emacs/doc/lispref/searching.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1995, 1998--1999, 2001--2025 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Searching and Matching
@chapter Searching and Matching
@cindex searching

  GNU Emacs provides two ways to search through a buffer for specified
text: exact string searches and regular expression searches.  After a
regular expression search, you can examine the @dfn{match data} to
determine which text matched the whole regular expression or various
portions of it.

@menu
* String Search::         Search for an exact match.
* Searching and Case::    Case-independent or case-significant searching.
* Regular Expressions::   Describing classes of strings.
* Regexp Search::         Searching for a match for a regexp.
* Longest Match::         Searching for the longest match.
* Match Data::            Finding out which part of the text matched,
                            after a string or regexp search.
* Search and Replace::    Commands that loop, searching and replacing.
* Standard Regexps::      Useful regexps for finding sentences, pages,...
* POSIX Regexps::         Emacs regexps vs POSIX regexps.
@end menu

  The @samp{skip-chars@dots{}} functions also perform a kind of searching.
@xref{Skipping Characters}.  To search for changes in character
properties, see @ref{Property Search}.

@node String Search
@section Searching for Strings
@cindex string search

  These are the primitive functions for searching through the text in a
buffer.  They are meant for use in programs, but you may call them
interactively.  If you do so, they prompt for the search string; the
arguments @var{limit} and @var{noerror} are @code{nil}, and @var{repeat}
is 1.  For more details on interactive searching, @pxref{Search,,
Searching and Replacement, emacs, The GNU Emacs Manual}.

  These search functions convert the search string to multibyte if the
buffer is multibyte; they convert the search string to unibyte if the
buffer is unibyte.  @xref{Text Representations}.

@deffn Command search-forward string &optional limit noerror count
This function searches forward from point for an exact match for
@var{string}.  If successful, it sets point to the end of the occurrence
found, and returns the new value of point.  If no match is found, the
value and side effects depend on @var{noerror} (see below).

In the following example, point is initially at the beginning of the
line.  Then @code{(search-forward "fox")} moves point after the last
letter of @samp{fox}:

@example
@group
---------- Buffer: foo ----------
@point{}The quick brown fox jumped over the lazy dog.
---------- Buffer: foo ----------
@end group

@group
(search-forward "fox")
     @result{} 20

---------- Buffer: foo ----------
The quick brown fox@point{} jumped over the lazy dog.
---------- Buffer: foo ----------
@end group
@end example

The argument @var{limit} specifies the bound to the search, and should
be a position in the current buffer.  No match extending after
that position is accepted.  If @var{limit} is omitted or @code{nil}, it
defaults to the end of the accessible portion of the buffer.

@kindex search-failed
What happens when the search fails depends on the value of
@var{noerror}.  If @var{noerror} is @code{nil}, a @code{search-failed}
error is signaled.  If @var{noerror} is @code{t}, @code{search-forward}
returns @code{nil} and does nothing.  If @var{noerror} is neither
@code{nil} nor @code{t}, then @code{search-forward} moves point to the
upper bound and returns @code{nil}.
@c I see no prospect of this ever changing, and frankly the current
@c behavior seems better, so there seems no need to mention this.
@ignore
(It would be more consistent now to return the new position of point
in that case, but some existing programs may depend on a value of
@code{nil}.)
@end ignore

The argument @var{noerror} only affects valid searches which fail to
find a match.  Invalid arguments cause errors regardless of
@var{noerror}.

If @var{count} is a positive number @var{n}, the search is done
@var{n} times; each successive search starts at the end of the
previous match.  If all these successive searches succeed, the
function call succeeds, moving point and returning its new value.
Otherwise the function call fails, with results depending on the value
of @var{noerror}, as described above.  If @var{count} is a negative
number @minus{}@var{n}, the search is done @var{n} times in the opposite
(backward) direction.
@end deffn

@deffn Command search-backward string &optional limit noerror count
This function searches backward from point for @var{string}.  It is
like @code{search-forward}, except that it searches backwards rather
than forwards.  Backward searches leave point at the beginning of the
match.
@end deffn

@deffn Command word-search-forward string &optional limit noerror count
This function searches forward from point for a word match for
@var{string}.  If it finds a match, it sets point to the end of the
match found, and returns the new value of point.

Word matching regards @var{string} as a sequence of words, disregarding
punctuation that separates them.  It searches the buffer for the same
sequence of words.  Each word must be distinct in the buffer (searching
for the word @samp{ball} does not match the word @samp{balls}), but the
details of punctuation and spacing are ignored (searching for @samp{ball
boy} does match @samp{ball.  Boy!}).

In this example, point is initially at the beginning of the buffer; the
search leaves it between the @samp{y} and the @samp{!}.

@example
@group
---------- Buffer: foo ----------
@point{}He said "Please!  Find
the ball boy!"
---------- Buffer: foo ----------
@end group

@group
(word-search-forward "Please find the ball, boy.")
     @result{} 39

---------- Buffer: foo ----------
He said "Please!  Find
the ball boy@point{}!"
---------- Buffer: foo ----------
@end group
@end example

If @var{limit} is non-@code{nil}, it must be a position in the current
buffer; it specifies the upper bound to the search.  The match found
must not extend after that position.

If @var{noerror} is @code{nil}, then @code{word-search-forward} signals
an error if the search fails.  If @var{noerror} is @code{t}, then it
returns @code{nil} instead of signaling an error.  If @var{noerror} is
neither @code{nil} nor @code{t}, it moves point to @var{limit} (or the
end of the accessible portion of the buffer) and returns @code{nil}.

If @var{count} is a positive number, it specifies how many successive
occurrences to search for.  Point is positioned at the end of the last
match.  If @var{count} is a negative number, the search is backward
and point is positioned at the beginning of the last match.

@findex word-search-regexp
Internally, @code{word-search-forward} and related functions use the
function @code{word-search-regexp} to convert @var{string} to a
regular expression that ignores punctuation.
@end deffn

@deffn Command word-search-forward-lax string &optional limit noerror count
This command is identical to @code{word-search-forward}, except that
the beginning or the end of @var{string} need not match a word
boundary, unless @var{string} begins or ends in whitespace.
For instance, searching for @samp{ball boy} matches @samp{ball boyee},
but does not match @samp{balls boy}.
@end deffn

@deffn Command word-search-backward string &optional limit noerror count
This function searches backward from point for a word match to
@var{string}.  This function is just like @code{word-search-forward}
except that it searches backward and normally leaves point at the
beginning of the match.
@end deffn

@deffn Command word-search-backward-lax string &optional limit noerror count
This command is identical to @code{word-search-backward}, except that
the beginning or the end of @var{string} need not match a word
boundary, unless @var{string} begins or ends in whitespace.
@end deffn

@node Searching and Case
@section Searching and Case
@cindex searching and case

  By default, searches in Emacs ignore the case of the text they are
searching through; if you specify searching for @samp{FOO}, then
@samp{Foo} or @samp{foo} is also considered a match.  This applies to
regular expressions, too; thus, @samp{[aB]} would match @samp{a} or
@samp{A} or @samp{b} or @samp{B}.

  If you do not want this feature, set the variable
@code{case-fold-search} to @code{nil}.  Then all letters must match
exactly, including case.  This is a buffer-local variable; altering the
variable affects only the current buffer.  (@xref{Intro to
Buffer-Local}.)  Alternatively, you may change the default value.
In Lisp code, you will more typically use @code{let} to bind
@code{case-fold-search} to the desired value.

  Note that the user-level incremental search feature handles case
distinctions differently.  When the search string contains only lower
case letters, the search ignores case, but when the search string
contains one or more upper case letters, the search becomes
case-sensitive.  But this has nothing to do with the searching
functions used in Lisp code.  @xref{Incremental Search,,, emacs,
The GNU Emacs Manual}.

@defopt case-fold-search
This buffer-local variable determines whether searches should ignore
case.  If the variable is @code{nil} they do not ignore case; otherwise
(and by default) they do ignore case.
@end defopt

@defopt case-replace
This variable determines whether the higher-level replacement
functions should preserve case.  If the variable is @code{nil}, that
means to use the replacement text verbatim.  A non-@code{nil} value
means to convert the case of the replacement text according to the
text being replaced.

This variable is used by passing it as an argument to the function
@code{replace-match}.  @xref{Replacing Match}.
@end defopt

@node Regular Expressions
@section Regular Expressions
@cindex regular expression
@cindex regexp

  A @dfn{regular expression}, or @dfn{regexp} for short, is a pattern that
denotes a (possibly infinite) set of strings.  Searching for matches for
a regexp is a very powerful operation.  This section explains how to write
regexps; the following section says how to search for them.

@findex re-builder
@cindex regular expressions, developing
  For interactive development of regular expressions, you
can use the @kbd{M-x re-builder} command.  It provides a convenient
interface for creating regular expressions, by giving immediate visual
feedback in a separate buffer.  As you edit the regexp, all its
matches in the target buffer are highlighted.  Each parenthesized
sub-expression of the regexp is shown in a distinct face, which makes
it easier to verify even very complex regexps.

  Note that by default Emacs search ignores case (@pxref{Searching and
Case}).  To enable case-sensitive regexp search and match, bind
@code{case-fold-search} to @code{nil} around the code you want to be
case-sensitive.

@menu
* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Example::          Illustrates regular expression syntax.
@ifnottex
* Rx Notation::             An alternative, structured regexp notation.
@end ifnottex
* Regexp Functions::        Functions for operating on regular expressions.
* Regexp Problems::         Some problems and how they may be avoided.
@end menu

@node Syntax of Regexps
@subsection Syntax of Regular Expressions
@cindex regexp syntax
@cindex syntax of regular expressions

  Regular expressions have a syntax in which a few characters are
special constructs and the rest are @dfn{ordinary}.  An ordinary
character is a simple regular expression that matches that character
and nothing else.  The special characters are @samp{.}, @samp{*},
@samp{+}, @samp{?}, @samp{[}, @samp{^}, @samp{$}, and @samp{\}; no new
special characters will be defined in the future.  The character
@samp{]} is special if it ends a bracket expression (see later).
The character @samp{-} is special inside a bracket expression.  A
@samp{[:} and balancing @samp{:]} enclose a character class inside a
bracket expression.  Any other character appearing in a regular
expression is ordinary, unless a @samp{\} precedes it.

  For example, @samp{f} is not a special character, so it is ordinary, and
therefore @samp{f} is a regular expression that matches the string
@samp{f} and no other string.  (It does @emph{not} match the string
@samp{fg}, but it does match a @emph{part} of that string.)  Likewise,
@samp{o} is a regular expression that matches only @samp{o}.

  Any two regular expressions @var{a} and @var{b} can be concatenated.  The
result is a regular expression that matches a string if @var{a} matches
some amount of the beginning of that string and @var{b} matches the rest of
the string.

  As a simple example, we can concatenate the regular expressions @samp{f}
and @samp{o} to get the regular expression @samp{fo}, which matches only
the string @samp{fo}.  Still trivial.  To do something more powerful, you
need to use one of the special regular expression constructs.

@menu
* Regexp Special::      Special characters in regular expressions.
* Char Classes::        Character classes used in regular expressions.
* Regexp Backslash::    Backslash-sequences in regular expressions.
@end menu

@node Regexp Special
@subsubsection Special Characters in Regular Expressions
@cindex regexp, special characters in

  Here is a list of the characters that are special in a regular
expression.

@need 800
@table @asis
@item @samp{.}@: @r{(Period)}
@cindex @samp{.} in regexp
is a special character that matches any single character except a newline.
Using concatenation, we can make regular expressions like @samp{a.b}, which
matches any three-character string that begins with @samp{a} and ends with
@samp{b}.

@item @samp{*}
@cindex @samp{*} in regexp
is not a construct by itself; it is a postfix operator that means to
match the preceding regular expression repetitively as many times as
possible.  Thus, @samp{o*} matches any number of @samp{o}s (including no
@samp{o}s).

@samp{*} always applies to the @emph{smallest} possible preceding
expression.  Thus, @samp{fo*} has a repeating @samp{o}, not a repeating
@samp{fo}.  It matches @samp{f}, @samp{fo}, @samp{foo}, and so on.

@cindex backtracking and regular expressions
The matcher processes a @samp{*} construct by matching, immediately, as
many repetitions as can be found.  Then it continues with the rest of
the pattern.  If that fails, backtracking occurs, discarding some of the
matches of the @samp{*}-modified construct in the hope that this will
make it possible to match the rest of the pattern.  For example, in
matching @samp{ca*ar} against the string @samp{caaar}, the @samp{a*}
first tries to match all three @samp{a}s; but the rest of the pattern is
@samp{ar} and there is only @samp{r} left to match, so this try fails.
The next alternative is for @samp{a*} to match only two @samp{a}s.  With
this choice, the rest of the regexp matches successfully.

@item @samp{+}
@cindex @samp{+} in regexp
is a postfix operator, similar to @samp{*} except that it must match
the preceding expression at least once.  So, for example, @samp{ca+r}
matches the strings @samp{car} and @samp{caaaar} but not the string
@samp{cr}, whereas @samp{ca*r} matches all three strings.

@item @samp{?}
@cindex @samp{?} in regexp
is a postfix operator, similar to @samp{*} except that it must match the
preceding expression either once or not at all.  For example,
@samp{ca?r} matches @samp{car} or @samp{cr}; nothing else.

@anchor{Non-greedy repetition}
@item @samp{*?}, @samp{+?}, @samp{??}
@cindex non-greedy repetition characters in regexp
are @dfn{non-greedy} variants of the operators @samp{*}, @samp{+}
and @samp{?}.  Where those operators match the largest possible
substring (consistent with matching the entire containing expression),
the non-greedy variants match the smallest possible substring
(consistent with matching the entire containing expression).

For example, the regular expression @samp{c[ad]*a} when applied to the
string @samp{cdaaada} matches the whole string; but the regular
expression @samp{c[ad]*?a}, applied to that same string, matches just
@samp{cda}.  (The smallest possible match here for @samp{[ad]*?} that
permits the whole expression to match is @samp{d}.)

@item @samp{[ @dots{} ]}
@cindex bracket expression (in regexp)
@cindex character alternative (in regexp)
@cindex @samp{[} in regexp
@cindex @samp{]} in regexp
is a @dfn{bracket expression} (a.k.a.@: @dfn{character alternative}),
which begins with @samp{[} and is terminated by @samp{]}.  In the
simplest case, the characters between the two brackets are what this
bracket expression can match.

Thus, @samp{[ad]} matches either one @samp{a} or one @samp{d}, and
@samp{[ad]*} matches any string composed of just @samp{a}s and @samp{d}s
(including the empty string).  It follows that @samp{c[ad]*r}
matches @samp{cr}, @samp{car}, @samp{cdr}, @samp{caddaar}, etc.

You can also include character ranges in a bracket expression, by
writing the starting and ending characters with a @samp{-} between them.
Thus, @samp{[a-z]} matches any lower-case @acronym{ASCII} letter.
Ranges may be intermixed freely with individual characters, as in
@samp{[a-z$%.]}, which matches any lower case @acronym{ASCII} letter
or @samp{$}, @samp{%} or period.  However, the ending character of one
range should not be the starting point of another one; for example,
@samp{[a-m-z]} should be avoided.

A bracket expression can also specify named character classes
(@pxref{Char Classes}).  For example, @samp{[[:ascii:]]} matches any
@acronym{ASCII} character.  Using a character class is equivalent to
mentioning each of the characters in that class; but the latter is not
feasible in practice, since some classes include thousands of
different characters.  A character class should not appear as the
lower or upper bound of a range.

The usual regexp special characters are not special inside a
bracket expression.  A completely different set of characters is
special: @samp{]}, @samp{-} and @samp{^}.
To include @samp{]} in a bracket expression, put it at the
beginning.  To include @samp{^}, put it anywhere but at the beginning.
To include @samp{-}, put it at the end.  Thus, @samp{[]^-]} matches
all three of these special characters.  You cannot use @samp{\} to
escape these three characters, since @samp{\} is not special here.

The following aspects of ranges are specific to Emacs, in that POSIX
allows but does not require this behavior and programs other than
Emacs may behave differently:

@enumerate
@item
If @code{case-fold-search} is non-@code{nil}, @samp{[a-z]} also
matches upper-case letters.

@item
A range is not affected by the locale's collation sequence: it always
represents the set of characters with codepoints ranging between those
of its bounds, so that @samp{[a-z]} matches only ASCII letters, even
outside the C or POSIX locale.

@item
If the lower bound of a range is greater than its upper bound, the
range is empty and represents no characters.  Thus, @samp{[z-a]}
always fails to match, and @samp{[^z-a]} matches any character,
including newline.  However, a reversed range should always be from
the letter @samp{z} to the letter @samp{a} to make it clear that it is
not a typo; for example, @samp{[+-*/]} should be avoided, because it
matches only @samp{/} rather than the likely-intended four characters.

@item
If the end points of a range are raw 8-bit bytes (@pxref{Text
Representations}), or if the range start is ASCII and the end is a raw
byte (as in @samp{[a-\377]}), the range will match only ASCII
characters and raw 8-bit bytes, but not non-ASCII characters.  This
feature is intended for searching text in unibyte buffers and strings.
@end enumerate

Some kinds of bracket expressions are not the best style even
though they have a well-defined meaning in Emacs.  They include:

@enumerate
@item
Although a range's bound can be almost any character, it is better
style to stay within natural sequences of ASCII letters and digits
because most people have not memorized character code tables.
For example, @samp{[.-9]} is less clear than @samp{[./0-9]},
and @samp{[`-~]} is less clear than @samp{[`a-z@{|@}~]}.
Unicode character escapes can help here; for example, for most programmers
@samp{[ก-ฺ฿-๛]} is less clear than @samp{[\u0E01-\u0E3A\u0E3F-\u0E5B]}.

@item
Although a bracket expression can include duplicates, it is better
style to avoid them.  For example, @samp{[XYa-yYb-zX]} is less clear
than @samp{[XYa-z]}.

@item
Although a range can denote just one, two, or three characters, it
is simpler to list the characters.  For example,
@samp{[a-a0]} is less clear than @samp{[a0]}, @samp{[i-j]} is less clear
than @samp{[ij]}, and @samp{[i-k]} is less clear than @samp{[ijk]}.

@item
Although a @samp{-} can appear at the beginning of a bracket
expression or as the upper bound of a range, it is better style to
put @samp{-} by itself at the end of a bracket expression.  For
example, although @samp{[-a-z]} is valid, @samp{[a-z-]} is better
style; and although @samp{[*--]} is valid, @samp{[*+,-]} is clearer.
@end enumerate

@item @samp{[^ @dots{} ]}
@cindex @samp{^} in regexp
@samp{[^} begins a @dfn{complemented bracket expression}, or
@dfn{complemented character alternative}.  This matches any character
except the ones specified.  Thus, @samp{[^a-z0-9A-Z]} matches all
characters @emph{except} ASCII letters and digits.

@samp{^} is not special in a bracket expression unless it is the first
character.  The character following the @samp{^} is treated as if it
were first (in other words, @samp{-} and @samp{]} are not special there).

A complemented bracket expression can match a newline, unless newline is
mentioned as one of the characters not to match.  This is in contrast to
the handling of regexps in programs such as @code{grep}.

You can specify named character classes, just like in bracket
expressions.  For instance, @samp{[^[:ascii:]]} matches any
non-@acronym{ASCII} character.  @xref{Char Classes}.

@item @samp{^}
@cindex beginning of line in regexp
When matching a buffer, @samp{^} matches the empty string, but only at the
beginning of a line in the text being matched (or the beginning of the
accessible portion of the buffer).  Otherwise it fails to match
anything.  Thus, @samp{^foo} matches a @samp{foo} that occurs at the
beginning of a line.

When matching a string instead of a buffer, @samp{^} matches at the
beginning of the string or after a newline character.

For historical compatibility, @samp{^} is special only at the beginning
of the regular expression, or after @samp{\(}, @samp{\(?:} or @samp{\|}.
Although @samp{^} is an ordinary character in other contexts,
it is good practice to use @samp{\^} even then.

@item @samp{$}
@cindex @samp{$} in regexp
@cindex end of line in regexp
is similar to @samp{^} but matches only at the end of a line (or the
end of the accessible portion of the buffer).  Thus, @samp{x+$}
matches a string of one @samp{x} or more at the end of a line.

When matching a string instead of a buffer, @samp{$} matches at the end
of the string or before a newline character.

For historical compatibility, @samp{$} is special only at the
end of the regular expression, or before @samp{\)} or @samp{\|}.
Although @samp{$} is an ordinary character in other contexts,
it is good practice to use @samp{\$} even then.

@item @samp{\}
@cindex @samp{\} in regexp
has two functions: it quotes the special characters (including
@samp{\}), and it introduces additional special constructs.

Because @samp{\} quotes special characters, @samp{\$} is a regular
expression that matches only @samp{$}, and @samp{\[} is a regular
expression that matches only @samp{[}, and so on.

Note that @samp{\} also has special meaning in the read syntax of Lisp
strings (@pxref{String Type}), and must be quoted with @samp{\}.  For
example, the regular expression that matches the @samp{\} character is
@samp{\\}.  To write a Lisp string that contains the characters
@samp{\\}, Lisp syntax requires you to quote each @samp{\} with another
@samp{\}.  Therefore, the read syntax for a regular expression matching
@samp{\} is @code{"\\\\"}.
@end table

For historical compatibility, a repetition operator is treated as ordinary
if it appears at the start of a regular expression
or after @samp{^}, @samp{\`}, @samp{\(}, @samp{\(?:} or @samp{\|}.
For example, @samp{*foo} is treated as @samp{\*foo}, and
@samp{two\|^\@{2\@}} is treated as @samp{two\|^@{2@}}.
It is poor practice to depend on this behavior; use proper backslash
escaping anyway, regardless of where the repetition operator appears.

As a @samp{\} is not special inside a bracket expression, it can
never remove the special meaning of @samp{-}, @samp{^} or @samp{]}.
You should not quote these characters when they have no special
meaning.  This would not clarify anything, since backslashes
can legitimately precede these characters where they @emph{have}
special meaning, as in @samp{[^\]} (@code{"[^\\]"} for Lisp string
syntax), which matches any single character except a backslash.

In practice, most @samp{]} that occur in regular expressions close a
bracket expression and hence are special.  However, occasionally a
regular expression may try to match a complex pattern of literal
@samp{[} and @samp{]}.  In such situations, it sometimes may be
necessary to carefully parse the regexp from the start to determine
which square brackets enclose a bracket expression.  For example,
@samp{[^][]]} consists of the complemented bracket expression
@samp{[^][]} (which matches any single character that is not a square
bracket), followed by a literal @samp{]}.

The exact rules are that at the beginning of a regexp, @samp{[} is
special and @samp{]} not.  This lasts until the first unquoted
@samp{[}, after which we are in a bracket expression; @samp{[} is
no longer special (except when it starts a character class) but @samp{]}
is special, unless it immediately follows the special @samp{[} or that
@samp{[} followed by a @samp{^}.  This lasts until the next special
@samp{]} that does not end a character class.  This ends the bracket
expression and restores the ordinary syntax of regular expressions;
an unquoted @samp{[} is special again and a @samp{]} not.

@node Char Classes
@subsubsection Character Classes
@cindex character classes in regexp
@cindex ascii character class, regexp
@cindex alnum character class, regexp
@cindex alpha character class, regexp
@cindex xdigit character class, regexp

  Below is a table of the classes you can use in a bracket expression
(@pxref{Regexp Special, bracket expression}), and what they mean.
Note that the @samp{[} and @samp{]} characters that enclose the class
name are part of the name, so a regular expression using these classes
needs one more pair of brackets.  For example, a regular expression
matching a sequence of one or more letters and digits would be
@samp{[[:alnum:]]+}, not @samp{[:alnum:]+}.

@table @samp
@item [:ascii:]
This matches any @acronym{ASCII} character (codes 0--127).
@item [:alnum:]
This matches any letter or digit.  For multibyte characters, it
matches characters whose Unicode @samp{general-category} property
(@pxref{Character Properties}) indicates they are alphabetic or
decimal number characters.
@item [:alpha:]
This matches any letter.  For multibyte characters, it matches
characters whose Unicode @samp{general-category} property
(@pxref{Character Properties}) indicates they are alphabetic
characters.
@item [:blank:]
This matches horizontal whitespace, as defined by Annex C of the
Unicode Technical Standard #18.  In particular, it matches spaces,
tabs, and other characters whose Unicode @samp{general-category}
property (@pxref{Character Properties}) indicates they are spacing
separators.
@item [:cntrl:]
This matches any character whose code is in the range 0--31.
@item [:digit:]
This matches @samp{0} through @samp{9}.  Thus, @samp{[-+[:digit:]]}
matches any digit, as well as @samp{+} and @samp{-}.
@cindex graphic characters
@item [:graph:]
This matches graphic characters---everything except spaces,
@acronym{ASCII} and non-@acronym{ASCII} control characters,
surrogates, and codepoints unassigned by Unicode, as indicated by the
Unicode @samp{general-category} property (@pxref{Character
Properties}).
@item [:lower:]
This matches any lower-case letter, as determined by the current case
table (@pxref{Case Tables}).  If @code{case-fold-search} is
non-@code{nil}, this also matches any upper-case letter.  Note that a
buffer can have its own local case table different from the default
one.
@item [:multibyte:]
This matches any multibyte character (@pxref{Text Representations}).
@item [:nonascii:]
This matches any non-@acronym{ASCII} character.
@cindex printable characters
@item [:print:]
This matches any printing character---either spaces or graphic
characters matched by @samp{[:graph:]}.
@item [:punct:]
This matches any punctuation character.  (At present, for multibyte
characters, it matches anything that has non-word syntax, and thus its
exact definition can vary from one major mode to another, since the
syntax of a character depends on the major mode.)
@item [:space:]
This matches any character that has whitespace syntax
(@pxref{Syntax Class Table}).  Note that the syntax of a character,
and thus which characters are considered ``whitespace'',
depends on the major mode.
@item [:unibyte:]
This matches any unibyte character (@pxref{Text Representations}).
@item [:upper:]
This matches any upper-case letter, as determined by the current case
table (@pxref{Case Tables}).  If @code{case-fold-search} is
non-@code{nil}, this also matches any lower-case letter.  Note that a
buffer can have its own local case table different from the default
one.
@item [:word:]
This matches any character that has word syntax (@pxref{Syntax Class
Table}).  Note that the syntax of a character, and thus which
characters are considered ``word-constituent'', depends on the major
mode.
@item [:xdigit:]
This matches the hexadecimal digits: @samp{0} through @samp{9}, @samp{a}
through @samp{f} and @samp{A} through @samp{F}.
@end table

The classes @samp{[:space:]}, @samp{[:word:]} and @samp{[:punct:]} use
the syntax-table of the current buffer but not any overriding syntax
text properties (@pxref{Syntax Properties}).

@node Regexp Backslash
@subsubsection Backslash Constructs in Regular Expressions
@cindex backslash in regular expressions

  For the most part, @samp{\} followed by any character matches only
that character.  However, there are several exceptions: certain
sequences starting with @samp{\} that have special meanings.  Here is
a table of the special @samp{\} constructs.

@table @samp
@item \|
@cindex @samp{|} in regexp
@cindex regexp alternative
specifies an alternative.
Two regular expressions @var{a} and @var{b} with @samp{\|} in
between form an expression that matches anything that either @var{a} or
@var{b} matches.

Thus, @samp{foo\|bar} matches either @samp{foo} or @samp{bar}
but no other string.

@samp{\|} applies to the largest possible surrounding expressions.  Only a
surrounding @samp{\( @dots{} \)} grouping can limit the grouping power of
@samp{\|}.

If you need full backtracking capability to handle multiple uses of
@samp{\|}, use the POSIX regular expression functions (@pxref{POSIX
Regexps}).

@item \@{@var{m}\@}
is a postfix operator that repeats the previous pattern exactly @var{m}
times.  Thus, @samp{x\@{5\@}} matches the string @samp{xxxxx}
and nothing else.  @samp{c[ad]\@{3\@}r} matches string such as
@samp{caaar}, @samp{cdddr}, @samp{cadar}, and so on.

@item \@{@var{m},@var{n}\@}
is a more general postfix operator that specifies repetition with a
minimum of @var{m} repeats and a maximum of @var{n} repeats.  If @var{m}
is omitted, the minimum is 0; if @var{n} is omitted, there is no
maximum.  For both forms, @var{m} and @var{n}, if specified, may be no
larger than
@ifnottex
2**16 @minus{} 1
@end ifnottex
@tex
@math{2^{16}-1}
@end tex
.

For example, @samp{c[ad]\@{1,2\@}r} matches the strings @samp{car},
@samp{cdr}, @samp{caar}, @samp{cadr}, @samp{cdar}, and @samp{cddr}, and
nothing else.@*
@samp{\@{0,1\@}} or @samp{\@{,1\@}} is equivalent to @samp{?}.@*
@samp{\@{0,\@}} or @samp{\@{,\@}} is equivalent to @samp{*}.@*
@samp{\@{1,\@}} is equivalent to @samp{+}.

@item \( @dots{} \)
@cindex @samp{(} in regexp
@cindex @samp{)} in regexp
@cindex regexp grouping
is a grouping construct that serves three purposes:

@enumerate
@item
To enclose a set of @samp{\|} alternatives for other operations.  Thus,
the regular expression @samp{\(foo\|bar\)x} matches either @samp{foox}
or @samp{barx}.

@item
To enclose a complicated expression for the postfix operators @samp{*},
@samp{+} and @samp{?} to operate on.  Thus, @samp{ba\(na\)*} matches
@samp{ba}, @samp{bana}, @samp{banana}, @samp{bananana}, etc., with any
number (zero or more) of @samp{na} strings.

@item
To record a matched substring for future reference with
@samp{\@var{digit}} (see below).
@end enumerate

This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature that was assigned as a
second meaning to the same @samp{\( @dots{} \)} construct because, in
practice, there was usually no conflict between the two meanings.  But
occasionally there is a conflict, and that led to the introduction of
shy groups.

@item \(?: @dots{} \)
@cindex shy groups
@cindex non-capturing group
@cindex unnumbered group
@cindex @samp{(?:} in regexp
is the @dfn{shy group} construct.  A shy group serves the first two
purposes of an ordinary group (controlling the nesting of other
operators), but it does not get a number, so you cannot refer back to
its value with @samp{\@var{digit}}.  Shy groups are particularly
useful for mechanically-constructed regular expressions, because they
can be added automatically without altering the numbering of ordinary,
non-shy groups.

Shy groups are also called @dfn{non-capturing} or @dfn{unnumbered
groups}.

@item \(?@var{num}: @dots{} \)
is the @dfn{explicitly numbered group} construct.  Normal groups get
their number implicitly, based on their position, which can be
inconvenient.  This construct allows you to force a particular group
number.  There is no particular restriction on the numbering,
e.g., you can have several groups with the same number in which case
the last one to match (i.e., the rightmost match) will win.
Implicitly numbered groups always get the smallest integer larger than
the one of any previous group.

@item \@var{digit}
matches the same text that matched the @var{digit}th occurrence of a
grouping (@samp{\( @dots{} \)}) construct.

In other words, after the end of a group, the matcher remembers the
beginning and end of the text matched by that group.  Later on in the
regular expression you can use @samp{\} followed by @var{digit} to
match that same text, whatever it may have been.

The strings matching the first nine grouping constructs appearing in
the entire regular expression passed to a search or matching function
are assigned numbers 1 through 9 in the order that the open
parentheses appear in the regular expression.  So you can use
@samp{\1} through @samp{\9} to refer to the text matched by the
corresponding grouping constructs.

For example, @samp{\(.*\)\1} matches any newline-free string that is
composed of two identical halves.  The @samp{\(.*\)} matches the first
half, which may be anything, but the @samp{\1} that follows must match
the same exact text.

If a @samp{\( @dots{} \)} construct matches more than once (which can
happen, for instance, if it is followed by @samp{*}), only the last
match is recorded.

If a particular grouping construct in the regular expression was never
matched---for instance, if it appears inside of an alternative that
wasn't used, or inside of a repetition that repeated zero times---then
the corresponding @samp{\@var{digit}} construct never matches
anything.  To use an artificial example, @samp{\(foo\(b*\)\|lose\)\2}
cannot match @samp{lose}: the second alternative inside the larger
group matches it, but then @samp{\2} is undefined and can't match
anything.  But it can match @samp{foobb}, because the first
alternative matches @samp{foob} and @samp{\2} matches @samp{b}.

@item \w
@cindex @samp{\w} in regexp
matches any word-constituent character.  The editor syntax table
determines which characters these are.  @xref{Syntax Tables}.

@item \W
@cindex @samp{\W} in regexp
matches any character that is not a word constituent.

@item \s@var{code}
@cindex @samp{\s} in regexp
matches any character whose syntax is @var{code}.  Here @var{code} is a
character that represents a syntax code: thus, @samp{w} for word
constituent, @samp{-} for whitespace, @samp{(} for open parenthesis,
etc.  To represent whitespace syntax, use either @samp{-} or a space
character.  @xref{Syntax Class Table}, for a list of syntax codes and
the characters that stand for them.

@item \S@var{code}
@cindex @samp{\S} in regexp
matches any character whose syntax is not @var{code}.

@cindex category, regexp search for
@item \c@var{code}
matches any character whose category is @var{code}.  Here @var{code}
is a character that represents a category: for example, in the standard
category table, @samp{c} stands for Chinese characters and @samp{g}
stands for Greek characters.  You can see the list of all the
currently defined categories with @w{@kbd{M-x describe-categories
@key{RET}}}.  You can also define your own categories in addition to
the standard ones using the @code{define-category} function
(@pxref{Categories}).

@item \C@var{code}
matches any character whose category is not @var{code}.
@end table

  The following regular expression constructs match the empty string---that is,
they don't consume any characters---but whether they match depends on the
context.  For all, the beginning and end of the accessible portion of
the buffer are treated as if they were the actual beginning and end of
the buffer.

@table @samp
@item \`
@cindex @samp{\`} in regexp
matches the empty string, but only at the beginning
of the buffer or string being matched against.

@item \'
@cindex @samp{\'} in regexp
matches the empty string, but only at the end of
the buffer or string being matched against.

@item \=
@cindex @samp{\=} in regexp
matches the empty string, but only at point.
(This construct is not defined when matching against a string.)

@item \b
@cindex @samp{\b} in regexp
matches the empty string, but only at the beginning or
end of a word.  Thus, @samp{\bfoo\b} matches any occurrence of
@samp{foo} as a separate word.  @samp{\bballs?\b} matches
@samp{ball} or @samp{balls} as a separate word.

@samp{\b} matches at the beginning or end of the buffer (or string)
regardless of what text appears next to it.

@item \B
@cindex @samp{\B} in regexp
matches the empty string, but @emph{not} at the beginning or
end of a word, nor at the beginning or end of the buffer (or string).

@item \<
@cindex @samp{\<} in regexp
matches the empty string, but only at the beginning of a word.
@samp{\<} matches at the beginning of the buffer (or string) only if a
word-constituent character follows.

@item \>
@cindex @samp{\>} in regexp
matches the empty string, but only at the end of a word.  @samp{\>}
matches at the end of the buffer (or string) only if the contents end
with a word-constituent character.

@item \_<
@cindex @samp{\_<} in regexp
matches the empty string, but only at the beginning of a symbol.  A
symbol is a sequence of one or more word or symbol constituent
characters.  @samp{\_<} matches at the beginning of the buffer (or
string) only if a symbol-constituent character follows.

@item \_>
@cindex @samp{\_>} in regexp
matches the empty string, but only at the end of a symbol.  @samp{\_>}
matches at the end of the buffer (or string) only if the contents end
with a symbol-constituent character.
@end table

@kindex invalid-regexp
  Not every string is a valid regular expression.  For example, a string
that ends inside a bracket expression without a terminating @samp{]}
is invalid, and so is a string that ends with a single @samp{\}.  If
an invalid regular expression is passed to any of the search functions,
an @code{invalid-regexp} error is signaled.

@node Regexp Example
@subsection Complex Regexp Example

  Here is a complicated regexp which was formerly used by Emacs to
recognize the end of a sentence together with any whitespace that
follows.  (Nowadays Emacs uses a similar but more complex default
regexp constructed by the function @code{sentence-end}.
@xref{Standard Regexps}.)

  Below, we show first the regexp as a string in Lisp syntax (to
distinguish spaces from tab characters), and then the result of
evaluating it.  The string constant begins and ends with a
double-quote.  @samp{\"} stands for a double-quote as part of the
string, @samp{\\} for a backslash as part of the string, @samp{\t} for a
tab and @samp{\n} for a newline.

@example
@group
"[.?!][]\"')@}]*\\($\\| $\\|\t\\|@ @ \\)[ \t\n]*"
     @result{} "[.?!][]\"')@}]*\\($\\| $\\|  \\|@ @ \\)[
]*"
@end group
@end example

@noindent
In the output, tab and newline appear as themselves.

  This regular expression contains four parts in succession and can be
deciphered as follows:

@table @code
@item [.?!]
The first part of the pattern is a bracket expression that matches
any one of three characters: period, question mark, and exclamation
mark.  The match must begin with one of these three characters.  (This
is one point where the new default regexp used by Emacs differs from
the old.  The new value also allows some non-@acronym{ASCII}
characters that end a sentence without any following whitespace.)

@item []\"')@}]*
The second part of the pattern matches any closing braces and quotation
marks, zero or more of them, that may follow the period, question mark
or exclamation mark.  The @code{\"} is Lisp syntax for a double-quote in
a string.  The @samp{*} at the end indicates that the immediately
preceding regular expression (a bracket expression, in this case) may be
repeated zero or more times.

@item \\($\\|@ $\\|\t\\|@ @ \\)
The third part of the pattern matches the whitespace that follows the
end of a sentence: the end of a line (optionally with a space), or a
tab, or two spaces.  The double backslashes mark the parentheses and
vertical bars as regular expression syntax; the parentheses delimit a
group and the vertical bars separate alternatives.  The dollar sign is
used to match the end of a line.

@item [ \t\n]*
Finally, the last part of the pattern matches any additional whitespace
beyond the minimum needed to end a sentence.
@end table

@ifnottex
In the @code{rx} notation (@pxref{Rx Notation}), the regexp could be written

@example
@group
(rx (any ".?!")                    ; Punctuation ending sentence.
    (zero-or-more (any "\"')]@}"))  ; Closing quotes or brackets.
    (or line-end
        (seq " " line-end)
        "\t"
        "  ")                      ; Two spaces.
    (zero-or-more (any "\t\n ")))  ; Optional extra whitespace.
@end group
@end example

Since @code{rx} regexps are just S-expressions, they can be formatted
and commented as such.
@end ifnottex

@ifnottex
@node Rx Notation
@subsection The @code{rx} Structured Regexp Notation
@cindex rx
@cindex regexp syntax

  As an alternative to the string-based syntax, Emacs provides the
structured @code{rx} notation based on Lisp S-expressions.  This
notation is usually easier to read, write and maintain than regexp
strings, and can be indented and commented freely.  It requires a
conversion into string form since that is what regexp functions
expect, but that conversion typically takes place during
byte-compilation rather than when the Lisp code using the regexp is
run.

  Here is an @code{rx} regexp@footnote{It could be written much
simpler with non-greedy operators (how?), but that would make the
example less interesting.} that matches a block comment in the C
programming language:

@example
@group
(rx "/*"                    ; Initial /*
    (zero-or-more
     (or (not "*")          ;  Either non-*,
         (seq "*"           ;  or * followed by
              (not "/"))))  ;     non-/
    (one-or-more "*")       ; At least one star,
    "/")                    ; and the final /
@end group
@end example

@noindent
or, using shorter synonyms and written more compactly,

@example
@group
(rx "/*"
    (* (| (not "*")
          (: "*" (not "/"))))
    (+ "*") "/")
@end group
@end example

@noindent
In conventional string syntax, it would be written

@example
"/\\*\\(?:[^*]\\|\\*[^/]\\)*\\*+/"
@end example

The @code{rx} notation is mainly useful in Lisp code; it cannot be
used in most interactive situations where a regexp is requested, such
as when running @code{query-replace-regexp} or in variable
customization.

@menu
* Rx Constructs::       Constructs valid in rx forms.
* Rx Functions::        Functions and macros that use rx forms.
* Extending Rx::        How to define your own rx forms.
@end menu

@node Rx Constructs
@subsubsection Constructs in @code{rx} regexps

The various forms in @code{rx} regexps are described below.  The
shorthand @var{rx} represents any @code{rx} form.  @var{rx}@dots{}
means zero or more @code{rx} forms and, unless stated otherwise,
matches these forms in sequence as if wrapped in a @code{(seq @dots{})}
subform.

These are all valid arguments to the @code{rx} macro.  All forms are
defined by their described semantics; the corresponding string regexps
are provided for ease of understanding only.  @var{A}, @var{B}, @dots{}
denote (suitably bracketed) string regexp subexpressions therein.

@subsubheading Literals

@table @asis
@item @code{"some-string"}
Match the string @samp{some-string} literally.  There are no
characters with special meaning, unlike in string regexps.

@item @code{?C}
Match the character @samp{C} literally.
@end table

@subsubheading Sequence and alternative

@table @asis
@item @code{(seq @var{rx}@dots{})}
@cindex @code{seq} in rx
@itemx @code{(sequence @var{rx}@dots{})}
@cindex @code{sequence} in rx
@itemx @code{(: @var{rx}@dots{})}
@cindex @code{:} in rx
@itemx @code{(and @var{rx}@dots{})}
@cindex @code{and} in rx
Match the @var{rx}s in sequence.  Without arguments, the expression
matches the empty string.@*
Corresponding string regexp: @samp{@var{A}@var{B}@dots{}}
(subexpressions in sequence).

@item @code{(or @var{rx}@dots{})}
@cindex @code{or} in rx
@itemx @code{(| @var{rx}@dots{})}
@cindex @code{|} in rx
Match exactly one of the @var{rx}s.
If all arguments are strings, characters, or @code{or} forms
so constrained, the longest possible match will always be used.
Otherwise, either the longest match or the
first (in left-to-right order) will be used.
Without arguments, the expression will not match anything at all.@*
Corresponding string regexp: @samp{@var{A}\|@var{B}\|@dots{}}.

@item @code{unmatchable}
@cindex @code{unmatchable} in rx
Refuse any match.  Equivalent to @code{(or)}.
@xref{regexp-unmatchable}.
@end table

@subsubheading Repetition

Normally, repetition forms are greedy, in that they attempt to match
as many times as possible.  Some forms are non-greedy; they try to
match as few times as possible (@pxref{Non-greedy repetition}).

@table @code
@item (zero-or-more @var{rx}@dots{})
@cindex @code{zero-or-more} in rx
@itemx (0+ @var{rx}@dots{})
@cindex @code{0+} in rx
Match the @var{rx}s zero or more times.  Greedy by default.@*
Corresponding string regexp: @samp{@var{A}*} (greedy),
@samp{@var{A}*?} (non-greedy)

@item (one-or-more @var{rx}@dots{})
@cindex @code{one-or-more} in rx
@itemx (1+ @var{rx}@dots{})
@cindex @code{1+} in rx
Match the @var{rx}s one or more times.  Greedy by default.@*
Corresponding string regexp: @samp{@var{A}+} (greedy),
@samp{@var{A}+?} (non-greedy)

@item (zero-or-one @var{rx}@dots{})
@cindex @code{zero-or-one} in rx
@itemx (optional @var{rx}@dots{})
@cindex @code{optional} in rx
@itemx (opt @var{rx}@dots{})
@cindex @code{opt} in rx
Match the @var{rx}s once or an empty string.  Greedy by default.@*
Corresponding string regexp: @samp{@var{A}?} (greedy),
@samp{@var{A}??} (non-greedy).

@item (* @var{rx}@dots{})
@cindex @code{*} in rx
Match the @var{rx}s zero or more times.  Greedy.@*
Corresponding string regexp: @samp{@var{A}*}

@item (+ @var{rx}@dots{})
@cindex @code{+} in rx
Match the @var{rx}s one or more times.  Greedy.@*
Corresponding string regexp: @samp{@var{A}+}

@item (? @var{rx}@dots{})
@cindex @code{?} in rx
Match the @var{rx}s once or an empty string.  Greedy.@*
Corresponding string regexp: @samp{@var{A}?}

@item (*? @var{rx}@dots{})
@cindex @code{*?} in rx
Match the @var{rx}s zero or more times.  Non-greedy.@*
Corresponding string regexp: @samp{@var{A}*?}

@item (+? @var{rx}@dots{})
@cindex @code{+?} in rx
Match the @var{rx}s one or more times.  Non-greedy.@*
Corresponding string regexp: @samp{@var{A}+?}

@item (?? @var{rx}@dots{})
@cindex @code{??} in rx
Match the @var{rx}s or an empty string.  Non-greedy.@*
Corresponding string regexp: @samp{@var{A}??}

@item (= @var{n} @var{rx}@dots{})
@cindex @code{=} in rx
@itemx (repeat @var{n} @var{rx})
Match the @var{rx}s exactly @var{n} times.@*
Corresponding string regexp: @samp{@var{A}\@{@var{n}\@}}

@item (>= @var{n} @var{rx}@dots{})
@cindex @code{>=} in rx
Match the @var{rx}s @var{n} or more times.  Greedy.@*
Corresponding string regexp: @samp{@var{A}\@{@var{n},\@}}

@item (** @var{n} @var{m} @var{rx}@dots{})
@cindex @code{**} in rx
@itemx (repeat @var{n} @var{m} @var{rx}@dots{})
@cindex @code{repeat} in rx
Match the @var{rx}s at least @var{n} but no more than @var{m} times.  Greedy.@*
Corresponding string regexp: @samp{@var{A}\@{@var{n},@var{m}\@}}
@end table

The greediness of some repetition forms can be controlled using the
following constructs.  However, it is usually better to use the
explicit non-greedy forms above when such matching is required.

@table @code
@item (minimal-match @var{rx})
@cindex @code{minimal-match} in rx
Match @var{rx}, with @code{zero-or-more}, @code{0+},
@code{one-or-more}, @code{1+}, @code{zero-or-one}, @code{opt} and
@code{optional} using non-greedy matching.

@item (maximal-match @var{rx})
@cindex @code{maximal-match} in rx
Match @var{rx}, with @code{zero-or-more}, @code{0+},
@code{one-or-more}, @code{1+}, @code{zero-or-one}, @code{opt} and
@code{optional} using greedy matching.  This is the default.
@end table

@subsubheading Matching single characters

@table @asis
@item @code{(any @var{set}@dots{})}
@cindex @code{any} in rx
@itemx @code{(char @var{set}@dots{})}
@cindex @code{char} in rx
@itemx @code{(in @var{set}@dots{})}
@cindex @code{in} in rx
@cindex character class in rx
Match a single character from one of the @var{set}s.  Each @var{set}
is a character, a string representing the set of its characters, a
range or a character class (see below).  A range is either a
hyphen-separated string like @code{"A-Z"}, or a cons of characters
like @code{(?A . ?Z)}.

Note that hyphen (@code{-}) is special in strings in this construct,
since it acts as a range separator.  To include a hyphen, add it as a
separate character or single-character string.@*
Corresponding string regexp: @samp{[@dots{}]}

@item @code{(not @var{charspec})}
@cindex @code{not} in rx
Match a character not included in @var{charspec}.  @var{charspec} can
be a character, a single-character string, an @code{any}, @code{not},
@code{or}, @code{intersection}, @code{syntax} or @code{category} form,
or a character class.
If @var{charspec} is an @code{or} form, its arguments have the same
restrictions as those of @code{intersection}; see below.@*
Corresponding string regexp: @samp{[^@dots{}]}, @samp{\S@var{code}},
@samp{\C@var{code}}

@item @code{(intersection @var{charset}@dots{})}
@cindex @code{intersection} in rx
Match a character included in all of the @var{charset}s.
Each @var{charset} can be a character, a single-character string, an
@code{any} form without character classes, or an @code{intersection},
@code{or} or @code{not} form whose arguments are also @var{charset}s.

@item @code{not-newline}, @code{nonl}
@cindex @code{not-newline} in rx
@cindex @code{nonl} in rx
Match any character except a newline.@*
Corresponding string regexp: @samp{.} (dot)

@item @code{anychar}, @code{anything}
@cindex @code{anychar} in rx
@cindex @code{anything} in rx
Match any character.@*
Corresponding string regexp: @samp{.\|\n} (for example)

@item character class
@cindex character class in rx
Match a character from a named character class:

@table @asis
@item @code{alpha}, @code{alphabetic}, @code{letter}
Match alphabetic characters.  More precisely, match characters whose
Unicode @samp{general-category} property indicates that they are
alphabetic.

@item @code{alnum}, @code{alphanumeric}
Match alphabetic characters and digits.  More precisely, match
characters whose Unicode @samp{general-category} property indicates
that they are alphabetic or decimal digits.

@item @code{digit}, @code{numeric}, @code{num}
Match the digits @samp{0}--@samp{9}.

@item @code{xdigit}, @code{hex-digit}, @code{hex}
Match the hexadecimal digits @samp{0}--@samp{9}, @samp{A}--@samp{F}
and @samp{a}--@samp{f}.

@item @code{cntrl}, @code{control}
Match any character whose code is in the range 0--31.

@item @code{blank}
Match horizontal whitespace.  More precisely, match characters whose
Unicode @samp{general-category} property indicates that they are
spacing separators.

@item @code{space}, @code{whitespace}, @code{white}
Match any character that has whitespace syntax
(@pxref{Syntax Class Table}).

@item @code{lower}, @code{lower-case}
Match anything lower-case, as determined by the current case table.
If @code{case-fold-search} is non-@code{nil}, this also matches any
upper-case letter.

@item @code{upper}, @code{upper-case}
Match anything upper-case, as determined by the current case table.
If @code{case-fold-search} is non-@code{nil}, this also matches any
lower-case letter.

@item @code{graph}, @code{graphic}
Match any character except whitespace, @acronym{ASCII} and
non-@acronym{ASCII} control characters, surrogates, and codepoints
unassigned by Unicode, as indicated by the Unicode
@samp{general-category} property.

@item @code{print}, @code{printing}
Match whitespace or a character matched by @code{graph}.

@item @code{punct}, @code{punctuation}
Match any punctuation character.  (At present, for multibyte
characters, anything that has non-word syntax.)

@item @code{word}, @code{wordchar}
Match any character that has word syntax (@pxref{Syntax Class Table}).

@item @code{ascii}
Match any @acronym{ASCII} character (codes 0--127).

@item @code{nonascii}
Match any non-@acronym{ASCII} character (but not raw bytes).
@end table

The classes @code{space}, @code{word} and @code{punct} use the
syntax-table of the current buffer but not any overriding syntax text
properties (@pxref{Syntax Properties}).@*
Corresponding string regexp: @samp{[[:@var{class}:]]}

@item @code{(syntax @var{syntax})}
@cindex @code{syntax} in rx
Match a character with syntax @var{syntax}, being one of the following
names:

@multitable {@code{close-parenthesis}} {Syntax character}
@headitem Syntax name          @tab Syntax character
@item @code{whitespace}        @tab @code{-}
@item @code{punctuation}       @tab @code{.}
@item @code{word}              @tab @code{w}
@item @code{symbol}            @tab @code{_}
@item @code{open-parenthesis}  @tab @code{(}
@item @code{close-parenthesis} @tab @code{)}
@item @code{expression-prefix} @tab @code{'}
@item @code{string-quote}      @tab @code{"}
@item @code{paired-delimiter}  @tab @code{$}
@item @code{escape}            @tab @code{\}
@item @code{character-quote}   @tab @code{/}
@item @code{comment-start}     @tab @code{<}
@item @code{comment-end}       @tab @code{>}
@item @code{string-delimiter}  @tab @code{|}
@item @code{comment-delimiter} @tab @code{!}
@end multitable

For details, @pxref{Syntax Class Table}.  Please note that
@code{(syntax punctuation)} is @emph{not} equivalent to the character class
@code{punctuation}.@*
Corresponding string regexp: @samp{\s@var{char}} where @var{char} is the
syntax character.

@item @code{(category @var{category})}
@cindex @code{category} in rx
Match a character in category @var{category}, which is either one of
the names below or its category character.

@multitable {@code{vowel-modifying-diacritical-mark}} {Category character}
@headitem Category name                       @tab Category character
@item @code{space-for-indent}                 @tab space
@item @code{base}                             @tab @code{.}
@item @code{consonant}                        @tab @code{0}
@item @code{base-vowel}                       @tab @code{1}
@item @code{upper-diacritical-mark}           @tab @code{2}
@item @code{lower-diacritical-mark}           @tab @code{3}
@item @code{tone-mark}                        @tab @code{4}
@item @code{symbol}                           @tab @code{5}
@item @code{digit}                            @tab @code{6}
@item @code{vowel-modifying-diacritical-mark} @tab @code{7}
@item @code{vowel-sign}                       @tab @code{8}
@item @code{semivowel-lower}                  @tab @code{9}
@item @code{not-at-end-of-line}               @tab @code{<}
@item @code{not-at-beginning-of-line}         @tab @code{>}
@item @code{alpha-numeric-two-byte}           @tab @code{A}
@item @code{chinese-two-byte}                 @tab @code{C}
@item @code{greek-two-byte}                   @tab @code{G}
@item @code{japanese-hiragana-two-byte}       @tab @code{H}
@item @code{indian-two-byte}                  @tab @code{I}
@item @code{japanese-katakana-two-byte}       @tab @code{K}
@item @code{strong-left-to-right}             @tab @code{L}
@item @code{korean-hangul-two-byte}           @tab @code{N}
@item @code{strong-right-to-left}             @tab @code{R}
@item @code{cyrillic-two-byte}                @tab @code{Y}
@item @code{combining-diacritic}              @tab @code{^}
@item @code{ascii}                            @tab @code{a}
@item @code{arabic}                           @tab @code{b}
@item @code{chinese}                          @tab @code{c}
@item @code{ethiopic}                         @tab @code{e}
@item @code{greek}                            @tab @code{g}
@item @code{korean}                           @tab @code{h}
@item @code{indian}                           @tab @code{i}
@item @code{japanese}                         @tab @code{j}
@item @code{japanese-katakana}                @tab @code{k}
@item @code{latin}                            @tab @code{l}
@item @code{lao}                              @tab @code{o}
@item @code{tibetan}                          @tab @code{q}
@item @code{japanese-roman}                   @tab @code{r}
@item @code{thai}                             @tab @code{t}
@item @code{vietnamese}                       @tab @code{v}
@item @code{hebrew}                           @tab @code{w}
@item @code{cyrillic}                         @tab @code{y}
@item @code{can-break}                        @tab @code{|}
@end multitable

For more information about currently defined categories, run the
command @kbd{M-x describe-categories @key{RET}}.  For how to define
new categories, @pxref{Categories}.@*
Corresponding string regexp: @samp{\c@var{char}} where @var{char} is the
category character.
@end table

@subsubheading Zero-width assertions

These all match the empty string, but only in specific places.

@table @asis
@item @code{line-start}, @code{bol}
@cindex @code{line-start} in rx
@cindex @code{bol} in rx
Match at the beginning of a line.@*
Corresponding string regexp: @samp{^}

@item @code{line-end}, @code{eol}
@cindex @code{line-end} in rx
@cindex @code{eol} in rx
Match at the end of a line.@*
Corresponding string regexp: @samp{$}

@item @code{string-start}, @code{bos}, @code{buffer-start}, @code{bot}
@cindex @code{string-start} in rx
@cindex @code{bos} in rx
@cindex @code{buffer-start} in rx
@cindex @code{bot} in rx
Match at the start of the string or buffer being matched against.@*
Corresponding string regexp: @samp{\`}

@item @code{string-end}, @code{eos}, @code{buffer-end}, @code{eot}
@cindex @code{string-end} in rx
@cindex @code{eos} in rx
@cindex @code{buffer-end} in rx
@cindex @code{eot} in rx
Match at the end of the string or buffer being matched against.@*
Corresponding string regexp: @samp{\'}

@item @code{point}
@cindex @code{point} in rx
Match at point.@*
Corresponding string regexp: @samp{\=}

@item @code{word-start}, @code{bow}
@cindex @code{word-start} in rx
@cindex @code{bow} in rx
Match at the beginning of a word.@*
Corresponding string regexp: @samp{\<}

@item @code{word-end}, @code{eow}
@cindex @code{word-end} in rx
@cindex @code{eow} in rx
Match at the end of a word.@*
Corresponding string regexp: @samp{\>}

@item @code{word-boundary}
@cindex @code{word-boundary} in rx
Match at the beginning or end of a word.@*
Corresponding string regexp: @samp{\b}

@item @code{not-word-boundary}
@cindex @code{not-word-boundary} in rx
Match anywhere but at the beginning or end of a word.@*
Corresponding string regexp: @samp{\B}

@item @code{symbol-start}
@cindex @code{symbol-start} in rx
Match at the beginning of a symbol.@*
Corresponding string regexp: @samp{\_<}

@item @code{symbol-end}
@cindex @code{symbol-end} in rx
Match at the end of a symbol.@*
Corresponding string regexp: @samp{\_>}
@end table

@subsubheading Capture groups

@table @code
@item (group @var{rx}@dots{})
@cindex @code{group} in rx
@itemx (submatch @var{rx}@dots{})
@cindex @code{submatch} in rx
Match the @var{rx}s, making the matched text and position accessible
in the match data.  The first group in a regexp is numbered 1;
subsequent groups will be numbered one above the previously
highest-numbered group in the pattern so far.@*
Corresponding string regexp: @samp{\(@dots{}\)}

@item (group-n @var{n} @var{rx}@dots{})
@cindex @code{group-n} in rx
@itemx (submatch-n @var{n} @var{rx}@dots{})
@cindex @code{submatch-n} in rx
Like @code{group}, but explicitly assign the group number @var{n}.
@var{n} must be positive.@*
Corresponding string regexp: @samp{\(?@var{n}:@dots{}\)}

@item (backref @var{n})
@cindex @code{backref} in rx
Match the text previously matched by group number @var{n}.
@var{n} must be in the range 1--9.@*
Corresponding string regexp: @samp{\@var{n}}
@end table

@subsubheading Dynamic inclusion

@table @code
@item (literal @var{expr})
@cindex @code{literal} in rx
Match the literal string that is the result from evaluating the Lisp
expression @var{expr}.  The evaluation takes place at call time, in
the current lexical environment.

@item (regexp @var{expr})
@cindex @code{regexp} in rx
@itemx (regex @var{expr})
@cindex @code{regex} in rx
Match the string regexp that is the result from evaluating the Lisp
expression @var{expr}.  The evaluation takes place at call time, in
the current lexical environment.

@item (eval @var{expr})
@cindex @code{eval} in rx
Match the rx form that is the result from evaluating the Lisp
expression @var{expr}.  The evaluation takes place at macro-expansion
time for @code{rx}, at call time for @code{rx-to-string},
in the current global environment.
@end table

@node Rx Functions
@subsubsection Functions and macros using @code{rx} regexps

@defmac rx rx-form@dots{}
Translate the @var{rx-form}s to a string regexp, as if they were the
body of a @code{(seq @dots{})} form.  The @code{rx} macro expands to a
string constant, or, if @code{literal} or @code{regexp} forms are
used, a Lisp expression that evaluates to a string.  Example:

@example
@group
(rx (+ alpha) "=" (+ digit))
  @result{} "[[:alpha:]]+=[[:digit:]]+"
@end group
@end example
@end defmac

@defun rx-to-string rx-expr &optional no-group
Translate @var{rx-expr} to a string regexp which is returned.
If @var{no-group} is absent or @code{nil}, bracket the result in a
non-capturing group, @samp{\(?:@dots{}\)}, if necessary to ensure that
a postfix operator appended to it will apply to the whole expression.
Example:

@example
@group
(rx-to-string '(seq (+ alpha) "=" (+ digit)) t)
  @result{} "[[:alpha:]]+=[[:digit:]]+"
@end group
@end example

Arguments to @code{literal} and @code{regexp} forms in @var{rx-expr}
must be string literals.
@end defun

The @code{pcase} macro can use @code{rx} expressions as patterns
directly; @pxref{rx in pcase}.

For mechanisms to add user-defined extensions to the @code{rx}
notation, @pxref{Extending Rx}.

@node Extending Rx
@subsubsection Defining new @code{rx} forms

The @code{rx} notation can be extended by defining new symbols and
parameterized forms in terms of other @code{rx} expressions.  This is
handy for sharing parts between several regexps, and for making
complex ones easier to build and understand by putting them together
from smaller pieces.

For example, you could define @code{name} to mean
@code{(one-or-more letter)}, and @code{(quoted @var{x})} to mean
@code{(seq ?' @var{x} ?')} for any @var{x}.  These forms could then be
used in @code{rx} expressions like any other: @code{(rx (quoted name))}
would match a nonempty sequence of letters inside single quotes.

The Lisp macros below provide different ways of binding names to
definitions.  Common to all of them are the following rules:

@itemize
@item
Built-in @code{rx} forms, like @code{digit} and @code{group}, cannot
be redefined.

@item
The definitions live in a name space of their own, separate from that
of Lisp variables.  There is thus no need to attach a suffix like
@code{-regexp} to names; they cannot collide with anything else.

@item
Definitions cannot refer to themselves recursively, directly or
indirectly.  If you find yourself needing this, you want a parser, not
a regular expression.

@item
Definitions are only ever expanded in calls to @code{rx} or
@code{rx-to-string}, not merely by their presence in definition
macros.  This means that the order of definitions doesn't matter, even
when they refer to each other, and that syntax errors only show up
when they are used, not when they are defined.

@item
User-defined forms are allowed wherever arbitrary @code{rx}
expressions are expected; for example, in the body of a
@code{zero-or-one} form, but not inside @code{any} or @code{category}
forms.  They are also allowed inside @code{not} and
@code{intersection} forms.
@end itemize

@defmac rx-define name [arglist] rx-form
Define @var{name} globally in all subsequent calls to @code{rx} and
@code{rx-to-string}.  If @var{arglist} is absent, then @var{name} is
defined as a plain symbol to be replaced with @var{rx-form}.  Example:

@example
@group
(rx-define haskell-comment (seq "--" (zero-or-more nonl)))
(rx haskell-comment)
     @result{} "--.*"
@end group
@end example

If @var{arglist} is present, it must be a list of zero or more
argument names, and @var{name} is then defined as a parameterized form.
When used in an @code{rx} expression as @code{(@var{name} @var{arg}@dots{})},
each @var{arg} will replace the corresponding argument name inside
@var{rx-form}.

@var{arglist} may end in @code{&rest} and one final argument name,
denoting a rest parameter.  The rest parameter will expand to all
extra actual argument values not matched by any other parameter in
@var{arglist}, spliced into @var{rx-form} where it occurs.  Example:

@example
@group
(rx-define moan (x y &rest r) (seq x (one-or-more y) r "!"))
(rx (moan "MOO" "A" "MEE" "OW"))
     @result{} "MOOA+MEEOW!"
@end group
@end example

Since the definition is global, it is recommended to give @var{name} a
package prefix to avoid name clashes with definitions elsewhere, as is
usual when naming non-local variables and functions.

Forms defined this way only perform simple template substitution.
For arbitrary computations, use them together with the @code{rx}
forms @code{eval}, @code{regexp} or @code{literal}.  Example:

@example
@group
(defun n-tuple-rx (n element)
  `(seq "<"
        (group-n 1 ,element)
        ,@@(mapcar (lambda (i) `(seq ?, (group-n ,i ,element)))
                  (number-sequence 2 n))
        ">"))
(rx-define n-tuple (n element) (eval (n-tuple-rx n 'element)))
(rx (n-tuple 3 (+ (in "0-9"))))
  @result{} "<\\(?1:[0-9]+\\),\\(?2:[0-9]+\\),\\(?3:[0-9]+\\)>"
@end group
@end example
@end defmac

@defmac rx-let (bindings@dots{}) body@dots{}
Make the @code{rx} definitions in @var{bindings} available locally for
@code{rx} macro invocations in @var{body}, which is then evaluated.

Each element of @var{bindings} is on the form
@w{@code{(@var{name} [@var{arglist}] @var{rx-form})}}, where the parts
have the same meaning as in @code{rx-define} above.  Example:

@example
@group
(rx-let ((comma-separated (item) (seq item (0+ "," item)))
         (number (1+ digit))
         (numbers (comma-separated number)))
  (re-search-forward (rx "(" numbers ")")))
@end group
@end example

The definitions are only available during the macro-expansion of
@var{body}, and are thus not present during execution of compiled
code.

@code{rx-let} can be used not only inside a function, but also at top
level to include global variable and function definitions that need
to share a common set of @code{rx} forms.  Since the names are local
inside @var{body}, there is no need for any package prefixes.
Example:

@example
@group
(rx-let ((phone-number (seq (opt ?+) (1+ (any digit ?-)))))
  (defun find-next-phone-number ()
    (re-search-forward (rx phone-number)))
  (defun phone-number-p (string)
    (string-match-p (rx bos phone-number eos) string)))
@end group
@end example

The scope of the @code{rx-let} bindings is lexical, which means that
they are not visible outside @var{body} itself, even in functions
called from @var{body}.
@end defmac

@defmac rx-let-eval bindings body@dots{}
Evaluate @var{bindings} to a list of bindings as in @code{rx-let},
and evaluate @var{body} with those bindings in effect for calls
to @code{rx-to-string}.

This macro is similar to @code{rx-let}, except that the @var{bindings}
argument is evaluated (and thus needs to be quoted if it is a list
literal), and the definitions are substituted at run time, which is
required for @code{rx-to-string} to work.  Example:

@example
@group
(rx-let-eval
    '((ponder (x) (seq "Where have all the " x " gone?")))
  (looking-at (rx-to-string
               '(ponder (or "flowers" "young girls"
                            "left socks")))))
@end group
@end example

Another difference from @code{rx-let} is that the @var{bindings} are
dynamically scoped, and thus also available in functions called from
@var{body}.  However, they are not visible inside functions defined in
@var{body}.
@end defmac

@end ifnottex

@node Regexp Functions
@subsection Regular Expression Functions

  These functions operate on regular expressions.

@cindex quote special characters in regexp
@defun regexp-quote string
This function returns a regular expression whose only exact match is
@var{string}.  Using this regular expression in @code{looking-at} will
succeed only if the next characters in the buffer are @var{string};
using it in a search function will succeed if the text being searched
contains @var{string}.  @xref{Regexp Search}.

This allows you to request an exact string match or search when calling
a function that wants a regular expression.

@example
@group
(regexp-quote "^The cat$")
     @result{} "\\^The cat\\$"
@end group
@end example

One use of @code{regexp-quote} is to combine an exact string match with
context described as a regular expression.  For example, this searches
for the string that is the value of @var{string}, surrounded by
whitespace:

@example
@group
(re-search-forward
 (concat "\\s-" (regexp-quote string) "\\s-"))
@end group
@end example

The returned string may be @var{string} itself if it does not contain
any special characters.
@end defun

@cindex optimize regexp
@defun regexp-opt strings &optional paren
This function returns an efficient regular expression that will match
any of the strings in the list @var{strings}.  This is useful when you
need to make matching or searching as fast as possible---for example,
for Font Lock mode@footnote{Note that @code{regexp-opt} does not
guarantee that its result is absolutely the most efficient form
possible.  A hand-tuned regular expression can sometimes be slightly
more efficient, but is almost never worth the effort.}.
@c E.g., see https://debbugs.gnu.org/2816

If @var{strings} is the empty list, the return value is a regexp that
never matches anything.

The optional argument @var{paren} can be any of the following:

@table @asis
@item a string
The resulting regexp is preceded by @var{paren} and followed by
@samp{\)}.  For example, use @samp{"\\(?1:"} to produce an explicitly
numbered group.

@item @code{words}
The resulting regexp is surrounded by @samp{\<\(} and @samp{\)\>}.

@item @code{symbols}
The resulting regexp is surrounded by @samp{\_<\(} and @samp{\)\_>}
(this is often appropriate when matching programming-language
keywords and the like).

@item non-@code{nil}
The resulting regexp is surrounded by @samp{\(} and @samp{\)}.

@item @code{nil}
The resulting regexp is surrounded by @samp{\(?:} and @samp{\)},
if it is necessary to ensure that a postfix operator appended to
it will apply to the whole expression.
@end table

The returned regexp is ordered in such a way that it will always match
the longest string possible.

Up to reordering, the resulting regexp of @code{regexp-opt} is
equivalent to but usually more efficient than that of a simplified
version:

@example
(defun simplified-regexp-opt (strings &optional paren)
 (let ((parens
        (cond
         ((stringp paren)       (cons paren "\\)"))
         ((eq paren 'words)    '("\\<\\(" . "\\)\\>"))
         ((eq paren 'symbols) '("\\_<\\(" . "\\)\\_>"))
         ((null paren)          '("\\(?:" . "\\)"))
         (t                       '("\\(" . "\\)")))))
   (concat (car parens)
           (mapconcat 'regexp-quote strings "\\|")
           (cdr parens))))
@end example
@end defun

@defun regexp-opt-depth regexp
This function returns the total number of grouping constructs
(parenthesized expressions) in @var{regexp}.  This does not include
shy groups (@pxref{Regexp Backslash}).
@end defun

@c Supposedly an internal regexp-opt function, but table.el uses it at least.
@defun regexp-opt-charset chars
This function returns a regular expression matching a character in the
list of characters @var{chars}.

@example
(regexp-opt-charset '(?a ?b ?c ?d ?e))
     @result{} "[a-e]"
@end example
@end defun

@c Internal functions: regexp-opt-group

@anchor{regexp-unmatchable}
@defvar regexp-unmatchable
This variable contains a regexp that is guaranteed not to match any
string at all.  It is particularly useful as default value for
variables that may be set to a pattern that actually matches
something.
@end defvar

@node Regexp Problems
@subsection Problems with Regular Expressions
@cindex regular expression problems
@cindex regexp stack overflow
@cindex stack overflow in regexp

The Emacs regexp implementation, like many of its kind, is generally
robust but occasionally causes trouble in either of two ways: matching
may run out of internal stack space and signal an error, and it can
take a long time to complete.  The advice below will make these
symptoms less likely and help alleviate problems that do arise.

@itemize
@item
Anchor regexps at the beginning of a line, string or buffer using
zero-width assertions (@samp{^} and @code{\`}).  This takes advantage
of fast paths in the implementation and can avoid futile matching
attempts.  Other zero-width assertions may also bring benefits by
causing a match to fail early.

@item
Avoid or-patterns in favor of bracket expressions: write
@samp{[ab]} instead of @samp{a\|b}.  Recall that @samp{\s-} and @samp{\sw}
are equivalent to @samp{[[:space:]]} and @samp{[[:word:]]}, respectively,
most of the time.

@item
Since the last branch of an or-pattern does not add a backtrack point
on the stack, consider putting the most likely matched pattern last.
For example, @samp{^\(?:a\|.b\)*c} will run out of stack if trying to
match a very long string of @samp{a}s, but the equivalent
@samp{^\(?:.b\|a\)*c} will not.

(It is a trade-off: successfully matched or-patterns run faster with
the most frequently matched pattern first.)

@item
Try to ensure that any part of the text can only match in a single
way.  For example, @samp{a*a*} will match the same set of strings as
@samp{a*}, but the former can do so in many ways and will therefore
cause slow backtracking if the match fails later on.  Make or-pattern
branches mutually exclusive if possible, so that matching will not go
far into more than one branch before failing.

Be especially careful with nested repetitions: they can easily result
in very slow matching in the presence of ambiguities.  For example,
@samp{\(?:a*b*\)+c} will take a long time attempting to match even a
moderately long string of @samp{a}s before failing.  The equivalent
@samp{\(?:a\|b\)*c} is much faster, and @samp{[ab]*c} better still.

@item
Don't use capturing groups unless they are really needed; that is, use
@samp{\(?:@dots{}\)} instead of @samp{\(@dots{}\)} for bracketing
purposes.

@ifnottex
@item
Consider using @code{rx} (@pxref{Rx Notation}); it can optimize some
or-patterns automatically and will never introduce capturing groups
unless explicitly requested.
@end ifnottex
@end itemize

If you run into regexp stack overflow despite following the above
advice, don't be afraid of performing the matching in multiple
function calls, each using a simpler regexp where backtracking can
more easily be contained.

@defun re--describe-compiled regexp &optional raw
To help diagnose problems in your regexps or in the regexp engine
itself, this function returns a string describing the compiled
form of @var{regexp}.  To make sense of it, it can be necessary
to read at least the description of the @code{re_opcode_t} type in the
@code{src/regex-emacs.c} file in Emacs's source code.

It is currently able to give a meaningful description only if Emacs
was compiled with @code{--enable-checking}.
@end defun

@node Regexp Search
@section Regular Expression Searching
@cindex regular expression searching
@cindex regexp searching
@cindex searching for regexp

  In GNU Emacs, you can search for the next match for a regular
expression (@pxref{Syntax of Regexps}) either incrementally or not.
For incremental search commands, see @ref{Regexp Search, , Regular
Expression Search, emacs, The GNU Emacs Manual}.  Here we describe
only the search functions useful in programs.  The principal one is
@code{re-search-forward}.

  These search functions convert the regular expression to multibyte if
the buffer is multibyte; they convert the regular expression to unibyte
if the buffer is unibyte.  @xref{Text Representations}.

@deffn Command re-search-forward regexp &optional limit noerror count
This function searches forward in the current buffer for a string of
text that is matched by the regular expression @var{regexp}.  The
function skips over any amount of text that is not matched by
@var{regexp}, and leaves point at the end of the first match found.
It returns the new value of point.

If @var{limit} is non-@code{nil}, it must be a position in the current
buffer.  It specifies the upper bound to the search.  No match
extending after that position is accepted.  If @var{limit} is omitted
or @code{nil}, it defaults to the end of the accessible portion of the
buffer.

What @code{re-search-forward} does when the search fails depends on
the value of @var{noerror}:

@table @asis
@item @code{nil}
Signal a @code{search-failed} error.
@item @code{t}
Do nothing and return @code{nil}.
@item anything else
Move point to @var{limit} (or the end of the accessible portion of the
buffer) and return @code{nil}.
@end table

The argument @var{noerror} only affects valid searches which fail to
find a match.  Invalid arguments cause errors regardless of
@var{noerror}.

If @var{count} is a positive number @var{n}, the search is done
@var{n} times; each successive search starts at the end of the
previous match.  If all these successive searches succeed, the
function call succeeds, moving point and returning its new value.
Otherwise the function call fails, with results depending on the value
of @var{noerror}, as described above.  If @var{count} is a negative
number @minus{}@var{n}, the search is done @var{n} times in the opposite
(backward) direction.

In the following example, point is initially before the @samp{T}.
Evaluating the search call moves point to the end of that line (between
the @samp{t} of @samp{hat} and the newline).

@example
@group
---------- Buffer: foo ----------
I read "@point{}The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
@end group

@group
(re-search-forward "[a-z]+" nil t 5)
     @result{} 27

---------- Buffer: foo ----------
I read "The cat in the hat@point{}
comes back" twice.
---------- Buffer: foo ----------
@end group
@end example
@end deffn

@c This anchor is referenced by re-search-backward's docstring.
@anchor{re-search-backward}
@deffn Command re-search-backward regexp &optional limit noerror count
This function searches backward in the current buffer for a string of
text that is matched by the regular expression @var{regexp}, leaving
point at the beginning of the first text found.

This function is analogous to @code{re-search-forward}, but they are not
simple mirror images.  @code{re-search-forward} finds the match whose
beginning is as close as possible to the starting point.  If
@code{re-search-backward} were a perfect mirror image, it would find the
match whose end is as close as possible.  However, in fact it finds the
match whose beginning is as close as possible (and yet ends before the
starting point).  The reason for this is that matching a regular
expression at a given spot always works from beginning to end, and
starts at a specified beginning position.

A true mirror-image of @code{re-search-forward} would require a special
feature for matching regular expressions from end to beginning.  It's
not worth the trouble of implementing that.
@end deffn

@defun string-match regexp string &optional start inhibit-modify
This function returns the index of the start of the first match for
the regular expression @var{regexp} in @var{string}, or @code{nil} if
there is no match.  If @var{start} is non-@code{nil}, the search starts
at that index in @var{string}.

For example,

@example
@group
(string-match
 "quick" "The quick brown fox jumped quickly.")
     @result{} 4
@end group
@group
(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     @result{} 27
@end group
@end example

@noindent
The index of the first character of the
string is 0, the index of the second character is 1, and so on.

By default, if this function finds a match, the index of the first
character beyond the match is available as @code{(match-end 0)}.
@xref{Match Data}.  If @var{inhibit-modify} is non-@code{nil}, the
match data isn't modified.

@example
@group
(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     @result{} 27
@end group

@group
(match-end 0)
     @result{} 32
@end group
@end example
@end defun

@defun string-match-p regexp string &optional start
This predicate function does what @code{string-match} does, but it
avoids modifying the match data.
@end defun

@defun looking-at regexp &optional inhibit-modify
This function determines whether the text in the current buffer directly
following point matches the regular expression @var{regexp}.  ``Directly
following'' means precisely that: the search is ``anchored'' and it can
succeed only starting with the first character following point.  The
result is @code{t} if so, @code{nil} otherwise.

This function does not move point, but it does update the match data
(if @var{inhibit-modify} is @code{nil} or missing, which is the
default).  @xref{Match Data}.  As a convenience, instead of using the
@var{inhibit-modify} argument, you can use @code{looking-at-p},
described below.

In this example, point is located directly before the @samp{T}.  If it
were anywhere else, the result would be @code{nil}.

@example
@group
---------- Buffer: foo ----------
I read "@point{}The cat in the hat
comes back" twice.
---------- Buffer: foo ----------

(looking-at "The cat in the hat$")
     @result{} t
@end group
@end example
@end defun

@defun looking-back regexp limit &optional greedy
This function returns @code{t} if @var{regexp} matches the text
immediately before point (i.e., ending at point), and @code{nil} otherwise.

Because regular expression matching works only going forward, this is
implemented by searching backwards from point for a match that ends at
point.  That can be quite slow if it has to search a long distance.
You can bound the time required by specifying a non-@code{nil} value
for @var{limit}, which says not to search before @var{limit}.  In this
case, the match that is found must begin at or after @var{limit}.
Here's an example:

@example
@group
---------- Buffer: foo ----------
I read "@point{}The cat in the hat
comes back" twice.
---------- Buffer: foo ----------

(looking-back "read \"" 3)
     @result{} t
(looking-back "read \"" 4)
     @result{} nil
@end group
@end example

If @var{greedy} is non-@code{nil}, this function extends the match
backwards as far as possible, stopping when a single additional
previous character cannot be part of a match for @var{regexp}.  When
the match is extended, its starting position is allowed to occur
before @var{limit}.

@c https://debbugs.gnu.org/5689
As a general recommendation, try to avoid using @code{looking-back}
wherever possible, since it is slow.  For this reason, there are no
plans to add a @code{looking-back-p} function.
@end defun

@defun looking-at-p regexp
This predicate function works like @code{looking-at}, but without
updating the match data.
@end defun

@defvar search-spaces-regexp
If this variable is non-@code{nil}, it should be a regular expression
that says how to search for whitespace.  In that case, any group of
spaces in a regular expression being searched for stands for use of
this regular expression.  However, spaces inside of constructs such as
@samp{[@dots{}]} and @samp{*}, @samp{+}, @samp{?} are not affected by
@code{search-spaces-regexp}.

Since this variable affects all regular expression search and match
constructs, you should bind it temporarily for as small as possible
a part of the code, and only where the Lisp code affected by the
binding performs searches whose regexp was produced from interactive
user input.  In other words, this variable should only be used to tell
regexp search primitives how to interpret whitespace typed by the
user.
@end defvar

@node Longest Match
@section Longest-match searching for regular expression matches

@cindex backtracking and POSIX regular expressions
  The usual regular expression functions do backtracking when necessary
to handle the @samp{\|} and repetition constructs, but they continue
this only until they find @emph{some} match.  Then they succeed and
report the first match found.

  This section describes alternative search functions which perform the
full backtracking specified by the POSIX standard for regular expression
matching.  They continue backtracking until they have tried all
possibilities and found all matches, so they can report the longest
match, as required by POSIX@.  This is much slower, so use these
functions only when you really need the longest match.

  Despite their names, the POSIX search and match functions
use Emacs regular expressions, not POSIX regular expressions.
@xref{POSIX Regexps}.  Also, they do not properly support the
non-greedy repetition operators (@pxref{Regexp Special, non-greedy}).
This is because POSIX backtracking conflicts with the semantics of
non-greedy repetition.

@deffn Command posix-search-forward regexp &optional limit noerror count
This is like @code{re-search-forward} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end deffn

@deffn Command posix-search-backward regexp &optional limit noerror count
This is like @code{re-search-backward} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end deffn

@defun posix-looking-at regexp &optional inhibit-modify
This is like @code{looking-at} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end defun

@defun posix-string-match regexp string &optional start inhibit-modify
This is like @code{string-match} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end defun

@node Match Data
@section The Match Data
@cindex match data

  Emacs keeps track of the start and end positions of the segments of
text found during a search; this is called the @dfn{match data}.
Thanks to the match data, you can search for a complex pattern, such
as a date in a mail message, and then extract parts of the match under
control of the pattern.

  Because the match data normally describe the most recent search only,
you must be careful not to do another search inadvertently between the
search you wish to refer back to and the use of the match data.  If you
can't avoid another intervening search, you must save and restore the
match data around it, to prevent it from being overwritten.

  Notice that all functions are allowed to overwrite the match data
unless they're explicitly documented not to do so.  A consequence is
that functions that are run implicitly in the background
(@pxref{Timers}, and @ref{Idle Timers}) should likely save and restore
the match data explicitly.

@menu
* Replacing Match::       Replacing a substring that was matched.
* Simple Match Data::     Accessing single items of match data,
                            such as where a particular subexpression started.
* Entire Match Data::     Accessing the entire match data at once, as a list.
* Saving Match Data::     Saving and restoring the match data.
@end menu

@node Replacing Match
@subsection Replacing the Text that Matched
@cindex replace matched text

  This function replaces all or part of the text matched by the last
search.  It works by means of the match data.

@cindex case in replacements
@defun replace-match replacement &optional fixedcase literal string subexp
This function performs a replacement operation on a buffer or string.

If you did the last search in a buffer, you should omit the
@var{string} argument or specify @code{nil} for it, and make sure that
the current buffer is the one in which you performed the last search.
Then this function edits the buffer, replacing the matched text with
@var{replacement}.  It leaves point at the end of the replacement
text.

If you performed the last search on a string, pass the same string as
@var{string}.  Then this function returns a new string, in which the
matched text is replaced by @var{replacement}.

If @var{fixedcase} is non-@code{nil}, then @code{replace-match} uses
the replacement text without case conversion; otherwise, it converts
the replacement text depending upon the capitalization of the text to
be replaced.  If the original text is all upper case, this converts
the replacement text to upper case.  If all words of the original text
are capitalized, this capitalizes all the words of the replacement
text.  If all the words are one-letter and they are all upper case,
they are treated as capitalized words rather than all-upper-case
words.

If @var{literal} is non-@code{nil}, then @var{replacement} is inserted
exactly as it is, the only alterations being case changes as needed.
If it is @code{nil} (the default), then the character @samp{\} is treated
specially.  If a @samp{\} appears in @var{replacement}, then it must be
part of one of the following sequences:

@table @asis
@item @samp{\&}
@cindex @samp{&} in replacement
This stands for the entire text being replaced.

@item @samp{\@var{n}}, where @var{n} is a digit
@cindex @samp{\@var{n}} in replacement
This stands for the text that matched the @var{n}th subexpression in
the original regexp.  Subexpressions are those expressions grouped
inside @samp{\(@dots{}\)}.  If the @var{n}th subexpression never
matched, an empty string is substituted.

@item @samp{\\}
@cindex @samp{\} in replacement
This stands for a single @samp{\} in the replacement text.

@item @samp{\?}
This stands for itself (for compatibility with @code{replace-regexp}
and related commands; @pxref{Regexp Replace,,, emacs, The GNU
Emacs Manual}).
@end table

@noindent
Any other character following @samp{\} signals an error.

The substitutions performed by @samp{\&} and @samp{\@var{n}} occur
after case conversion, if any.  Therefore, the strings they substitute
are never case-converted.

If @var{subexp} is non-@code{nil}, that says to replace just
subexpression number @var{subexp} of the regexp that was matched, not
the entire match.  For example, after matching @samp{foo \(ba*r\)},
calling @code{replace-match} with 1 as @var{subexp} means to replace
just the text that matched @samp{\(ba*r\)}.
@end defun

@defun match-substitute-replacement replacement &optional fixedcase literal string subexp
This function returns the text that would be inserted into the buffer
by @code{replace-match}, but without modifying the buffer.  It is
useful if you want to present the user with actual replacement result,
with constructs like @samp{\@var{n}} or @samp{\&} substituted with
matched groups.  Arguments @var{replacement} and optional
@var{fixedcase}, @var{literal}, @var{string} and @var{subexp} have the
same meaning as for @code{replace-match}.
@end defun

@node Simple Match Data
@subsection Simple Match Data Access

  This section explains how to use the match data to find out what was
matched by the last search or match operation, if it succeeded.

  You can ask about the entire matching text, or about a particular
parenthetical subexpression of a regular expression.  The @var{count}
argument in the functions below specifies which.  If @var{count} is
zero, you are asking about the entire match.  If @var{count} is
positive, it specifies which subexpression you want.

  Recall that the subexpressions of a regular expression are those
expressions grouped with escaped parentheses, @samp{\(@dots{}\)}.  The
@var{count}th subexpression is found by counting occurrences of
@samp{\(} from the beginning of the whole regular expression.  The first
subexpression is numbered 1, the second 2, and so on.  Only regular
expressions can have subexpressions---after a simple string search, the
only information available is about the entire match.

  Every successful search sets the match data.  Therefore, you should
query the match data immediately after searching, before calling any
other function that might perform another search.  Alternatively, you
may save and restore the match data (@pxref{Saving Match Data}) around
the call to functions that could perform another search.  Or use the
functions that explicitly do not modify the match data;
e.g., @code{string-match-p}.

@c This is an old comment and presumably there is no prospect of this
@c changing now.  But still the advice stands.
  A search which fails may or may not alter the match data.  In the
current implementation, it does not, but we may change it in the
future.  Don't try to rely on the value of the match data after a
failing search.

@defun match-string count &optional in-string
This function returns, as a string, the text matched in the last search
or match operation.  It returns the entire text if @var{count} is zero,
or just the portion corresponding to the @var{count}th parenthetical
subexpression, if @var{count} is positive.

If the last such operation was done against a string with
@code{string-match}, then you should pass the same string as the
argument @var{in-string}.  After a buffer search or match,
you should omit @var{in-string} or pass @code{nil} for it; but you
should make sure that the current buffer when you call
@code{match-string} is the one in which you did the searching or
matching.  Failure to follow this advice will lead to incorrect results.

The value is @code{nil} if @var{count} is out of range, or for a
subexpression inside a @samp{\|} alternative that wasn't used or a
repetition that repeated zero times.
@end defun

@defun match-string-no-properties count &optional in-string
This function is like @code{match-string} except that the result
has no text properties.
@end defun

@defun match-beginning count
If the last regular expression search found a match, this function
returns the position of the start of the matching text or of a
subexpression of it.

If @var{count} is zero, then the value is the position of the start of
the entire match.  Otherwise, @var{count} specifies a subexpression in
the regular expression, and the value of the function is the starting
position of the match for that subexpression.

The value is @code{nil} for a subexpression inside a @samp{\|}
alternative that wasn't used or a repetition that repeated zero times.
@end defun

@defun match-end count
This function is like @code{match-beginning} except that it returns the
position of the end of the match, rather than the position of the
beginning.
@end defun

  Here is an example of using the match data, with a comment showing the
positions within the text:

@example
@group
(string-match "\\(qu\\)\\(ick\\)"
              "The quick fox jumped quickly.")
              ;0123456789
     @result{} 4
@end group

@group
(match-string 0 "The quick fox jumped quickly.")
     @result{} "quick"
(match-string 1 "The quick fox jumped quickly.")
     @result{} "qu"
(match-string 2 "The quick fox jumped quickly.")
     @result{} "ick"
@end group

@group
(match-beginning 1)       ; @r{The beginning of the match}
     @result{} 4                 ;   @r{with @samp{qu} is at index 4.}
@end group

@group
(match-beginning 2)       ; @r{The beginning of the match}
     @result{} 6                 ;   @r{with @samp{ick} is at index 6.}
@end group

@group
(match-end 1)             ; @r{The end of the match}
     @result{} 6                 ;   @r{with @samp{qu} is at index 6.}

(match-end 2)             ; @r{The end of the match}
     @result{} 9                 ;   @r{with @samp{ick} is at index 9.}
@end group
@end example

  Here is another example.  Point is initially located at the beginning
of the line.  Searching moves point to between the space and the word
@samp{in}.  The beginning of the entire match is at the 9th character of
the buffer (@samp{T}), and the beginning of the match for the first
subexpression is at the 13th character (@samp{c}).

@example
@group
(list
  (re-search-forward "The \\(cat \\)")
  (match-beginning 0)
  (match-beginning 1))
    @result{} (17 9 13)
@end group

@group
---------- Buffer: foo ----------
I read "The cat @point{}in the hat comes back" twice.
        ^   ^
        9  13
---------- Buffer: foo ----------
@end group
@end example

@noindent
(In this case, the index returned is a buffer position; the first
character of the buffer counts as 1.)

@node Entire Match Data
@subsection Accessing the Entire Match Data

  The functions @code{match-data} and @code{set-match-data} read or
write the entire match data, all at once.

@defun match-data &optional integers reuse reseat
This function returns a list of positions (markers or integers) that
record all the information on the text that the last search matched.
Element zero is the position of the beginning of the match for the
whole expression; element one is the position of the end of the match
for the expression.  The next two elements are the positions of the
beginning and end of the match for the first subexpression, and so on.
In general, element
@ifnottex
number 2@var{n}
@end ifnottex
@tex
number {\mathsurround=0pt $2n$}
@end tex
corresponds to @code{(match-beginning @var{n})}; and
element
@ifnottex
number 2@var{n} + 1
@end ifnottex
@tex
number {\mathsurround=0pt $2n+1$}
@end tex
corresponds to @code{(match-end @var{n})}.

Normally all the elements are markers or @code{nil}, but if
@var{integers} is non-@code{nil}, that means to use integers instead
of markers.  (In that case, the buffer itself is appended as an
additional element at the end of the list, to facilitate complete
restoration of the match data.)  If the last match was done on a
string with @code{string-match}, then integers are always used,
since markers can't point into a string.

If @var{reuse} is non-@code{nil}, it should be a list.  In that case,
@code{match-data} stores the match data in @var{reuse}.  That is,
@var{reuse} is destructively modified.  @var{reuse} does not need to
have the right length.  If it is not long enough to contain the match
data, it is extended.  If it is too long, the length of @var{reuse}
stays the same, but the elements that were not used are set to
@code{nil}.  The purpose of this feature is to reduce the need for
garbage collection.

If @var{reseat} is non-@code{nil}, all markers on the @var{reuse} list
are reseated to point to nowhere.

As always, there must be no possibility of intervening searches between
the call to a search function and the call to @code{match-data} that is
intended to access the match data for that search.

@example
@group
(match-data)
     @result{}  (#<marker at 9 in foo>
          #<marker at 17 in foo>
          #<marker at 13 in foo>
          #<marker at 17 in foo>)
@end group
@end example
@end defun

@defun set-match-data match-list &optional reseat
This function sets the match data from the elements of @var{match-list},
which should be a list that was the value of a previous call to
@code{match-data}.  (More precisely, anything that has the same format
will work.)

If @var{match-list} refers to a buffer that doesn't exist, you don't get
an error; that sets the match data in a meaningless but harmless way.

If @var{reseat} is non-@code{nil}, all markers on the @var{match-list} list
are reseated to point to nowhere.

@c TODO Make it properly obsolete.
@findex store-match-data
@code{store-match-data} is a semi-obsolete alias for @code{set-match-data}.
@end defun

@node Saving Match Data
@subsection Saving and Restoring the Match Data

  When you call a function that may search, you may need to save
and restore the match data around that call, if you want to preserve the
match data from an earlier search for later use.  Here is an example
that shows the problem that arises if you fail to save the match data:

@example
@group
(re-search-forward "The \\(cat \\)")
     @result{} 48
(foo)                   ; @r{@code{foo} does more searching.}
(match-end 0)
     @result{} 61              ; @r{Unexpected result---not 48!}
@end group
@end example

  You can save and restore the match data with @code{save-match-data}:

@defmac save-match-data body@dots{}
This macro executes @var{body}, saving and restoring the match
data around it.  The return value is the value of the last form in
@var{body}.
@end defmac

  You could use @code{set-match-data} together with @code{match-data} to
imitate the effect of the special form @code{save-match-data}.  Here is
how:

@example
@group
(let ((data (match-data)))
  (unwind-protect
      @dots{}   ; @r{Ok to change the original match data.}
    (set-match-data data)))
@end group
@end example

  Emacs automatically saves and restores the match data when it runs
process filter functions (@pxref{Filter Functions}) and process
sentinels (@pxref{Sentinels}).

@ignore
  Here is a function which restores the match data provided the buffer
associated with it still exists.

@smallexample
@group
(defun restore-match-data (data)
@c It is incorrect to split the first line of a doc string.
@c If there's a problem here, it should be solved in some other way.
  "Restore the match data DATA unless the buffer is missing."
  (catch 'foo
    (let ((d data))
@end group
      (while d
        (and (car d)
             (null (marker-buffer (car d)))
@group
             ;; @file{match-data} @r{buffer is deleted.}
             (throw 'foo nil))
        (setq d (cdr d)))
      (set-match-data data))))
@end group
@end smallexample
@end ignore

@node Search and Replace
@section Search and Replace
@cindex replacement after search
@cindex searching and replacing

  If you want to find all matches for a regexp in part of the buffer
and replace them, the most flexible way is to write an explicit loop
using @code{re-search-forward} and @code{replace-match}, like this:

@example
(while (re-search-forward "foo[ \t]+bar" nil t)
  (replace-match "foobar"))
@end example

@noindent
@xref{Replacing Match,, Replacing the Text that Matched}, for a
description of @code{replace-match}.

  It may be more convenient to limit the replacements to a specific
region.  The function @code{replace-regexp-in-region} does that.

@defun replace-regexp-in-region regexp replacement &optional start end
This function replaces all the occurrences of @var{regexp} with
@var{replacement} in the region of buffer text between @var{start} and
@var{end}; @var{start} defaults to position of point, and @var{end}
defaults to the last accessible position of the buffer.  The search
for @var{regexp} is case-sensitive, and @var{replacement} is inserted
without changing its letter-case.  The @var{replacement} string can
use the same special elements starting with @samp{\} as
@code{replace-match} does.  The function returns the number of
replaced occurrences, or @code{nil} if @var{regexp} is not found.  The
function preserves the position of point.

@example
(replace-regexp-in-region "foo[ \t]+bar" "foobar")
@end example
@end defun

@defun replace-string-in-region string replacement &optional start end
  This function works similarly to @code{replace-regexp-in-region},
but searches for, and replaces, literal @var{string}s instead of
regular expressions.
@end defun

  Emacs also has special functions for replacing matches in a string.

@defun replace-regexp-in-string regexp rep string &optional fixedcase literal subexp start
This function copies @var{string} and searches it for matches for
@var{regexp}, and replaces them with @var{rep}.  It returns the
modified copy.  If @var{start} is non-@code{nil}, the search for
matches starts at that index in @var{string}, and the returned value
does not include the first @var{start} characters of @var{string}.
To get the whole transformed string, concatenate the first
@var{start} characters of @var{string} with the return value.

This function uses @code{replace-match} to do the replacement, and it
passes the optional arguments @var{fixedcase}, @var{literal} and
@var{subexp} along to @code{replace-match}.

Instead of a string, @var{rep} can be a function.  In that case,
@code{replace-regexp-in-string} calls @var{rep} for each match,
passing the text of the match as its sole argument.  It collects the
value @var{rep} returns and passes that to @code{replace-match} as the
replacement string.  The match data at this point are the result
of matching @var{regexp} against a substring of @var{string}.
@end defun

@defun string-replace from-string to-string in-string
This function replaces all occurrences of @var{from-string} with
@var{to-string} in @var{in-string} and returns the result.  It may
return one of its arguments unchanged, a constant string or a new
string.  Case is significant, and text properties are ignored.
@end defun

  If you want to write a command along the lines of @code{query-replace},
you can use @code{perform-replace} to do the work.

@defun perform-replace from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map start end backward region-noncontiguous-p
This function is the guts of @code{query-replace} and related
commands.  It searches for occurrences of @var{from-string} in the
text between positions @var{start} and @var{end} and replaces some or
all of them.  If @var{start} is @code{nil} (or omitted), point is used
instead, and the end of the buffer's accessible portion is used for
@var{end}.  (If the optional argument @var{backward} is
non-@code{nil}, the search starts at @var{end} and goes backward.)

If @var{query-flag} is @code{nil}, it replaces all
occurrences; otherwise, it asks the user what to do about each one.

If @var{regexp-flag} is non-@code{nil}, then @var{from-string} is
considered a regular expression; otherwise, it must match literally.  If
@var{delimited-flag} is non-@code{nil}, then only replacements
surrounded by word boundaries are considered.

The argument @var{replacements} specifies what to replace occurrences
with.  If it is a string, that string is used.  It can also be a list of
strings, to be used in cyclic order.

If @var{replacements} is a cons cell, @w{@code{(@var{function}
. @var{data})}}, this means to call @var{function} after each match to
get the replacement text.  This function is called with two arguments:
@var{data}, and the number of replacements already made.

If @var{repeat-count} is non-@code{nil}, it should be an integer.  Then
it specifies how many times to use each of the strings in the
@var{replacements} list before advancing cyclically to the next one.

If @var{from-string} contains upper-case letters, then
@code{perform-replace} binds @code{case-fold-search} to @code{nil}, and
it uses the @var{replacements} without altering their case.

Normally, the keymap @code{query-replace-map} defines the possible
user responses for queries.  The argument @var{map}, if
non-@code{nil}, specifies a keymap to use instead of
@code{query-replace-map}.

Non-@code{nil} @var{region-noncontiguous-p} means that the region
between @var{start} and @var{end} is composed of noncontiguous pieces.
The most common example of this is a rectangular region, where the
pieces are separated by newline characters.

This function uses one of two functions to search for the next
occurrence of @var{from-string}.  These functions are specified by the
values of two variables: @code{replace-re-search-function} and
@code{replace-search-function}.  The former is called when the
argument @var{regexp-flag} is non-@code{nil}, the latter when it is
@code{nil}.
@end defun

@defvar query-replace-map
This variable holds a special keymap that defines the valid user
responses for @code{perform-replace} and the commands that use it, as
well as @code{y-or-n-p} and @code{map-y-or-n-p}.  This map is unusual
in two ways:

@itemize @bullet
@item
The key bindings are not commands, just symbols that are meaningful
to the functions that use this map.

@item
Prefix keys are not supported; each key binding must be for a
single-event key sequence.  This is because the functions don't use
@code{read-key-sequence} to get the input; instead, they read a single
event and look it up ``by hand''.
@end itemize
@end defvar

Here are the meaningful bindings for @code{query-replace-map}.
Several of them are meaningful only for @code{query-replace} and
friends.

@table @code
@item act
Do take the action being considered---in other words, ``yes''.

@item skip
Do not take action for this question---in other words, ``no''.

@item exit
Answer this question ``no'', and give up on the entire series of
questions, assuming that the answers will be ``no''.

@item exit-prefix
Like @code{exit}, but add the key that was pressed to
@code{unread-command-events} (@pxref{Event Input Misc}).

@item act-and-exit
Answer this question ``yes'', and give up on the entire series of
questions, assuming that subsequent answers will be ``no''.

@item act-and-show
Answer this question ``yes'', but show the results---don't advance yet
to the next question.

@item automatic
Answer this question and all subsequent questions in the series with
``yes'', without further user interaction.

@item backup
Move back to the previous place that a question was asked about.

@item undo
Undo last replacement and move back to the place where that
replacement was performed.

@item undo-all
Undo all replacements and move back to the place where the first
replacement was performed.

@item edit
Enter a recursive edit to deal with this question---instead of any
other action that would normally be taken.

@item edit-replacement
Edit the replacement for this question in the minibuffer.

@item delete-and-edit
Delete the text being considered, then enter a recursive edit to replace
it.

@item recenter
@itemx scroll-up
@itemx scroll-down
@itemx scroll-other-window
@itemx scroll-other-window-down
Perform the specified window scroll operation, then ask the same
question again.  Only @code{y-or-n-p} and related functions use this
answer.

@item quit
Perform a quit right away.  Only @code{y-or-n-p} and related functions
use this answer.

@item help
Display some help, then ask again.
@end table

@defvar multi-query-replace-map
This variable holds a keymap that extends @code{query-replace-map} by
providing additional key bindings that are useful in multi-buffer
replacements.  The additional bindings are:

@table @code
@item automatic-all
Answer this question and all subsequent questions in the series with
``yes'', without further user interaction, for all remaining buffers.

@item exit-current
Answer this question ``no'', and give up on the entire series of
questions for the current buffer.  Continue to the next buffer in the
sequence.
@end table
@end defvar

@defvar replace-search-function
This variable specifies a function that @code{perform-replace} calls
to search for the next string to replace.  Its default value is
@code{search-forward}.  Any other value should name a function of 3
arguments: the first 3 arguments of @code{search-forward}
(@pxref{String Search}).
@end defvar

@defvar replace-re-search-function
This variable specifies a function that @code{perform-replace} calls
to search for the next regexp to replace.  Its default value is
@code{re-search-forward}.  Any other value should name a function of 3
arguments: the first 3 arguments of @code{re-search-forward}
(@pxref{Regexp Search}).
@end defvar

@node Standard Regexps
@section Standard Regular Expressions Used in Editing
@cindex regexps used standardly in editing
@cindex standard regexps used in editing

  This section describes some variables that hold regular expressions
used for certain purposes in editing:

@defopt page-delimiter
This is the regular expression describing line-beginnings that separate
pages.  The default value is @code{"^\014"} (i.e., @code{"^^L"} or
@code{"^\C-l"}); this matches a line that starts with a formfeed
character.
@end defopt

  The following two regular expressions should @emph{not} assume the
match always starts at the beginning of a line; they should not use
@samp{^} to anchor the match.  Most often, the paragraph commands do
check for a match only at the beginning of a line, which means that
@samp{^} would be superfluous.  When there is a nonzero left margin,
they accept matches that start after the left margin.  In that case, a
@samp{^} would be incorrect.  However, a @samp{^} is harmless in modes
where a left margin is never used.

@defopt paragraph-separate
This is the regular expression for recognizing the beginning of a line
that separates paragraphs.  (If you change this, you may have to
change @code{paragraph-start} also.)  The default value is
@w{@code{"[@ \t\f]*$"}}, which matches a line that consists entirely of
spaces, tabs, and form feeds (after its left margin).
@end defopt

@defopt paragraph-start
This is the regular expression for recognizing the beginning of a line
that starts @emph{or} separates paragraphs.  The default value is
@w{@code{"\f\\|[ \t]*$"}}, which matches a line containing only
whitespace or starting with a form feed (after its left margin).
@end defopt

@defopt sentence-end
If non-@code{nil}, the value should be a regular expression describing
the end of a sentence, including the whitespace following the
sentence.  (All paragraph boundaries also end sentences, regardless.)

If the value is @code{nil}, as it is by default, then the function
@code{sentence-end} constructs the regexp.  That is why you
should always call the function @code{sentence-end} to obtain the
regexp to be used to recognize the end of a sentence.
@end defopt

@defun sentence-end
This function returns the value of the variable @code{sentence-end},
if non-@code{nil}.  Otherwise it returns a default value based on the
values of the variables @code{sentence-end-double-space}
(@pxref{Definition of sentence-end-double-space}),
@code{sentence-end-without-period}, and
@code{sentence-end-without-space}.
@end defun

@node POSIX Regexps
@section Emacs versus POSIX Regular Expressions
@cindex POSIX regular expressions

Regular expression syntax varies significantly among computer programs.
When writing Elisp code that generates regular expressions for use by other
programs, it is helpful to know how syntax variants differ.
To give a feel for the variation, this section discusses how
Emacs regular expressions differ from two syntax variants standarded by POSIX:
basic regular expressions (BREs) and extended regular expressions (EREs).
Plain @command{grep} uses BREs, and @samp{grep -E} uses EREs.

Emacs regular expressions have a syntax closer to EREs than to BREs,
with some extensions.  Here is a summary of how POSIX BREs and EREs
differ from Emacs regular expressions.

@itemize @bullet
@item
In POSIX BREs @samp{+} and @samp{?} are not special.
The only backslash escape sequences are @samp{\(@dots{}\)},
@samp{\@{@dots{}\@}}, @samp{\1} through @samp{\9}, along with the
escaped special characters @samp{\$}, @samp{\*}, @samp{\.}, @samp{\[},
@samp{\\}, and @samp{\^}.
Therefore @samp{\(?:} acts like @samp{\([?]:}.
POSIX does not define how other BRE escapes behave;
for example, GNU @command{grep} treats @samp{\|} like Emacs does,
but does not support all the Emacs escapes.

@item
In POSIX BREs, it is an implementation option whether @samp{^} is special
after @samp{\(}; GNU @command{grep} treats it like Emacs does.
In POSIX EREs, @samp{^} is always special outside of bracket expressions,
which means the ERE @samp{x^} never matches.
In Emacs regular expressions, @samp{^} is special only at the
beginning of the regular expression, or after @samp{\(}, @samp{\(?:}
or @samp{\|}.

@item
In POSIX BREs, it is an implementation option whether @samp{$} is
special before @samp{\)}; GNU @command{grep} treats it like Emacs
does.  In POSIX EREs, @samp{$} is always special outside of bracket
expressions (@pxref{Regexp Special, bracket expressions}), which means
the ERE @samp{$x} never matches.  In Emacs regular expressions,
@samp{$} is special only at the end of the regular expression, or
before @samp{\)} or @samp{\|}.

@item
In POSIX EREs @samp{@{}, @samp{(} and @samp{|} are special,
and @samp{)} is special when matched with a preceding @samp{(}.
These special characters do not use preceding backslashes;
@samp{(?} produces undefined results.
The only backslash escape sequences are the escaped special characters
@samp{\$}, @samp{\(}, @samp{\)}, @samp{\*}, @samp{\+}, @samp{\.},
@samp{\?}, @samp{\[}, @samp{\\}, @samp{\^}, @samp{\@{} and @samp{\|}.
POSIX does not define how other ERE escapes behave;
for example, GNU @samp{grep -E} treats @samp{\1} like Emacs does,
but does not support all the Emacs escapes.

@item
In POSIX BREs and EREs, undefined results are produced by repetition
operators at the start of a regular expression or subexpression
(possibly preceded by @samp{^}), except that the repetition operator
@samp{*} has the same behavior in BREs as in Emacs.
In Emacs, these operators are treated as ordinary.

@item
In BREs and EREs, undefined results are produced by two repetition
operators in sequence.  In Emacs, these have well-defined behavior,
e.g., @samp{a**} is equivalent to @samp{a*}.

@item
In BREs and EREs, undefined results are produced by empty regular
expressions or subexpressions.  In Emacs these have well-defined
behavior, e.g., @samp{\(\)*} matches the empty string,

@item
In BREs and EREs, undefined results are produced for the named
character classes @samp{[:ascii:]}, @samp{[:multibyte:]},
@samp{[:nonascii:]}, @samp{[:unibyte:]}, and @samp{[:word:]}.

@item
BREs and EREs can contain collating symbols and equivalence
class expressions within bracket expressions, e.g., @samp{[[.ch.]d[=a=]]}.
Emacs regular expressions do not support this.

@item
BREs, EREs, and the strings they match cannot contain encoding errors
or NUL bytes.  In Emacs these constructs simply match themselves.

@item
BRE and ERE searching always finds the longest match.
Emacs searching by default does not necessarily do so.
@xref{Longest Match}.
@end itemize