Elisp: Text Properties

By Xah Lee. Date: . Last updated: .

Text in a buffer can have “text properties”.

Text properties is used to color text, or add keybinding to that text (for example, pressing Enter on a link in HTML opens the link instead of insert newline.), and other purposes.

Here's a command that makes a region red, by adding a text property of 'font-lock-face to it.

(defun x-make-word-red (begin end)
  "make current region colored red, using text properties"
  (interactive (list (region-beginning) (region-end)))
  (put-text-property begin end 'font-lock-face '(:foreground "red")))

Select a region, then Alt+x x-make-word-red

Here's example of adding a keymap to text, so that when Enter is pressed, it tries to open the file, taking current line as file name/path.

(setq x-keymap (make-sparse-keymap))
(define-key x-keymap (kbd "RET") (lambda () (find-file (thing-at-point 'filename) )))

(defun x-add-prop (begin end)
  "add text properties to a region."
  (interactive "r")
    (put-text-property begin end 'font-lock-face '(:foreground "blue"))
    (put-text-property begin end 'keymap x-keymap))

try it:

  1. Open a new file, insert some text that contains file path.
  2. Go to a line that's a file path, select it, then Alt+x x-add-prop. The region will be colored blue, and pressing Enter key on it will call x-open-me

What is Text Property

Text Property is a property list attached to text.

A property list is a list but interpreted as key and value pairs. Like this:

'(key1 val1 key2 val2 )

[see Elisp: Property List]

When a property list is attached to text, some keys have special meaning that are predefined by emacs.

Here are some of the most useful property keys for writing a major/minor mode:

For syntax coloring. Value is a face. [see Elisp: How to Define Face]
For syntax coloring. Value is a face. It overrides the 'font-lock-face property. This property is automatically updated/managed by font-lock-mode. [see Elisp: Font Lock Mode Basics]
Add syntax table to text. Value is a syntax table. This is useful for example, in a major mode for programing language, you want to make regex delimiter character such as SOLIDUS / to have string syntax, so regex string are colored as string. [see Elisp: Syntax Table]
Add keybinding to text. Value is a keymap. When cursor is on the text, the keymap has priority in determining keystroke action. [see Elisp: How to Create Keymap for Major Mode]

For a complete list, see (info "(elisp) Special Properties")

Functions for Working with Text Property

There are many functions to get values of text properties, add/change/delete them, or find out where a property begins, etc. See

(info "(elisp) Text Properties")

Video Tutorial

XahTV 2020-08-21 emacs lisp, write command to highlight text

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