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Like other editors, Emacs has commands for searching for occurrences of a string. The principal search command is unusual in that it is incremental; it begins to search before you have finished typing the search string. There are also nonincremental search commands more like those of other editors.
Besides the usual replace-string command that finds all
occurrences of one string and replaces them with another, Emacs has a fancy
replacement command called query-replace which asks interactively
which occurrences to replace.
An incremental search begins searching as soon as you type the first character of the search string. As you type in the search string, Emacs shows you where the string (as you have typed it so far) 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 RET.
isearch-forward).
isearch-backward).
C-s starts an incremental search. C-s reads characters from the keyboard and positions the cursor at the first occurrence of the characters that you have typed. If you type C-s and then F, the cursor moves right after the first `F'. Type an O, and see the cursor move to after the first `FO'. After another O, the cursor is after the first `FOO' after the place where you started the search. Meanwhile, the search string `FOO' has been echoed in the echo area.
If you make a mistake in typing the search string, you can cancel characters with DEL. Each DEL cancels the last character of search string. This does not happen until Emacs is ready to read another input character; first it must either find, or fail to find, the character you want to erase. If you do not want to wait for this to happen, use C-g as described below.
When you are satisfied with the place you have reached, you can type RET, which 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 C-a would exit the search and then move to the beginning of the line. RET is necessary only if the next command you want to type is a printing character, DEL, RET, or another control character that is special within searches (C-q, C-w, C-r, C-s, C-y, M-y, M-r, or M-s).
Sometimes you search for `FOO' and find it, but not the one you expected to find. There was a second `FOO' that you forgot about, before the one you were looking for. In this event, type another C-s to move to the next occurrence of the search string. This can be done any number of times. If you overshoot, you can cancel some C-s characters with DEL.
After you exit a search, you can search for the same string again by typing just C-s C-s: the first C-s is the key that invokes incremental search, and the second C-s means "search again".
To reuse earlier search strings, use the search ring. The commands M-p and 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 C-s or C-r to terminate editing the string and search for it.
If your string is not found at all, the echo area says `Failing I-Search'. The cursor is after the place where Emacs found as much of your string as it could. Thus, if you search for `FOOT', and there is no `FOOT', you might see the cursor after the `FOO' in `FOOL'. At this point there are several things you can do. If your string was mistyped, you can rub some of it out and correct it. If you like the place you have found, you can type RET or some other Emacs command to "accept what the search offered". Or you can type C-g, which removes from the search string the characters that could not be found (the `T' in `FOOT'), leaving those that were found (the `FOO' in `FOOT'). A second C-g at that point cancels the search entirely, returning point to where it was when the search started.
An upper-case letter in the search string makes the search case-sensitive. If you delete the upper-case character from the search string, it ceases to have this effect. See section Searching and Case.
If a search is failing and you ask to repeat it by typing another
C-s, it starts again from the beginning of the buffer. Repeating
a failing reverse search with C-r starts again from the end. This
is called wrapping around. `Wrapped' appears in the search
prompt once this has happened.
The C-g "quit" character does special things during searches; just what it does depends on the status of the search. If the search has found what you specified and is waiting for input, C-g cancels the entire search. The cursor moves back to where you started the search. If 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 C-g will cancel the entire search.
To search for a newline, type LFD (also known as C-j). To search for another control character such as control-S or carriage return, you must quote it by typing C-q first. This function of C-q is analogous to its meaning as an Emacs command: it causes the following character to be treated the way a graphic character would normally be treated in the same context. You can also specify a character by its octal code: enter C-q followed by three octal digits.
You can change to searching backwards with C-r. If a search fails because the place you started was too late in the file, you should do this. Repeated C-r keeps looking for more occurrences backwards. A C-s starts going forwards again. C-r in a search can be cancelled with DEL.
If you know initially that you want to search backwards, you can
use C-r instead of C-s to start the search, because C-r
is also a key running a command (isearch-backward) to search
backward.
The characters C-y and C-w can be used in incremental search to grab text from the buffer into the search string. This makes it convenient to search for another occurrence of text at point. C-w copies the word after point as part of the search string, advancing point over that word. Another C-s to repeat the search will then search for a string including that word. C-y is similar to C-w but copies all the rest of the current line into the search string. Both C-y and C-w convert the text they copy to lower case if the search is current not case-sensitive; this is so the search remains case-insensitive.
The character M-y copies the most recent killed text into the
search string.
To customize the special characters that incremental search understands,
alter their bindings in the keymap isearch-mode-map.
Incremental search on a slow terminal uses a modified style of display that is designed to take less time. Instead of redisplaying the buffer at each place the search gets to, it creates a new single-line window and uses that to display the line that the search has found. The single-line window comes into play as soon as point gets outside of the text that is already on the screen.
When you terminate the search, the single-line window is removed. Then Emacs redisplays the window in which the search was done, to show its new position of point.
The slow terminal style of display is used when the terminal baud rate is
less than or equal to the value of the variable search-slow-speed,
initially 1200.
The number of lines to use in slow terminal search display is controlled
by the variable search-slow-window-lines. 1 is its normal value.
Emacs also has conventional nonincremental search commands, which require you to type the entire search string before searching begins.
To do a nonincremental search, first type C-s RET. This enters the minibuffer to read the search string; terminate the string with RET, and then the search takes place. If the string is not found, the search command gets an error.
The way C-s RET works is that the C-s invokes incremental search, which is specially programmed to invoke nonincremental search if the argument you give it is empty. (Such an empty argument would otherwise be useless.) C-r RET also works this way.
However, nonincremental searches performed using C-s RET do
not call search-forward right away. The first thing done is to see
if the next character is C-w, which requests a word search.
Forward and backward nonincremental searches are implemented by the
commands search-forward and search-backward. These
commands may be bound to keys in the usual manner. The feature that you
can get to them via the incremental search commands exists for
historical reasons, and to avoid the need to find suitable key sequences
for them.
Word search searches for a sequence of words without regard to how the words are separated. More precisely, you type a string of many words, using single spaces to separate them, and the string can be found even if there are multiple spaces, newlines or other punctuation between the words.
Word search is useful for editing a printed document made with a text formatter. If you edit while looking at the printed, formatted version, you can't tell where the line breaks are in the source file. With word search, you can search without having to know them.
Word search is a special case of nonincremental search and is invoked with C-s RET C-w. This is followed by the search string, which must always be terminated with RET. Being nonincremental, this search does not start until the argument is terminated. It works by constructing a regular expression and searching for that; see section Regular Expression Search.
Use C-r RET C-w to do backward word search.
Forward and backward word searches are implemented by the commands
word-search-forward and word-search-backward. These
commands may be bound to keys in the usual manner. The feature that you
can get to them via the incremental search commands exists for historical
reasons, and to avoid the need to find suitable key sequences for them.
A regular expression (regexp, for short) is a pattern that
denotes a class of alternative strings to match, possibly infinitely
many. In GNU Emacs, you can search for the next match for a regexp
either incrementally or not.
Incremental search for a regexp is done by typing C-M-s
(isearch-forward-regexp). This command reads a search string
incrementally just like 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
in the buffer, use C-M-r (isearch-backward-regexp).
All of the control characters that do special things within an ordinary incremental search have the same function in incremental regexp search. Typing C-s or C-r immediately after starting the search retrieves the last incremental search regexp used; that is to say, incremental regexp and non-regexp searches have independent defaults. They also have separate search rings that you can access with M-p and M-n.
If you type SPC in incremental regexp search, it matches any sequence of whitespace characters, including newlines. If you want to match just a space, type C-q SPC.
Note that 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 `foo' and you add `\|bar', the cursor
backs up in case the first `bar' precedes the first `foo'.
Nonincremental search for a regexp is done by the functions
re-search-forward and re-search-backward. You can invoke
these with M-x, or bind them to keys, or invoke them by way of
incremental regexp search with C-M-s RET and C-M-r
RET.
Regular expressions have a syntax in which a few characters are special constructs and the rest are ordinary. An ordinary character is a simple regular expression which matches that same character and nothing else. The special characters are `$', `^', `.', `*', `+', `?', `[', `]' and `\'. Any other character appearing in a regular expression is ordinary, unless a `\' precedes it.
For example, `f' is not a special character, so it is ordinary, and therefore `f' is a regular expression that matches the string `f' and no other string. (It does not match the string `ff'.) Likewise, `o' is a regular expression that matches only `o'. (When case distinctions are being ignored, these regexps also match `F' and `O', but we consider this a generalization of "the same string", rather than an exception.)
Any two regular expressions a and b can be concatenated. The result is a regular expression which matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.
As a simple example, we can concatenate the regular expressions `f' and `o' to get the regular expression `fo', which matches only the string `fo'. Still trivial. To do something nontrivial, you need to use one of the special characters. Here is a list of them.
`*' always applies to the smallest possible preceding expression. Thus, `fo*' has a repeating `o', not a repeating `fo'. It matches `f', `fo', `foo', and so on.
The matcher processes a `*' 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 `*'-modified construct in case that makes it possible to match the rest of the pattern. For example, matching `ca*ar' against the string `caaar', the `a*' first tries to match all three `a's; but the rest of the pattern is `ar' and there is only `r' left to match, so this try fails. The next alternative is for `a*' to match only two `a's. With this choice, the rest of the regexp matches successfully.
Thus, `[ad]' matches either one `a' or one `d', and `[ad]*' matches any string composed of just `a's and `d's (including the empty string), from which it follows that `c[ad]*r' matches `cr', `car', `cdr', `caddaar', etc.
You can also include character ranges a character set, by writing two characters with a `-' between them. Thus, `[a-z]' matches any lower-case letter. Ranges may be intermixed freely with individual characters, as in `[a-z$%.]', which matches any lower case letter or `$', `%' or period.
Note that the usual special characters are not special any more inside a character set. A completely different set of special characters exists inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first character. For example, `[]a]' matches `]' or `a'. To include a `-', write `-' at the beginning or end of a range. To include `^', make it other than the first character in the set.
`^' is not special in a character set unless it is the first character. The character following the `^' is treated as if it were first (`-' and `]' are not special there).
A complemented character set can match a newline, unless newline is
mentioned as one of the characters not to match. This is in contrast to
the handling of regexps in programs such as grep.
Because `\' quotes special characters, `\$' is a regular expression which matches only `$', and `\[' is a regular expression which matches only `[', etc.
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, `*foo' treats `*' as ordinary since there is no preceding expression on which the `*' can act. It is poor practice to depend on this behavior; better to quote the special character anyway, regardless of where is appears.
For the most part, `\' followed by any character matches only that character. However, there are several exceptions: two-character sequences starting with `\' which have special meanings. The second character in the sequence is always an ordinary character on their own. Here is a table of `\' constructs.
Thus, `foo\|bar' matches either `foo' or `bar' but no other string.
`\|' applies to the largest possible surrounding expressions. Only a surrounding `\( ... \)' grouping can limit the scope of `\|'.
Full backtracking capability exists to handle multiple uses of `\|'.
This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature which is assigned as a second meaning to the same `\( ... \)' construct. In practice there is no conflict between the two meanings. Here is an explanation of this feature:
The strings matching the first nine `\( ... \)' constructs appearing in a regular expression are assigned numbers 1 through 9 in order that the open-parentheses appear in the regular expression. `\1' through `\9' refer to the text previously matched by the corresponding `\( ... \)' construct.
For example, `\(.*\)\1' matches any newline-free string that is composed of two identical halves. The `\(.*\)' matches the first half, which may be anything, but the `\1' that follows must match the same exact text.
If a particular `\( ... \)' construct matches more than once (which can easily happen if it is followed by `*'), only the last match is recorded.
The constructs that pertain to words and syntax are controlled by the setting of the syntax table (see section The Syntax Table).
Here is a complicated regexp, used by Emacs to recognize the end of a sentence together with any whitespace that follows. It is given in Lisp syntax to enable you to distinguish the spaces from the tab characters. In Lisp syntax, the string constant begins and ends with a double-quote. `\"' stands for a double-quote as part of the regexp, `\\' for a backslash as part of the regexp, `\t' for a tab and `\n' for a newline.
"[.?!][]\"')]*\\($\\|\t\\| \\)[ \t\n]*"
This contains four parts in succession: a character set matching period, `?', or `!'; a character set matching close-brackets, quotes, or parentheses, repeated any number of times; an alternative in backslash-parentheses that matches end-of-line, a tab, or two spaces; and a character set matching whitespace characters, repeated any number of times.
To enter the same regexp interactively, you would type TAB to enter a tab, and C-q C-j to enter a newline. You would also type single slashes as themselves, instead of doubling them for Lisp syntax.
Incremental 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 `foo', then `Foo' and `foo' are also considered a match. Regexps, and in particular character sets, are included: `[ab]' would match `a' or `A' or `b' or `B'.
An upper-case letter in the incremental search string makes the search case-sensitive. Thus, searching for `Foo' does not find `foo' or `FOO'. This applies to regular expression search as well as to string search. If you delete the upper-case character from the search string, it ceases to have this effect.
If you set the variable case-fold-search to nil, then
all letters must match exactly, including case. This is a per-buffer
variable; altering the variable affects only the current buffer, but
there is a default value which you can change as well. See section Local Variables.
This variable applies to nonincremental searches also, including those
performed by the replace commands (see section Replacement Commands).
Global search-and-replace operations are not needed as often in Emacs as they are in other editors(1), but they are available. In addition to the simple M-x replace-string command which is like that found in most editors, there is a M-x query-replace command which asks you, for each occurrence of the pattern, whether to replace it.
The replace commands all replace one string (or regexp) with one
replacement string. It is possible to perform several replacements in
parallel using the command expand-region-abbrevs. See section Controlling Abbrev Expansion.
To replace every instance of `foo' after point with `bar', use the command M-x replace-string with the two arguments `foo' and `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, narrow to that part of the buffer before doing the replacement (see section Narrowing).
When replace-string exits, point is left at the last occurrence
replaced. The position of point where the replace-string command
was issued is remembered on the mark ring; use C-u C-SPC to
move back there.
A numeric argument restricts replacement to matches that are surrounded by word boundaries.
The M-x replace-string command replaces exact matches for a single string. The similar command M-x replace-regexp replaces any match for a specified pattern.
In replace-regexp, the newstring need not be constant: it
can refer to all or part of what is matched by the regexp.
`\&' in newstring stands for the entire text being replaced.
`\d' in newstring, where d is a digit, stands for
whatever matched the dth parenthesized grouping in regexp.
To include a `\' in the text to replace with, you must give
`\\'. For example,
M-x replace-regexp RET c[ad]+r RET \&-safe RET
replaces (for example) `cadr' with `cadr-safe' and `cddr' with `cddr-safe'.
M-x replace-regexp RET \(c[ad]+r\)-safe RET \1 RET
performs the inverse transformation.
If the arguments to a replace command are in lower case, it preserves case when it makes a replacement. Thus, the command
M-x replace-string RET foo RET bar RET
replaces a lower case `foo' with a lower case `bar',
`FOO' with `BAR', and `Foo' with `Bar'. If upper
case letters are used in the second argument, they remain upper case
every time that argument is inserted. If upper case letters are used in
the first argument, the second argument is always substituted exactly as
given, with no case conversion. Likewise, if the variable
case-replace is set to nil, replacement is done without
case conversion. If case-fold-search is set to nil, case
is significant in matching occurrences of `foo' to replace; this
also inhibits case conversion of the replacement string.
If you want to change only some of the occurrences of `foo' to
`bar', not all of them, then you cannot use an ordinary
replace-string. Instead, use M-% (query-replace).
This command finds occurrences of `foo' one by one, displays each
occurrence and asks you whether to replace it. A numeric argument to
query-replace tells it to consider only occurrences that are
bounded by word-delimiter characters. This preserves case, just like
replace-string, provided case-replace is non-nil,
as it normally is.
Aside from querying, query-replace works just like
replace-string, and query-replace-regexp works just like
replace-regexp. The shortest way to type this command name is
M-x que SPC SPC SPC RET.
The things you can type when you are shown an occurrence of string or a match for regexp are:
query-replace, so if you want to do further replacement you must use
C-x ESC ESC RET to restart (see section Repeating Minibuffer Commands).
query-replace.
Some other characters are aliases for the ones listed above: y, n and q are equivalent to SPC, DEL and RET.
Aside from this, any other character exits the query-replace,
and is then reread as part of a key sequence. Thus, if you type
C-k, it exits the query-replace and then kills to end of
line.
To restart a query-replace once it is exited, use C-x
ESC ESC, which repeats the query-replace because it
used the minibuffer to read its arguments. See section Repeating Minibuffer Commands.
See also section Transforming File Names in Dired, for Dired commands to rename, copy, or link files by replacing regexp matches in file names.
Here are some other commands that find matches for a regular expression.
They all operate from point to the end of the buffer.
The buffer `*Occur*' containing the output serves as a menu for finding the occurrences in their original context. Find an occurrence as listed in `*Occur*', position point there and type C-c C-c; this switches to the buffer that was searched and moves point to the original of the same occurrence.
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