Unicode control characters
Encyclopedia
Many Unicode control characters are used to control the interpretation or display of text, but these characters themselves have no visual or spatial representation. For example, the null character is used in C-programming application environments to indicate the end of a string of characters. In this way, these programs only require a single starting memory address for a string (as opposed to a starting address and a length), since the string ends once the program reads the null character.
. Additionally, U+0080–U+009F were used in conjunction with ISO 8859
character sets (among others). They are specified in ISO 6429 and often referred to as C0 and C1 control codes
respectively.
Most of these characters play no explicit role in Unicode text handling. The characters , , , , and are commonly used in text processing as formatting characters.
characters used in legacy text, UCS
introduces its own newline characters to separate either lines or paragraphs: and . These characters are text formatting only, and not characters.
These language tag characters would not be displayed themselves. However, they would provide information for text processing or even for the display of other characters. For example the display of Unihan ideographs might substitute different glyphs if the language tags indicated Korean than if the tags indicated Japanese. Another example, might influence the display of decimal digits 0 through 9 differently depending on the language they appeared in.
The tag characters have become deprecated in Unicode 5.1 (2008).
However, for other glyph substitution, the author's intent may need to be encoded with the text and cannot be determined contextually. This is the case with character/glyphs referred to as gaiji where different glyphs are used for the same character either historically or for ideographs for family names. This is one of the gray areas in distinguishing between a glyph and a character. If a family name differs slightly from the ideograph character it derives from, then is that a simple glyph variant or a character variant. As of Unicode 3.2 and 4.0, the character set now includes 256 variation selectors so that these combining mark characters can select from 256 possible character/glyph variations for the preceding character.
ISO 6429 control characters (C0 and C1)
The control characters U+0000–U+001F and U+007F come from ASCIIASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. Additionally, U+0080–U+009F were used in conjunction with ISO 8859
ISO/IEC 8859
ISO/IEC 8859 is a joint ISO and IEC series of standards for 8-bit character encodings. The series of standards consists of numbered parts, such as ISO/IEC 8859-1, ISO/IEC 8859-2, etc. There are 15 parts, excluding the abandoned ISO/IEC 8859-12...
character sets (among others). They are specified in ISO 6429 and often referred to as C0 and C1 control codes
C0 and C1 control codes
Most character encodings, in addition to representing printable characters, may also represent additional information about the text, such as the position of a cursor, an instruction to start a new line, or a message that the text has been received...
respectively.
Most of these characters play no explicit role in Unicode text handling. The characters , , , , and are commonly used in text processing as formatting characters.
Unicode introduced separators
In an attempt to simplify the several newlineNewline
In computing, a newline, also known as a line break or end-of-line marker, is a special character or sequence of characters signifying the end of a line of text. The name comes from the fact that the next character after the newline will appear on a new line—that is, on the next line below the...
characters used in legacy text, UCS
Universal Character Set
The Universal Character Set , defined by the International Standard ISO/IEC 10646, Information technology — Universal multiple-octet coded character set , is a standard set of characters upon which many character encodings are based...
introduces its own newline characters to separate either lines or paragraphs: and . These characters are text formatting only, and not
Language tags
Unicode includes 128 characters for language tags. These characters essentially mirror the 128 ASCII characters but are used to identify the subsequent text as belonging to a particular language according to BCP 47. For example, to indicate subsequent text as the variant of English as written in the United States, the initiating ‘Language Tag character’ (U+E0001) followed by the sequence ‘Tag Small Letter e’ (U+E0065), ‘Tag Small Letter n’ (U+E006E), ‘Tag Hyphen-minus’ (U+E002D), ‘Tag Small Letter u’ (U+E0075) and ‘Tag Small Letter s’ (U+E0073) would be used.These language tag characters would not be displayed themselves. However, they would provide information for text processing or even for the display of other characters. For example the display of Unihan ideographs might substitute different glyphs if the language tags indicated Korean than if the tags indicated Japanese. Another example, might influence the display of decimal digits 0 through 9 differently depending on the language they appeared in.
The tag characters have become deprecated in Unicode 5.1 (2008).
Interlinear annotation
Three formatting characters provide support for interlinear annotation (U+FFF9, U+FFFA, U+FFFB). This may be used for providing notes that would typically be displayed between the lines of other text. Unicode considers such annotation to be rich text and recommends using other protocols for such annotation. The W3C Ruby markup recommendation is an example of an alternate protocol supporting more advanced interlinear annotation.Bidirectional text control
Unicode supports standard bidirectional text without any special characters. In other words Unicode conforming software should display right-to-left characters such as Hebrew letters as right-to-left simply from the properties of those characters. Similarly, Unicode handles the mixture of left-to-right-text alongside right-to-left text without any special characters. For example, one can quote Arabic (“بسم الله”) (translated into English as "Bismillah") right alongside English and the Arabic letters will flow from right-to-left and the Latin letters left-to-right. However, support for bidirectional text becomes more complicated when text flowing in opposite directions is embedded hierarchically, for example if one quotes an Arabic phrase that in turn quotes an English phrase. Other situations may also complicate this, such as when an author wants the left-to-right characters overridden so that they flow from right-to-left. While these situations are fairly rare, Unicode provides seven characters (U+200E, U+200F, U+202A, U+202B, U+202C, U+202D, U+202E) to help control these embedded bidirectional text levels up to 61 levels deep.Variation selectors
Many characters map to alternate glyphs depending on the context. For example Arabic and Latin cursive characters substitute different glyphs to connect glyphs together depending on whether the character is the initial character in a word, the final character, a medial character or an isolated character. These types of glyph substitution are easily handled by the context of the character with no other authoring input involved. Authors may also use special-purpose characters such as joiners and non-joiners to force an alternate form of glyph where it would not otherwise appear. Ligatures are similar instances where glyphs may be substituted simply by turning ligatures on or off as a rich text attribute.However, for other glyph substitution, the author's intent may need to be encoded with the text and cannot be determined contextually. This is the case with character/glyphs referred to as gaiji where different glyphs are used for the same character either historically or for ideographs for family names. This is one of the gray areas in distinguishing between a glyph and a character. If a family name differs slightly from the ideograph character it derives from, then is that a simple glyph variant or a character variant. As of Unicode 3.2 and 4.0, the character set now includes 256 variation selectors so that these combining mark characters can select from 256 possible character/glyph variations for the preceding character.