9.6.4 Active Regions

By default, both the text you select in an Emacs buffer using the click-and-drag mechanism and text you select by setting point and the mark is highlighted. You can use Emacs region commands as well as the Cut and Copy commands on the highlighted region you selected with the mouse.

If you prefer, you can make a distinction between text selected with the mouse and text selected with point and the mark by setting the variable zmacs-regions to nil. In that case:

• The text selected with the mouse becomes both the X selection and the Emacs selected region. You can use menu-bar commands as well as Emacs region commands on it.
• The text selected with point and the mark is not highlighted. You can only use Emacs region commands on it, not the menu-bar items.

Active regions originally come from Zmacs, the Lisp Machine editor. The idea behind them is that commands can only operate on a region when the region is in an "active" state. Put simply, you can only operate on a region that is highlighted.

The variable zmacs-regions checks whether LISPM-style active regions should be used. This means that commands that operate on the region (the area between point and the mark) only work while the region is in the active state, which is indicated by highlighting. Most commands causes the region to not be in the active state; for example, C-w only works immediately after activating the region.

More specifically:

• Commands that operate on the region only work if the region is active.
• Only a very small set of commands causes the region to become active— those commands whose semantics are to mark an area, such as mark-defun.
• The region is deactivated after each command that is executed, except that motion commands do not change whether the region is active or not.

set-mark-command (C-SPC) pushes a mark and activates the region. Moving the cursor with normal motion commands (C-n, C-p, etc.) will cause the region between point and the recently-pushed mark to be highlighted. It will remain highlighted until some non-motion command is executed.

exchange-point-and-mark (C-x C-x) activates the region. So if you mark a region and execute a command that operates on it, you can reactivate the same region with C-x C-x (or perhaps C-x C-x C-x C-x) to operate on it again.

Generally, commands that push marks as a means of navigation, such as beginning-of-buffer (M-<) and end-of-buffer (M->), do not activate the region. However, commands that push marks as a means of marking an area of text, such as mark-defun (M-C-h), mark-word (M-@), and mark-whole-buffer (C-x h), do activate the region.

When zmacs-regions is t, there is no distinction between the primary X selection and the active region selected by point and the mark. To see this, set the mark (C-SPC) and move the cursor with any cursor-motion command: the region between point and mark is highlighted, and you can watch it grow and shrink as you move the cursor.

Any other commands besides cursor-motion commands (such as inserting or deleting text) will cause the region to no longer be active; it will no longer be highlighted, and will no longer be the primary selection. Region can be explicitly deactivated with C-g.

Commands that require a region (such as C-w) signal an error if the region is not active. Certain commands cause the region to be in its active state. The most common ones are push-mark (C-SPC) and exchange-point-and-mark (C-x C-x).

When zmacs-regions is t, programs can be non-intrusive on the state of the region by setting the variable zmacs-region-stays to a non-nil value. If you are writing a new Emacs command that is conceptually a “motion” command and should not interfere with the current highlightedness of the region, then you may set this variable. It is reset to nil after each user command is executed.

When zmacs-regions is t, programs can make the region between point and mark go into the active (highlighted) state by using the function zmacs-activate-region. Only a small number of commands should ever do this.

When zmacs-regions is t, programs can deactivate the region between point and the mark by using zmacs-deactivate-region. Note: you should not have to call this function; the command loop calls it when appropriate.