* term.c: Fix minor fdopen-related file descriptor leaks.

[emacs.git] / doc / emacs / search.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2013 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 a tags table
(@pxref{Tags 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.
* 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
@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

@menu
* Basic Isearch::        Basic incremental search commands.
* Repeat Isearch::       Searching for the same string again.
* Error in Isearch::     When your string is not found.
* Special Isearch::      Special input in incremental search.
* Isearch Yank::         Commands that grab text into the search string
                           or else edit the search string.
* 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.  Then 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
  At each step, Emacs highlights the @dfn{current match}---the buffer
text that matches the search string---using the @code{isearch} face
(@pxref{Faces}).  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.

  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.
@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).

  As a special exception, entering @key{RET} when the search string is
empty launches nonincremental search (@pxref{Nonincremental Search}).

  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}} to return to where you were before beginning the
search.  @xref{Mark Ring}.  It only does this if the mark was not
already active.

@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
@vindex isearch-lazy-highlight
  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}.

  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 ``search
again''.  Similarly, @kbd{C-r C-r} searches backward for the last
search string.  In determining the last search string, it doesn't
matter whether the 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.

  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)}
  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.

@kindex M-e @r{(Incremental search)}
  To edit the current search string in the minibuffer without
replacing it with items from the search ring, type @kbd{M-e}.  Type @key{RET},
@kbd{C-s} or @kbd{C-r} to finish editing the string and search for it.

@node Error in Isearch
@subsection Errors in Incremental Search

  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.  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

  Some of the characters you type during incremental search have
special effects.

@cindex lax space matching
@kindex M-s SPC @r{(Incremental search)}
@kindex SPC @r{(Incremental search)}
@findex isearch-toggle-lax-whitespace
@vindex search-whitespace-regexp
  By default, incremental search performs @dfn{lax space matching}:
each space, or sequence of spaces, matches any sequence of one or more
spaces in the text.  Hence, @samp{foo bar} matches @samp{foo bar},
@samp{foo  bar}, @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, set it to
@samp{"[[:space:]\n]+"} to make spaces match sequences of newlines as
well as spaces.  To toggle lax space matching, type @kbd{M-s SPC}
(@code{isearch-toggle-lax-whitespace}).  To disable this feature
entirely, change @code{search-whitespace-regexp} to @code{nil}; then
each space in the search string matches exactly one space

  If the search string you entered contains only lower-case letters,
the search is case-insensitive; as long as an upper-case letter exists
in the search string, the search becomes case-sensitive.  If you
delete the upper-case character from the search string, it ceases to
have this effect.  @xref{Search Case}.

  To search for a newline character, type @kbd{C-j}.

  To search for other control characters, such as @key{control-S},
quote it by typing @kbd{C-q} first (@pxref{Inserting Text}).  To
search for non-@acronym{ASCII} characters, you can either use
@kbd{C-q} and enter its octal code, or use an input method
(@pxref{Input Methods}).  If an input method is enabled in the current
buffer when you start the search, you can use it in the search string
also.  While typing the search string, you can toggle the input method
with the command @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
@findex isearch-toggle-input-method
@findex isearch-toggle-specified-input-method
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.

@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.  @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 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}).

@vindex isearch-mode-map
  When incremental search is active, you can type @kbd{C-h C-h} 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 Isearch Yank
@subsection Isearch Yanking

@kindex C-y @r{(Incremental search)}
@kindex M-y @r{(Incremental search)}
@findex isearch-yank-kill
@findex isearch-yank-pop
  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}).  @kbd{Mouse-2} appends
the current X selection (@pxref{Primary Selection}).

@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.

  If the search is currently case-insensitive, both @kbd{C-w} and
@kbd{M-s C-e} convert the text they copy to lower case, so that the
search remains case-insensitive.

@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}
at the end of the search string in the minibuffer.

@node Not Exiting Isearch
@subsection Not Exiting Incremental Search

This subsection 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.

@table @asis
@item Prefix Arguments
@vindex isearch-allow-prefix
  In incremental search, when you enter 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 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.

@item Scrolling Commands
@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.

  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.
@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.

@findex search-forward
@findex search-backward
  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, @code{search-forward}, if the
string you specify is empty.  (Such an empty argument would otherwise
be useless.)  @kbd{C-r @key{RET}} does likewise, invoking the command
@code{search-backward}.

@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.

@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.
@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, the last word
in the search string must exactly match a whole word.  In an
incremental word search, the matching is more lax: the last word in
the search string can match part of a word, so that the matching
proceeds incrementally as you type.  This additional laxity does not
apply to the lazy highlight, which always matches whole words.

@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 _
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 _ @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 _
@findex isearch-forward-symbol
  To begin a forward incremental symbol search, type @kbd{M-s _}.  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, only the beginning of the search string is required to match
the beginning of a symbol.

  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.

@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.

  All of the special key sequences in an ordinary incremental search
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}.

  Just as in ordinary incremental search, any @key{SPC} typed in
incremental regexp search matches any sequence of one or more
whitespace characters.  The variable @code{search-whitespace-regexp}
specifies the regexp for the lax space matching, and @kbd{M-s SPC}
(@code{isearch-toggle-lax-whitespace}) toggles the feature.
@xref{Special Isearch}.

  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}}.

  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 manual 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 later).  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.  For example, concatenating the
regular expressions @samp{f} and @samp{o} gives the regular expression
@samp{fo}, which matches only the string @samp{fo}.  Still trivial.
To do something nontrivial, 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.@refill

@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.

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.@refill

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

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

@item \( @dots{} \)
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.@refill

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

This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature that is assigned as a
second meaning to the same @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.

@item \(?: @dots{} \)
@cindex shy group, in regexp
specifies a ``shy'' group that does not record the matched substring;
you can't refer back to it with @samp{\@var{d}}.  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.@refill

@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.

@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 Search Case
@section Searching and Case

  Searches in Emacs normally ignore the case of the text they are
searching through, if you specify the text 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}.@refill

  An upper-case letter anywhere in the incremental 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 string search.  The effect ceases if you delete the
upper-case letter from the search string.

  Typing @kbd{M-c} within an incremental search toggles the case
sensitivity of that search.  The effect does not extend beyond the
current incremental search to the next one, but it does override the
effect of adding or removing an upper-case letter in the current
search.

@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}).

  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}}.

@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 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}).

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

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

@node Unconditional Replace
@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 Case}, for details about case-sensitivity 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 pattern.

@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, 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{n}} refer here, as usual, to the entire
match as a string, and to a submatch as a string.  @var{n} may be
multiple digits, and the value of @samp{\@var{n}} is @code{nil} if
subexpression @var{n} did not match.  You can also use @samp{\#&} and
@samp{\#@var{n}} 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 Case
@subsection Replace Commands and Case

  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.

@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 Case}).  A numeric argument means to consider
only occurrences that are bounded by word-delimiter characters.

@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}.

@cindex faces for highlighting query replace
  These commands highlight the current match using the face
@code{query-replace}.  They highlight other matches using
@code{lazy-highlight} just like incremental search (@pxref{Incremental
Search}).  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.

  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}
to replace the occurrence with @var{newstring}.

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

@item , @r{(Comma)}
to replace this occurrence and display the result.  You are then 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}
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 Y @r{(Upper-case)}
to replace all remaining occurrences in all remaining buffers in
multi-buffer replacements (like the Dired `Q' command which 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 ^
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 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 C-h
to display a message summarizing these options.  Then you must type
another character to specify what to do with this occurrence.
@end table

  Some other characters are aliases for the ones listed above: @kbd{y},
@kbd{n} and @kbd{q} are equivalent to @key{SPC}, @key{DEL} and
@key{RET}.

  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.

  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 ESC
ESC}.

  @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{occur} and its variants, all operate on the text from
point to the end of the buffer, or on the region if it is active.

@findex list-matching-lines
@findex occur
@findex multi-occur
@findex multi-occur-in-matching-buffers
@findex how-many
@findex flush-lines
@findex keep-lines

@table @kbd
@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.

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

@cindex Occur mode
@cindex mode, Occur
@item M-x occur
Prompt for a regexp, and display a list showing each line in the
buffer that contains a match for it.  To limit the search to part of
the buffer, narrow to that part (@pxref{Narrowing}).  A numeric
argument @var{n} specifies that @var{n} lines of context are to be
displayed before and after each matching line.

@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.

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

@kindex M-s o
@item M-s o
Run @code{occur} using the search string of the last incremental
string search.  You can also run @kbd{M-s o} when an incremental
search is active; this uses the current search string.

@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.

@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.

@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.

@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.

@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