Changes in Basic Editing Functions

• There are two new primitives for putting text in the kill ring: kill-new and kill-append. The function kill-new adds a string to the front of the kill ring. Use kill-append to add a string to a previous kill. The second argument before-p, if non-nil, says to add the string at the beginning; otherwise, it goes at the end. Both of these functions apply interprogram-cut-function to the entire string of killed text that ends up at the beginning of the kill ring.
• The new 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, it doesn't actually move the yanking point; it just returns the nth kill forward. If n is zero, indicating a request for the latest kill, current-kill calls interprogram-paste-function (documented below) before consulting the kill ring. All Emacs Lisp programs should either use current-kill, kill-new, and kill-append to manipulate the kill ring, or be sure to call interprogram-paste-function and interprogram-cut-function as appropriate.
• The variables interprogram-paste-function and interprogram-cut-function exist so that you can provide functions to transfer killed text to and from other programs.
• The kill-region function can now be used in read-only buffers. It beeps, but adds the region to the kill ring without deleting it.
• The new function compare-buffer-substrings lets you compare two substrings of the same buffer or two different buffers. Its arguments look like this:

  (compare-buffer-substrings buf1 beg1 end1 buf2 beg2 end2)
  

  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. 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 different characters.
• Overwrite mode treats tab and newline characters specially. You can now turn off this special treatment by setting overwrite-binary-mode to t.
• Once the mark "exists" in a buffer, it normally never ceases to exist. However, in Transient Mark mode, it may become inactive. The variable mark-active, which is always local in all buffers, indicates whether the mark is active: non-nil means yes. When the mark is inactive, the function mark normally gets an error. However, (mark t) returns the position of the inactive mark. The function push-mark normally does not activate the mark. However, it accepts an optional third argument activate which, if non-nil, says to activate. A command can request deactivation of the mark upon return to the editor command loop by setting deactivate-mark to a non-nil value. Transient Mark mode works by causing the command loop to take note of deactivate-mark and actually deactivate the mark. Transient Mark mode enables highlighting of the region when the mark is active. This is currently implemented only under the X Window System. A few other commands vary their behavior slightly in this case, by testing transient-mark-mode. More specifically, they avoid special display actions such as moving the cursor temporarily, which are not needed when the region is shown by highlighting. The variables activate-mark-hook and deactivate-mark-hook are normal hooks run, respectively, when the mark becomes active and when it becomes inactive. The hook activate-mark-hook is also run at the end of a command if the mark is active and the region may have changed.
• The function move-to-column now accepts a second optional argument force, in addition to column; if the requested column column is in the middle of a tab character and force is non-nil, move-to-column replaces the tab with the appropriate sequence of spaces so that it can place point exactly at column.
• The search functions when successful now return the value of point rather than just t. This affects the functions search-forward, search-backward, word-search-forward, word-search-backward, re-search-forward, and re-search-backward.
• When you do regular expression searching or matching, there is no longer a limit to how many `\(...\)' pairs you can get information about with match-beginning and match-end. Also, these parenthetical groupings may now be nested to any degree.
• In a regular expression, when you use an asterisk after a parenthetical grouping, and then ask about what range was matched by the grouping, Emacs 19 reports just its last occurrence. Emacs 18 used to report the range of all the repetitions put together. For example,

  (progn
   (string-match "f\\(o\\)*" "foo")
   (list (match-beginning 1)
         (match-end 1)))
  

  returns (2 3) in Emacs 19, corresponding to just the last repetition of `\(o\)'. In Emacs 18, that expression returns (1 3), encompassing both repetitions. If you want the Emacs 18 behavior, use a grouping containing the asterisk: "f\\(o*\\)".
• The new special form save-match-data preserves the regular expression match status. Usage: (save-match-data body...).
• The function translate-region applies a translation table to the characters in a part of the buffer. Invoke it as (translate-region start end table); start and end bound the region to translate. 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. translate-region returns 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.
• There are two new hook variables that let you notice all changes in all buffers (or in a particular buffer, if you make them buffer-local): before-change-function and after-change-function. If before-change-function is non-nil, then it 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. If after-change-function is non-nil, then it is called after any buffer modification. It takes 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 has just changed 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 hooks to be run recursively, write your hook functions to bind these variables back to their usual values.
• The hook first-change-hook is run using run-hooks whenever a buffer is changed that was previously in the unmodified state.
• The second argument to insert-abbrev-table-description is now optional.