1 Text

This chapter describes the functions that deal with the text in a buffer. Most examine, insert or delete text in the current buffer, often in the vicinity of point. Many are interactive. All the functions that change the text provide for undoing the changes (see section Undo).

Many text-related functions operate on a region of text defined by two buffer positions passed in arguments named start and end. These arguments should be either markers (@pxref{Markers}) or or numeric character positions (@pxref{Positions}). The order of these arguments does not matter; it is all right for start to be the end of the region and end the beginning. For example, (delete-region 1 10) and (delete-region 10 1) perform identically. An args-out-of-range error is signaled if either start or end is outside the accessible portion of the buffer. In an interactive call, point and the mark are used for these arguments.

Throughout this chapter, “text” refers to the characters in the buffer.

1.1 Examining Text Near Point

Many functions are provided to look at the characters around point. Several simple functions are described here. See also looking-at in @ref{Regexp Search}.

Function: char-after position

This function returns the character in the current buffer at (i.e., immediately after) position position. If position is out of range for this purpose, either before the beginning of the buffer, or at or beyond the end, then the value is nil.

Remember that point is always between characters, and the terminal cursor normally appears over the character following point. Therefore, the character returned by char-after is the character the cursor is over.

In the following example, assume that the first character in the buffer is ‘@’:

(char-to-string (char-after 1))
     ⇒ "@"

Function: following-char

This function returns the character following point in the current buffer. This is similar to (char-after (point)). However, if point is at the end of the buffer, then the result of following-char is 0.

In this example, point is between the ‘a’ and the ‘c’.

---------- Buffer: foo ----------
Gentlemen may cry ``Pea∗ce! Peace!,''
but there is no peace.
---------- Buffer: foo ----------

(char-to-string (preceding-char))
     ⇒ "a"
(char-to-string (following-char))
     ⇒ "c"

Function: preceding-char

This function returns the character preceding point in the current buffer. See above, under following-char, for an example. If point is at the beginning of the buffer, then the result of preceding-char is 0.

Function: bobp

This function returns t if point is at the beginning of the buffer. If narrowing is in effect, this means the beginning of the accessible portion of the text. See also point-min in @ref{Point}.

Function: eobp

This function returns t if point is at the end of the buffer. If narrowing is in effect, this means the end of accessible portion of the text. See also point-max in @xref{Point}.

Function: bolp

This function returns t if point is at the beginning of a line. @xref{Text Lines}.

Function: eolp

This function returns t if point is at the end of a line. The end of the buffer is always considered the end of a line.

1.2 Examining Buffer Contents

This section describes two functions that allow a Lisp program to convert any portion of the text in the buffer into a string.

Function: buffer-substring start end

This function returns a string containing a copy of the text of the region defined by positions start and end in the current buffer. If the arguments are not positions in the accessible portion of the buffer, Emacs signals an args-out-of-range error.

It is not necessary for start to be less than end; the arguments can be given in either order. But most often the smaller argument is written first.

---------- Buffer: foo ----------
This is the contents of buffer foo

---------- Buffer: foo ----------

(buffer-substring 1 10)
⇒ "This is t"

(buffer-substring (point-max) 10)
⇒ "he contents of buffer foo
"

Function: buffer-string

This function returns the contents of the accessible portion of the current buffer as a string. This is the portion between (point-min) and (point-max) (@pxref{Narrowing}).

---------- Buffer: foo ----------
This is the contents of buffer foo

---------- Buffer: foo ----------

(buffer-string)
     ⇒ "This is the contents of buffer foo
"

1.3 Comparing Text

This function lets you compare portions of the text in a buffer, without copying them into strings first.

Function: compare-buffer-substrings buffer1 start1 end1 buffer2 start2 end2

This function lets you compare two substrings of the same buffer or two different buffers. The first three arguments specify one substring, giving a buffer and two positions within the buffer. The last three arguments specify the other substring in the same way. You can use nil for buffer1, buffer2 or both to stand for the current buffer.

The value is negative if the first substring is less, positive if the first is greater, and zero if they are equal. The absolute value of the result is one plus the index of the first differing characters within the substrings.

This function ignores case when comparing characters if case-fold-search is non-nil.

Suppose the current buffer contains the text ‘foobarbar haha!rara!’; then in this example the two substrings are ‘rbar ’ and ‘rara!’. The value is 2 because the first substring is greater at the second character.

(compare-buffer-substring nil 6 11 nil 16 21)
     ⇒ 2

This function does not exist in Emacs version 18 and earlier.

1.4 Insertion

Insertion takes place at point. Markers pointing at positions after the insertion point are relocated with the surrounding text (@pxref{Markers}). When a marker points at the place of insertion, it is normally not relocated, so that it points to the beginning of the inserted text; however, when insert-before-markers is used, all such markers are relocated to point after the inserted text.

Point may end up either before or after inserted text, depending on the function used. If point is left after the inserted text, we speak of insertion before point.

Each of these functions signals an error if the current buffer is read-only.

Function: insert &rest args

This function inserts the strings and/or characters args into the current buffer, at point, moving point forward. An error is signaled unless all args are either strings or characters. The value is nil.

Function: insert-before-markers &rest args

This function inserts the strings and/or characters args into the current buffer, at point, moving point forward. An error is signaled unless all args are either strings or characters. The value is nil.

This function is unlike the other insertion functions in that a marker whose position initially equals point is relocated to come after the newly inserted text.

Function: insert-char character count

This function inserts count instances of character into the current buffer before point. count must be a number, and character must be a character. The value is nil.

Function: insert-buffer-substring from-buffer-or-name &optional start end

This function inserts a substring of the contents of buffer from-buffer-or-name (which must already exist) into the current buffer before point. The text inserted consists of the characters in the region defined by start and end (These arguments default to the beginning and end of the accessible portion of that buffer). The function returns nil.

In this example, the form is executed with buffer ‘bar’ as the current buffer. We assume that buffer ‘bar’ is initially empty.

---------- Buffer: foo ----------
We hold these truths to be self-evident, that all
---------- Buffer: foo ----------

(insert-buffer-substring "foo" 1 20)
     ⇒ nil

---------- Buffer: bar ----------
We hold these truth
---------- Buffer: bar ----------

1.5 User-Level Insertion Commands

This section describes higher-level commands for inserting text, commands intended primarily for the user but useful also in Lisp programs.

Command: insert-buffer from-buffer-or-name

This function inserts the entire contents of from-buffer-or-name (which must exist) into the current buffer after point. It leaves the mark after the inserted text. The value is nil.

Command: self-insert-command count

This function inserts the last character typed count times and returns nil. Most printing characters are bound to this command. In routine use, self-insert-command is the most frequently called function in Emacs, but programs rarely use it except to install it on a keymap.

In an interactive call, count is the numeric prefix argument.

This function calls auto-fill-function if the current column number is greater than the value of fill-column and the character inserted is a space (see section Auto Filling).

This function performs abbrev expansion if Abbrev mode is enabled and the inserted character does not have word-constituent syntax. (@xref{Abbrevs}, and @ref{Syntax Class Table}.)

This function is also responsible for calling blink-paren-function when the inserted character has close parenthesis syntax (@pxref{Blinking}).

Command: newline &optional number-of-newlines

This function inserts newlines into the current buffer before point. If number-of-newlines is supplied, that many newline characters are inserted.

In Auto Fill mode, newline can break the preceding line if number-of-newlines is not supplied. When this happens, it actually inserts two newlines at different places: one at point, and another earlier in the line. newline does not auto-fill if number-of-newlines is non-nil.

The value returned is nil. In an interactive call, count is the numeric prefix argument.

Command: split-line

This function splits the current line, moving the portion of the line after point down vertically, so that it is on the next line directly below where it was before. Whitespace is inserted as needed at the beginning of the lower line, using the indent-to function. split-line returns the position of point.

Programs hardly ever use this function.

Variable: overwrite-mode

This variable controls whether overwrite mode is in effect: a non-nil value enables the mode. It is automatically made buffer-local when set in any fashion.

1.6 Deletion of Text

All of the deletion functions operate on the current buffer, and all return a value of nil. In addition to these functions, you can also delete text using the “kill” functions that save it in the kill ring; some of these functions save text in the kill ring in some cases but not in the usual case. See section The Kill Ring.

Function: erase-buffer

This function deletes the entire text of the current buffer, leaving it empty. If the buffer is read-only, it signals a buffer-read-only error. Otherwise, it deletes the text without asking for any confirmation. The value is always nil.

Normally, deleting a large amount of text from a buffer inhibits further auto-saving of that buffer “because it has shrunk”. However, erase-buffer does not do this, the idea being that the future text is not really related to the former text, and its size should not be compared with that of the former text.

Command: delete-region start end

This function deletes the text in the current buffer in the region defined by start and end. The value is nil.

Command: delete-char count &optional killp

This function deletes count characters directly after point, or before point if count is negative. If killp is non-nil, then it saves the deleted characters in the kill ring.

In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

The value returned is always nil.

Command: delete-backward-char count &optional killp

This function deletes count characters directly before point, or after point if count is negative. If killp is non-nil, then it saves the deleted characters in the kill ring.

In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

The value returned is always nil.

Command: backward-delete-char-untabify count &optional killp

This function deletes count characters backward, changing tabs into spaces. When the next character to be deleted is a tab, it is first replaced with the proper number of spaces to preserve alignment and then one of those spaces is deleted instead of the tab. If killp is non-nil, then the command saves the deleted characters in the kill ring.

If count is negative, then tabs are not changed to spaces, and the characters are deleted by calling delete-backward-char with count.

In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

The value returned is always nil.

1.7 User-Level Deletion Commands

This section describes higher-level commands for deleting text, commands intended primarily for the user but useful also in Lisp programs.

Command: delete-horizontal-space

This function deletes all spaces and tabs around point. It returns nil.

In the following examples, assume that delete-horizontal-space is called four times, once on each line, with point between the second and third characters on the line.

---------- Buffer: foo ----------
I ∗thought
I ∗     thought
We∗ thought
Yo∗u thought
---------- Buffer: foo ----------

(delete-horizontal-space)   ; Four times.
     ⇒ nil

---------- Buffer: foo ----------
Ithought
Ithought
Wethought
You thought
---------- Buffer: foo ----------

Command: delete-indentation &optional join-following-p

This function joins the line point is on to the previous line, deleting any whitespace at the join and in some cases replacing it with one space. If join-following-p is non-nil, delete-indentation joins this line to the following line instead. The value is nil.

If there is a fill prefix, and the second of the lines being joined starts with the prefix, then delete-indentation deletes the fill prefix before joining the lines.

In the example below, point is located on the line starting ‘events’, and it makes no difference if there are trailing spaces in the preceding line.

---------- Buffer: foo ----------
When in the course of human
∗    events, it becomes necessary
---------- Buffer: foo ----------

(delete-indentation)
     ⇒ nil

---------- Buffer: foo ----------
When in the course of human∗ events, it becomes necessary
---------- Buffer: foo ----------

After the lines are joined, the function fixup-whitespace is responsible for deciding whether to leave a space at the junction.

Function: fixup-whitespace

This function replaces white space between the objects on either side of point with either one space or no space as appropriate. It returns nil.

The appropriate amount of space is none at the beginning or end of the line. Otherwise, it is one space except when point is before a character with close parenthesis syntax or after a character with open parenthesis or expression-prefix syntax. @xref{Syntax Class Table}.

In the example below, when fixup-whitespace is called the first time, point is before the word ‘spaces’ in the first line. It is located directly after the ‘(’ for the second invocation.

---------- Buffer: foo ----------
This has too many     ∗spaces
This has too many spaces at the start of (∗   this list)
---------- Buffer: foo ----------

(fixup-whitespace)
     ⇒ nil
(fixup-whitespace)
     ⇒ nil

---------- Buffer: foo ----------
This has too many spaces
This has too many spaces at the start of (this list)
---------- Buffer: foo ----------

Command: just-one-space

This command replaces any spaces and tabs around point with a single space. It returns nil.

Command: delete-blank-lines

This function deletes blank lines surrounding point. If point is on a blank line with one or more blank lines before or after it, then all but one of them are deleted. If point is on an isolated blank line, then it is deleted. If point is on a nonblank line, the command deletes all blank lines following it.

A blank line is defined as a line containing only tabs and spaces.

delete-blank-lines returns nil.

1.8 The Kill Ring

Kill functions delete text like the deletion functions, but save it so that the user can reinsert it by yanking. Most of these functions have ‘kill-’ in their name. By contrast, the functions whose names start with ‘delete-’ normally do not save text for yanking (though they can still be undone); these are “deletion” functions.

Most of the kill commands are primarily for interactive use, and are not described here. What we do describe are the functions provided for use in writing such commands. When deleting text for internal purposes within a Lisp function, you should normally use deletion functions, so as not to disturb the kill ring contents. See section Deletion of Text.

Emacs saves the last several batches of killed text in a list. We call it the kill ring because, in yanking, the elements are considered to be in a cyclic order. The list is kept in the variable kill-ring, and can be operated on with the usual functions for lists; there are also specialized functions, described in this section, which treat it as a ring.

Some people think use of the word “kill” in Emacs is unfortunate, since it refers to processes which specifically do not destroy the entities “killed”. This is in sharp contrast to ordinary life, in which death is permanent and “killed” entities do not come back to life. Therefore, other metaphors have been proposed. For example, the term “cut ring” makes sense to people who, in pre-computer days, used scissors and paste to cut up and rearrange manuscripts. However, it would be difficult to change now.

1.8.1 Kill Ring Concepts

The kill ring records killed text as strings in a list. A short kill ring, for example, might look like this:

("some text" "a different piece of text" "yet more text")

New entries in the kill ring go at the front of the list. When the list reaches kill-ring-max entries in length, adding a new entry automatically deletes the last entry.

When kill commands are interwoven with other commands, the killed portions of text are put into separate entries in the kill ring. But when two or more kill commands are executed in succession, the text they kill forms a single entry, because the second and subsequent consecutive kill commands append to the entry made by the first one.

The user can reinsert or yank text from any element in the kill ring. One of the entries in the ring is considered the “front”, and the simplest yank command yanks that entry. Other yank commands “rotate” the ring by designating other entries as the “front”.

1.8.2 Functions for Killing

kill-region is the usual subroutine for killing text. Any command that calls this function is a “kill command” (and should probably have ‘kill’ in its name). kill-region puts the newly killed text in a new element at the beginning of the kill ring or adds it to the most recent element. It uses the last-command variable to keep track of whether the previous was a kill command, and in such cases appends the killed text to the most recent entry.

Command: kill-region start end

This function kills the text in the region defined by start and end. The text is deleted but saved in the kill ring. The value is always nil.

In an interactive call, start and end are point and the mark.

If the buffer is read-only, kill-region modifies the kill ring just the same, then signals an error without modifying the buffer. This is convenient because it lets the user use all the kill commands to copy text into the kill ring from a read-only buffer.

Command: copy-region-as-kill start end

This function saves the region defined by start and end on the kill ring, but does not delete the text from the buffer. It returns nil. It also indicates the extent of the text copied by moving the cursor momentarily, or by displaying a message in the echo area.

Don’t use this command in Lisp programs; use kill-new or kill-append instead. See section Low Level Kill Ring.

In an interactive call, start and end are point and the mark.

1.8.3 Functions for Yanking

Command: yank &optional arg

This function inserts the text in the first entry in the kill ring directly before point. After the yank, the mark is positioned at the beginning and point is positioned after the end of the inserted text.

If arg is a list (which occurs interactively when the user types C-u with no digits), then yank inserts the text as described above, but puts point before the yanked text and puts the mark after it. If arg is a number, then yank inserts the argth most recently killed text.

yank does not alter the contents of the kill ring or rotate it. It returns nil.

Command: yank-pop arg

This function replaces the just-yanked text with another batch of killed text—another element of the kill ring.

This command is allowed only immediately after a yank or a yank-pop. At such a time, the region contains text that was just inserted by the previous yank. yank-pop deletes that text and inserts in its place a different stretch of killed text. The text that is deleted is not inserted into the kill ring, since it is already in the kill ring somewhere.

If arg is nil, then the existing region contents are replaced with the previous element of the kill ring. If arg is numeric, then the argth previous kill is the replacement. If arg is negative, a more recent kill is the replacement.

The sequence of kills in the kill ring wraps around, so that after the oldest one comes the newest one, and before the newest one goes the oldest.

The value is always nil.

1.8.4 Low Level Kill Ring

These functions and variables provide access to the kill ring at a lower level, but still convenient for use in Lisp programs. They take care of interaction with X Window selections. They do not exist in Emacs version 18.

Function: current-kill n &optional do-not-move

The function current-kill rotates the yanking pointer in the kill ring by n places, and returns the text at that place in the ring.

If the optional second argument do-not-move is non-nil, then current-kill doesn’t alter the yanking pointer; it just returns the nth kill forward from the current yanking pointer.

If n is zero, indicating a request for the latest kill, current-kill calls the value of interprogram-paste-function (documented below) before consulting the kill ring.

Function: kill-new string

This function puts the text string into the kill ring as a new entry at the front of the ring. It also discards the oldest entry if appropriate. It also invokes the value of interprogram-cut-function (see below).

Function: kill-append string before-p

This function appends the text string to the first entry in the kill ring. Normally string goes at the end of the entry, but if before-p is non-nil, it goes at the beginning. This function also invokes the value of interprogram-cut-function (see below).

Variable: interprogram-paste-function

This variable provides a way of transferring killed text from other programs, when you are using a window system. Its value should be nil or a function of no arguments.

If the value is a function, it is called when the “most recent kill” value is called for. If the function returns a non-nil values, then that value is used as the “most recent kill”. If it returns nil, then the first element of the kill ring is used.

Variable: interprogram-cut-function

This variable provides a way of communicating killed text to and from other programs, when you are using a window system. Its value should be nil or a function of one argument.

If the value is a function, it is called whenever the “most recent kill” is changed, with the new string of killed text as an argument.

1.8.5 Internals of the Kill Ring

The variable kill-ring holds the kill ring contents, in the form of a list of strings. The most recent kill is always at the front of the list.

The kill-ring-yank-pointer variable points to a link in the kill ring list, whose CAR is the text that yank functions should copy. Moving kill-ring-yank-pointer to a different link is called rotating the kill ring. We call the kill ring a “ring” because the functions that move the yank pointer wrap around from the end of the list to the beginning, or vice-versa. Rotating the ring does not change the value of kill-ring.

Both kill-ring and kill-ring-yank-pointer are Lisp variables whose values are normally lists. The word “pointer” in the name of the kill-ring-yank-pointer indicates that the variable’s purpose is to identify one element of the list for use by the next yank command.

The value of kill-ring-yank-pointer is always eq to one of the links in the kill ring list. The element it identifies is the CAR of that link. Commands which change the text in the kill ring also set this variable from kill-ring. The effect is to rotate the ring so that the newly killed text is at front.

Here is a diagram that shows the variable kill-ring-yank-pointer pointing to the second entry in the kill ring ("some text" "a different piece of text" "yet more text").

kill-ring       kill-ring-yank-pointer
  |               |
  |     ___ ___    --->  ___ ___      ___ ___
   --> |___|___|------> |___|___|--> |___|___|--> nil
         |                |            |            
         |                |            |            
         |                |             -->"yet more text" 
         |                |
         |                 --> "a different piece of text" 
         |
          --> "some text"

This circumstance might occur after C-y (yank) immediately followed by M-y (yank-pop).

Variable: kill-ring

List of killed text sequences, most recently killed first.

Variable: kill-ring-yank-pointer

This variable’s value indicates which element of the kill ring is at the “front” of the ring for yanking. More precisely, the value is a sublist of the value of kill-ring, and its CAR is the kill string that C-y should yank.

User Option: kill-ring-max

The value of this variable is the maximum length to which the kill ring can grow, before elements are thrown away at the end. The default value for kill-ring-max is 30.

1.9 Undo

Most buffers have an undo list which records all changes made to the buffer’s text so that they can be undone. (The buffers which don’t have one are usually special-purpose buffers for which Emacs assumes that undoing is not useful.) All the primitives which modify the text in the buffer automatically add elements to the front of the undo list, which you can find in the variable buffer-undo-list.

Variable: buffer-undo-list

This variable’s value is the undo list of the current buffer. A value of t disables the recording of undo information.

Here are the kinds of elements an undo list can have:

integer

This kind of element records a previous value of point. Ordinary cursor motion does not get any sort of undo record, but these entries are used to record where point was before a deletion.

(beg . end)

This kind of element indicates how to delete text that was inserted. Upon insertion, the text occupied the range beg–end in the buffer.

(pos . deleted)

This kind of element indicates how to reinsert text that was deleted. The deleted text itself is the string deleted. The place to reinsert it is pos.

(t high . low)

This kind of element indicates that an unmodified buffer became modified. The elements high and low are two integers, each recording 16 bits of the visited file’s modification time as of when it was previously visited or saved. primitive-undo uses those values to determine whether to mark the buffer as unmodified once again; it does so only if the file’s modification time matches those numbers.

(nil property value beg . end)

This kind of element records a change in a text property. Here’s how you might undo the change:

(put-text-property beg end
                   property value)

nil

This element is a boundary. The function undo-boundary adds these elements. The elements between two boundaries are called a change group; normally, each change group corresponds to one keyboard command, and undo commands normally undo an entire group as a unit.

Function: undo-boundary

This function places a boundary element in the undo list. The undo command stops at such a boundary, and successive undo commands undo to earlier and earlier boundaries. This function returns nil.

The editor command loop automatically creates an undo boundary between keystroke commands. Thus, each undo normally undoes the effects of one command. Calling this function explicitly is useful for splitting the effects of a command into more than one unit. For example, query-replace calls this function after each replacement so that the user can undo individual replacements one by one.

Function: primitive-undo count list

This is the basic function for undoing elements of an undo list. It undoes the first count elements of list, returning the rest of list. You could write this function in Lisp, but it is convenient to have it in C.

primitive-undo adds elements to the buffer’s undo list. Undo commands avoid confusion by saving the undo list value at the beginning of a sequence of undo operations. Then the undo operations use and update the saved value. The new elements added by undoing never get into the saved value, so they don’t cause any trouble.

1.10 Maintaining Undo Lists

This section describes how to enable and disable undo information for a given buffer. It also explains how data from the undo list is discarded automatically so it doesn’t get too big.

Recording of undo information in a newly created buffer is normally enabled to start with; but if the buffer name starts with a space, the undo recording is initially disabled. You can explicitly enable or disable undo recording with the following two functions, or by setting buffer-undo-list yourself.

Command: buffer-enable-undo &optional buffer-or-name

This function enables recording undo information for buffer buffer-or-name, so that subsequent changes can be undone. If no argument is supplied, then the current buffer is used. This function does nothing if undo recording is already enabled in the buffer. It returns nil.

In an interactive call, buffer-or-name is the current buffer. You cannot specify any other buffer.

Function: buffer-disable-undo buffer

Function: buffer-flush-undo buffer

This function discards the undo list of buffer, and disables further recording of undo information. As a result, it is no longer possible to undo either previous changes or any subsequent changes. If the undo list of buffer is already disabled, this function has no effect.

This function returns nil. It cannot be called interactively.

The name buffer-flush-undo is not considered obsolete, but the preferred name buffer-disable-undo was not provided in Emacs versions 18 and earlier.

As editing continues, undo lists get longer and longer. To prevent them from using up all available memory space, garbage collection trims them back to size limits you can set. (For this purpose, the “size” of an undo list measures the cons cells that make up the list, plus the strings of deleted text.) Two variables control the range of acceptable sizes: undo-limit and undo-strong-limit.

Variable: undo-limit

This is the soft limit for the acceptable size of an undo list. The change group at which this size is exceeded is the last one kept.

Variable: undo-strong-limit

The upper limit for the acceptable size of an undo list. The change group at which this size is exceeded is discarded itself (along with all subsequent changes). There is one exception: garbage collection always keeps the very last change group no matter how big it is.

1.11 Filling

Filling means adjusting the lengths of lines (by moving words between them) so that they are nearly (but no greater than) a specified maximum width. Additionally, lines can be justified, which means that spaces are inserted between words to make the line exactly the specified width. The width is controlled by the variable fill-column. For ease of reading, lines should be no longer than 70 or so columns.

You can use Auto Fill mode (see section Auto Filling) to fill text automatically as you insert it, but changes to existing text may leave it improperly filled. Then you must fill the text explicitly.

Most of the functions in this section return values that are not meaningful.

Command: fill-paragraph justify-flag

This function fills the paragraph at or after point. If justify-flag is non-nil, each line is justified as well. It uses the ordinary paragraph motion commands to find paragraph boundaries.

Command: fill-region start end &optional justify-flag

This function fills each of the paragraphs in the region from start to end. It justifies as well if justify-flag is non-nil. (In an interactive call, this is true if there is a prefix argument.)

The variable paragraph-separate controls how to distinguish paragraphs.

Command: fill-individual-paragraphs start end &optional justify-flag mail-flag

This function fills each paragraph in the region according to its individual fill prefix. Thus, if the lines of a paragraph are indented with spaces, the filled paragraph will continue to be indented in the same fashion.

The first two arguments, start and end, are the beginning and end of the region that will be filled. The third and fourth arguments, justify-flag and mail-flag, are optional. If justify-flag is non-nil, the paragraphs are justified as well as filled. If mail-flag is non-nil, the function is told that it is operating on a mail message and therefore should not fill the header lines.

Ordinarily, fill-individual-paragraphs regards each change in indentation as starting a new paragraph. If fill-individual-varying-indent is non-nil, then only separator lines separate paragraphs. That mode can handle paragraphs with extra indentation on the first line.

User Option: fill-individual-varying-indent

This variable alters the action of fill-individual-paragraphs as described above.

Command: fill-region-as-paragraph start end &optional justify-flag

This function considers a region of text as a paragraph and fills it. If the region was made up of many paragraphs, the blank lines between paragraphs are removed. This function justifies as well as filling when justify-flag is non-nil. In an interactive call, any prefix argument requests justification.

In Adaptive Fill mode, which is enabled by default, fill-region-as-paragraph on an indented paragraph when there is no fill prefix uses the indentation of the second line of the paragraph as the fill prefix.

Command: justify-current-line

This function inserts spaces between the words of the current line so that the line ends exactly at fill-column. It returns nil.

User Option: fill-column

This buffer-local variable specifies the maximum width of filled lines. Its value should be an integer, which is a number of columns. All the filling, justification and centering commands are affected by this variable, including Auto Fill mode (see section Auto Filling).

As a practical matter, if you are writing text for other people to read, you should set fill-column to no more than 70. Otherwise the line will be too long for people to read comfortably, and this can make the text seem clumsy.

Variable: default-fill-column

The value of this variable is the default value for fill-column in buffers that do not override it. This is the same as (default-value 'fill-column).

The default value for default-fill-column is 70.

1.12 Auto Filling

Filling breaks text into lines that are no more than a specified number of columns wide. Filled lines end between words, and therefore may have to be shorter than the maximum width.

Auto Fill mode is a minor mode in which Emacs fills lines automatically as text as inserted. This section describes the hook and the two variables used by Auto Fill mode. For a description of functions that you can call manually to fill and justify text, see Filling.

Variable: auto-fill-function

The value of this variable should be a function (of no arguments) to be called after self-inserting a space at a column beyond fill-column. It may be nil, in which case nothing special is done.

The default value for auto-fill-function is do-auto-fill, a function whose sole purpose is to implement the usual strategy for breaking a line.

In older Emacs versions, this variable was named auto-fill-hook, but since it is not called with the standard convention for hooks, it was renamed to auto-fill-function in version 19.

1.13 Sorting Text

The sorting commands described in this section all rearrange text in a buffer. This is in contrast to the function sort, which rearranges the order of the elements of a list (@pxref{Rearrangement}). The values returned by these commands are not meaningful.

Command: sort-regexp-fields reverse record-regexp key-regexp start end

This command sorts the region between start and end alphabetically as specified by record-regexp and key-regexp. If reverse is a negative integer, then sorting is in reverse order.

Alphabetical sorting means that two sort keys are compared by comparing the first characters of each, the second characters of each, and so on. If a mismatch is found, it means that the sort keys are unequal; the sort key whose character is less at the point of first mismatch is the lesser sort key. The individual characters are compared according to their numerical values. Since Emacs uses the ASCII character set, the ordering in that set determines alphabetical order.

The value of the record-regexp argument specifies the textual units or records that should be sorted. At the end of each record, a search is done for this regular expression, and the text that matches it is the next record. For example, the regular expression ‘^.+$’, which matches lines with at least one character besides a newline, would make each such line into a sort record. @xref{Regular Expressions}, for a description of the syntax and meaning of regular expressions.

The value of the key-regexp argument specifies what part of each record is to be compared against the other records. The key-regexp could match the whole record, or only a part. In the latter case, the rest of the record has no effect on the sorted order of records, but it is carried along when the record moves to its new position.

The key-regexp argument can refer to the text matched by a subexpression of record-regexp, or it can be a regular expression on its own.

If key-regexp is:

‘\digit’

then the text matched by the digitth ‘\(...\)’ parenthesis grouping in record-regexp is used for sorting.

‘\&’

then the whole record is used for sorting.

a regular expression

then the function searches for a match for the regular expression within the record. If such a match is found, it is used for sorting. If a match for key-regexp is not found within a record then that record is ignored, which means its position in the buffer is not changed. (The other records may move around it.)

For example, if you plan to sort all the lines in the region by the first word on each line starting with the letter ‘f’, you should set record-regexp to ‘^.*$’ and set key-regexp to ‘\<f\w*\>’. The resulting expression looks like this:

(sort-regexp-fields nil "^.*$" "\\<f\\w*\\>"
                    (region-beginning)
                    (region-end))

If you call sort-regexp-fields interactively, you are prompted for record-regexp and key-regexp in the minibuffer.

Command: sort-subr reverse nextrecfun endrecfun &optional startkeyfun endkeyfun

This command is the general text sorting routine that divides a buffer into records and sorts them. The functions sort-lines, sort-paragraphs, sort-pages, sort-fields, sort-regexp-fields and sort-numeric-fields all use sort-subr.

To understand how sort-subr works, consider the whole accessible portion of the buffer as being divided into disjoint pieces called sort records. A portion of each sort record (perhaps all of it) is designated as the sort key. The records are rearranged in the buffer in order by their sort keys. The records may or may not be contiguous.

Usually, the records are rearranged in order of ascending sort key. If the first argument to the sort-subr function, reverse, is non-nil, the sort records are rearranged in order of descending sort key.

The next four arguments to sort-subr are functions that are called to move point across a sort record. They are called many times from within sort-subr.

1. nextrecfun is called with point at the end of a record. This function moves point to the start of the next record. The first record is assumed to start at the position of point when sort-subr is called. (Therefore, you should usually move point to the beginning of the buffer before calling sort-subr.)

   This function can indicate there are no more sort records by leaving point at the end of the buffer.

2. endrecfun is called with point within a record. It moves point to the end of the record.
3. startkeyfun is called to move point from the start of a record to the start of the sort key. This argument is optional. If supplied, the function should either return a non-nil value to be used as the sort key, or return nil to indicate that the sort key is in the buffer starting at point. In the latter case, endkeyfun is called to find the end of the sort key.
4. endkeyfun is called to move point from the start of the sort key to the end of the sort key. This argument is optional. If startkeyfun returns nil and this argument is omitted (or nil), then the sort key extends to the end of the record. There is no need for endkeyfun if startkeyfun returns a non-nil value.

As an example of sort-subr, here is the complete function definition for sort-lines:

;; Note that the first two lines of doc string
;; are effectively one line when viewed by a user.
(defun sort-lines (reverse beg end)
  "Sort lines in region alphabetically;\
 argument means descending order.
Called from a program, there are three arguments:

REVERSE (non-nil means reverse order),
and BEG and END (the region to sort)."
  (interactive "P\nr")
  (save-restriction
    (narrow-to-region beg end)
    (goto-char (point-min))
    (sort-subr reverse
               'forward-line
               'end-of-line)))

Here forward-line moves point to the start of the next record, and end-of-line moves point to the end of record. We do not pass the arguments startkeyfun and endkeyfun, because the entire record is used as the sort key.

The sort-paragraphs function is very much the same, except that its sort-subr call looks like this:

(sort-subr reverse
           (function 
            (lambda () 
              (skip-chars-forward "\n \t\f")))
           'forward-paragraph)

Command: sort-lines reverse start end

This command sorts lines in the region between start and end alphabetically. If reverse is non-nil, the sort is in reverse order.

Command: sort-paragraphs reverse start end

This command sorts paragraphs in the region between start and end alphabetically. If reverse is non-nil, the sort is in reverse order.

Command: sort-pages reverse start end

This command sorts pages in the region between start and end alphabetically. If reverse is non-nil, the sort is in reverse order.

Command: sort-fields field start end

This command sorts lines in the region between start and end, comparing them alphabetically by the fieldth field of each line. Fields are separated by whitespace and numbered starting from 1. If field is negative, sorting is by the -fieldth field from the end of the line. This command is useful for sorting tables.

Command: sort-numeric-fields field start end

This command sorts lines in the region between start and end, comparing them numerically by the fieldth field of each line. Fields are separated by whitespace and numbered starting from 1. The specified field must contain a number in each line of the region. If field is negative, sorting is by the -fieldth field from the end of the line. This command is useful for sorting tables.

Command: sort-columns reverse &optional beg end

This command sorts the lines in the region between beg and end, comparing them alphabetically by a certain range of columns. The column positions of beg and end bound the range of columns to sort on.

If reverse is non-nil, the sort is in reverse order.

One unusual thing about this command is that the entire line containing position beg, and the entire line containing position end, are included in the region sorted.

Note that sort-columns uses the sort utility program, and so cannot work properly on text containing tab characters. Use M-x untabify to convert tabs to spaces before sorting.

The sort-columns function did not work on VMS prior to Emacs 19.

1.14 Indentation

The indentation functions are used to examine, move to, and change whitespace that is at the beginning of a line. Some of the functions can also change whitespace elsewhere on a line. Indentation always counts from zero at the left margin.

1.14.1 Indentation Primitives

This section describes the primitive functions used to count and insert indentation. The functions in the following sections use these primitives.

Function: current-indentation

This function returns the indentation of the current line, which is the horizontal position of the first nonblank character. If the contents are entirely blank, then this is the horizontal position of the end of the line.

Command: indent-to column &optional minimum

This function indents from point with tabs and spaces until column is reached. If minimum is specified and non-nil, then at least that many spaces are inserted even if this requires going beyond column. The value is the column at which the inserted indentation ends.

User Option: indent-tabs-mode

If this variable is non-nil, indentation functions can insert tabs as well as spaces. Otherwise, they insert only spaces. Setting this variable automatically makes it local to the current buffer.

1.14.2 Indentation Controlled by Major Mode

An important function of each major mode is to customize the <TAB> key to indent properly for the language being edited. This section describes the mechanism of the <TAB> key and how to control it. The functions in this section return unpredictable values.

Variable: indent-line-function

This variable’s value is the function to be used by <TAB> (and various commands) to indent the current line. The command indent-according-to-mode does no more than call this function.

In Lisp mode, the value is the symbol lisp-indent-line; in C mode, c-indent-line; in Fortran mode, fortran-indent-line. In Fundamental mode, Text mode, and many other modes with no standard for indentation, the value is indent-to-left-margin (which is the default value).

Command: indent-according-to-mode

This command calls the function in indent-line-function to indent the current line in a way appropriate for the current major mode.

Command: indent-for-tab-command

This command calls the function in indent-line-function to indent the current line, except that if that function is indent-to-left-margin, insert-tab is called instead. (That is a trivial command which inserts a tab character.)

Variable: left-margin

This variable is the column to which the default indent-line-function will indent. (That function is indent-to-left-margin.) In Fundamental mode, <LFD> indents to this column. This variable automatically becomes buffer-local when set in any fashion.

Function: indent-to-left-margin

This is the default indent-line-function, used in Fundamental mode, Text mode, etc. Its effect is to adjust the indentation at the beginning of the current line to the value specified by the variable left-margin. This may involve either inserting or deleting whitespace.

Command: newline-and-indent

This function inserts a newline, then indents the new line (the one following the newline just inserted) according to the major mode.

Indentation is done using the current indent-line-function. In programming language modes, this is the same thing <TAB> does, but in some text modes, where <TAB> inserts a tab, newline-and-indent indents to the column specified by left-margin.

Command: reindent-then-newline-and-indent

This command reindents the current line, inserts a newline at point, and then reindents the new line (the one following the newline just inserted).

Indentation of both lines is done according to the current major mode; this means that the current value of indent-line-function is called. In programming language modes, this is the same thing <TAB> does, but in some text modes, where <TAB> inserts a tab, reindent-then-newline-and-indent indents to the column specified by left-margin.

1.14.3 Indenting an Entire Region

This section describes commands which indent all the lines in the region. They return unpredictable values.

Command: indent-region start end to-column

This command indents each nonblank line starting between start (inclusive) and end (exclusive). If to-column is nil, indent-region indents each nonblank line by calling the current mode’s indentation function, the value of indent-line-function.

If to-column is non-nil, it should be an integer specifying the number of columns of indentation; then this function gives each line exactly that much indentation, by either adding or deleting whitespace.

If there is a fill prefix, indent-region indents each line by making it start with the fill prefix.

Variable: indent-region-function

The value of this variable is a function that can be used by indent-region as a short cut. You should design the function so that it will produce the same results as indenting the lines of the region one by one (but presumably faster).

If the value is nil, there is no short cut, and indent-region actually works line by line.

A short cut function is useful in modes such as C mode and Lisp mode, where the indent-line-function must scan from the beginning of the function: applying it to each line would be quadratic in time. The short cut can update the scan information as it moves through the lines indenting them; this takes linear time. If indenting a line individually is fast, there is no need for a short cut.

indent-region with a non-nil argument has a different definition and does not use this variable.

Command: indent-rigidly start end count

This command indents all lines starting between start (inclusive) and end (exclusive) sideways by count columns. This “preserves the shape” of the affected region, moving it as a rigid unit. Consequently, this command is useful not only for indenting regions of unindented text, but also for indenting regions of formatted code.

For example, if count is 3, this command adds 3 columns of indentation to each of the lines beginning in the region specified.

In Mail mode, C-c C-y (mail-yank-original) uses indent-rigidly to indent the text copied from the message being replied to.

Function: indent-code-rigidly start end columns &optional nochange-regexp

This is like indent-rigidly, except that it doesn’t alter lines that start within strings or comments.

In addition, it doesn’t alter a line if nochange-regexp matches at the beginning of the line (if nochange-regexp is non-nil).

1.14.4 Indentation Relative to Previous Lines

This section describes two commands which indent the current line based on the contents of previous lines.

Command: indent-relative &optional unindented-ok

This function inserts whitespace at point, extending to the same column as the next indent point of the previous nonblank line. An indent point is a non-whitespace character following whitespace. The next indent point is the first one at a column greater than the current column of point. For example, if point is underneath and to the left of the first non-blank character of a line of text, it moves to that column by inserting whitespace.

If the previous nonblank line has no next indent point (i.e., none at a great enough column position), this function either does nothing (if unindented-ok is non-nil) or calls tab-to-tab-stop. Thus, if point is underneath and to the right of the last column of a short line of text, this function moves point to the next tab stop by inserting whitespace.

This command returns an unpredictable value.

In the following example, point is at the beginning of the second line:

            This line is indented twelve spaces.
∗The quick brown fox jumped.

Evaluation of the expression (indent-relative nil) produces the following:

            This line is indented twelve spaces.
            ∗The quick brown fox jumped.

In this example, point is between the ‘m’ and ‘p’ of ‘jumped’:

            This line is indented twelve spaces.
The quick brown fox jum∗ped.

Evaluation of the expression (indent-relative nil) produces the following:

            This line is indented twelve spaces.
The quick brown fox jum  ∗ped.

Command: indent-relative-maybe

This command indents the current line like the previous nonblank line. The function consists of a call to indent-relative with a non-nil value passed to the unindented-ok optional argument. The value is unpredictable.

If the previous line has no indentation, the current line is given no indentation (any existing indentation is deleted); if the previous nonblank line has no indent points beyond the column at which point starts, nothing is changed.

1.14.5 Adjustable “Tab Stops”

This section explains the mechanism for user-specified “tab stops” and the mechanisms which use and set them. The name “tab stops” is used because the feature is similar to that of the tab stops on a typewriter. The feature works by inserting an appropriate number of spaces and tab characters to reach the designated position, like the other indentation functions; it does not affect the display of tab characters in the buffer (@pxref{Usual Display}). Note that the <TAB> character as input uses this tab stop feature only in a few major modes, such as Text mode.

Function: tab-to-tab-stop

This function inserts spaces or tabs up to the next tab stop column defined by tab-stop-list. It searches the list for an element greater than the current column number, and uses that element as the column to indent to. If no such element is found, then nothing is done.

User Option: tab-stop-list

This variable is the list of tab stop columns used by tab-to-tab-stops. The elements should be integers in increasing order. The tab stop columns need not be evenly spaced.

Use M-x edit-tab-stops to edit the location of tab stops interactively.

1.14.6 Indentation-Based Motion Commands

These commands, primarily for interactive use, act based on the indentation in the text.

Command: back-to-indentation

This command moves point to the first non-whitespace character in the current line (which is the line in which point is located). It returns nil.

Command: backward-to-indentation arg

This command moves point backward arg lines and then to the first nonblank character on that line. It returns nil.

Command: forward-to-indentation arg

This command moves point forward arg lines and then to the first nonblank character on that line. It returns nil.

1.15 Counting Columns

The column functions convert between a character position (counting characters from the beginning of the buffer) and a column position (counting screen characters from the beginning of a line).

Column number computations ignore the width of the window and the amount of horizontal scrolling. Consequently, a column value can be arbitrarily high. The first (or leftmost) column is numbered 0.

A character counts according to the number of columns it occupies on the screen. This means control characters count as occupying 2 or 4 columns, depending upon the value of ctl-arrow, and tabs count as occupying a number of columns that depends on the value of tab-width and on the column where the tab begins. @xref{Usual Display}.

Function: current-column

This function returns the horizontal position of point, measured in columns, counting from 0 at the left margin. The column count is calculated by adding together the widths of all the displayed representations of the characters between the start of the current line and point.

For a more complicated example of the use of current-column, see the description of count-lines in @ref{Text Lines}.

Function: move-to-column column &optional force

This function moves point to column in the current line. The calculation of column takes into account the widths of all the displayed representations of the characters between the start of the line and point.

If the argument column is greater than the column position of the end of the line, point moves to the end of the line. If column is negative, point moves to the beginning of the line.

If it is impossible to move to column column because that is in the middle of a multicolumn character such as a tab, point moves to the end of that character. However, if force is non-nil, and column is in the middle of a tab, then move-to-column converts the tab into spaces so that it can move precisely to column column.

The argument force also has an effect if the line isn’t long enough to reach column column; in that case, it says to indent at the end of the line to reach that column.

If column is not an integer, an error is signaled.

The return value is the column number actually moved to.

1.16 Case Changes

The case change commands described here work on text in the current buffer. @xref{Character Case}, for case conversion commands that work on strings and characters. @xref{Case Table}, for how to customize which characters are upper or lower case and how to convert them.

Command: capitalize-region start end

This function capitalizes all words in the region defined by start and end. To capitalize means to convert each word’s first character to upper case and convert the rest of each word to lower case. The function returns nil.

If one end of the region is in the middle of a word, the part of the word within the region is treated as an entire word.

When capitalize-region is called interactively, start and end are point and the mark, with the smallest first.

---------- Buffer: foo ----------
This is the contents of the 5th foo.
---------- Buffer: foo ----------

(capitalize-region 1 44)
⇒ nil

---------- Buffer: foo ----------
This Is The Contents Of The 5th Foo.
---------- Buffer: foo ----------

Command: downcase-region start end

This function converts all of the letters in the region defined by start and end to lower case. The function returns nil.

When downcase-region is called interactively, start and end are point and the mark, with the smallest first.

Command: upcase-region start end

This function converts all of the letters in the region defined by start and end to upper case. The function returns nil.

When upcase-region is called interactively, start and end are point and the mark, with the smallest first.

Command: capitalize-word count

This function capitalizes count words after point, moving point over as it does. To capitalize means to convert each word’s first character to upper case and convert the rest of each word to lower case. If count is negative, the function capitalizes the -count previous words but does not move point. The value is nil.

If point is in the middle of a word, the part of word the before point (if moving forward) or after point (if operating backward) is ignored. The rest is treated as an entire word.

When capitalize-word is called interactively, count is set to the numeric prefix argument.

Command: downcase-word count

This function converts the count words after point to all lower case, moving point over as it does. If count is negative, it converts the -count previous words but does not move point. The value is nil.

When downcase-word is called interactively, count is set to the numeric prefix argument.

Command: upcase-word count

This function converts the count words after point to all upper case, moving point over as it does. If count is negative, it converts the -count previous words but does not move point. The value is nil.

When upcase-word is called interactively, count is set to the numeric prefix argument.

1.17 Text Properties

Each character position in a buffer or a string can have a text property list, much like the property list of a symbol. The properties belong to a particular character at a particular place, such as, the letter ‘T’ at the beginning of this sentence or the first ‘o’ in ‘foo’—if the same character occurs in two different places, the two occurrences generally have different properties.

Each property has a name, which is usually a symbol, and an associated value, which can be any Lisp object—just as for properties of symbols (@pxref{Property Lists}).

If a character has a category property, we call it the category of the character. It should be a symbol. The properties of the symbol serve as defaults for the properties of the character.

Copying text between strings and buffers preserves the properties along with the characters; this includes such diverse functions as substring, insert, and buffer-substring.

1.17.1 Examining Text Properties

The simplest way to examine text properties is to ask for the value of a particular property of a particular character. For that, use get-text-property. Use text-properties-at to get the entire property list of a character. See section Property Search Functions, for functions to examine the properties of a number of characters at once.

These functions handle both strings and buffers. Keep in mind that positions in a string start from 0, whereas positions in a buffer start from 1.

Function: get-text-property pos prop &optional object

This function returns the value of the prop property of the character after position pos in object (a buffer or string). The argument object is optional and defaults to the current buffer.

If there is no prop property strictly speaking, but the character has a category which is a symbol, then get-text-property returns the prop property of that symbol.

Function: text-properties-at position &optional object

This function returns the list of properties held by the character at position in the string or buffer object. If object is nil, it defaults to the current buffer.

1.17.2 Changing Text Properties

The primitives for changing properties apply to a specified range of text. The function set-text-properties (see end of section) sets the entire property list of the text in that range; more often, it is useful to add, change, or delete just certain properties specified by name.

Since text properties are considered part of the buffer’s contents, and can affect how the buffer looks on the screen, any change in the text properties is considered a buffer modification. Buffer text property changes are undoable.

Function: add-text-properties start end props &optional object

This function modifies the text properties for the text between start and end in the string or buffer object. If object is nil, it defaults to the current buffer.

The argument props specifies which properties to change. It should have the form of a property list (@pxref{Property Lists}): a list whose elements include the property names followed alternately by the corresponding values.

The return value is t if the function actually changed some property’s value; nil otherwise (if props is nil or its values agree with those in the text).

For example, here is how to set the comment property to t for a range of text:

(add-text-properties (region-beginning)
                     (region-end)
                     (list 'comment t))

Function: put-text-property start end prop value &optional object

This function sets the prop property to value for the text between start and end in the string or buffer object. If object is nil, it defaults to the current buffer.

Function: remove-text-properties start end props &optional object

This function deletes specified text properties from the text between start and end in the string or buffer object. If object is nil, it defaults to the current buffer.

The argument props specifies which properties to delete. It should have the form of a property list (@pxref{Property Lists}): a list whose elements include the property names followed by the corresponding values. The property names mentioned in props are the ones deleted from the text. The values associated in props with these names do not matter.

The return value is t if the function actually changed some property’s value; nil otherwise (if props is nil or if none of the text had any of those properties).

Function: set-text-properties start end props &optional object

This function completely replaces the text property list for the text between start and end in the string or buffer object. If object is nil, it defaults to the current buffer.

The argument props is the new property list. It should have the form of a list whose elements include the property names followed by the corresponding values.

After set-text-properties returns, all the characters in the specified range have identical properties.

If props is nil, the effect is to get rid of all properties from the specified range of text. Here’s an example:

(set-text-properties (region-beginning)
                     (region-end)
                     nil)

1.17.3 Property Search Functions

In typical use of text properties, most of the time several or many consecutive characters have the same value for a property. Rather than writing your programs to examine characters one by one, it is much faster to process chunks of text that have the same property value.

Here are functions you can use to do this. In all cases, object defaults to the current buffer.

Function: next-property-change pos &optional object

The function scans the text forward from position pos in the string or buffer object till it finds a change in some text property, then returns the position of the change. In other words, it returns the position of the first character beyond pos whose properties are not identical to those of the character just after pos.

The value is nil if the properties remain unchanged all the way to the end of object. If the value is non-nil, it is a position greater than pos, never equal.

Here is an example of how to scan the buffer by chunks of text within which all properties are constant:

(while (not (eobp))
  (let ((plist (text-properties-at (point)))
        (next-change
         (or (next-property-change (point) (current-buffer))
             (point-max))))
    Process text from point to next-change…
    (goto-char next-change)))

Function: next-single-property-change pos prop &optional object

The function scans the text forward from position pos in the string or buffer object till it finds a change in the prop property, then returns the position of the change. In other words, it returns the position of the first character beyond pos whose prop property differs from that of the character just after pos.

The value is nil if the properties remain unchanged all the way to the end of object. If the value is non-nil, it is a position greater than pos, never equal.

Function: previous-property-change pos &optional object

This is like next-property-change, but scans back from pos instead of forward. If the value is non-nil, it is a position always strictly less than pos.

Function: previous-single-property-change pos prop &optional object

This is like next-property-change, but scans back from pos instead of forward. If the value is non-nil, it is a position always strictly less than pos.

1.17.4 Special Properties

If a character has a category property, we call it the category of the character. It should be a symbol. The properties of the symbol serve as defaults for the properties of the character.

You can use the property face to control the font and color of text. @xref{Faces}, for more information. This feature is temporary; in the future, we may replace it with other ways of specifying how to display text.

The property mouse-face is used instead of face when the mouse is on or near the character. For this purpose, “near” means that all text between the character and where the mouse is have the same mouse-face property value.

You can specify a different keymap for a portion of the text by means of a local-map property. The property’s value, for the character after point, replaces the buffer’s local map. @xref{Active Keymaps}.

If a character has the property read-only, then modifying that character is not allowed. Any command that would do so gets an error.

If a character has the property modification-hooks, then its value should be a list of functions; modifying that character calls all of those functions. Each function receives two arguments: the beginning and end of the part of the buffer being modified. Note that if a particular modification hook function appears on several characters being modified by a single primitive, you can’t predict how many times the function will be called.

Insertion of text does not, strictly speaking, change any existing character, so there is a special rule for insertion. It compares the read-only properties of the two surrounding characters; if they are non-nil and eq to each other, then the insertion is not allowed. Assuming insertion is allowed, it then gets the modification-hooks properties of those characters and calls all the functions in each of them. (If a function appears on both characters, it may be called once or twice.)

See also Change Hooks, for other hooks that are called when you change text in a buffer.

The special properties point-entered and point-left record hook functions that report motion of point. Each time point moves, Emacs compares these two property values:

• the point-left property of the character after the old location, and
• the point-entered property of the character after the new location.

If these two values differ, each of them is called (if not nil) with two arguments: the old value of point, and the new one.

The same comparison is made for the characters before the old and new locations. The result may be to execute two point-left functions (which may be the same function) and/or two point-entered functions (which may be the same function). The point-left functions are always called before the point-entered functions.

A primitive function may examine characters at various positions without moving point to those positions. Only an actual change in the value of point runs these hook functions.

1.17.5 Why Text Properties are not Intervals

Some editors that support adding attributes to text in the buffer do so by letting the user specify “intervals” within the text, and adding the properties to the intervals. Those editors permit the user or the programmer to determine where individual intervals start and end. We deliberately provided a different sort of interface in Emacs Lisp to avoid certain paradoxical behavior associated with text modification.

If the actual subdivision into intervals is meaningful, that means you can distinguish between a buffer that is just one interval with a certain property, and a buffer containing the same text subdivided into two intervals, both of which have that property.

Suppose you take the buffer with just one interval and kill part of the text. The text remaining in the buffer is one interval, and the copy in the kill ring (and the undo list) becomes a separate interval. Then if you undo the kill, you get two intervals with the same properties. Thus, the distinction can’t be preserved when editing happens.

But suppose we “fix” this problem by coalescing the two intervals when the text is inserted. That works fine if the buffer originally was a single interval. But if it was two intervals, and the killed text equals one of them, then undoing the kill yields just one interval. Again, the distinction can’t be preserved.

Insertion of text at the border between intervals also raises questions that have no satisfactory answer.

However, it is easy to arrange for editing to behave consistently for questions of the form, “What are the properties of this character?” So we have decided these are the only questions that make sense; we have not implemented asking questions about where intervals start or end.

For practical purposes, the property search functions serve in place of explicit interval boundaries. You can think of them as finding the boundaries of intervals, assuming that intervals are always coalesced whenever possible. See section Property Search Functions.

Emacs also provides explicit intervals as a presentation feature; see @ref{Overlays}.

1.18 Substituting for a Character Code

The following functions replace characters within a specified region based on their character codes.

Function: subst-char-in-region start end old-char new-char &optional noundo

This function replaces all occurrences of the character old-char with the character new-char in the region of the current buffer defined by start and end.

If noundo is non-nil, then subst-char-in-region does not record the change for undo and does not mark the buffer as modified. This feature is useful for changes which are not considered significant, such as when Outline mode changes visible lines to invisible lines and vice versa.

subst-char-in-region does not move point and returns nil.

---------- Buffer: foo ----------
This is the contents of the buffer before.
---------- Buffer: foo ----------

(subst-char-in-region 1 20 ?i ?X)
     ⇒ nil

---------- Buffer: foo ----------
ThXs Xs the contents of the buffer before.
---------- Buffer: foo ----------

Function: translate-region start end table

This function applies a translation table to the characters in the buffer between positions start and end.

The translation table table is a string; (aref table ochar) gives the translated character corresponding to ochar. If the length of table is less than 256, any characters with codes larger than the length of table are not altered by the translation.

The return value of translate-region is the number of characters which were actually changed by the translation. This does not count characters which were mapped into themselves in the translation table.

This function is available in Emacs versions 19 and later.

1.19 Underlining

The underlining commands are somewhat obsolete. The underline-region function actually inserts ‘_^H’ before each appropriate character in the region. This command provides a minimal text formatting feature that might work on your printer; however, we recommend instead that you use more powerful text formatting facilities, such as Texinfo.

Command: underline-region start end

This function underlines all nonblank characters in the region defined by start and end. That is, an underscore character and a backspace character are inserted just before each non-whitespace character in the region. The backspace characters are intended to cause overstriking, but in Emacs they display as either ‘\010’ or ‘^H’, depending on the setting of ctl-arrow. There is no way to see the effect of the overstriking within Emacs. The value is nil.

Command: ununderline-region start end

This function removes all underlining (overstruck underscores) in the region defined by start and end. The value is nil.

1.20 Registers

A register is a sort of variable used in Emacs editing that can hold a marker, a string, a rectangle, a window configuration (of one frame), or a frame configuration (of all frames). Each register is named by a single character. All characters, including control and meta characters (but with the exception of C-g), can be used to name registers. Thus, there are 255 possible registers. A register is designated in Emacs Lisp by a character which is its name.

The functions in this section return unpredictable values unless otherwise stated.

Variable: register-alist

This variable is an alist of elements of the form (name . contents). Normally, there is one element for each Emacs register that has been used.

The object name is a character (an integer) identifying the register. The object contents is a string, marker, or list representing the register contents. A string represents text stored in the register. A marker represents a position. A list represents a rectangle; its elements are strings, one per line of the rectangle.

Command: view-register reg

This command displays what is contained in register reg.

Function: get-register reg

This function returns the contents of the register reg, or nil if it has no contents.

Function: set-register reg value

This function sets the contents of register reg to value. A register can be set to any value, but the other register functions expect only certain data types. The return value is value.

Command: point-to-register reg

This command stores both the current location of point and the current buffer in register reg as a marker.

Command: jump-to-register reg

Command: register-to-point reg

This command restores the status recorded in register reg.

If reg contains a marker, it moves point to the position stored in the marker. Since both the buffer and the location within the buffer are stored by the point-to-register function, this command can switch you to another buffer.

If reg contains a window configuration or a frame configuration. jump-to-register restores that configuration.

Command: insert-register reg &optional beforep

This command inserts contents of register reg into the current buffer.

Normally, this command puts point before the inserted text, and the mark after it. However, if the optional second argument beforep is non-nil, it puts the mark before and point after. You can pass a non-nil second argument beforep to this function interactively by supplying any prefix argument.

If the register contains a rectangle, then the rectangle is inserted with its upper left corner at point. This means that text is inserted in the current line and underneath it on successive lines.

If the register contains something other than saved text (a string) or a rectangle (a list), currently useless things happen. This may be changed in the future.

Command: copy-to-register reg start end &optional delete-flag

This command copies the region from start to end into register reg. If delete-flag is non-nil, it deletes the region from the buffer after copying it into the register.

Command: prepend-to-register reg start end &optional delete-flag

This command prepends the region from start to end into register reg. If delete-flag is non-nil, it deletes the region from the buffer after copying it to the register.

Command: append-to-register reg start end &optional delete-flag

This command appends the region from start to end to the text already in register reg. If delete-flag is non-nil, it deletes the region from the buffer after copying it to the register.

Command: copy-rectangle-to-register reg start end &optional delete-flag

This command copies a rectangular region from start to end into register reg. If delete-flag is non-nil, it deletes the region from the buffer after copying it to the register.

Command: window-configuration-to-register reg

This function stores the window configuration of the selected frame in register reg.

Command: frame-configuration-to-register reg

This function stores the current frame configuration in register reg.

1.21 Change Hooks

These hook variables let you arrange to take notice of all changes in all buffers (or in a particular buffer, if you make them buffer-local). See also Special Properties, for how to detect changes to specific parts of the text.

Variable: before-change-function

If this variable is non-nil, then it should be a function; the function is called before any buffer modification. Its arguments are the beginning and end of the region that is going to change, represented as integers. The buffer that’s about to change is always the current buffer.

Variable: after-change-function

If this variable is non-nil, then it should be a function; the function is called after any buffer modification. It receives three arguments: the beginning and end of the region just changed, and the length of the text that existed before the change. (To get the current length, subtract the region beginning from the region end.) All three arguments are integers. The buffer that’s about to change is always the current buffer.

Both of these variables are temporarily bound to nil during the time that either of these hooks is running. This means that if one of these functions changes the buffer, that change won’t run these functions. If you do want the hook function to be run recursively, write your hook functions to bind these variables back to their usual values.

Variable: first-change-hook

This variable is a normal hook; its hook functions are run using run-hooks whenever a buffer is changed that was previously in the unmodified state.

The variables described in this section are meaningful only starting with Emacs version 19.

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