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Search and Replace

If you want to find all matches for a regexp in part of the buffer, and replace them, the best way is to write an explicit loop using re-search-forward and replace-match, like this:

(while (re-search-forward "foo[ \t]+bar" nil t)
  (replace-match "foobar"))

See Replacing the Text that Matched, for a description of replace-match.

However, replacing matches in a string is more complex, especially if you want to do it efficiently. So Emacs provides a function to do this.

Function: replace-regexp-in-string regexp rep string &optional fixedcase literal subexp start

This function copies string and searches it for matches for regexp, and replaces them with rep. It returns the modified copy. If start is non-nil, the search for matches starts at that index in string, and the returned value does not include the first start characters of string. To get the whole transformed string, concatenate the first start characters of string with the return value.

This function uses replace-match to do the replacement, and it passes the optional arguments fixedcase, literal and subexp along to replace-match.

Instead of a string, rep can be a function. In that case, replace-regexp-in-string calls rep for each match, passing the text of the match as its sole argument. It collects the value rep returns and passes that to replace-match as the replacement string. The match data at this point are the result of matching regexp against a substring of string.

If you want to write a command along the lines of query-replace, you can use perform-replace to do the work.

Function: perform-replace from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map start end backward region-noncontiguous-p

This function is the guts of query-replace and related commands. It searches for occurrences of from-string in the text between positions start and end and replaces some or all of them. If start is nil (or omitted), point is used instead, and the end of the buffer’s accessible portion is used for end. (If the optional argument backward is non-nil, the search starts at end and goes backward.)

If query-flag is nil, it replaces all occurrences; otherwise, it asks the user what to do about each one.

If regexp-flag is non-nil, then from-string is considered a regular expression; otherwise, it must match literally. If delimited-flag is non-nil, then only replacements surrounded by word boundaries are considered.

The argument replacements specifies what to replace occurrences with. If it is a string, that string is used. It can also be a list of strings, to be used in cyclic order.

If replacements is a cons cell, (function . data), this means to call function after each match to get the replacement text. This function is called with two arguments: data, and the number of replacements already made.

If repeat-count is non-nil, it should be an integer. Then it specifies how many times to use each of the strings in the replacements list before advancing cyclically to the next one.

If from-string contains upper-case letters, then perform-replace binds case-fold-search to nil, and it uses the replacements without altering their case.

Normally, the keymap query-replace-map defines the possible user responses for queries. The argument map, if non-nil, specifies a keymap to use instead of query-replace-map.

Non-nil region-noncontiguous-p means that the region between start and end is composed of noncontiguous pieces. The most common example of this is a rectangular region, where the pieces are separated by newline characters.

This function uses one of two functions to search for the next occurrence of from-string. These functions are specified by the values of two variables: replace-re-search-function and replace-search-function. The former is called when the argument regexp-flag is non-nil, the latter when it is nil.

Variable: query-replace-map

This variable holds a special keymap that defines the valid user responses for perform-replace and the commands that use it, as well as y-or-n-p and map-y-or-n-p. This map is unusual in two ways:

The key bindings are not commands, just symbols that are meaningful to the functions that use this map.
Prefix keys are not supported; each key binding must be for a single-event key sequence. This is because the functions don’t use read-key-sequence to get the input; instead, they read a single event and look it up “by hand”.

Here are the meaningful bindings for query-replace-map. Several of them are meaningful only for query-replace and friends.

act: Do take the action being considered—in other words, “yes”.
skip: Do not take action for this question—in other words, “no”.
exit: Answer this question “no”, and give up on the entire series of questions, assuming that the answers will be “no”.
exit-prefix: Like exit, but add the key that was pressed to unread-command-events (see Event Input Misc).
act-and-exit: Answer this question “yes”, and give up on the entire series of questions, assuming that subsequent answers will be “no”.
act-and-show: Answer this question “yes”, but show the results—don’t advance yet to the next question.
automatic: Answer this question and all subsequent questions in the series with “yes”, without further user interaction.
backup: Move back to the previous place that a question was asked about.
undo: Undo last replacement and move back to the place where that replacement was performed.
undo-all: Undo all replacements and move back to the place where the first replacement was performed.
edit: Enter a recursive edit to deal with this question—instead of any other action that would normally be taken.
edit-replacement: Edit the replacement for this question in the minibuffer.
delete-and-edit: Delete the text being considered, then enter a recursive edit to replace it.
recenter
scroll-up
scroll-down
scroll-other-window
scroll-other-window-down: Perform the specified window scroll operation, then ask the same question again. Only y-or-n-p and related functions use this answer.
quit: Perform a quit right away. Only y-or-n-p and related functions use this answer.
help: Display some help, then ask again.

Variable: multi-query-replace-map

This variable holds a keymap that extends query-replace-map by providing additional keybindings that are useful in multi-buffer replacements. The additional bindings are:

automatic-all: Answer this question and all subsequent questions in the series with “yes”, without further user interaction, for all remaining buffers.
exit-current: Answer this question “no”, and give up on the entire series of questions for the current buffer. Continue to the next buffer in the sequence.

Variable: replace-search-function: This variable specifies a function that perform-replace calls to search for the next string to replace. Its default value is search-forward. Any other value should name a function of 3 arguments: the first 3 arguments of search-forward (see String Search).

Variable: replace-re-search-function: This variable specifies a function that perform-replace calls to search for the next regexp to replace. Its default value is re-search-forward. Any other value should name a function of 3 arguments: the first 3 arguments of re-search-forward (see Regexp Search).