normally, it modifies a category set by adding @var{category} to it.
But if @var{reset} is non-@code{nil}, then it deletes @var{category}
instead.
@end defun

@deffn Command describe-categories &optional buffer-or-name
This function describes the category specifications in the current
category table.  It inserts the descriptions in a buffer, and then
displays that buffer.  If @var{buffer-or-name} is non-@code{nil}, it
describes the category table of that buffer instead.
@end deffn