Searching and Replacement
*************************

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 non-incremental 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.

Incremental Search
==================

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

C-s
     Incremental search forward (isearch-forward).

C-r
     Incremental search backward (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.

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

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

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

Sometimes you search for FOO and find it, but were actually looking for a different occurrence of it.  To move to the next occurrence of the search string, type another C-s.  Do this as often as necessary.  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''.

If the specified string is not found at all, the echo area displays the text 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, the cursor may be after the FOO in FOOL.  At this point there are several things you can do.  If you mistyped the search string, correct it.  If you like the place you have found, you can type 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.

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 backward 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.  The search is now successful and waiting for more input, so a second C-g cancels the entire search.

To search for a control character such as C-s or DEL or ESC, 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.

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

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 the rest of the current line into the search string.

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

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

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

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

These are the default bindings of isearch-mode:

DEL
     Delete a character from the incremental search string (isearch-delete-char).

RET
     Exit incremental search (isearch-exit).

C-q
     Quote special characters for incremental search (isearch-quote-char).

C-s
     Repeat incremental search forward (isearch-repeat-forward).

C-r
     Repeat incremental search backward (isearch-repeat-backward).

C-y
     Pull rest of line from buffer into search string (isearch-yank-line).

C-w
     Pull next word from buffer into search string (isearch-yank-word).

C-g
     Cancels input back to what has been found successfully, or aborts the isearch (isearch-abort).

M-p
     Recall the previous element in the isearch history ring (isearch-ring-retreat).

M-n
     Recall the next element in the isearch history ring (isearch-ring-advance).

M-TAB
     Do completion on the elements in the isearch history ring (isearch-complete).

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

Slow Terminal Incremental Search
--------------------------------

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

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

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

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.  Its normal value is 1.

Non-Incremental Search
======================

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

C-s RET STRING RET
     Search for STRING.

C-r RET STRING RET
     Search backward for STRING.

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

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

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

Non-incremental searches performed using C-s RET do not call search-forward right away.  They first check if the next character is C-w, which requests a word search. See Word Search.

Word Search
===========

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

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

C-s RET C-w WORDS RET
     Search for WORDS, ignoring differences in punctuation.

C-r RET C-w WORDS RET
     Search backward for WORDS, ignoring differences in punctuation.

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

You can do a backward word search with C-r RET C-w.

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

Regular Expression Search
=========================

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

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

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

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

Incremental search for a regexp is done by typing M-C-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.  A reverse regexp search command isearch-backward-regexp also exists, bound to M-C-r.

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

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

Syntax of Regular Expressions
=============================

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 that matches that character and nothing else.  The special characters are ., *, +, ?, [, ], ^, $, and \; no new special characters will be defined in the future.  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.

Any two regular expressions A and B can be concatenated.  The result is a regular expression that 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 more powerful, you need to use one of the special characters.  Here is a list of them:

. (Period)
     is a special character that matches any single character except a newline. Using concatenation, we can make regular expressions like a.b, which matches any three-character string that begins with a and ends with b.

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

     * always applies to the smallest possible preceding expression.  Thus, fo* has a repeating o, not a repeating fo.

     The matcher processes a * construct by matching, immediately, as many repetitions as can be found; it is "greedy".  Then it continues with the rest of the pattern.  If that fails, backtracking occurs, discarding some of the matches of the *-modified construct in case that makes it possible to match the rest of the pattern.  For example, in matching ca*ar against the string caaar, the a* first tries to match all three as; 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 as.  With this choice, the rest of the regexp matches successfully.

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

+
     is a quantifying suffix operator similar to * except that the preceding expression must match at least once.  It is also "greedy". So, for example, ca+r matches the strings car and caaaar but not the string cr, whereas ca*r matches all three strings.

?
     is a quantifying suffix operator similar to *, except that the preceding expression can match either once or not at all.  For example, ca?r matches car or cr, but does not match anything else.

*?
     works just like *, except that rather than matching the longest match, it matches the shortest match.  *? is known as a non-greedy quantifier, a regexp construct borrowed from Perl.

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

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

+?
     is the non-greedy version of +.

??
     is the non-greedy version of ?.

\{n,m\}
     serves as an interval quantifier, analogous to * or +, but specifies that the expression must match at least N times, but no more than M times.  This syntax is supported by most Unix regexp utilities, and has been introduced to XEmacs for the version 20.3.

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

[ ... ]
     [ begins a character set, which is terminated by a ].  In the simplest case, the characters between the two brackets form the set.  Thus, [ad] matches either one a or one d, and [ad]* matches any string composed of just as and ds (including the empty string), from which it follows that c[ad]*r matches cr, car, cdr, caddaar, etc.

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

     - is used for ranges of characters.  To write a range, write 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 a period.

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

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

[^ ... ]
     [^ begins a complement character set, which matches any character except the ones specified.  Thus, [^a-z0-9A-Z] matches all characters except letters and digits.

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

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

^
     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, ^foo matches a foo that occurs at the beginning of a line.

     When matching a string instead of a buffer, ^ matches at the beginning of the string or after a newline character \n.

$
     is similar to ^ but matches only at the end of a line.  Thus, x+$ matches a string of one x or more at the end of a line.

     When matching a string instead of a buffer, $ matches at the end of the string or before a newline character \n.

\
     has two functions: it quotes the special characters (including \), and it introduces additional special constructs.

     Because \ quotes special characters, \$ is a regular expression that matches only $, and \[ is a regular expression that matches only [, and so on.

Please note: For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense.  For example, *foo treats * as ordinary since there is no preceding expression on which the * can act.  It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears.

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

\|
     specifies an alternative. Two regular expressions A and B with \| in between form an expression that matches anything that either A or B matches.

     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 grouping power of \|.

     Full backtracking capability exists to handle multiple uses of \|.

\( ... \)
     is a grouping construct that serves three purposes:

      1. To enclose a set of \| alternatives for other operations. Thus, \(foo\|bar\)x matches either foox or barx.

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

      3. To record a matched substring for future reference.

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

\DIGIT
     matches the same text that matched the DIGITth occurrence of a \( ... \) construct.

     In other words, after the end of a \( ... \) 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 \ followed by DIGIT to match that same text, whatever it may have been.

     The strings matching the first nine \( ... \) 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 \1 through \9 to refer to the text matched by the corresponding \( ... \) constructs.

     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.

\(?: ... \)
     is called a shy grouping operator, and it is used just like \( ... \), except that it does not cause the matched substring to be recorded for future reference.

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

     Using \(?: ... \) rather than \( ... \) when you don't need the captured substrings ought to speed up your programs some, since it shortens the code path followed by the regular expression engine, as well as the amount of memory allocation and string copying it must do.  The actual performance gain to be observed has not been measured or quantified as of this writing.

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

\w
     matches any word-constituent character.  The editor syntax table determines which characters these are.  See Syntax.

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

\sCODE
     matches any character whose syntax is CODE.  Here CODE is a character that represents a syntax code: thus, w for word constituent, - for whitespace, ( for open parenthesis, etc.  See Syntax, for a list of syntax codes and the characters that stand for them.

\SCODE
     matches any character whose syntax is not CODE.

The following regular expression constructs match the empty string---that is, they don't use up any characters---but whether they match depends on the context.

\`
     matches the empty string, but only at the beginning of the buffer or string being matched against.

\'
     matches the empty string, but only at the end of the buffer or string being matched against.

\=
     matches the empty string, but only at point. (This construct is not defined when matching against a string.)

\b
     matches the empty string, but only at the beginning or end of a word.  Thus, \bfoo\b matches any occurrence of foo as a separate word.  \bballs?\b matches ball or balls as a separate word.

\B
     matches the empty string, but not at the beginning or end of a word.

\<
     matches the empty string, but only at the beginning of a word.

\>
     matches the empty string, but only at the end of a word.

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 regexp contains four parts: 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.

Searching and Case
==================

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

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

Replacement Commands
====================

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

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

Unconditional Replacement
-------------------------

M-x replace-string RET STRING RET NEWSTRING RET
     Replace every occurrence of STRING with NEWSTRING.

M-x replace-regexp RET REGEXP RET NEWSTRING RET
     Replace every match for REGEXP with NEWSTRING.

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 occurs only after point: if you want to cover the whole buffer you must go to the beginning first.  By default, all occurrences up to the end of the buffer are replaced.  To limit replacement to part of the buffer, narrow to that part of the buffer before doing the replacement (see Narrowing).

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

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

Regexp Replacement
------------------

replace-string replaces exact matches for a single string.  The similar command 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 D'th parenthesized grouping in REGEXP. For example,

     M-x replace-regexp RET c[ad]+r RET \&-safe RET

would replace (for example) cadr with cadr-safe and cddr with cddr-safe.

     M-x replace-regexp RET \(c[ad]+r\)-safe RET \1 RET

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

Replace Commands and Case
-------------------------

If the arguments to a replace command are in lower case, the command preserves case when it makes a replacement.  Thus, the following 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; also, case conversion of the replacement string is not done.

Query Replace
-------------

M-% STRING RET NEWSTRING RET
M-x query-replace RET STRING RET NEWSTRING RET
     Replace some occurrences of STRING with NEWSTRING.

M-x query-replace-regexp RET REGEXP RET NEWSTRING RET
     Replace some matches for REGEXP with NEWSTRING.

If you want to change only some of the occurrences of foo to bar, not all of them, you can use query-replace instead of M-%.  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.

Aside from querying, query-replace works just like replace-string, and query-replace-regexp works just like replace-regexp.

The things you can type when you are shown an occurrence of STRING or a match for REGEXP are:

SPC
     to replace the occurrence with NEWSTRING.  This preserves case, just like replace-string, provided case-replace is non-nil, as it normally is.

DEL
     to skip to the next occurrence without replacing this one.

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

ESC
     to exit without doing any more replacements.

. (Period)
     to replace this occurrence and then exit.

!
     to replace all remaining occurrences without asking again.

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

C-r
     to enter a recursive editing level, in case the occurrence needs to be edited rather than just replaced with NEWSTRING.  When you are done, exit the recursive editing level with C-M-c and the next occurrence will be displayed.  See Recursive Edit.

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

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

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

If you type any other character, Emacs exits the query-replace, and executes the character as a command.  To restart the query-replace, use C-x ESC, which repeats the query-replace because it used the minibuffer to read its arguments.  See C-x ESC.

Other Search-and-Loop Commands
==============================

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

M-x occur
     Print each line that follows point and contains a match for the specified regexp.  A numeric argument specifies the number of context lines to print before and after each matching line; the default is none.

     The buffer *Occur* containing the output serves as a menu for finding 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.

M-x list-matching-lines
     Synonym for M-x occur.

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

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

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