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Overview to Text-Editing Programing In Emacs

Xah Lee, 2005-10, 2006, 2009-11-19

In emacs, a user can program it using the embedded language (called Emacs Lisp, or elisp) so that he can have custome functions to insert texts, templates, process files, and many other features of emacs. This page gives a overview of how this environment works. If you don't know elisp, first take a look at Emacs Lisp Basics.

Emacs provides you with a text-edit programing framework. More specifically, a set of functions for text editing and manipulation.

For example, there is a lisp function that returns the cursor position in a buffer. A function that returns the beginning (or ending) position of the current selected text (aka active region). Functions that move the cursor to a given position, or delete a block of text of given beginning and ending positions. Functions that insert a string at a given position. Functions that open or save files. Functions that list opened files. Functions that list buffer names. Function that returns the current mode of a given buffer. Functions that activate modes. Functions that make a section of text into a particular color or font, ... and much more.

So, programing emacs to manipulate text, is a matter of calling these functions provided in Emacs.

Example of Simple Elisp Functions

Here are some examples of simple elisp functions.

Cursor Position

; current cursor position is called “point”.
; The first char in buffer is 1
; This returns the current cursor position
(point)

; returns the position of the beginning/end of region
(region-beginning)
(region-end)

; returns the position for the end of buffer
; (taking account of narrow-to-region)
(point-max) ; (there's also point-min)

; absolute end position of buffer
; regardless of narrow-to-region
(buffer-end 1)

Moving Cursor and Searching

; move cursor to position 392
(goto-char 392)

; move cursor by n chars
(forward-char n)
(backward-char n)

; move cursor to the first char that's not a newline or tab
; Returns the distance traveled
(skip-chars-forward "\n\t")
(skip-chars-backward "\n\t")

; move cursor to the location of myStr
; returns the new position
(search-forward myStr)
(search-backward myStr)

; move cursor to the location matched by a regex
; returns the new position
(re-search-forward myRegex)
(re-search-backward myRegex)

Text Editing

; delete 9 chars starting at current cursor pos
(delete-char 9)

; deleting text
(delete-region myStartPos myEndPos)

; insert string at current cursor position
(insert "hi i ♥ u.")

; get the string from buffer
(setq myStr (buffer-substring myStartPos myEndPos))

; change case
(capitalize-region myStartPos myEndPos)

Strings

; length
(length "abc") ; returns 3

; gets a substring
(substring myStr startIndex endIndex)

; change a given string using regex
(replace-regexp-in-string myRegex myReplacement myStr)

Buffers

; return the name of current buffer
(buffer-name)

; return the full path of current file
(buffer-file-name)

; switch to the buffer named myBufferName
(set-buffer myBufferName)

; save current buffer
(save-buffer)

; close a buffer
(kill-buffer myBuffName)

; close the current buffer
(kill-this-buffer)

; temporarily sets a buffer as current to work with
(with-current-buffer myBuffer
  ;; do something here ...
)

Files

; open a file (in a buffer)
(find-file myPath)

; same as “Save As”.
; close current buffer and open the new saved
(write-file myPath)

; insert file into current position
(insert-file-contents myPath)

; append a text block to file
(append-to-file myStartPos myEndPos myPath)

; renaming file
(rename-file fileName newName)

; copying file
(copy-file oldName newName)

; deleting file
(delete-file fileName)

; get dir path
(file-name-directory myFullPath)

; get filename part
(file-name-nondirectory myFullPath)

; get filename's suffix
(file-name-extension myFileName)

; get filename sans suffix
(file-name-sans-extension myFileName)

A Simple Example

This code illustrates how to insert a string, then position cursor somewhere inside.

(defun insert-p-tag ()
  "Insert <p></p> at cursor point."
  (interactive)
  (insert "<p></p>")
  (backward-char 4))

Type the above in any file, then select the whole code, type “Alt+x eval-region”. To execute the command, type “Alt+x insert-p-tag”.

For many simple elisp examples that illustrate how text manipulation programing is done, see Elisp Examples.

Programing A Major Mode

The above gave you a introduction of how to do text processing programing in elisp. In summary, it is done by calling functions in emacs, and gave you many examples of such functions that manipulate text and open save files.

The other area of elisp programing, is to program emacs itself, to create major or minor modes and their interface. For example, creating menus, interpreting keyboard inputs, changing behavior of mouse buttons, syntax coloring, manipulating windows, displaying image files, creating a file manager (dired), creating a interactive command line interface (shell), programing network clients (e.g. ftp, irc, http), creating user interface (menus, buttons, toolbars, status bar), creating keyword completion, etc. In summary, these type of tasks we can group it under “Programing A Major Mode”.

Tasks of writing a mode is slightly more complex, because it involves understanding many of emacs's systems. For example, keyboard/mouse input event system, display system (windows and fonts), user interface system (menu, windows, scroll bar, tool bar), major/minor mode structure.

For most people, text edit programing is far more useful to learn, because that's what text editor users need to do in their jobs, and most major modes have already been written for them.

For writing a major mode, you should have good experience of text edit programing in emacs, because it is simpler, and most major modes will need to manipulate text too. For those curious, see: How To Write A Emacs Major Mode For Syntax Coloring.