Unicode control characters: Difference between revisions

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The [[ISO/IEC 8859]] series of encodings conforms to [[ISO/IEC 4873]] (ECMA-43) level 1, a subset of ISO/IEC 2022 designed for 8-bit character encodings, and therefore reserves the range 0x80–0x9F for use as non-printing codes by C1 control code sets such as ISO/IEC 6429.<ref>{{citation|mode=cs1 |quotation=This set of coded graphic characters may be regarded as a version of an 8-bit code according to ISO/IEC 2022 or ISO/IEC 4873 at level 1. […] The shaded positions in the code table correspond to bit combinations that do not represent graphic characters. Their use is outside the scope of ISO/IEC 8859; it is specified in other International Standards, for example ISO/IEC 6429. |url=http://www.open-std.org/JTC1/sc2/wg3/docs/n411.pdf |title=Final Text of DIS 8859-1, 8-bit single-byte coded graphic character sets—Part 1: Latin alphabet No.1 |author=ISO/IEC JTC 1/SC 2/WG 3 |author-link=ISO/IEC JTC 1/SC 2 |id=[[ISO]]/[[International Electrotechnical Commission|IEC]] [[International Organization for Standardization#Standardization process|FDIS]] 8859-1:1998; JTC1/SC2/N2988; WG3/N411 |date=1998-02-12}}</ref> Unicode inherits its [[Basic Latin (Unicode block)|first]] and [[Latin-1 Supplement (Unicode block)|second]] blocks (comprising U+0000 through U+00FF) from ASCII and [[ISO/IEC 8859-1]], thus incorporating the C0 and C1 control code ranges (U+0000&ndash;U+001F, U+007F&ndash;U+009F) as general category "Cc". It does not assign normative names to these control codes, though it does assign them normative aliases.<ref name="aliases" />

Category "Cc" control codes can serve a variety of purposes, not limited to format effectors: for example, the default ASCII C0 set includes six format effectors ({{ctrl|BS}}, {{ctrl|HT}}, {{ctrl|LF}}, {{ctrl|VT}}, {{ctrl|FF}} and {{ctrl|CR}}), ten transmission controls, four device controls, four information separators and eight other control codes.<ref name="ir001">{{citation|mode=cs1cite iso-ir |authorsponsor=ISO/TC 97/SC 2 |authorsponsor-link=ISO/IEC JTC 1/SC 2#History |title=The set of control characters of the ISO 646 |date=1975 |publishernumber=ITSCJ/[[Information Processing Society of Japan|IPSJ]] |id=ISO-IR-1 |url=https://www.itscj.ipsj.or.jp/iso-ir/001.pdf}}</ref> Most of these characters play no explicit role in Unicode text handling, and are used only by higher-level protocols such as those used by [[terminal emulator]]s. Certain characters are commonly used for formatting or [[sentinel value|sentinel]] purposes:

* {{tt|U+0000}}&nbsp;{{unichar/name|na=NULL}} (used in [[null-terminated string]]s)

* {{tt|U+0009}}&nbsp;{{unichar/name|na=HORIZONTAL TABULATION (HT)}} (inserted by the [[tab key]])

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* {{tt|U+000D}}&nbsp;{{unichar/name|na=CARRIAGE RETURN (CR)}} (used in some line-breaking conventions)

* {{tt|U+0085}}&nbsp;{{unichar/name|na=NEXT LINE (NEL)}} (sometimes used as a line break in text transcoded from [[EBCDIC]])

Unicode only specifies semantics for {{tt|U+0009&mdash;U+000D}}, {{tt|U+001C&mdash;U+001F}}, and {{tt|U+0085}} (the ASCII format effectors except for {{ctrl|BS}}, plus the ASCII information separators and the C1 {{ctrl|NEL}}). The rest of the "Cc" control codes are transparent to Unicode and their meanings are left to higher-level protocols, although interpretation as defined in ISO/IEC 6429 is suggested as a default.<ref name="unicode-23-1">{{cite bookweb |url=https://www.unicode.org/versions/Unicode12.0.0/ch23.pdf#page=3 |title=23.1: Control Codes |work=The Unicode Standard |edition=12.0.0 |date=2019 |author=Unicode Consortium |author-link=Unicode Consortium |isbn=978-1-936213-22-1 |pages=868–870}}</ref> Furthermore, certain specialised higher-level protocols, such as transcoded [[Teletext]], may include a [[Teletext character set#Control characters|different interpretation]] of the entire C0 control code range.<ref>{{cite web |url=https://corp.unicode.org/pipermail/unicode/2020-October/009120.html |title=Teletext separated mosaic graphics |work=Unicode Mailing List Archive |last=Ewell |first=Doug |date=2020-10-16 |publisher=[[Unicode Consortium]] |quotation=I reiterate that it was UTC {{bracket|[[Unicode Technical Committee]]}} and Script Ad Hoc who provided the guidance to the group writing the [[Symbols for Legacy Computing]] proposal (and there is a second on the way) that 0x00 through 0x1F in the original teletext set should map to U+0000 through U+001F when converting to Unicode.}}</ref>

== Unicode introduced separators ==

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== Language tags ==

{{main|Tags (Unicode block)}}

Unicode previously includedincludes 128 characters, now deprecated, forpreviously intended as language tags. These characters essentially mirrored the 128 ASCII characters but were 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 sequence {{unichar|E0001|LANGUAGE TAG}}, {{unichar|E0065|Tag Latin Small Letter e}}, {{unichar|E006E|Tag Latin Small Letter n}}, {{unichar|E002D|Tag Hyphen-minus}}, {{unichar|E0075|Tag Latin Small Letter u}} and {{unichar|E0073|Tag Latin Small Letter s}} would have been 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 have substituted different glyphs if the language tags indicated Korean than if the tags indicated Japanese. Another example, might have influenced the display of decimal digits 0 through 9 differently depending on the language they appeared in.

The tag characters {{unichar|E0001|LANGUAGE TAG}} and {{unichar|E007F|CANCEL TAG}} were deprecated in Unicode 5.1 (2008) and should not be used for language information.<ref>{{cite documentweb |url=http://tools.ietf.org/html/rfc6082|title=RFC6082: Deprecating Unicode Language Tag Characters: RFC 2482 is Historic | publisher=Internet Engineering Task Force (IETF)|date=November 2010|last1=Klensin |first1=John C. |last2=Presuhn |first2=Randy |last3=Whistler |first3=Ken |last4=Dürst |first4=Martin J. |last5=Adams |first5=Glenn |editor-first1=R. |editor-last1=Presuhn |doi=10.17487/RFC6082 |doi-access=free }}</ref> The characters {{tt|U+E0020—U+E0073}} were also deprecated, but were restored with the release of Unicode 8.0 (2015). The change was made "to clear the way for the potential future use of tag characters for a purpose other than to represent language tags".<ref name="migration">{{cite web|url=http://unicode.org/versions/Unicode8.0.0/#Migration|title=Unicode 8.0.0, Implications for Migration | publisher=Unicode Consortium}}</ref>

Unicode states that "the use of tag characters to represent language tags in a plain text stream is still a deprecated mechanism for conveying language information about text.<ref name="migration" />

== Interlinear annotation ==

Three formatting characters provide support for [[RubyInterlinear textgloss|interlinear annotation]] ({{unichar|FFF9|INTERLINEAR ANNOTATION ANCHOR}}, {{unichar|FFFA|INTERLINEAR ANNOTATION SEPARATOR}}, {{unichar|FFFB|INTERLINEAR ANNOTATION TERMINATOR}}). 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 character#Ruby markup|Ruby markup]] recommendation is an example of an alternate protocol supporting more advanced interlinear annotation.

== Bidirectional text control ==

{{main|Bi-directionalBidirectional text}}

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.

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{{unicode navigation}}

[[Category:Unicode special code points|Control characters]]