@c -*- coding: utf-8 -*-
@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2018 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Search
@chapter Searching and Replacement
@cindex searching
@cindex finding strings within text

  Like other editors, Emacs has commands to search for occurrences of
a string.  Emacs also has commands to replace occurrences of a string
with a different string.  There are also commands that do the same
thing, but search for patterns instead of fixed strings.

  You can also search multiple files under the control of @code{xref}
(@pxref{Identifier Search}) or through the Dired @kbd{A} command
(@pxref{Operating on Files}), or ask the @code{grep} program to do it
(@pxref{Grep Searching}).

@menu
* Incremental Search::        Search happens as you type the string.
* Nonincremental Search::     Specify entire string and then search.
* Word Search::               Search for sequence of words.
* Symbol Search::             Search for a source code symbol.
* Regexp Search::             Search for match for a regexp.
* Regexps::                   Syntax of regular expressions.
* Regexp Backslash::          Regular expression constructs starting with `\'.
* Regexp Example::            A complex regular expression explained.
* Lax Search::                Search ignores some distinctions among
                                similar characters, like letter-case.
* Replace::                   Search, and replace some or all matches.
* Other Repeating Search::    Operating on all matches for some regexp.
* Search Customizations::     Various search customizations.
@end menu

@node Incremental Search
@section Incremental Search
@cindex incremental search
@cindex isearch

  The principal search command in Emacs is @dfn{incremental}: it
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) would be found.  When you have
typed enough characters to identify the place you want, you can stop.
Depending on what you plan to do next, you may or may not need to
terminate the search explicitly with @key{RET}.

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

You can also invoke incremental search from the menu bar's
@samp{Edit->Search} menu.

@menu
* Basic Isearch::        Basic incremental search commands.
* Repeat Isearch::       Searching for the same string again.
* Isearch Yank::         Commands that grab text into the search string
                           or else edit the search string.
* Error in Isearch::     When your string is not found.
* Special Isearch::      Special input in incremental search.
* Not Exiting Isearch::  Prefix argument and scrolling commands.
* Isearch Minibuffer::   Incremental search of the minibuffer history.
@end menu

@node Basic Isearch
@subsection Basics of Incremental Search

@table @kbd
@item C-s
Begin incremental search (@code{isearch-forward}).
@item C-r
Begin reverse incremental search (@code{isearch-backward}).
@end table

@kindex C-s
@findex isearch-forward
  @kbd{C-s} (@code{isearch-forward}) starts a forward incremental
search.  It reads characters from the keyboard, and moves point just
past the end of the next occurrence of those characters in the buffer.

  For instance, if you type @kbd{C-s} and then @kbd{F}, that puts the
cursor after the first @samp{F} that occurs in the buffer after the
starting point.  If you then type @kbd{O}, the cursor moves to just
after the first @samp{FO}; the @samp{F} in that @samp{FO} might not be
the first @samp{F} previously found.  After another @kbd{O}, the
cursor moves to just after the first @samp{FOO}.

@cindex faces for highlighting search matches
@cindex @code{isearch} face
  At each step, Emacs highlights the @dfn{current match}---the buffer
text that matches the search string---using the @code{isearch} face
(@pxref{Faces}).  @xref{Search Customizations}, for various options
that customize this highlighting.  The current search string is also
displayed in the echo area.

  If you make a mistake typing the search string, type @key{DEL}.
Each @key{DEL} cancels the last character of the search string.
@xref{Error in Isearch}, for more about dealing with unsuccessful
search.

@cindex exit incremental search
@cindex incremental search, exiting
  When you are satisfied with the place you have reached, type
@key{RET}.  This stops searching, leaving the cursor where the search
brought it.  Also, any command not specially meaningful in searches
stops the searching and is then executed.  Thus, typing @kbd{C-a}
exits the search and then moves to the beginning of the line; typing
one of the arrow keys exits the search and performs the respective
movement command; etc.  @key{RET} is necessary only if the next
command you want to type is a printing character, @key{DEL},
@key{RET}, or another character that is special within searches
(@kbd{C-q}, @kbd{C-w}, @kbd{C-r}, @kbd{C-s}, @kbd{C-y}, @kbd{M-y},
@kbd{M-r}, @kbd{M-c}, @kbd{M-e}, and some others described below).
You can fine-tune the commands that exit the search; see @ref{Not
Exiting Isearch}.

  As a special exception, entering @key{RET} when the search string is
empty launches nonincremental search (@pxref{Nonincremental Search}).
(This can be customized; see @ref{Search Customizations}.)

  To abandon the search and return to the place where you started,
type @kbd{@key{ESC} @key{ESC} @key{ESC}} (@code{isearch-cancel}) or
@kbd{C-g C-g} (@code{isearch-abort}).

  When you exit the incremental search, it adds the original value of
point to the mark ring, without activating the mark; you can thus use
@kbd{C-u C-@key{SPC}} or @kbd{C-x C-x} to return to where you were
before beginning the search.  @xref{Mark Ring}.  (Emacs only does this
if the mark was not already active; if the mark was active when you
started the search, both @kbd{C-u C-@key{SPC}} and @kbd{C-x C-x} will
go to the mark.)

@kindex C-r
@findex isearch-backward
  To search backwards, use @kbd{C-r} (@code{isearch-backward}) instead
of @kbd{C-s} to start the search.  A backward search finds matches
that end before the starting point, just as a forward search finds
matches that begin after it.

@node Repeat Isearch
@subsection Repeating Incremental Search

  Suppose you search forward for @samp{FOO} and find a match, but not
the one you expected to find: the @samp{FOO} you were aiming for
occurs later in the buffer.  In this event, type another @kbd{C-s} to
move to the next occurrence of the search string.  You can repeat this
any number of times.  If you overshoot, you can cancel some @kbd{C-s}
characters with @key{DEL}.  Similarly, each @kbd{C-r} in a backward
incremental search repeats the backward search.

@cindex lazy search highlighting
  If you pause for a little while during incremental search, Emacs
highlights all the other possible matches for the search string that
are present on the screen.  This helps you anticipate where you can
get to by typing @kbd{C-s} or @kbd{C-r} to repeat the search.  The
other matches are highlighted differently from the current match,
using the customizable face @code{lazy-highlight} (@pxref{Faces}).  If
you don't like this feature, you can disable it by setting
@code{isearch-lazy-highlight} to @code{nil}.  For other customizations
related to highlighting matches, see @ref{Search Customizations}.

  After exiting 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 to search
again for the last search string.  Similarly, @kbd{C-r C-r} searches
backward for the last search string.  In determining the last search
string, it doesn't matter whether that string was searched for with
@kbd{C-s} or @kbd{C-r}.

  If you are searching forward but you realize you were looking for
something before the starting point, type @kbd{C-r} to switch to a
backward search, leaving the search string unchanged.  Similarly,
@kbd{C-s} in a backward search switches to a forward search.

@cindex search, wrapping around
@cindex search, overwrapped
@cindex wrapped search
@cindex overwrapped search
  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 reverse search with @kbd{C-r} starts again from
the end.  This is called @dfn{wrapping around}, and @samp{Wrapped}
appears in the search prompt once this has happened.  If you keep on
going past the original starting point of the search, it changes to
@samp{Overwrapped}, which means that you are revisiting matches that
you have already seen.

@cindex search ring
@kindex M-n @r{(Incremental search)}
@kindex M-p @r{(Incremental search)}
@vindex search-ring-max
  To reuse earlier search strings, use the @dfn{search ring}.  The
commands @kbd{M-p} and @kbd{M-n} move through the ring to pick a
search string to reuse.  These commands leave the selected search ring
element in the minibuffer, where you can edit it.  Type
@kbd{C-s}/@kbd{C-r} or @key{RET} to accept the string and start
searching for it.  The number of most recently used search strings
saved in the search ring is specified by the variable
@code{search-ring-max}, 16 by default.

@cindex incremental search, edit search string
@cindex interactively edit search string
@kindex M-e @r{(Incremental search)}
@kindex mouse-1 @r{in the minibuffer (Incremental Search)}
  To edit the current search string in the minibuffer without
replacing it with items from the search ring, type @kbd{M-e} or click
@kbd{mouse-1} in the minibuffer.  Type @key{RET}, @kbd{C-s} or
@kbd{C-r} to finish editing the string and search for it.  Type
@kbd{C-f} or @kbd{@key{RIGHT}} to add to the search string characters
following point from the buffer from which you started the search.

@node Isearch Yank
@subsection Isearch Yanking

  In many cases, you will want to use text at or near point as your
search string.  The commands described in this subsection let you do
that conveniently.

@kindex C-w @r{(Incremental search)}
@findex isearch-yank-word-or-char
  @kbd{C-w} (@code{isearch-yank-word-or-char}) appends the next
character or word at point to the search string.  This is an easy way
to search for another occurrence of the text at point.  (The decision
of whether to copy a character or a word is heuristic.)

@kindex M-s C-e @r{(Incremental search)}
@findex isearch-yank-line
  Similarly, @kbd{M-s C-e} (@code{isearch-yank-line}) appends the rest
of the current line to the search string.  If point is already at the
end of a line, it appends the next line.  With a prefix argument
@var{n}, it appends the next @var{n} lines.

@kindex C-y @r{(Incremental search)}
@kindex M-y @r{(Incremental search)}
@kindex mouse-2 @r{in the minibuffer (Incremental search)}
@findex isearch-yank-kill
@findex isearch-yank-pop
@findex isearch-yank-x-selection
  Within incremental search, @kbd{C-y} (@code{isearch-yank-kill})
appends the current kill to the search string.  @kbd{M-y}
(@code{isearch-yank-pop}), if called after @kbd{C-y}, replaces that
appended text with an earlier kill, similar to the usual @kbd{M-y}
(@code{yank-pop}) command (@pxref{Yanking}).  Clicking @kbd{mouse-2}
in the echo area appends the current X selection (@pxref{Primary
Selection}) to the search string (@code{isearch-yank-x-selection}).

@kindex C-M-w @r{(Incremental search)}
@kindex C-M-y @r{(Incremental search)}
@findex isearch-del-char
@findex isearch-yank-char
  @kbd{C-M-w} (@code{isearch-del-char}) deletes the last character
from the search string, and @kbd{C-M-y} (@code{isearch-yank-char})
appends the character after point to the search string.  An
alternative method to add the character after point is to enter the
minibuffer with @kbd{M-e} (@pxref{Repeat Isearch}) and type @kbd{C-f}
or @kbd{@key{RIGHT}} at the end of the search string in the
minibuffer.  Each @kbd{C-f} or @kbd{@key{RIGHT}} you type adds another
character following point to the search string.

  Normally, when the search is case-insensitive, text yanked into the
search string is converted to lower case, so that the search remains
case-insensitive (@pxref{Lax Search, case folding}).  However, if the
value of the variable @code{search-upper-case} (@pxref{Lax Search,
search-upper-case}) is other than @code{not-yanks}, that disables this
down-casing.

@node Error in Isearch
@subsection Errors in Incremental Search

@cindex @code{isearch-fail} face
  If your string is not found at all, the echo area says @samp{Failing
I-Search}, and the cursor moves past 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}, you might see the cursor after the
@samp{FOO} in @samp{FOOL}.  In the echo area, the part of the search
string that failed to match is highlighted using the face
@code{isearch-fail}.

  At this point, there are several things you can do.  If your string
was mistyped, you can use @key{DEL} to erase some of it and correct
it, or you can type @kbd{M-e} and edit it.  If you like the place you
have found, you can type @key{RET} to remain there.  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.

@cindex quitting (in search)
@kindex C-g @r{(Incremental search)}
  The quit command, @kbd{C-g}, 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, moving the cursor 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.  With them gone, the search is now successful and
waiting for more input, so a second @kbd{C-g} will cancel the entire
search.

@node Special Isearch
@subsection Special Input for Incremental Search

  In addition to characters described in the previous subsections,
some of the other characters you type during incremental search have
special effects.  They are described here.

  To toggle lax space matching (@pxref{Lax Search, lax space
matching}), type @kbd{M-s @key{SPC}}.

  To toggle case sensitivity of the search, type @kbd{M-c} or
@kbd{M-s c}.  @xref{Lax Search, case folding}.  If the search string
includes upper-case letters, the search is case-sensitive by default.

  To toggle whether or not the search will consider similar and
equivalent characters as a match, type @kbd{M-s '}.  @xref{Lax Search,
character folding}.  If the search string includes accented
characters, that disables character folding during that search.

@cindex invisible text, searching for
@kindex M-s i @r{(Incremental search)}
@findex isearch-toggle-invisible
  To toggle whether or not invisible text is searched, type
@kbd{M-s i} (@code{isearch-toggle-invisible}).  @xref{Outline Search}.

@kindex M-r @r{(Incremental Search)}
@kindex M-s r @r{(Incremental Search)}
@findex isearch-toggle-regexp
  To toggle between non-regexp and regexp incremental search, type
@kbd{M-r} or @kbd{M-s r} (@code{isearch-toggle-regexp}).
@xref{Regexp Search}.

  To toggle symbol mode, type @kbd{M-s _}.  @xref{Symbol Search}.

  To search for a newline character, type @kbd{C-j} as part of the
search string.

  To search for non-@acronym{ASCII} characters, use one of the
following methods:

@itemize @bullet
@item
Type @kbd{C-q}, followed by a non-graphic character or a sequence of
octal digits.  This adds a character to the search string, similar to
inserting into a buffer using @kbd{C-q} (@pxref{Inserting Text}).  For
example, @kbd{C-q C-s} during incremental search adds the
@samp{control-S} character to the search string.

@item
Type @kbd{C-x 8 @key{RET}}, followed by a Unicode name or code-point
in hex.  This adds the specified character into the search string,
similar to the usual @code{insert-char} command (@pxref{Inserting
Text}).

@item
@kindex C-^ @r{(Incremental Search)}
@findex isearch-toggle-input-method
@findex isearch-toggle-specified-input-method
Use an input method (@pxref{Input Methods}).  If an input method is
enabled in the current buffer when you start the search, the same
method will be active in the minibuffer when you type the search
string.  While typing the search string, you can toggle the input
method with @kbd{C-\} (@code{isearch-toggle-input-method}).  You can
also turn on a non-default input method with @kbd{C-^}
(@code{isearch-toggle-specified-input-method}), which prompts for the
name of the input method.  When an input method is active during
incremental search, the search prompt includes the input method
mnemonic, like this:

@example
I-search [@var{im}]:
@end example

@noindent
where @var{im} is the mnemonic of the active input method.  Any input
method you enable during incremental search remains enabled in the
current buffer afterwards.
@end itemize

@kindex M-s o @r{(Incremental Search)}
@findex isearch-occur
  Typing @kbd{M-s o} in incremental search invokes
@code{isearch-occur}, which runs @code{occur} with the current search
string.  @xref{Other Repeating Search, occur}.

@kindex M-% @r{(Incremental search)}
  Typing @kbd{M-%} in incremental search invokes @code{query-replace}
or @code{query-replace-regexp} (depending on search mode) with the
current search string used as the string to replace.  A negative
prefix argument means to replace backward.  @xref{Query Replace}.

@kindex M-TAB @r{(Incremental search)}
  Typing @kbd{M-@key{TAB}} in incremental search invokes
@code{isearch-complete}, which attempts to complete the search string
using the search ring (the previous search strings you used) as a list
of completion alternatives.  @xref{Completion}.  In many operating
systems, the @kbd{M-@key{TAB}} key sequence is captured by the window
manager; you then need to rebind @code{isearch-complete} to another
key sequence if you want to use it (@pxref{Rebinding}).

@kindex M-s h r @r{(Incremental Search)}
@findex isearch-highlight-regexp
  You can exit the search while leaving the matches for the last
search string highlighted on display.  To this end, type @kbd{M-s h r}
(@code{isearch-highlight-regexp}), which will run
@code{highlight-regexp} (@pxref{Highlight Interactively}) passing
it the regexp derived from the last search string and prompting you
for the face to use for highlighting.  To remove the highlighting,
type @kbd{M-s h u} (@code{unhighlight-regexp}).

@cindex incremental search, help on special keys
@kindex C-h C-h @r{(Incremental Search)}
@findex isearch-help-map
@vindex isearch-mode-map
  When incremental search is active, you can type @kbd{C-h C-h}
(@code{isearch-help-map}) to access interactive help options,
including a list of special key bindings.  These key bindings are part
of the keymap @code{isearch-mode-map} (@pxref{Keymaps}).

@node Not Exiting Isearch
@subsection Not Exiting Incremental Search

This subsection describes how to control whether typing a command not
specifically meaningful in searches exits the search before executing
the command.  It also describes two categories of commands which you
can type without exiting the current incremental search, even though
they are not themselves part of incremental search.

@vindex search-exit-option
  Normally, typing a command that is not bound by the incremental
search exits the search before executing the command.  Thus, the
command operates on the buffer from which you invoked the search.
However, if you customize the variable @code{search-exit-option} to
@code{nil}, the characters which you type that are not interpreted by
the incremental search are simply appended to the search string.  This
is so you could include in the search string control characters, such
as @kbd{C-a}, that would normally exit the search and invoke the
command bound to them on the buffer.

@table @asis
@item Prefix Arguments
@cindex prefix argument commands, during incremental search
@vindex isearch-allow-prefix
  In incremental search, when you type a command that specifies a
prefix argument (@pxref{Arguments}), by default it will apply either
to the next action in the search or to the command that exits the
search.  In other words, entering a prefix argument will not by itself
terminate the search.

  In previous versions of Emacs, entering a prefix argument always
terminated the search.  You can revert to this behavior by setting the
variable @code{isearch-allow-prefix} to @code{nil}.

  When @code{isearch-allow-scroll} is non-@code{nil} (see below),
prefix arguments always have the default behavior described above,
i.e., they don't terminate the search, even if
@code{isearch-allow-prefix} is @code{nil}.

@item Scrolling Commands
@cindex scrolling commands, during incremental search
@vindex isearch-allow-scroll
  Normally, scrolling commands exit incremental search.  If you change
the variable @code{isearch-allow-scroll} to a non-@code{nil} value,
that enables the use of the scroll-bar, as well as keyboard scrolling
commands like @kbd{C-v}, @kbd{M-v}, and @kbd{C-l} (@pxref{Scrolling}).
This applies only to calling these commands via their bound key
sequences---typing @kbd{M-x} will still exit the search.  You can give
prefix arguments to these commands in the usual way.  This feature
won't let you scroll the current match out of visibility, however.

  The @code{isearch-allow-scroll} feature also affects some other
commands, such as @kbd{C-x 2} (@code{split-window-below}) and
@kbd{C-x ^} (@code{enlarge-window}), which don't exactly scroll but do
affect where the text appears on the screen.  It applies to any
command whose name has a non-@code{nil} @code{isearch-scroll}
property.  So you can control which commands are affected by changing
these properties.

@cindex prevent commands from exiting incremental search
  For example, to make @kbd{C-h l} usable within an incremental search
in all future Emacs sessions, use @kbd{C-h c} to find what command it
runs (@pxref{Key Help}), which is @code{view-lossage}.  Then you can
put the following line in your init file (@pxref{Init File}):

@example
(put 'view-lossage 'isearch-scroll t)
@end example

@noindent
This feature can be applied to any command that doesn't permanently
change point, the buffer contents, the match data, the current buffer,
or the selected window and frame.  The command must not itself attempt
an incremental search.  This feature is disabled if
@code{isearch-allow-scroll} is @code{nil} (which it is by default).
@end table

@node Isearch Minibuffer
@subsection Searching the Minibuffer
@cindex minibuffer history, searching

If you start an incremental search while the minibuffer is active,
Emacs searches the contents of the minibuffer.  Unlike searching an
ordinary buffer, the search string is not shown in the echo area,
because that is used to display the minibuffer.

If an incremental search fails in the minibuffer, it tries searching
the minibuffer history.  @xref{Minibuffer History}.  You can visualize
the minibuffer and its history as a series of pages, with the
earliest history element on the first page and the current minibuffer
on the last page.  A forward search, @kbd{C-s}, searches forward to
later pages; a reverse search, @kbd{C-r}, searches backwards to
earlier pages.  Like in ordinary buffer search, a failing search can
wrap around, going from the last page to the first page or vice versa.

When the current match is on a history element, that history element
is pulled into the minibuffer.  If you exit the incremental search
normally (e.g., by typing @key{RET}), it remains in the minibuffer
afterwards.  Canceling the search, with @kbd{C-g}, restores the
contents of the minibuffer when you began the search.

@node Nonincremental Search
@section Nonincremental Search
@cindex nonincremental search

  Emacs also has conventional nonincremental search commands, which require
you to 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 start a nonincremental search, first type @kbd{C-s @key{RET}}.
This enters the minibuffer to read the search string; terminate the
string with @key{RET}, and then the search takes place.  If the string
is not found, the search command signals an error.

  When you type @kbd{C-s @key{RET}}, the @kbd{C-s} invokes incremental
search as usual.  That command is specially programmed to invoke the
command for nonincremental search, if the string you specify is empty.
(Such an empty argument would otherwise be useless.)  @kbd{C-r
@key{RET}} does likewise, invoking the nonincremental
backward-searching command.

  Nonincremental search can also be invoked from the menu bar's
@samp{Edit->Search} menu.

@findex search-forward
@findex search-backward
  You can also use two simpler commands, @kbd{M-x search-forward} and
@kbd{M-x search-backward}.  These commands look for the literal
strings you specify, and don't support any of the lax-search features
(@pxref{Lax Search}) except case folding.

@node Word Search
@section Word Search
@cindex word search

  A @dfn{word search} finds a sequence of words without regard to the
type of punctuation between them.  For instance, if you enter a search
string that consists of two words separated by a single space, the
search matches any sequence of those two words separated by one or
more spaces, newlines, or other punctuation characters.  This is
particularly useful for searching text documents, because you don't
have to worry whether the words you are looking for are separated by
newlines or spaces.  Note that major modes for programming languages
or other specialized modes can modify the definition of a word to suit
their syntactic needs.

@table @kbd
@item M-s w
  If incremental search is active, toggle word search mode
(@code{isearch-toggle-word}); otherwise, begin an incremental forward
word search (@code{isearch-forward-word}).
@item M-s w @key{RET} @var{words} @key{RET}
Search for @var{words}, using a forward nonincremental word search.
@item M-s w C-r @key{RET} @var{words} @key{RET}
Search backward for @var{words}, using a nonincremental word search.
@item M-s M-w
Search the Web for the text in region.
@end table

@kindex M-s w
@findex isearch-forward-word
  To begin a forward incremental word search, type @kbd{M-s w}.  If
incremental search is not already active, this runs the command
@code{isearch-forward-word}.  If incremental search is already active
(whether a forward or backward search), @kbd{M-s w} switches to a word
search while keeping the direction of the search and the current
search string unchanged.  You can toggle word search back off by
typing @kbd{M-s w} again.

@findex word-search-forward
@findex word-search-backward
  To begin a nonincremental word search, type @kbd{M-s w @key{RET}}
for a forward search, or @kbd{M-s w C-r @key{RET}} for a backward search.
These run the commands @code{word-search-forward} and
@code{word-search-backward} respectively.

  Incremental and nonincremental word searches differ slightly in the
way they find a match.  In a nonincremental word search, each word in
the search string must exactly match a whole word.  In an incremental
word search, the matching is more lax: while you are typing the search
string, its first and last words need not match whole words.  This is
so that the matching can proceed incrementally as you type.  This
additional laxity does not apply to the lazy highlight
(@pxref{Incremental Search}), which always matches whole words.
While you are typing the search string, @samp{Pending} appears in the
search prompt until you use a search repeating key like @kbd{C-s}.

  The word search commands don't perform character folding, and
toggling lax whitespace matching (@pxref{Lax Search, lax space
matching}) has no effect on them.

@kindex M-s M-w
@findex eww-search-words
@vindex eww-search-prefix
  To search the Web for the text in region, type @kbd{M-s M-w}.  This
command performs an Internet search for the words in region using the
search engine whose @acronym{URL} is specified by the variable
@code{eww-search-prefix} (@pxref{Basics, EWW, , eww, The Emacs Web
Wowser Manual}).  If the region is not active, or doesn't contain any
words, this command prompts the user for a URL or keywords to search.


@node Symbol Search
@section Symbol Search
@cindex symbol search

  A @dfn{symbol search} is much like an ordinary search, except that
the boundaries of the search must match the boundaries of a symbol.
The meaning of @dfn{symbol} in this context depends on the major mode,
and usually refers to a source code token, such as a Lisp symbol in
Emacs Lisp mode.  For instance, if you perform an incremental symbol
search for the Lisp symbol @code{forward-word}, it would not match
@code{isearch-forward-word}.  This feature is thus mainly useful for
searching source code.

@table @kbd
@item M-s _
@findex isearch-toggle-symbol
If incremental search is active, toggle symbol search mode
(@code{isearch-toggle-symbol}); otherwise, begin an incremental
forward symbol search (@code{isearch-forward-symbol}).
@item M-s .
Start a symbol incremental search forward with the symbol found near
point added to the search string initially.
@item M-s _ @key{RET} @var{symbol} @key{RET}
Search forward for @var{symbol}, nonincrementally.
@item M-s _ C-r @key{RET} @var{symbol} @key{RET}
Search backward for @var{symbol}, nonincrementally.
@end table

@kindex M-s _
@kindex M-s .
@findex isearch-forward-symbol
@findex isearch-forward-symbol-at-point
  To begin a forward incremental symbol search, type @kbd{M-s _} (or
@kbd{M-s .} if the symbol to search is near point).  If incremental
search is not already active, this runs the command
@code{isearch-forward-symbol}.  If incremental search is already
active, @kbd{M-s _} switches to a symbol search, preserving the
direction of the search and the current search string; you can disable
symbol search by typing @kbd{M-s _} again.  In incremental symbol
search, while you are typing the search string, only the beginning
of the search string is required to match the beginning of a symbol,
and @samp{Pending} appears in the search prompt until you use a search
repeating key like @kbd{C-s}.

  To begin a nonincremental symbol search, type @kbd{M-s _ @key{RET}}
for a forward search, or @kbd{M-s _ C-r @key{RET}} or a backward
search.  In nonincremental symbol searches, the beginning and end of
the search string are required to match the beginning and end of a
symbol, respectively.

  The symbol search commands don't perform character folding, and
toggling lax whitespace matching (@pxref{Lax Search, lax space
matching}) has no effect on them.

@node Regexp Search
@section Regular Expression Search
@cindex regexp search
@cindex search for a regular expression

  A @dfn{regular expression} (or @dfn{regexp} for short) is a pattern
that denotes a class of alternative strings to match.  Emacs
provides both incremental and nonincremental ways to search for a
match for a regexp.  The syntax of regular expressions is explained in
the next section.

@table @kbd
@item C-M-s
Begin incremental regexp search (@code{isearch-forward-regexp}).
@item C-M-r
Begin reverse incremental regexp search (@code{isearch-backward-regexp}).
@end table

@kindex C-M-s
@findex isearch-forward-regexp
@kindex C-M-r
@findex isearch-backward-regexp
  Incremental search for a regexp is done by typing @kbd{C-M-s}
(@code{isearch-forward-regexp}), by invoking @kbd{C-s} with a
prefix argument (whose value does not matter), or by typing @kbd{M-r}
within a forward incremental search.  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.  To search backward for a regexp, use @kbd{C-M-r}
(@code{isearch-backward-regexp}), @kbd{C-r} with a prefix argument,
or @kbd{M-r} within a backward incremental search.

@vindex regexp-search-ring-max
  All of the special key sequences in an ordinary incremental search
(@pxref{Special Isearch}) do similar things in an incremental regexp
search.  For instance, typing @kbd{C-s} immediately after starting the
search retrieves the last incremental search regexp used and searches
forward for it.  Incremental regexp and non-regexp searches have
independent defaults.  They also have separate search rings, which you
can access with @kbd{M-p} and @kbd{M-n}.  The maximum number of search
regexps saved in the search ring is determined by the value of
@code{regexp-search-ring-max}, 16 by default.

  Unlike ordinary incremental search, incremental regexp search
does not use lax space matching by default.  To toggle this feature
use @kbd{M-s @key{SPC}} (@code{isearch-toggle-lax-whitespace}).
Then any @key{SPC} typed in incremental regexp search will match
any sequence of one or more whitespace characters.  The variable
@code{search-whitespace-regexp} specifies the regexp for the lax
space matching.  @xref{Special Isearch}.

  Also unlike ordinary incremental search, incremental regexp search
cannot use character folding (@pxref{Lax Search}).  (If you toggle
character folding during incremental regexp search with @kbd{M-s '},
the search becomes a non-regexp search and the search pattern you
typed is interpreted as a literal string.)

  In some cases, adding characters to the regexp in an incremental
regexp search can make the cursor move back and start again.  For
example, if you have searched for @samp{foo} and you add @samp{\|bar},
the cursor backs up in case the first @samp{bar} precedes the first
@samp{foo}.  @xref{Regexps}.

  Forward and backward regexp search are not symmetrical, because
regexp matching in Emacs always operates forward, starting with the
beginning of the regexp.  Thus, forward regexp search scans forward,
trying a forward match at each possible starting position.  Backward
regexp search scans backward, trying a forward match at each possible
starting position.  These search methods are not mirror images.

@findex re-search-forward
@findex re-search-backward
  Nonincremental search for a regexp is done with the commands
@code{re-search-forward} and @code{re-search-backward}.  You can
invoke these with @kbd{M-x}, or by way of incremental regexp search
with @kbd{C-M-s @key{RET}} and @kbd{C-M-r @key{RET}}.  When you invoke
these commands with @kbd{M-x}, they search for the exact regexp you
specify, and thus don't support any lax-search features (@pxref{Lax
Search}) except case folding.

  If you use the incremental regexp search commands with a prefix
argument, they perform ordinary string search, like
@code{isearch-forward} and @code{isearch-backward}.  @xref{Incremental
Search}.

@node Regexps
@section Syntax of Regular Expressions
@cindex syntax of regexps
@cindex regular expression
@cindex regexp

  This section (and this manual in general) describes regular
expression features that users typically use.  @xref{Regular
Expressions,,, elisp, The Emacs Lisp Reference Manual}, for additional
features used mainly in Lisp programs.

  Regular expressions have a syntax in which a few characters are
special constructs and the rest are @dfn{ordinary}.  An ordinary
character matches that same character and nothing else.  The special
characters are @samp{$^.*+?[\}.  The character @samp{]} is special if
it ends a character alternative (see below).  The character @samp{-}
is special inside a character alternative.  Any other character
appearing in a regular expression is ordinary, unless a @samp{\}
precedes it.  (When you use regular expressions in a Lisp program,
each @samp{\} must be doubled, see the example near the end of this
section.)

  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}.  (When case distinctions are being ignored, these regexps
also match @samp{F} and @samp{O}, but we consider this a generalization
of ``the same string'', rather than an exception.)

  Any two regular expressions @var{a} and @var{b} can be concatenated.
The result is a regular expression which 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 trivial example, concatenating
the regular expressions @samp{f} and @samp{o} gives the regular
expression @samp{fo}, which matches only the string @samp{fo}.  To do
something less trivial, you need to use one of the special characters.
Here is a list of them.

@table @asis
@item @kbd{.}@: @r{(Period)}
is a special character that matches any single character except a
newline.  For example, the regular expressions @samp{a.b} matches any
three-character string that begins with @samp{a} and ends with
@samp{b}.

@item @kbd{*}
is not a construct by itself; it is a postfix operator that means to
match the preceding regular expression repetitively any number of
times, 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.

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 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.

@item @kbd{+}
is a postfix operator, similar to @samp{*} except that it must match
the preceding expression at least once.  Thus, @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 @kbd{?}
is a postfix operator, similar to @samp{*} except that it can match
the preceding expression either once or not at all.  Thus, @samp{ca?r}
matches @samp{car} or @samp{cr}, and nothing else.

@item @kbd{*?}, @kbd{+?}, @kbd{??}
@cindex non-greedy regexp matching
are non-@dfn{greedy} variants of the operators above.  The normal
operators @samp{*}, @samp{+}, @samp{?} match as much as they can, as
long as the overall regexp can still match.  With a following
@samp{?}, they will match as little as possible.

Thus, both @samp{ab*} and @samp{ab*?} can match the string @samp{a}
and the string @samp{abbbb}; but if you try to match them both against
the text @samp{abbb}, @samp{ab*} will match it all (the longest valid
match), while @samp{ab*?}  will match just @samp{a} (the shortest
valid match).

Non-greedy operators match the shortest possible string starting at a
given starting point; in a forward search, though, the earliest
possible starting point for match is always the one chosen.  Thus, if
you search for @samp{a.*?$} against the text @samp{abbab} followed by
a newline, it matches the whole string.  Since it @emph{can} match
starting at the first @samp{a}, it does.

@item @kbd{\@{@var{n}\@}}
is a postfix operator specifying @var{n} repetitions---that is, the
preceding regular expression must match exactly @var{n} times in a
row.  For example, @samp{x\@{4\@}} matches the string @samp{xxxx} and
nothing else.

@item @kbd{\@{@var{n},@var{m}\@}}
is a postfix operator specifying between @var{n} and @var{m}
repetitions---that is, the preceding regular expression must match at
least @var{n} times, but no more than @var{m} times.  If @var{m} is
omitted, then there is no upper limit, but the preceding regular
expression must match at least @var{n} times.@* @samp{\@{0,1\@}} is
equivalent to @samp{?}. @* @samp{\@{0,\@}} is equivalent to
@samp{*}. @* @samp{\@{1,\@}} is equivalent to @samp{+}.

@item @kbd{[ @dots{} ]}
is a @dfn{character set}, beginning with @samp{[} and terminated by
@samp{]}.

In the simplest case, the characters between the two brackets are what
this set 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 set, 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.  As another example, @samp{[α-ωί]} matches all lower-case
Greek letters.

You can also include certain special @dfn{character classes} in a
character set.  A @samp{[:} and balancing @samp{:]} enclose a
character class inside a character alternative.  For instance,
@samp{[[:alnum:]]} matches any letter or digit.  @xref{Char Classes,,,
elisp, The Emacs Lisp Reference Manual}, for a list of character
classes.

To include a @samp{]} in a character set, 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
set, or put it after a range.  Thus, @samp{[]-]} matches both @samp{]}
and @samp{-}.

To include @samp{^} in a set, put it anywhere but at the beginning of
the set.  (At the beginning, it complements the set---see below.)

When you use a range in case-insensitive search, you should write both
ends of the range in upper case, or both in lower case, or both should
be non-letters.  The behavior of a mixed-case range such as @samp{A-z}
is somewhat ill-defined, and it may change in future Emacs versions.

@item @kbd{[^ @dots{} ]}
@samp{[^} begins a @dfn{complemented character set}, which matches any
character except the ones specified.  Thus, @samp{[^a-z0-9A-Z]} matches
all characters @emph{except} @acronym{ASCII} letters and digits.

@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 (in other words, @samp{-} and @samp{]} are not special there).

A complemented character set 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}.

@item @kbd{^}
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.

For historical compatibility reasons, @samp{^} can be used with this
meaning only at the beginning of the regular expression, or after
@samp{\(} or @samp{\|}.

@item @kbd{$}
is similar to @samp{^} but matches only at the end of a line.  Thus,
@samp{x+$} matches a string of one @samp{x} or more at the end of a line.

For historical compatibility reasons, @samp{$} can be used with this
meaning only at the end of the regular expression, or before @samp{\)}
or @samp{\|}.

@item @kbd{\}
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.

See the following section for the special constructs that begin
with @samp{\}.
@end table

  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; it is better to 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.

@node Regexp Backslash
@section Backslash in Regular Expressions

  For the most part, @samp{\} followed by any character matches only
that character.  However, there are several exceptions: two-character
sequences starting with @samp{\} that have special meanings.  The
second character in the sequence is always an ordinary character when
used on its own.  Here is a table of @samp{\} constructs.

@table @kbd
@item \|
specifies an alternative.  Two regular expressions @var{a} and @var{b}
with @samp{\|} in between form an expression that matches some text if
either @var{a} matches it or @var{b} matches it.  It works by trying to
match @var{a}, and if that fails, by trying to match @var{b}.

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{\|}.

Full backtracking capability exists to handle multiple uses of @samp{\|}.

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

@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 is assigned as a
second meaning to the same @w{@samp{\( @dots{} \)}} construct.  In practice
there is usually no conflict between the two meanings; when there is
a conflict, you can use a shy group, described below.

@item \(?: @dots{} \)
@cindex shy group, in regexp
specifies a @dfn{shy group} that does not record the matched substring;
you can't refer back to it with @samp{\@var{d}} (see below).  This is
useful in mechanically combining regular expressions, so that you can
add groups for syntactic purposes without interfering with the
numbering of the groups that are meant to be referred to.

@item \@var{d}
@cindex back reference, in regexp
matches the same text that matched the @var{d}th occurrence of a
@samp{\( @dots{} \)} construct.  This is called a @dfn{back
reference}.

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 the
digit @var{d} to mean ``match the same text matched the @var{d}th time
by the @samp{\( @dots{} \)} construct''.

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.

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

@item \`
matches the empty string, but only at the beginning of the string or
buffer (or its accessible portion) being matched against.

@item \'
matches the empty string, but only at the end of the string or buffer
(or its accessible portion) being matched against.

@item \=
matches the empty string, but only at point.

@item \b
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
regardless of what text appears next to it.

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

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

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

@item \w
matches any word-constituent character.  The syntax table determines
which characters these are.  @xref{Syntax Tables,, Syntax Tables,
elisp, The Emacs Lisp Reference Manual}.

@item \W
matches any character that is not a word-constituent.

@item \_<
matches the empty string, but only at the beginning of a symbol.
A symbol is a sequence of one or more symbol-constituent characters.
A symbol-constituent character is a character whose syntax is either
@samp{w} or @samp{_}.  @samp{\_<} matches at the beginning of the
buffer only if a symbol-constituent character follows.  As with words,
the syntax table determines which characters are symbol-constituent.

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

@item \s@var{c}
matches any character whose syntax is @var{c}.  Here @var{c} is a
character that designates a particular syntax class: thus, @samp{w}
for word constituent, @samp{-} or @samp{ } for whitespace, @samp{.}
for ordinary punctuation, etc.  @xref{Syntax Tables,, Syntax Tables,
elisp, The Emacs Lisp Reference Manual}.

@item \S@var{c}
matches any character whose syntax is not @var{c}.

@cindex categories of characters
@cindex characters which belong to a specific language
@findex describe-categories
@item \c@var{c}
matches any character that belongs to the category @var{c}.  For
example, @samp{\cc} matches Chinese characters, @samp{\cg} matches
Greek characters, etc.  For the description of the known categories,
type @kbd{M-x describe-categories @key{RET}}.

@item \C@var{c}
matches any character that does @emph{not} belong to category
@var{c}.
@end table

  The constructs that pertain to words and syntax are controlled by
the setting of the syntax table.  @xref{Syntax Tables,, Syntax Tables,
elisp, The Emacs Lisp Reference Manual}.

@node Regexp Example
@section Regular Expression Example

  Here is an example of a regexp---similar to the regexp that Emacs
uses, by default, to recognize the end of a sentence, not including
the following space (i.e., the variable @code{sentence-end-base}):

@example
@verbatim
[.?!][]\"')}]*
@end verbatim
@end example

@noindent
This contains two parts in succession: a character set matching
period, @samp{?}, or @samp{!}, and a character set matching
close-brackets, quotes, or parentheses, repeated zero or more times.

@node Lax Search
@section Lax Matching During Searching

@cindex lax search
@cindex character equivalence in search
  Normally, you'd want search commands to disregard certain minor
differences between the search string you type and the text being
searched.  For example, sequences of whitespace characters of
different length are usually perceived as equivalent; letter-case
differences usually don't matter; etc.  This is known as
@dfn{character equivalence}.

  This section describes the Emacs lax search features, and how to
tailor them to your needs.

@cindex lax space matching in search
@kindex M-s SPC @r{(Incremental search)}
@kindex SPC @r{(Incremental search)}
@findex isearch-toggle-lax-whitespace
@vindex search-whitespace-regexp
  By default, search commands perform @dfn{lax space matching}:
each space, or sequence of spaces, matches any sequence of one or more
whitespace characters in the text.  (Incremental regexp search has a
separate default; see @ref{Regexp Search}.)  Hence, @w{@samp{foo bar}}
matches @w{@samp{foo bar}}, @w{@samp{foo@ @ bar}},
@w{@samp{foo@ @ @ bar}}, and so on (but not @samp{foobar}).  More
precisely, Emacs matches each sequence of space characters in the
search string to a regular expression specified by the variable
@code{search-whitespace-regexp}.  For example, to make spaces match
sequences of newlines as well as spaces, set it to
@samp{"[[:space:]\n]+"}.  The default value of this variable depends
on the buffer's major mode; most major modes classify spaces, tabs,
and formfeed characters as whitespace.

  If you want whitespace characters to match exactly, you can turn lax
space matching off by typing @kbd{M-s @key{SPC}}
(@code{isearch-toggle-lax-whitespace}) within an incremental search.
Another @kbd{M-s @key{SPC}} turns lax space matching back on.  To
disable lax whitespace matching for all searches, change
@code{search-whitespace-regexp} to @code{nil}; then each space in the
search string matches exactly one space.

@cindex case folding in search
@cindex case-sensitivity and search
  Searches in Emacs by default ignore the case of the text they are
searching through, if you specify the search string in lower case.
Thus, if you specify searching for @samp{foo}, then @samp{Foo} and
@samp{fOO} also match.  Regexps, and in particular character sets,
behave likewise: @samp{[ab]} matches @samp{a} or @samp{A} or @samp{b}
or @samp{B}.  This feature is known as @dfn{case folding}, and it is
supported in both incremental and non-incremental search modes.

@vindex search-upper-case
  An upper-case letter anywhere in the search string makes the search
case-sensitive.  Thus, searching for @samp{Foo} does not find
@samp{foo} or @samp{FOO}.  This applies to regular expression search
as well as to literal string search.  The effect ceases if you delete
the upper-case letter from the search string.  The variable
@code{search-upper-case} controls this: if it is non-@code{nil} (the
default), an upper-case character in the search string makes the
search case-sensitive; setting it to @code{nil} disables this effect
of upper-case characters.

@vindex case-fold-search
  If you set the variable @code{case-fold-search} to @code{nil}, then
all letters must match exactly, including case.  This is a per-buffer
variable; altering the variable normally affects only the current buffer,
unless you change its default value.  @xref{Locals}.
This variable applies to nonincremental searches also, including those
performed by the replace commands (@pxref{Replace}) and the minibuffer
history matching commands (@pxref{Minibuffer History}).

@kindex M-c @r{(Incremental search)}
@kindex M-s c @r{(Incremental search)}
@findex isearch-toggle-case-fold
  Typing @kbd{M-c} or @kbd{M-s c} (@code{isearch-toggle-case-fold})
within an incremental search toggles the case sensitivity of that
search.  The effect does not extend beyond the current incremental
search, but it does override the effect of adding or removing an
upper-case letter in the current search.

  Several related variables control case-sensitivity of searching and
matching for specific commands or activities.  For instance,
@code{tags-case-fold-search} controls case sensitivity for
@code{find-tag}.  To find these variables, do @kbd{M-x
apropos-variable @key{RET} case-fold-search @key{RET}}.

@cindex character folding in search
@cindex equivalent character sequences
  Case folding disregards case distinctions among characters, making
upper-case characters match lower-case variants, and vice versa.  A
generalization of case folding is @dfn{character folding}, which
disregards wider classes of distinctions among similar characters.
For instance, under character folding the letter @code{a} matches all
of its accented cousins like @code{@"a} and @code{@'a}, i.e., the
match disregards the diacritics that distinguish these
variants.  In addition, @code{a} matches other characters that
resemble it, or have it as part of their graphical representation,
such as @sc{u+249c parenthesized latin small letter a} and @sc{u+2100
account of} (which looks like a small @code{a} over @code{c}).
Similarly, the @acronym{ASCII} double-quote character @code{"} matches
all the other variants of double quotes defined by the Unicode
standard.  Finally, character folding can make a sequence of one or
more characters match another sequence of a different length: for
example, the sequence of two characters @code{ff} matches @sc{u+fb00
latin small ligature ff}.  Character sequences that are not identical,
but match under character folding are known as @dfn{equivalent
character sequences}.

@kindex M-s ' @r{(Incremental Search)}
@findex isearch-toggle-char-fold
  Generally, search commands in Emacs do not by default perform
character folding in order to match equivalent character sequences.
You can enable this behavior by customizing the variable
@code{search-default-mode} to @code{char-fold-to-regexp}.
@xref{Search Customizations}.  Within an incremental search, typing
@kbd{M-s '} (@code{isearch-toggle-char-fold}) toggles character
folding, but only for that search.  (Replace commands have a different
default, controlled by a separate option; see @ref{Replacement and Lax
Matches}.)

  Like with case folding, typing an explicit variant of a character,
such as @code{@"a}, as part of the search string disables character
folding for that search.  If you delete such a character from the
search string, this effect ceases.

@node Replace
@section Replacement Commands
@cindex replacement
@cindex search-and-replace commands
@cindex string substitution
@cindex global substitution

  Emacs provides several commands for performing search-and-replace
operations.  In addition to the simple @kbd{M-x replace-string}
command, there is @kbd{M-%} (@code{query-replace}), which presents
each occurrence of the search pattern and asks you whether to replace
it.

  The replace commands normally operate on the text from point to the
end of the buffer.  When the region is active, they operate on it
instead (@pxref{Mark}).  The basic replace commands replace one
@dfn{search string} (or regexp) with one @dfn{replacement string}.  It
is possible to perform several replacements in parallel, using the
command @code{expand-region-abbrevs} (@pxref{Expanding Abbrevs}).

@menu
* Unconditional Replace::   Replacing all matches for a string.
* Regexp Replace::          Replacing all matches for a regexp.
* Replacement and Lax Matches::
                            Lax searching for text to replace.
* Query Replace::           How to use querying.
@end menu

@node Unconditional Replace
@subsection Unconditional Replacement
@findex replace-string

@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}.
@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 happens only in the text after
point, so if you want to cover the whole buffer you must go to the
beginning first.  All occurrences up to the end of the buffer are
replaced; to limit replacement to part of the buffer, activate the
region around that part.  When the region is active, replacement is
limited to the region (@pxref{Mark}).

  When @code{replace-string} exits, it leaves point at the last
occurrence replaced.  It adds the prior position of point (where the
@code{replace-string} command was issued) to the mark ring, without
activating the mark; use @kbd{C-u C-@key{SPC}} to move back there.
@xref{Mark Ring}.

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

  @xref{Replacement and Lax Matches}, for details about
case-sensitivity and character folding in replace commands.

@node Regexp Replace
@subsection Regexp Replacement
@findex replace-regexp

  The @kbd{M-x replace-string} command replaces exact matches for a
single string.  The similar command @kbd{M-x replace-regexp} replaces
any match for a specified regular expression pattern (@pxref{Regexps}).

@table @kbd
@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

@cindex back reference, in regexp replacement
  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 match being
replaced.  @samp{\@var{d}} in @var{newstring}, where @var{d} is a
digit starting from 1, stands for whatever matched the @var{d}th
parenthesized grouping in @var{regexp}.  (This is called a ``back
reference''.)  @samp{\#} refers to the count of replacements already
made in this command, as a decimal number.  In the first replacement,
@samp{\#} stands for @samp{0}; in the second, for @samp{1}; and so on.
For example,

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

@noindent
replaces (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
performs the inverse transformation.  To include a @samp{\} in the
text to replace with, you must enter @samp{\\}.

  If you want to enter part of the replacement string by hand each
time, use @samp{\?} in the replacement string.  Each replacement will
ask you to edit the replacement string in the minibuffer, putting
point where the @samp{\?} was.

  The remainder of this subsection is intended for specialized tasks
and requires knowledge of Lisp.  Most readers can skip it.

  You can use Lisp expressions to calculate parts of the
replacement string.  To do this, write @samp{\,} followed by the
expression in the replacement string.  Each replacement calculates the
value of the expression and converts it to text without quoting (if
it's a string, this means using the string's contents), and uses it in
the replacement string in place of the expression itself.  If the
expression is a symbol, one space in the replacement string after the
symbol name goes with the symbol name, so the value replaces them
both.

  Inside such an expression, you can use some special sequences.
@samp{\&} and @samp{\@var{d}} refer here, as usual, to the entire
match as a string, and to a submatch as a string.  @var{d} may be
multiple digits, and the value of @samp{\@var{d}} is @code{nil} if the
@var{d}'th parenthesized grouping did not match.  You can also use
@samp{\#&} and @samp{\#@var{d}} to refer to those matches as numbers
(this is valid when the match or submatch has the form of a numeral).
@samp{\#} here too stands for the number of already-completed
replacements.

  Repeating our example to exchange @samp{x} and @samp{y}, we can thus
do it also this way:

@example
M-x replace-regexp @key{RET} \(x\)\|y @key{RET}
\,(if \1 "y" "x") @key{RET}
@end example

  For computing replacement strings for @samp{\,}, the @code{format}
function is often useful (@pxref{Formatting Strings,,, elisp, The Emacs
Lisp Reference Manual}).  For example, to add consecutively numbered
strings like @samp{ABC00042} to columns 73 @w{to 80} (unless they are
already occupied), you can use

@example
M-x replace-regexp @key{RET} ^.\@{0,72\@}$ @key{RET}
\,(format "%-72sABC%05d" \& \#) @key{RET}
@end example

@node Replacement and Lax Matches
@subsection Replace Commands and Lax Matches

  This subsection describes the behavior of replace commands with
respect to lax matches (@pxref{Lax Search}) and how to customize it.
In general, replace commands mostly default to stricter matching than
their search counterparts.

@cindex lax space matching in replace commands
@vindex replace-lax-whitespace
  Unlike incremental search, the replacement commands do not use lax
space matching (@pxref{Lax Search, lax space matching}) by default.
To enable lax space matching for replacement, change the variable
@code{replace-lax-whitespace} to non-@code{nil}.  (This only affects
how Emacs finds the text to replace, not the replacement text.)

@vindex replace-regexp-lax-whitespace
  A companion variable @code{replace-regexp-lax-whitespace} controls
whether @code{query-replace-regexp} uses lax whitespace matching when
searching for patterns.

@cindex case folding in replace commands
  If the first argument of a replace command is all lower case, the
command ignores case while searching for occurrences to
replace---provided @code{case-fold-search} is non-@code{nil}.  If
@code{case-fold-search} is set to @code{nil}, case is always significant
in all searches.

@vindex case-replace
  In addition, when the @var{newstring} argument is all or partly lower
case, replacement commands try to preserve the case pattern of each
occurrence.  Thus, the 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}, an
all-caps @samp{FOO} with @samp{BAR}, and a capitalized @samp{Foo} with
@samp{Bar}.  (These three alternatives---lower case, all caps, and
capitalized, are the only ones that @code{replace-string} can
distinguish.)

  If upper-case letters are used in the replacement string, they remain
upper case every time that text 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 either
@code{case-replace} or @code{case-fold-search} is set to @code{nil},
replacement is done without case conversion.

@cindex character folding in replace commands
  The replacement commands by default do not use character folding
(@pxref{Lax Search, character folding}) when looking for the text to
replace.  To enable character folding for matching in
@code{query-replace} and @code{replace-string}, set the variable
@code{replace-char-fold} to a non-@code{nil} value.  (This
setting does not affect the replacement text, only how Emacs finds the
text to replace.  It also doesn't affect @code{replace-regexp}.)

@node Query Replace
@subsection Query Replace
@cindex query replace

@table @kbd
@item M-% @var{string} @key{RET} @var{newstring} @key{RET}
Replace some occurrences of @var{string} with @var{newstring}.
@item C-M-% @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, use @kbd{M-%} (@code{query-replace}).
This command finds occurrences of @samp{foo} one by one, displays each
occurrence and asks you whether to replace it.  Aside from querying,
@code{query-replace} works just like @code{replace-string}
(@pxref{Unconditional Replace}).  In particular, it preserves case
provided @code{case-replace} is non-@code{nil}, as it normally is
(@pxref{Replacement and Lax Matches}).  A numeric argument means to
consider only occurrences that are bounded by word-delimiter
characters.  A negative prefix argument replaces backward.

@kindex C-M-%
@findex query-replace-regexp
  @kbd{C-M-%} performs regexp search and replace (@code{query-replace-regexp}).
It works like @code{replace-regexp} except that it queries
like @code{query-replace}.

@vindex query-replace-from-to-separator
  You can reuse earlier replacements with these commands.  When
@code{query-replace} or @code{query-replace-regexp} prompts for the
search string, use @kbd{M-p} and @kbd{M-n} to show previous
replacements in the form @samp{@var{from} -> @var{to}}, where
@var{from} is the search pattern, @var{to} is its replacement, and the
separator between them is determined by the value of the variable
@code{query-replace-from-to-separator}.  Type @key{RET} to select the
desired replacement.  If the value of this variable is @code{nil},
replacements are not added to the command history, and cannot be
reused.

@cindex faces for highlighting query replace
@cindex @code{query-replace} face
@cindex @code{lazy-highlight} face, in replace
@vindex query-replace-highlight
@vindex query-replace-lazy-highlight
@vindex query-replace-show-replacement
  These commands highlight the current match using the face
@code{query-replace}.  You can disable this highlight by setting the
variable @code{query-replace-highlight} to @code{nil}.  They highlight
other matches using @code{lazy-highlight} just like incremental search
(@pxref{Incremental Search}); this can be disabled by setting
@code{query-replace-lazy-highlight} to @code{nil}.  By default,
@code{query-replace-regexp} will show the substituted replacement
string for the current match in the minibuffer.  If you want to keep
special sequences @samp{\&} and @samp{\@var{n}} unexpanded, customize
@code{query-replace-show-replacement} variable.

@vindex query-replace-skip-read-only
  The variable @code{query-replace-skip-read-only}, if set
non-@code{nil}, will cause replacement commands to ignore matches in
read-only text.  The default is not to ignore them.

  The characters you can type when you are shown a match for the string
or regexp are:

@ignore @c Not worth it.
@kindex SPC @r{(query-replace)}
@kindex DEL @r{(query-replace)}
@kindex , @r{(query-replace)}
@kindex RET @r{(query-replace)}
@kindex . @r{(query-replace)}
@kindex ! @r{(query-replace)}
@kindex ^ @r{(query-replace)}
@kindex C-r @r{(query-replace)}
@kindex C-w @r{(query-replace)}
@kindex C-l @r{(query-replace)}
@end ignore

@c WideCommands
@table @kbd
@item @key{SPC}
@itemx y
to replace the occurrence with @var{newstring}.

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

@item , @r{(Comma)}
to replace this occurrence and display the result.  You are then asked
for another input character to say what to do next.  Since the
replacement has already been made, @key{DEL} and @key{SPC} are
equivalent in this situation; both move to the next occurrence.

You can type @kbd{C-r} at this point (see below) to alter the replaced
text.  You can also type @kbd{C-x u} to undo the replacement; this exits
the @code{query-replace}, so if you want to do further replacement you
must use @kbd{C-x @key{ESC} @key{ESC} @key{RET}} to restart
(@pxref{Repetition}).

@item @key{RET}
@itemx q
to exit without doing any more replacements.

@item .@: @r{(Period)}
to replace this occurrence and then exit without searching for more
occurrences.

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

@item ^
to go back to the position of the previous occurrence (or what used to
be an occurrence), in case you changed it by mistake or want to
reexamine it.

@item u
to undo the last replacement and go back to where that replacement was
made.

@item U
to undo all the replacements and go back to where the first
replacement was made.

@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} to proceed to
the next occurrence.  @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} to proceed to the next occurrence.

@item e
to edit the replacement string in the minibuffer.  When you exit the
minibuffer by typing @key{RET}, the minibuffer contents replace the
current occurrence of the pattern.  They also become the new
replacement string for any further occurrences.

@item C-l
to redisplay the screen.  Then you must type another character to
specify what to do with this occurrence.

@item Y @r{(Upper-case)}
to replace all remaining occurrences in all remaining buffers in
multi-buffer replacements (like the Dired @kbd{Q} command that performs
query replace on selected files).  It answers this question and all
subsequent questions in the series with ``yes'', without further
user interaction.

@item N @r{(Upper-case)}
to skip to the next buffer in multi-buffer replacements without
replacing remaining occurrences in the current buffer.  It answers
this question ``no'', gives up on the questions for the current buffer,
and continues to the next buffer in the sequence.

@item C-h
@itemx ?
@itemx @key{F1}
to display a message summarizing these options.  Then you must type
another character to specify what to do with this occurrence.
@end table

  Aside from this, any other character exits the @code{query-replace},
and is then reread as part of a key sequence.  Thus, if you type
@kbd{C-k}, it exits the @code{query-replace} and then kills to end of
line.  In particular, @kbd{C-g} simply exits the @code{query-replace}.

  To restart a @code{query-replace} once it is exited, 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 @key{ESC}
@key{ESC}}.

@cindex invisible text, and query-replace
  The option @code{search-invisible} determines how @code{query-replace}
treats invisible text.  @xref{Outline Search}.

  @xref{Operating on Files}, for the Dired @kbd{Q} command which
performs query replace on selected files.  See also @ref{Transforming
File Names}, for Dired commands to rename, copy, or link files by
replacing regexp matches in file names.

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

  Here are some other commands that find matches for a regular
expression.  They all ignore case in matching, if the pattern contains
no upper-case letters and @code{case-fold-search} is non-@code{nil}.
Aside from @code{multi-occur} and @code{multi-occur-in-matching-buffers},
which always search the whole buffer, all operate on the text from point
to the end of the buffer, or on the region if it is active.

@table @kbd
@findex multi-isearch-buffers
@cindex isearch multiple buffers
@cindex multiple-buffer isearch
@item M-x multi-isearch-buffers
Prompt for one or more buffer names, ending with @key{RET}; then,
begin a multi-buffer incremental search in those buffers.  (If the
search fails in one buffer, the next @kbd{C-s} tries searching the
next specified buffer, and so forth.)  With a prefix argument, prompt
for a regexp and begin a multi-buffer incremental search in buffers
matching that regexp.

@findex multi-isearch-buffers-regexp
@item M-x multi-isearch-buffers-regexp
This command is just like @code{multi-isearch-buffers}, except it
performs an incremental regexp search.

@findex multi-isearch-files
@cindex isearch multiple files
@cindex multiple-file isearch
@item M-x multi-isearch-files
Prompt for one or more file names, ending with @key{RET}; then,
begin a multi-file incremental search in those files.  (If the
search fails in one file, the next @kbd{C-s} tries searching the
next specified file, and so forth.)  With a prefix argument, prompt
for a regexp and begin a multi-file incremental search in files
matching that regexp.

@findex multi-isearch-files-regexp
@item M-x multi-isearch-files-regexp
This command is just like @code{multi-isearch-files}, except it
performs an incremental regexp search.

In some modes that set the buffer-local variable
@code{multi-isearch-next-buffer-function} (e.g., in Change Log mode)
a multi-file incremental search is activated automatically.

@cindex Occur mode
@cindex mode, Occur
@cindex match (face name)
@vindex list-matching-lines-default-context-lines
@vindex list-matching-lines-jump-to-current-line
@c Too long.
@c @cindex list-matching-lines-current-line-face (face name)
@kindex M-s o
@findex occur
@item M-x occur
@itemx M-s o
Prompt for a regexp, and display a list showing each line in the
buffer that contains a match for it.  If you type @kbd{M-n} at the
prompt, you can reuse search strings from previous incremental
searches.  The text that matched is highlighted using the @code{match}
face.  A numeric argument @var{n} specifies that @var{n} lines of
context are to be displayed before and after each matching line.

The default number of context lines is specified by the variable
@code{list-matching-lines-default-context-lines}.  When
@code{list-matching-lines-jump-to-current-line} is non-@code{nil} the
current line is shown highlighted with face
@code{list-matching-lines-current-line-face} and the point is set at
the first match after such line.

You can also run @kbd{M-s o} when an incremental search is active;
this uses the current search string.

Note that matches for the regexp you type are extended to include
complete lines, and a match that starts before the previous match ends
is not considered a match.

@kindex RET @r{(Occur mode)}
@kindex o @r{(Occur mode)}
@kindex C-o @r{(Occur mode)}
In the @file{*Occur*} buffer, you can click on each entry, or move
point there and type @key{RET}, to visit the corresponding position in
the buffer that was searched.  @kbd{o} and @kbd{C-o} display the match
in another window; @kbd{C-o} does not select it.  Alternatively, you
can use the @kbd{C-x `} (@code{next-error}) command to visit the
occurrences one by one (@pxref{Compilation Mode}).

@cindex Occur Edit mode
@cindex mode, Occur Edit
Typing @kbd{e} in the @file{*Occur*} buffer switches to Occur Edit
mode, in which edits made to the entries are also applied to the text
in the originating buffer.  Type @kbd{C-c C-c} to return to Occur
mode.

@findex list-matching-lines
The command @kbd{M-x list-matching-lines} is a synonym for @kbd{M-x
occur}.

@findex multi-occur
@item M-x multi-occur
This command is just like @code{occur}, except it is able to search
through multiple buffers.  It asks you to specify the buffer names one
by one.

@findex multi-occur-in-matching-buffers
@item M-x multi-occur-in-matching-buffers
This command is similar to @code{multi-occur}, except the buffers to
search are specified by a regular expression that matches visited file
names.  With a prefix argument, it uses the regular expression to
match buffer names instead.

@findex how-many
@item M-x how-many
Prompt for a regexp, and print the number of matches for it in the
buffer after point.  If the region is active, this operates on the
region instead.

@findex flush-lines
@item M-x flush-lines
Prompt for a regexp, and delete each line that contains a match for
it, operating on the text after point.  This command deletes the
current line if it contains a match starting after point.  If the
region is active, it operates on the region instead; if a line
partially contained in the region contains a match entirely contained
in the region, it is deleted.

If a match is split across lines, @code{flush-lines} deletes all those
lines.  It deletes the lines before starting to look for the next
match; hence, it ignores a match starting on the same line at which
another match ended.

@findex keep-lines
@item M-x keep-lines
Prompt for a regexp, and delete each line that @emph{does not} contain
a match for it, operating on the text after point.  If point is not at
the beginning of a line, this command always keeps the current line.
If the region is active, the command operates on the region instead;
it never deletes lines that are only partially contained in the region
(a newline that ends a line counts as part of that line).

If a match is split across lines, this command keeps all those lines.
@end table

@node Search Customizations
@section Tailoring Search to Your Needs
@cindex search customizations

  This section describes miscellaneous search-related customizations
not described elsewhere.

@cindex default search mode
@cindex search mode, default
  The default search mode for the incremental search is specified by
the variable @code{search-default-mode}.  It can be @code{nil},
@code{t}, or a function.  If it is @code{nil}, the default mode is to
do literal searches without character folding, but with case folding
and lax-whitespace matches as determined by @code{case-fold-search}
and @code{search-whitespace-regexp}, respectively (@pxref{Lax
Search}).  If the value is @code{t}, incremental search defaults to
regexp searches.  The default value specifies a function that only
performs case folding and lax-whitespace matching.

@vindex search-highlight
  The current match of an on-going incremental search is highlighted
using the @code{isearch} face.  This highlighting can be disabled by
setting the variable @code{search-highlight} to @code{nil}.

@cindex lazy highlighting customizations
@vindex isearch-lazy-highlight
@cindex @code{lazy-highlight} face
  The other matches for the search string that are visible on display
are highlighted using the @code{lazy-highlight} face.  Setting the
variable @code{isearch-lazy-highlight} to @code{nil} disables this
highlighting.  Here are some other variables that customize the lazy
highlighting:

@table @code
@item lazy-highlight-initial-delay
Time in seconds to wait before highlighting visible matches.

@item lazy-highlight-interval
Time in seconds between highlighting successive matches.

@item lazy-highlight-max-at-a-time
The maximum number of matches to highlight before checking for input.
A large number can take some time to highlight, so if you want to
continue searching and type @kbd{C-s} or @kbd{C-r} during that time,
Emacs will not respond until it finishes highlighting all those
matches.  Thus, smaller values make Emacs more responsive.
@end table

@vindex search-nonincremental-instead
  Normally, entering @key{RET} within incremental search when the
search string is empty launches a nonincremental search.  (Actually,
it lets you edit the search string, and the next @key{RET} does the
search.)  However, if you customize the variable
@code{search-nonincremental-instead} to @code{nil}, typing @key{RET}
will always exit the incremental search, even if the search string is
empty.

@vindex isearch-hide-immediately
  By default, incremental search and query-replace commands match
invisible text, but hide any such matches as soon as the current match
moves off the invisible text.  If you customize the variable
@code{isearch-hide-immediately} to @code{nil}, any invisible text
where matches were found stays on display until the search or the
replace command exits.

@cindex search display on slow terminals
@vindex search-slow-speed
@vindex search-slow-window-lines
  Searching incrementally on slow terminals, such as displays
connected to remote machines over slow connection, could be annoying
due to the need to redraw large portions of the display as the search
proceeds.  Emacs provides a special display mode for slow terminals,
whereby search pops up a separate small window and displays the text
surrounding the match in that window.  Small windows display faster,
so the annoying effect of slow speed is alleviated.  The variable
@code{search-slow-speed} determines the baud rate threshold below
which Emacs will use this display mode.  The variable
@code{search-slow-window-lines} controls the number of lines in the
window Emacs pops up for displaying the search results; the default is
1 line.  Normally, this window will pop up at the bottom of the window
that displays the buffer where you start searching, but if the value
of @code{search-slow-window-lines} is negative, that means to put the
window at the top and give it the number of lines that is the absolute
value of @code{search-slow-window-lines}.
