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@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 SXEmacs 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.  SXEmacs' version of @cite{regex} has recently been
extended with some Perl--like capabilities, described in the next
section.

 In SXEmacs, 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