The Unicode standard calls for characters to be ordered 'logically', i.e. in the sequence they are intended to be interpreted, as opposed to 'visually', the sequence they appear. This distinction is relevant for bidi support because at any bidi transition, the visual presentation ceases to be the 'logical' one. Thus, in order to offer bidi support, Unicode prescribes an algorithm for how to convert the logical sequence of characters into the correct visual presentation. For this purpose, the Unicode encoding standard divides all its characters into one of four types: 'strong', 'weak', 'neutral', and 'explicit formatting'.
Strong characters Strong characters are those with a definite direction. Examples of this type of character include most alphabetic characters, syllabic characters, Han ideographs, non-European or non-Arabic digits, and punctuation characters that are specific to only those scripts.
Weak characters Weak characters are those with vague direction. Examples of this type of character include European digits, Eastern Arabic-Indic digits, arithmetic symbols, and currency symbols.
Neutral characters Neutral characters have direction indeterminable without context. Examples include paragraph separators, tabs, and most other whitespace characters. Punctuation symbols that are common to many scripts, such as the colon, comma, full-stop, and the no-break-space also fall within this category.
Explicit formatting Explicit formatting characters, also referred to as "directional formatting characters", are special Unicode sequences that direct the algorithm to modify its default behavior. These characters are subdivided into "marks", "embeddings", "isolates", and "overrides". Their effects continue until the occurrence of either a paragraph separator, or a "pop" character.
Marks If a "weak" character is followed by another "weak" character, the algorithm will look at the first neighbouring "strong" character. Sometimes this leads to unintentional display errors. These errors are corrected or prevented with "pseudo-strong" characters. Such
Unicode control characters are called
marks. The mark ( or ) is to be inserted into a location to make an enclosed weak character inherit its writing direction. For example, to correctly display the for an English name brand (LTR) in an Arabic (RTL) passage, an LRM mark is inserted after the trademark symbol if the symbol is not followed by LTR text (e.g. ""). If the LRM mark is not added, the weak character ™ will be neighbored by a strong LTR character and a strong RTL character. Hence, in an RTL context, it will be considered to be RTL, and displayed in an incorrect order (e.g. "").
Embeddings The "embedding" directional formatting characters are the classical Unicode method of explicit formatting, and as of Unicode 6.3, are being discouraged in favor of "isolates". An "embedding" signals that a piece of text is to be treated as directionally distinct. The text within the scope of the embedding formatting characters is not independent of the surrounding text. Also, characters within an embedding can affect the ordering of characters outside. Unicode 6.3 recognized that directional embeddings usually have too strong an effect on their surroundings and are thus unnecessarily difficult to use.
Isolates The "isolate" directional formatting characters signal that a piece of text is to be treated as directionally isolated from its surroundings. As of Unicode 6.3, these are the formatting characters that are being encouraged in new documents – once target platforms are known to support them. These formatting characters were introduced after it became apparent that directional embeddings usually have too strong an effect on their surroundings and are thus unnecessarily difficult to use. Unlike the legacy 'embedding' directional formatting characters, 'isolate' characters have no effect on the ordering of the text outside their scope. Isolates can be nested, and may be placed within embeddings and overrides.
Overrides The "override" directional formatting characters allow for special cases, such as for part numbers (e.g. to force a part number made of mixed English, digits and Hebrew letters to be written from right to left), and are recommended to be avoided wherever possible. As is true of the other directional formatting characters, "overrides" can be nested one inside another, and in embeddings and isolates.
Using Unicode to override Using will switch the text direction from left-to-right to right-to-left. Similarly, using will switch the text direction from right-to-left to left-to-right based on the
Unicode Bidirectional Algorithm.
Pops The "pop" directional formatting character, encoded at , terminates the scope of the most recent "embedding", "override", or "isolate".
Runs In the algorithm, each sequence of concatenated strong characters is called a "run". A "weak" character that is located between two "strong" characters with the same orientation will inherit their orientation. A "weak" character that is located between two "strong" characters with a different writing direction will inherit the main context's writing direction (in an LTR document the character will become LTR, in an RTL document, it will become RTL).
Table of possible BiDi character types Security Unicode bidirectional characters are used in the
Trojan Source vulnerability.
Visual Studio Code highlights BiDi control characters since version 1.62 released in October 2021.
Visual Studio highlights BiDi control characters since version 17.0.3 released on December 14, 2021. == Scripts using bidirectional text ==