Emacs supports editing text written in scripts, such as Arabic and Hebrew, whose natural ordering of horizontal text for display is from right to left. However, digits and Latin text embedded in these scripts are still displayed left to right. It is also not uncommon to have small portions of text in Arabic or Hebrew embedded in an otherwise Latin document; e.g., as comments and strings in a program source file. For these reasons, text that uses these scripts is actually bidirectional: a mixture of runs of left-to-right and right-to-left characters.
This section describes the facilities and options provided by Emacs for editing bidirectional text.
Emacs stores right-to-left and bidirectional text in the so-called logical (or reading) order: the buffer or string position of the first character you read precedes that of the next character. Reordering of bidirectional text into the visual order happens at display time. As result, character positions no longer increase monotonically with their positions on display. Emacs implements the Unicode Bidirectional Algorithm described in the Unicode Standard Annex #9, for reordering of bidirectional text for display.
The buffer-local variable bidi-display-reordering controls
whether text in the buffer is reordered for display. If its value is
nil, Emacs reorders characters that have right-to-left
directionality when they are displayed. The default value is
Each paragraph of bidirectional text can have its own base direction, either right-to-left or left-to-right. (Paragraph boundaries are empty lines, i.e., lines consisting entirely of whitespace characters.) Text in left-to-right paragraphs begins on the screen at the left margin of the window and is truncated or continued when it reaches the right margin. By contrast, text in right-to-left paragraphs is displayed starting at the right margin and is continued or truncated at the left margin.
Emacs determines the base direction of each paragraph dynamically,
based on the text at the beginning of the paragraph. However,
sometimes a buffer may need to force a certain base direction for its
paragraphs. The variable bidi-paragraph-direction, if
nil, disables the dynamic determination of the base
direction, and instead forces all paragraphs in the buffer to have the
direction specified by its buffer-local value. The value can be either
left-to-right. Any other value is
Alternatively, you can control the base direction of a paragraph by
inserting special formatting characters in front of the paragraph.
The special character
RIGHT-TO-LEFT MARK, or RLM, forces
the right-to-left direction on the following paragraph, while
LEFT-TO-RIGHT MARK, or LRM forces the left-to-right
direction. (You can use C-x 8 RET to insert these characters.)
In a GUI session, the LRM and RLM characters display as very
thin blank characters; on text terminals they display as blanks.
Because characters are reordered for display, Emacs commands that
operate in the logical order or on stretches of buffer positions may
produce unusual effects. For example, C-f and C-b
commands move point in the logical order, so the cursor will sometimes
jump when point traverses reordered bidirectional text. Similarly, a
highlighted region covering a contiguous range of character positions
may look discontinuous if the region spans reordered text. This is
normal and similar to the behavior of other programs that support
bidirectional text. If you set visual-order-cursor-movement to
nil value, cursor motion by the arrow keys follows the
visual order on screen (see visual-order movement).