@node Search, Fixit, Display, Top
@chapter Searching and Replacement
@cindex searching

  Like other editors, Emacs has commands for searching for occurrences of
a string.  The principal search command is unusual in that it is
@dfn{incremental}: it begins to search before you have finished typing the
search string.  There are also non-incremental search commands more like
those of other editors.

  Besides the usual @code{replace-string} command that finds all
occurrences of one string and replaces them with another, Emacs has a fancy
replacement command called @code{query-replace} which asks interactively
which occurrences to replace.

@menu
* Incremental Search::     Search happens as you type the string.
* Non-Incremental Search:: Specify entire string and then search.
* Word Search::            Search for sequence of words.
* Regexp Search::          Search for match for a regexp.
* Regexps::                Syntax of regular expressions.
* Search Case::            To ignore case while searching, or not.
* Replace::                Search, and replace some or all matches.
* Other Repeating Search:: Operating on all matches for some regexp.
@end menu

@node Incremental Search, Non-Incremental Search, Search, Search
@section Incremental Search

  An incremental search begins searching as soon as you type the first
character of the search string.  As you type in the search string, Emacs
shows you where the string (as you have typed it so far) is found.
When you have typed enough characters to identify the place you want, you
can stop.  Depending on what you do next, you may or may not need to
terminate the search explicitly with a @key{RET}.

@c WideCommands
@table @kbd
@item C-s
Incremental search forward (@code{isearch-forward}).
@item C-r
Incremental search backward (@code{isearch-backward}).
@end table

@kindex C-s
@kindex C-r
@findex isearch-forward
@findex isearch-backward
  @kbd{C-s} starts an incremental search.  @kbd{C-s} reads characters from
the keyboard and positions the cursor at the first occurrence of the
characters that you have typed.  If you type @kbd{C-s} and then @kbd{F},
the cursor moves right after the first @samp{F}.  Type an @kbd{O}, and see
the cursor move to after the first @samp{FO}.  After another @kbd{O}, the
cursor is after the first @samp{FOO} after the place where you started the
search.  Meanwhile, the search string @samp{FOO} has been echoed in the
echo area.@refill

  The echo area display ends with three dots when actual searching is going
on.  When search is waiting for more input, the three dots are removed.
(On slow terminals, the three dots are not displayed.)

  If you make a mistake in typing the search string, you can erase
characters with @key{DEL}.  Each @key{DEL} cancels the last character of the
search string.  This does not happen until Emacs is ready to read another
input character; first it must either find, or fail to find, the character
you want to erase.  If you do not want to wait for this to happen, use
@kbd{C-g} as described below.@refill

  When you are satisfied with the place you have reached, you can type
@key{RET} (or @key{C-m}), which stops searching, leaving the cursor where 
the search brought it.  Any command not specially meaningful in searches also
stops the search and is then executed.  Thus, typing @kbd{C-a} exits the
search and then moves to the beginning of the line.  @key{RET} is necessary
only if the next command you want to type is a printing character,
@key{DEL}, @key{ESC}, or another control character that is special
within searches (@kbd{C-q}, @kbd{C-w}, @kbd{C-r}, @kbd{C-s}, or @kbd{C-y}).

  Sometimes you search for @samp{FOO} and find it, but were actually
looking for a different occurrence of it.  To move to the next occurrence
of the search string, type another @kbd{C-s}.  Do this as often as
necessary.  If you overshoot, you can cancel some @kbd{C-s}
characters with @key{DEL}.

  After you exit a search, you can search for the same string again by
typing just @kbd{C-s C-s}: the first @kbd{C-s} is the key that invokes
incremental search, and the second @kbd{C-s} means ``search again''.

  If the specified string is not found at all, the echo area displays
the text @samp{Failing I-Search}.  The cursor is after the place where
Emacs found as much of your string as it could.  Thus, if you search for
@samp{FOOT}, and there is no @samp{FOOT}, the cursor may be after the
@samp{FOO} in @samp{FOOL}.  At this point there are several things you
can do.  If you mistyped the search string, correct it.  If you like the
place you have found, you can type @key{RET} or some other Emacs command
to ``accept what the search offered''.  Or you can type @kbd{C-g}, which
removes from the search string the characters that could not be found
(the @samp{T} in @samp{FOOT}), leaving those that were found (the
@samp{FOO} in @samp{FOOT}).  A second @kbd{C-g} at that point cancels
the search entirely, returning point to where it was when the search
started.

  If a search is failing and you ask to repeat it by typing another
@kbd{C-s}, it starts again from the beginning of the buffer.  Repeating
a failing backward search with @kbd{C-r} starts again from the end.  This
is called @dfn{wrapping around}.  @samp{Wrapped} appears in the search
prompt once this has happened.

@cindex quitting (in search)
  The @kbd{C-g} ``quit'' character does special things during searches;
just what it does depends on the status of the search.  If the search has
found what you specified and is waiting for input, @kbd{C-g} cancels the
entire search.  The cursor moves back to where you started the search.  If
@kbd{C-g} is typed when there are characters in the search string that have
not been found---because Emacs is still searching for them, or because it
has failed to find them---then the search string characters which have not
been found are discarded from the search string.  The
search is now successful and waiting for more input, so a second @kbd{C-g}
cancels the entire search.

  To search for a control character such as @kbd{C-s} or @key{DEL} or
@key{ESC}, you must quote it by typing @kbd{C-q} first.  This function
of @kbd{C-q} is analogous to its meaning as an Emacs command: it causes
the following character to be treated the way a graphic character would
normally be treated in the same context.

 To search backwards, you can use @kbd{C-r} instead of @kbd{C-s} to
start the search; @kbd{C-r} is the key that runs the command
(@code{isearch-backward}) to search backward.  You can also use
@kbd{C-r} to change from searching forward to searching backwards.  Do
this if a search fails because the place you started was too far down in the
file.  Repeated @kbd{C-r} keeps looking for more occurrences backwards.
@kbd{C-s} starts going forward again.  You can cancel @kbd{C-r} in a
search with @key{DEL}.

  The characters @kbd{C-y} and @kbd{C-w} can be used in incremental search
to grab text from the buffer into the search string.  This makes it
convenient to search for another occurrence of text at point.  @kbd{C-w}
copies the word after point as part of the search string, advancing
point over that word.  Another @kbd{C-s} to repeat the search will then
search for a string including that word.  @kbd{C-y} is similar to @kbd{C-w}
but copies the rest of the current line into the search string.

  The characters @kbd{M-p} and @kbd{M-n} can be used in an incremental
search to recall things which you have searched for in the past.  A
list of the last 16 things you have searched for is retained, and 
@kbd{M-p} and @kbd{M-n} let you cycle through that ring.

The character @kbd{M-@key{TAB}} does completion on the elements in 
the search history ring.  For example, if you know that you have
recently searched for the string @code{POTATOE}, you could type
@kbd{C-s P O M-@key{TAB}}.  If you had searched for other strings
beginning with @code{PO} then you would be shown a list of them, and
would need to type more to select one. 

  You can change any of the special characters in incremental search via
the normal keybinding mechanism: simply add a binding to the 
@code{isearch-mode-map}.  For example, to make the character
@kbd{C-b} mean ``search backwards'' while in isearch-mode, do this:

@example
(define-key isearch-mode-map "\C-b" 'isearch-repeat-backward)
@end example

These are the default bindings of isearch-mode:

@findex isearch-delete-char
@findex isearch-exit
@findex isearch-quote-char
@findex isearch-repeat-forward
@findex isearch-repeat-backward
@findex isearch-yank-line
@findex isearch-yank-word
@findex isearch-abort
@findex isearch-ring-retreat
@findex isearch-ring-advance
@findex isearch-complete

@kindex DEL (isearch-mode)
@kindex RET (isearch-mode)
@kindex C-q (isearch-mode)
@kindex C-s (isearch-mode)
@kindex C-r (isearch-mode)
@kindex C-y (isearch-mode)
@kindex C-w (isearch-mode)
@kindex C-g (isearch-mode)
@kindex M-p (isearch-mode)
@kindex M-n (isearch-mode)
@kindex M-TAB (isearch-mode)

@table @kbd
@item DEL
Delete a character from the incremental search string (@code{isearch-delete-char}).
@item RET
Exit incremental search (@code{isearch-exit}).
@item C-q
Quote special characters for incremental search (@code{isearch-quote-char}).
@item C-s
Repeat incremental search forward (@code{isearch-repeat-forward}).
@item C-r
Repeat incremental search backward (@code{isearch-repeat-backward}).
@item C-y
Pull rest of line from buffer into search string (@code{isearch-yank-line}).
@item C-w
Pull next word from buffer into search string (@code{isearch-yank-word}).
@item C-g
Cancels input back to what has been found successfully, or aborts the 
isearch (@code{isearch-abort}).
@item M-p
Recall the previous element in the isearch history ring 
(@code{isearch-ring-retreat}).
@item M-n
Recall the next element in the isearch history ring 
(@code{isearch-ring-advance}).
@item M-@key{TAB}
Do completion on the elements in the isearch history ring 
(@code{isearch-complete}).

@end table

Any other character which is normally inserted into a buffer when typed
is automatically added to the search string in isearch-mode.

@subsection Slow Terminal Incremental Search

  Incremental search on a slow terminal uses a modified style of display
that is designed to take less time.  Instead of redisplaying the buffer at
each place the search gets to, it creates a new single-line window and uses
that to display the line the search has found.  The single-line window
appears as soon as point gets outside of the text that is already
on the screen.

  When the search is terminated, the single-line window is removed.  Only
at this time the window in which the search was done is redisplayed to show
its new value of point.

  The three dots at the end of the search string, normally used to indicate
that searching is going on, are not displayed in slow style display.

@vindex search-slow-speed
  The slow terminal style of display is used when the terminal baud rate is
less than or equal to the value of the variable @code{search-slow-speed},
initially 1200.

@vindex search-slow-window-lines
  The number of lines to use in slow terminal search display is controlled
by the variable @code{search-slow-window-lines}.  Its normal value is 1.

@node Non-Incremental Search, Word Search, Incremental Search, Search
@section Non-Incremental Search
@cindex non-incremental search

  Emacs also has conventional non-incremental search commands, which require
you type the entire search string before searching begins.

@table @kbd
@item C-s @key{RET} @var{string} @key{RET}
Search for @var{string}.
@item C-r @key{RET} @var{string} @key{RET}
Search backward for @var{string}.
@end table

  To do a non-incremental search, first type @kbd{C-s @key{RET}}
(or @kbd{C-s C-m}).  This enters the minibuffer to read the search string.
Terminate the string with @key{RET} to start the search.  If the string
is not found, the search command gets an error.

 By default, @kbd{C-s} invokes incremental search, but if you give it an
empty argument, which would otherwise be useless, it invokes non-incremental
search.  Therefore, @kbd{C-s @key{RET}} invokes non-incremental search. 
@kbd{C-r @key{RET}} also works this way.

@findex search-forward
@findex search-backward
  Forward and backward non-incremental searches are implemented by the
commands @code{search-forward} and @code{search-backward}.  You can bind
these commands to keys.  The reason that incremental
search is programmed to invoke them as well is that @kbd{C-s @key{RET}}
is the traditional sequence of characters used in Emacs to invoke
non-incremental search.

 Non-incremental searches performed using @kbd{C-s @key{RET}} do
not call @code{search-forward} right away.  They first check
if the next character is @kbd{C-w}, which requests a word search.
@ifinfo
@xref{Word Search}.
@end ifinfo

@node Word Search, Regexp Search, Non-Incremental Search, Search
@section Word Search
@cindex word search

  Word search looks for a sequence of words without regard to how the
words are separated.  More precisely, you type a string of many words,
using single spaces to separate them, and the string is found even if
there are multiple spaces, newlines or other punctuation between the words.

  Word search is useful in editing documents formatted by text formatters.
If you edit while looking at the printed, formatted version, you can't tell
where the line breaks are in the source file.  Word search, allows you
to search  without having to know the line breaks.

@table @kbd
@item C-s @key{RET} C-w @var{words} @key{RET}
Search for @var{words}, ignoring differences in punctuation.
@item C-r @key{RET} C-w @var{words} @key{RET}
Search backward for @var{words}, ignoring differences in punctuation.
@end table

  Word search is a special case of non-incremental search.  It is invoked
with @kbd{C-s @key{RET} C-w} followed by the search string, which
must always be terminated with another @key{RET}.  Being non-incremental, this
search does not start until the argument is terminated.  It works by
constructing a regular expression and searching for that.  @xref{Regexp
Search}.

 You can do a backward word search with @kbd{C-r @key{RET} C-w}.

@findex word-search-forward
@findex word-search-backward
  Forward and backward word searches are implemented by the commands
@code{word-search-forward} and @code{word-search-backward}.  You can
bind these commands to keys.  The reason that incremental
search is programmed to invoke them as well is that @kbd{C-s @key{RET} C-w}
is the traditional Emacs sequence of keys for word search.

@node Regexp Search, Regexps, Word Search, Search
@section Regular Expression Search
@cindex regular expression
@cindex regexp

  A @dfn{regular expression} (@dfn{regexp}, for short) is a pattern that
denotes a (possibly infinite) set of strings.  Searching for matches
for a regexp is a powerful operation that editors on Unix systems have
traditionally offered.

 To gain a thorough understanding of regular expressions and how to use
them to best advantage, we recommend that you study @cite{Mastering
Regular Expressions, by Jeffrey E.F. Friedl, O'Reilly and Associates,
1997}. (It's known as the "Hip Owls" book, because of the picture on its
cover.)  You might also read the manuals to @ref{(gawk)Top},
@ref{(ed)Top}, @cite{sed}, @cite{grep}, @ref{(perl)Top},
@ref{(regex)Top}, @ref{(rx)Top}, @cite{pcre}, and @ref{(flex)Top}, which
also make good use of regular expressions.

 The XEmacs regular expression syntax most closely resembles that of
@cite{ed}, or @cite{grep}, the GNU versions of which all utilize the GNU
@cite{regex} library.  XEmacs' version of @cite{regex} has recently been
extended with some Perl--like capabilities, described in the next
section.

 In XEmacs, you can search for the next match for a regexp either
incrementally or not.

@kindex M-C-s
@kindex M-C-r
@findex isearch-forward-regexp
@findex isearch-backward-regexp
  Incremental search for a regexp is done by typing @kbd{M-C-s}
(@code{isearch-forward-regexp}).  This command reads a search string
incrementally just like @kbd{C-s}, but it treats the search string as a
regexp rather than looking for an exact match against the text in the
buffer.  Each time you add text to the search string, you make the regexp
longer, and the new regexp is searched for.  A reverse regexp search command
@code{isearch-backward-regexp} also exists, bound to @kbd{M-C-r}.

  All of the control characters that do special things within an ordinary
incremental search have the same functionality in incremental regexp search.
Typing @kbd{C-s} or @kbd{C-r} immediately after starting a search
retrieves the last incremental search regexp used:
incremental regexp and non-regexp searches have independent defaults.

@findex re-search-forward
@findex re-search-backward
  Non-incremental search for a regexp is done by the functions
@code{re-search-forward} and @code{re-search-backward}.  You can invoke
them with @kbd{M-x} or bind them to keys.  You can also call
@code{re-search-forward} by way of incremental regexp search with
@kbd{M-C-s @key{RET}}; similarly for @code{re-search-backward} with
@kbd{M-C-r @key{RET}}.

@node Regexps, Search Case, Regexp Search, Search
@section 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{^}, @samp{$}, and @samp{\}; no new
special characters will be defined in the future.  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{ff}.)  Likewise, @samp{o} is a regular expression that matches
only @samp{o}.@refill

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.@refill

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 characters.  Here is a list of them:

@need 1200
@table @kbd
@item .@: @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}.@refill

@item *
@cindex @samp{*} in regexp
is not a construct by itself; it is a quantifying suffix operator that
means to repeat the preceding regular expression as many times as
possible.  In @samp{fo*}, the @samp{*} applies to the @samp{o}, so
@samp{fo*} matches one @samp{f} followed by any number of @samp{o}s.
The case of zero @samp{o}s is allowed: @samp{fo*} does match
@samp{f}.@refill

@samp{*} always applies to the @emph{smallest} possible preceding
expression.  Thus, @samp{fo*} has a repeating @samp{o}, not a
repeating @samp{fo}.@refill

The matcher processes a @samp{*} construct by matching, immediately, as
many repetitions as can be found; it is "greedy".  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
case that makes 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.@refill

Nested repetition operators can be extremely slow if they specify
backtracking loops.  For example, it could take hours for the regular
expression @samp{\(x+y*\)*a} to match the sequence
@samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz}.  The slowness is because
Emacs must try each imaginable way of grouping the 35 @samp{x}'s before
concluding that none of them can work.  To make sure your regular
expressions run fast, check nested repetitions carefully.

@item +
@cindex @samp{+} in regexp
is a quantifying suffix operator similar to @samp{*} except that the
preceding expression must match at least once.  It is also "greedy".
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 ?
@cindex @samp{?} in regexp
is a quantifying suffix operator similar to @samp{*}, except that the
preceding expression can match either once or not at all.  For example,
@samp{ca?r} matches @samp{car} or @samp{cr}, but does not match anything
else.

@item *?
@cindex @samp{*?} in regexp
works just like @samp{*}, except that rather than matching the longest
match, it matches the shortest match.  @samp{*?} is known as a
@dfn{non-greedy} quantifier, a regexp construct borrowed from Perl.
@c Did perl get this from somewhere?  What's the real history of *? ?

This construct is very useful for when you want to match the text inside
a pair of delimiters.  For instance, @samp{/\*.*?\*/} will match C
comments in a string.  This could not easily be achieved without the use
of a non-greedy quantifier.

This construct has not been available prior to XEmacs 20.4.  It is not
available in FSF Emacs.

@item +?
@cindex @samp{+?} in regexp
is the non-greedy version of @samp{+}.

@item ??
@cindex @samp{??} in regexp
is the non-greedy version of @samp{?}.

@item \@{n,m\@}
@c Note the spacing after the close brace is deliberate.
@cindex @samp{\@{n,m\@} }in regexp
serves as an interval quantifier, analogous to @samp{*} or @samp{+}, but
specifies that the expression must match at least @var{n} times, but no
more than @var{m} times.  This syntax is supported by most Unix regexp
utilities, and has been introduced to XEmacs for the version 20.3.

Unfortunately, the non-greedy version of this quantifier does not exist
currently, although it does in Perl.

@item [ @dots{} ]
@cindex character set (in regexp)
@cindex @samp{[} in regexp
@cindex @samp{]} in regexp
@samp{[} begins a @dfn{character set}, which is terminated by a
@samp{]}.  In the simplest case, the characters between the two brackets
form the set.  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), from which it follows that
@samp{c[ad]*r} matches @samp{cr}, @samp{car}, @samp{cdr},
@samp{caddaar}, etc.@refill

The usual regular expression special characters are not special inside a
character set.  A completely different set of special characters exists
inside character sets: @samp{]}, @samp{-} and @samp{^}.@refill

@samp{-} is used for ranges of characters.  To write a range, write two
characters with a @samp{-} between them.  Thus, @samp{[a-z]} matches any
lower case letter.  Ranges may be intermixed freely with individual
characters, as in @samp{[a-z$%.]}, which matches any lower case letter
or @samp{$}, @samp{%}, or a period.@refill

To include a @samp{]} in a character set, make it the first character.
For example, @samp{[]a]} matches @samp{]} or @samp{a}.  To include a
@samp{-}, write @samp{-} as the first character in the set, or put it
immediately after a range.  (You can replace one individual character
@var{c} with the range @samp{@var{c}-@var{c}} to make a place to put the
@samp{-}.)  There is no way to write a set containing just @samp{-} and
@samp{]}.

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

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

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

Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.

@item ^
@cindex @samp{^} in regexp
@cindex beginning of line in regexp
is a special character that matches the empty string, but only at the
beginning of a line in the text being matched.  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 @samp{\n}.

@item $
@cindex @samp{$} in regexp
is similar to @samp{^} but matches only at the end of a line.  Thus,
@samp{x+$} 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 @samp{\n}.

@item \
@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.

@c Removed a paragraph here in lispref about doubling backslashes inside
@c of Lisp strings.

@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.@refill

For the most part, @samp{\} followed by any character matches only
that character.  However, there are several exceptions: characters
that, when preceded by @samp{\}, are special constructs.  Such
characters are always ordinary when encountered on their own.  Here
is a table of @samp{\} constructs:

@table @kbd
@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.@refill

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

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

Full backtracking capability exists to handle multiple uses of @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, @samp{\(foo\|bar\)x} matches either @samp{foox} or @samp{barx}.

@item
To enclose an expression for a suffix operator such as @samp{*} to act
on.  Thus, @samp{ba\(na\)*} matches @samp{bananana}, etc., with any
(zero or more) number of @samp{na} strings.@refill

@item
To record a matched substring for future reference.
@end enumerate

This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature that happens to be
assigned as a second meaning to the same @samp{\( @dots{} \)} construct
because there is no conflict in practice between the two meanings.
Here is an explanation of this feature:

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

In other words, after the end of a @samp{\( @dots{} \)} construct.  the
matcher remembers the beginning and end of the text matched by that
construct.  Then, 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 @samp{\( @dots{} \)} constructs
appearing in a regular expression 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 @samp{\( @dots{} \)} 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.

@item \(?: @dots{} \)
@cindex @samp{\(?:} in regexp
@cindex regexp grouping
is called a @dfn{shy} grouping operator, and it is used just like
@samp{\( @dots{} \)}, except that it does not cause the matched
substring to be recorded for future reference.

This is useful when you need a lot of grouping @samp{\( @dots{} \)}
constructs, but only want to remember one or two -- or if you have
more than nine groupings and need to use backreferences to refer to
the groupings at the end.

Using @samp{\(?: @dots{} \)} rather than @samp{\( @dots{} \)} when you
don't need the captured substrings ought to speed up your programs some,
since it shortens the code path followed by the regular expression
engine, as well as the amount of memory allocation and string copying it
must do.  The actual performance gain to be observed has not been
measured or quantified as of this writing.
@c This is used to good advantage by the font-locking code, and by
@c `regexp-opt.el'.

The shy grouping operator has been borrowed from Perl, and has not been
available prior to XEmacs 20.3, nor is it available in FSF Emacs.

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

@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.  @xref{Syntax}, 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}.
@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.

@table @kbd
@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.@refill

@item \B
@cindex @samp{\B} in regexp
matches the empty string, but @emph{not} at the beginning or
end of a word.

@item \<
@cindex @samp{\<} in regexp
matches the empty string, but only at the beginning of a word.

@item \>
@cindex @samp{\>} in regexp
matches the empty string, but only at the end of a word.
@end table

  Here is a complicated regexp used by Emacs to recognize the end of a
sentence together with any whitespace that follows.  It is given in Lisp
syntax to enable you to distinguish the spaces from the tab characters.  In
Lisp syntax, the string constant begins and ends with a double-quote.
@samp{\"} stands for a double-quote as part of the regexp, @samp{\\} for a
backslash as part of the regexp, @samp{\t} for a tab and @samp{\n} for a
newline.

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

@noindent
This regexp contains four parts: a character set matching
period, @samp{?} or @samp{!}; a character set matching close-brackets,
quotes or parentheses, repeated any number of times; an alternative in
backslash-parentheses that matches end-of-line, a tab or two spaces; and
a character set matching whitespace characters, repeated any number of
times.

@node Search Case, Replace, Regexps, Search
@section Searching and Case

@vindex case-fold-search
  All searches in Emacs normally ignore the case of the text they
are searching through; if you specify searching for @samp{FOO},
@samp{Foo} and @samp{foo} are also considered a match.  Regexps, and in
particular character sets, are included: @samp{[aB]} matches @samp{a}
or @samp{A} or @samp{b} or @samp{B}.@refill

  If you want a case-sensitive search, set the variable
@code{case-fold-search} to @code{nil}.  Then all letters must match
exactly, including case. @code{case-fold-search} is a per-buffer
variable; altering it affects only the current buffer, but
there is a default value which you can change as well.  @xref{Locals}. 
You can also use @b{Case Sensitive Search} from the @b{Options} menu 
on your screen.

@node Replace, Other Repeating Search, Search Case, Search
@section Replacement Commands
@cindex replacement
@cindex string substitution
@cindex global substitution

  Global search-and-replace operations are not needed as often in Emacs as
they are in other editors, but they are available.  In addition to the
simple @code{replace-string} command which is like that found in most
editors, there is a @code{query-replace} command which asks you, for each
occurrence of a pattern, whether to replace it.

  The replace commands all replace one string (or regexp) with one
replacement string.  It is possible to perform several replacements in
parallel using the command @code{expand-region-abbrevs}.  @xref{Expanding
Abbrevs}.

@menu
* Unconditional Replace::  Replacing all matches for a string.
* Regexp Replace::         Replacing all matches for a regexp.
* Replacement and Case::   How replacements preserve case of letters.
* Query Replace::          How to use querying.
@end menu

@node Unconditional Replace, Regexp Replace, Replace, Replace
@subsection Unconditional Replacement
@findex replace-string
@findex replace-regexp

@table @kbd
@item M-x replace-string @key{RET} @var{string} @key{RET} @var{newstring} @key{RET}
Replace every occurrence of @var{string} with @var{newstring}.
@item M-x replace-regexp @key{RET} @var{regexp} @key{RET} @var{newstring} @key{RET}
Replace every match for @var{regexp} with @var{newstring}.
@end table

  To replace every instance of @samp{foo} after point with @samp{bar},
use the command @kbd{M-x replace-string} with the two arguments
@samp{foo} and @samp{bar}.  Replacement occurs only after point: if you
want to cover the whole buffer you must go to the beginning first.  By
default, all occurrences up to the end of the buffer are replaced.  To
limit replacement to part of the buffer, narrow to that part of the
buffer before doing the replacement (@pxref{Narrowing}).

  When @code{replace-string} exits, point is left at the last occurrence
replaced.  The value of point when the @code{replace-string} command was
issued is remembered on the mark ring; @kbd{C-u C-@key{SPC}} moves back
there.

  A numeric argument restricts replacement to matches that are surrounded
by word boundaries.

@node Regexp Replace, Replacement and Case, Unconditional Replace, Replace
@subsection Regexp Replacement

  @code{replace-string} replaces exact matches for a single string.  The
similar command @code{replace-regexp} replaces any match for a specified
pattern.

  In @code{replace-regexp}, the @var{newstring} need not be constant.  It
can refer to all or part of what is matched by the @var{regexp}.  @samp{\&}
in @var{newstring} stands for the entire text being replaced.
@samp{\@var{d}} in @var{newstring}, where @var{d} is a digit, stands for
whatever matched the @var{d}'th parenthesized grouping in @var{regexp}.
For example,@refill

@example
M-x replace-regexp @key{RET} c[ad]+r @key{RET} \&-safe @key{RET}
@end example

@noindent
would replace (for example) @samp{cadr} with @samp{cadr-safe} and @samp{cddr}
with @samp{cddr-safe}.

@example
M-x replace-regexp @key{RET} \(c[ad]+r\)-safe @key{RET} \1 @key{RET}
@end example

@noindent
would perform exactly the opposite replacements.  To include a @samp{\}
in the text to replace with, you must give @samp{\\}.

@node Replacement and Case, Query Replace, Regexp Replace, Replace
@subsection Replace Commands and Case

@vindex case-replace
@vindex case-fold-search
  If the arguments to a replace command are in lower case, the command
preserves case when it makes a replacement.  Thus, the following command:

@example
M-x replace-string @key{RET} foo @key{RET} bar @key{RET}
@end example

@noindent
replaces a lower-case @samp{foo} with a lower case @samp{bar}, @samp{FOO}
with @samp{BAR}, and @samp{Foo} with @samp{Bar}.  If upper-case letters are
used in the second argument, they remain upper-case every time that
argument is inserted.  If upper-case letters are used in the first
argument, the second argument is always substituted exactly as given, with
no case conversion.  Likewise, if the variable @code{case-replace} is set
to @code{nil}, replacement is done without case conversion.  If
@code{case-fold-search} is set to @code{nil}, case is significant in
matching occurrences of @samp{foo} to replace; also, case conversion of the
replacement string is not done.

@node Query Replace,, Replacement and Case, Replace
@subsection Query Replace
@cindex query replace

@table @kbd
@item M-% @var{string} @key{RET} @var{newstring} @key{RET}
@itemx M-x query-replace @key{RET} @var{string} @key{RET} @var{newstring} @key{RET}
Replace some occurrences of @var{string} with @var{newstring}.
@item M-x query-replace-regexp @key{RET} @var{regexp} @key{RET} @var{newstring} @key{RET}
Replace some matches for @var{regexp} with @var{newstring}.
@end table

@kindex M-%
@findex query-replace
  If you want to change only some of the occurrences of @samp{foo} to
@samp{bar}, not all of them, you can use @code{query-replace} instead of
@kbd{M-%}.  This command finds occurrences of @samp{foo} one by one,
displays each occurrence, and asks you whether to replace it.  A numeric
argument to @code{query-replace} tells it to consider only occurrences
that are bounded by word-delimiter characters.@refill

@findex query-replace-regexp
  Aside from querying, @code{query-replace} works just like
@code{replace-string}, and @code{query-replace-regexp} works
just like @code{replace-regexp}.@refill

  The things you can type when you are shown an occurrence of @var{string}
or a match for @var{regexp} are:

@kindex SPC (query-replace)
@kindex DEL (query-replace)
@kindex , (query-replace)
@kindex ESC (query-replace)
@kindex . (query-replace)
@kindex ! (query-replace)
@kindex ^ (query-replace)
@kindex C-r (query-replace)
@kindex C-w (query-replace)
@kindex C-l (query-replace)

@c WideCommands
@table @kbd
@item @key{SPC}
to replace the occurrence with @var{newstring}.  This preserves case, just
like @code{replace-string}, provided @code{case-replace} is non-@code{nil},
as it normally is.@refill

@item @key{DEL}
to skip to the next occurrence without replacing this one.

@item , @r{(Comma)}
to replace this occurrence and display the result.  You are then
prompted for another input character.  However, since the replacement has
already been made, @key{DEL} and @key{SPC} are equivalent.  At this
point, you can type @kbd{C-r} (see below) to alter the replaced text.  To
undo the replacement, you can type @kbd{C-x u}. 
This exits the @code{query-replace}.  If you want to do further
replacement you must use @kbd{C-x @key{ESC} @key{ESC}} to restart (@pxref{Repetition}).

@item @key{ESC}
to exit without doing any more replacements.

@item .@: @r{(Period)}
to replace this occurrence and then exit.

@item !
to replace all remaining occurrences without asking again.

@item ^
to go back to the location of the previous occurrence (or what used to
be an occurrence), in case you changed it by mistake.  This works by
popping the mark ring.  Only one @kbd{^} in a row is allowed, because
only one previous replacement location is kept during @code{query-replace}.

@item C-r
to enter a recursive editing level, in case the occurrence needs to be
edited rather than just replaced with @var{newstring}.  When you are
done, exit the recursive editing level with @kbd{C-M-c} and the next
occurrence will be displayed.  @xref{Recursive Edit}.

@item C-w
to delete the occurrence, and then enter a recursive editing level as
in @kbd{C-r}.  Use the recursive edit to insert text to replace the
deleted occurrence of @var{string}.  When done, exit the recursive
editing level with @kbd{C-M-c} and the next occurrence will be
displayed.

@item C-l
to redisplay the screen and then give another answer.

@item C-h
to display a message summarizing these options, then give another
answer.
@end table

  If you type any other character, Emacs exits the @code{query-replace}, and
executes the character as a command.  To restart the @code{query-replace},
use @kbd{C-x @key{ESC} @key{ESC}}, which repeats the @code{query-replace} because it
used the minibuffer to read its arguments.  @xref{Repetition, C-x ESC ESC}.

@node Other Repeating Search,, Replace, Search
@section Other Search-and-Loop Commands

  Here are some other commands that find matches for a regular expression.
They all operate from point to the end of the buffer.

@findex list-matching-lines
@findex occur
@findex count-matches
@findex delete-non-matching-lines
@findex delete-matching-lines
@c grosscommands
@table @kbd
@item M-x occur
Print each line that follows point and contains a match for the
specified regexp.  A numeric argument specifies the number of context
lines to print before and after each matching line; the default is
none.

@kindex C-c C-c (Occur mode)
The buffer @samp{*Occur*} containing the output serves as a menu for
finding occurrences in their original context.  Find an occurrence
as listed in @samp{*Occur*}, position point there, and type @kbd{C-c
C-c}; this switches to the buffer that was searched and moves point to
the original of the same occurrence.

@item M-x list-matching-lines
Synonym for @kbd{M-x occur}.

@item M-x count-matches
Print the number of matches following point for the specified regexp.

@item M-x delete-non-matching-lines
Delete each line that follows point and does not contain a match for
the specified regexp.

@item M-x delete-matching-lines
Delete each line that follows point and contains a match for the
specified regexp.
@end table