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, 2001-2018 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.
* POSIX Regexps::         Searching POSIX-style 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,...
@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 -@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.

@menu
* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Example::          Illustrates regular expression syntax.
* Regexp Functions::        Functions for operating on regular expressions.
@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 character alternative (see later).
The character @samp{-} is special inside a character alternative.  A
@samp{[:} and balancing @samp{:]} enclose a character class inside a
character alternative.  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.

@strong{Warning:} Nested repetition operators can run for an
indefinitely long time, if they lead to ambiguous matching.  For
example, trying to match the regular expression @samp{\(x+y*\)*a}
against the string @samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz} could
take hours before it ultimately fails.  Emacs must try each way of
grouping the @samp{x}s before concluding that none of them can work.
Even worse, @samp{\(x*\)*} can match the null string in infinitely
many ways, so it causes an infinite loop.  To avoid these problems,
check nested repetitions carefully, to make sure that they do not
cause combinatorial explosions in backtracking.

@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.

@item @samp{*?}, @samp{+?}, @samp{??}
@cindex non-greedy repetition characters in regexp
These 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 character alternative (in regexp)
@cindex @samp{[} in regexp
@cindex @samp{]} in regexp
is 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 character alternative 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 character alternative, 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.

If @code{case-fold-search} is non-@code{nil}, @samp{[a-z]} also
matches upper-case letters.  Note that a range like @samp{[a-z]} is
not affected by the locale's collation sequence, it always represents
a sequence in @acronym{ASCII} order.
@c This wasn't obvious to me, since, e.g., the grep manual "Character
@c Classes and Bracket Expressions" specifically notes the opposite
@c behavior.  But by experiment Emacs seems unaffected by LC_COLLATE
@c in this regard.

Note also that the usual regexp special characters are not special inside a
character alternative.  A completely different set of characters is
special inside character alternatives: @samp{]}, @samp{-} and @samp{^}.

To include a @samp{]} in a character alternative, you must make it the
first character.  For example, @samp{[]a]} matches @samp{]} or @samp{a}.
To include a @samp{-}, write @samp{-} as the first or last character of
the character alternative, or put it after a range.  Thus, @samp{[]-]}
matches both @samp{]} and @samp{-}.  (As explained below, you cannot
use @samp{\]} to include a @samp{]} inside a character alternative,
since @samp{\} is not special there.)

To include @samp{^} in a character alternative, put it anywhere but at
the beginning.

@c What if it starts with a multibyte and ends with a unibyte?
@c That doesn't seem to match anything...?
If a range starts with a unibyte character @var{c} and ends with a
multibyte character @var{c2}, the range is divided into two parts: one
spans the unibyte characters @samp{@var{c}..?\377}, the other the
multibyte characters @samp{@var{c1}..@var{c2}}, where @var{c1} is the
first character of the charset to which @var{c2} belongs.

A character alternative can also specify named character classes
(@pxref{Char Classes}).  This is a POSIX feature.  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.

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

@samp{^} is not special in a character alternative 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 character alternative 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 character
alternatives.  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 reasons, @samp{^} can be used only at the
beginning of the regular expression, or after @samp{\(}, @samp{\(?:}
or @samp{\|}.

@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 reasons, @samp{$} can be used only at the
end of the regular expression, or before @samp{\)} or @samp{\|}.

@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

@strong{Please note:} For historical compatibility, special characters
are treated as ordinary ones if they are in contexts where their special
meanings make no sense.  For example, @samp{*foo} treats @samp{*} as
ordinary since there is no preceding expression on which the @samp{*}
can act.  It is poor practice to depend on this behavior; quote the
special character anyway, regardless of where it appears.

As a @samp{\} is not special inside a character alternative, it can
never remove the special meaning of @samp{-} or @samp{]}.  So you
should not quote these characters when they have no special meaning
either.  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
character alternative 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 character alternative.  For example,
@samp{[^][]]} consists of the complemented character alternative
@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 character alternative; @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 character
alternative 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

  Here is a table of the classes you can use in a character alternative,
and what they mean:

@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 @acronym{ASCII} control character.
@item [:digit:]
This matches @samp{0} through @samp{9}.  Thus, @samp{[-+[:digit:]]}
matches any digit, as well as @samp{+} and @samp{-}.
@item [:graph:]
This matches graphic characters---everything 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 (@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.
@item [:multibyte:]
This matches any multibyte character (@pxref{Text Representations}).
@item [:nonascii:]
This matches any non-@acronym{ASCII} character.
@item [:print:]
This matches any printing character---either whitespace, or a graphic
character matched by @samp{[:graph:]}.
@item [:punct:]
This matches any punctuation character.  (At present, for multibyte
characters, it matches anything that has non-word syntax.)
@item [:space:]
This matches any character that has whitespace syntax
(@pxref{Syntax Class Table}).
@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.
@item [:word:]
This matches any character that has word syntax (@pxref{Syntax Class
Table}).
@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

@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{c}
matches any character whose category is @var{c}.  Here @var{c} is a
character that represents a category: thus, @samp{c} for Chinese
characters or @samp{g} for Greek characters in the standard category
table.  You can see the list of all the currently defined categories
with @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{c}
matches any character whose category is not @var{c}.
@end table

  The following regular expression constructs match the empty string---that is,
they don't use up 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 character alternative 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 character alternative 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 character alternative, 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

@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
@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

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{\)}, e.g. 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 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 paren)
           (mapconcat 'regexp-quote strings "\\|")
           (cdr paren))))
@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

@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 -@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

@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
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.

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}.

@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
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.
@xref{Match Data}.  If you need to test for a match without modifying
the match data, 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.
@end defvar

@node POSIX Regexps
@section POSIX Regular Expression Searching

@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.

  The POSIX search and match functions 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
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
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 best 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}.

  However, replacing matches in a string is more complex, especially
if you want to do it efficiently.  So Emacs provides a function to do
this.

@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}, so matches starting
before that index are not changed.

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

  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
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 @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}.

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 keybindings 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