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22.20 Bidirectional Editing

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 non-nil, Emacs reorders characters that have right-to-left directionality when they are displayed. The default value is t.

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 non-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 right-to-left or left-to-right. Any other value is interpreted as nil.

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.