Network Working Group K. Whistler
Request for Comments: 2482 Sybase
Category: Informational G. Adams
Spyglass
January 1999
Language Tagging in Unicode Plain Text
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
IESG Note:
This document has been accepted by ISO/IEC JTC1/SC2/WG2 in meeting
#34 to be submitted as a recommendation from WG2 for inclusion in
Plane 14 in part 2 of ISO/IEC 10646.
1. Abstract
This document proposed a mechanism for language tagging in [UNICODE]
plain text. A set of special-use tag characters on Plane 14 of
[ISO10646] (accessible through UTF-8, UTF-16, and UCS-4 encoding
forms) are proposed for encoding to enable the spelling out of
ASCII-based string tags using characters which can be strictly
separated from ordinary text content characters in ISO10646 (or
UNICODE).
One tag identification character and one cancel tag character are
also proposed. In particular, a language tag identification character
is proposed to identify a language tag string specifically; the
language tag itself makes use of [RFC1766] language tag strings
spelled out using the Plane 14 tag characters. Provision of a
specific, low-overhead mechanism for embedding language tags in plain
text is aimed at meeting the need of Internet Protocols such as ACAP,
which require a standard mechanism for marking language in UTF-8
strings.
The tagging mechanism as well the characters proposed in this
document have been approved by the Unicode Consortium for inclusion
in The Unicode Standard. However, implementation of this decision
Whistler & Adams Informational [Page 1]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 awaits formal acceptance by ISO JTC1/SC2/WG2, the working group responsible for ISO10646. Potential implementers should be aware that until this formal acceptance occurs, any usage of the characters proposed herein is strictly experimental and not sanctioned for standardized character data interchange. 2. Definitions and Notation No attempt is made to define all terms used in this document. In particular, the terminology pertaining to the subject of coded character systems is not explicitly specified. See [UNICODE], [ISO10646], and [RFC2130] for additional definitions in this area. 2.1 Requirements Notation This document occasionally uses terms that appear in capital letters. When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate particular requirements of this specification. A discussion of the meanings of these terms appears in [RFC2119]. 2.2 Definitions The terms defined below are used in special senses and thus warrant some clarification. 2.2.1 Tagging The association of attributes of text with a point or range of the primary text. (The value of a particular tag is not generally considered to be a part of the "content" of the text. Typical examples of tagging is to mark language or font of a portion of text.) 2.2.2 Annotation The association of secondary textual content with a point or range of the primary text. (The value of a particular annotation *is* considered to be a part of the "content" of the text. Typical examples include glossing, citations, exemplication, Japanese yomi, etc.) 2.2.3 Out-of-band An out-of-band channel conveys a tag in such a way that the textual content, as encoded, is completely untouched and unmodified. This is typically done by metadata or hyperstructure of some sort. Whistler & Adams Informational [Page 2]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 2.2.4 In-band An in-band channel conveys a tag along with the textual content, using the same basic encoding mechanism as the text itself. This is done by various means, but an obvious example is SGML markup, where the tags are encoded in the same character set as the text and are interspersed with and carried along with the text data. 3.0 Background There has been much discussion over the last 8 years of language tagging and of other kinds of tagging of Unicode plain text. It is fair to say that there is more-or-less universal agreement that language tagging of Unicode plain text is required for certain textual processes. For example, language "hinting" of multilingual text is necessary for multilingual spell-checking based on multiple dictionaries to work well. Language tagging provides a minimum level of required information for text-to-speech processes to work correctly. Language tagging is regularly done on web pages, to enable selection of alternate content, for example. However, there has been a great deal of controversy regarding the appropriate placement of language tags. Some have held that the only appropriate placement of language tags (or other kinds of tags) is out-of-band, making use of attributed text structures or metadata. Others have argued that there are requirements for lower-complexity in-band mechanisms for language tags (or other tags) in plain text. The controversy has been muddied by the existence and widespread use of a number of in-band text markup mechanisms (HTML, text/enriched, etc.) which enable language tagging, but which imply the use of general parsing mechanisms which are deemed too "heavyweight" for protocol developers and a number of other applications. The difficulty of using general in-band text markup for simple protocols derives from the fact that some characters are used both for textual content and for the text markup; this makes it more difficult to write simple, fast algorithms to find only the textual content and ignore the tags, or vice versa. (Think of this as the algorithmic equivalent of the difficulty the human reader has attempting to read just the content of raw HTML source text without a browser interpreting all the markup tags.) The Plane 14 proposal addresses the recurrent and persistent call for a lighter-weight mechanism for text tagging than typical text markup mechanisms in Unicode. It proposes a special set of characters used *only* for tagging. These tag characters can be embedded into plain Whistler & Adams Informational [Page 3]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 text and can be identified and/or ignored with trivial algorithms, since there is no overloading of usage for these tag characters--they can only express tag values and never textual content itself. The Plane 14 proposal is not intended for general annotation of text, such as textual citations, phonetic readings (e.g. Japanese Yomi), etc. In its present form, its use is intended to be restriced solely to specifying in-line language tags. Future extensions may widen this scope of intended usage. 4.0 Proposal This proposal suggests the use of 97 dedicated tag characters encoded at the start of Plane 14 of ISO/IEC 10646 consisting of a clone of the 94 printable 7-bit ASCII graphic characters and ASCII SPACE, as well as a tag identification character and a tag cancel character. These tag characters are to be used to spell out any ASCII-based tagging scheme which needs to be embedded in Unicode plain text. In particular, they can be used to spell out language tags in order to meet the expressed requirements of the ACAP protocol and the likely requirements of other new protocols following the guidelines of the IAB character workshop (RFC 2130). The suggested range in Plane 14 for the block reserved for tag characters is as follows, expressed in each of the three most generally used encoding schemes for ISO/IEC 10646: UCS-4 U-000E0000 .. U-000E007F UTF-16 U+DB40 U+DC00 .. U+DB40 U+DC7F UTF-8 0xF3 0xA0 0x80 0x80 .. 0xF3 0xA0 0x81 0xBF Of this range, U-000E0020 .. U-000E007E is the suggested range for the ASCII clone tag characters themselves. 4.1 Names for the Tag Characters The names for the ASCII clone tag characters should be exactly the ISO 10646 names for 7-bit ASCII, prefixed with the word "TAG". Whistler & Adams Informational [Page 4]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 In addition, there is one tag identification character and a CANCEL TAG character. The use and syntax of these characters is described in detail below. The entire encoding for the proposed Plane 14 tag characters and names of those characters can be derived from the following list. (The encoded values here and throughout this proposal are listed in UCS-4 form, which is easiest to interpret. It is assumed that most Unicode applications will, however, be making use either of UTF-16 or UTF-8 encoding forms for actual implementation.) U-000E0000 <reserved> U-000E0001 LANGUAGE TAG U-000E0002 <reserved> U-000E001F <reserved> U-000E0020 TAG SPACE U-000E0021 TAG EXCLAMATION MARK U-000E0041 TAG LATIN CAPITAL LETTER A U-000E007A TAG LATIN SMALL LETTER Z U-000E007E TAG TILDE U-000E007F CANCEL TAG 4.2 Range Checking for Tag Characters The range checks required for code testing for tag characters would be as follows. The same range check is expressed here in C for each of the three significant encoding forms for 10646. Range check expressed in UCS-4: if ( ( *s >= 0xE0000 ) || ( *s <= 0xE007F ) ) Range check expressed in UTF-16 (Unicode): if ( ( *s == 0xDB40 ) && ( *(s+1) >= 0xDC00 ) && ( *(s+1) <= 0xDC7F ) ) Expressed in UTF-8: if ( ( *s == 0xF3 ) && ( *(s+1) == 0xA0 ) && ( *(s+2) & 0xE0 == 0x80 ) Because of the choice of the range for the tag characters, it would also be possible to express the range check for UCS-4 or UTF-16 in terms of bitmask operations, as well. Whistler & Adams Informational [Page 5]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 4.3 Syntax for Embedding Tags The use of the Plane 14 tag characters is very simple. In order to embed any ASCII-derived tag in Unicode plain text, the tag is simply spelled out with the tag characters instead, prefixed with the relevant tag identification character. The resultant string is embedded directly in the text. The tag identification character is used as a mechanism for identifying tags of different types. This enables multiple types of tags to coexist amicably embedded in plain text and solves the problem of delimitation if a tag is concatenated directly onto another tag. Although only one type of tag is currently specified, namely the language tag, the encoding of other tag identification characters in the future would allow for distinct tag types to be used. No termination character is required for a tag. A tag terminates either when the first non Plane 14 Tag Character (i.e. any other normal Unicode value) is encountered, or when the next tag identification character is encountered. All tag arguments must be encoded only with the tag characters U- 000E0020 .. U-000E007E. No other characters are valid for expressing the tag argument. A detailed BNF syntax for tags is listed below. 4.4 Tag Scope and Nesting The value of an established tag continues from the point the tag is embedded in text until either: A. The text itself goes out of scope, as defined by the application. (E.g. for line-oriented protocols, when reaching the end-of-line or end-of-string; for text streams, when reaching the end-of-stream; etc.) or B. The tag is explicitly cancelled by the CANCEL TAG character. Tags of the same type cannot be nested in any way. The appearance of a new embedded language tag, for example, after text which was already language tagged, simply changes the tagged value for subsequent text to that specified in the new tag. Whistler & Adams Informational [Page 6]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 Tags of different type can have interdigitating scope, but not hierarchical scope. In effect, tags of different type completely ignore each other, so that the use of language tags can be completely asynchronous with the use of character set source tags (or any other tag type) in the same text in the future. 4.5 Cancelling Tag Values U-000E007F CANCEL TAG is provided to allow the specific cancelling of a tag value. The use of CANCEL TAG has the following syntax. To cancel a tag value of a particular type, prefix the CANCEL TAG character with the tag identification character of the appropriate type. For example, the complete string to cancel a language tag is: U-000E0001 U-000E007F The value of the relevant tag type returns to the default state for that tag type, namely: no tag value specified, the same as untagged text. The use of CANCEL TAG without a prefixed tag identification character cancels *any* Plane 14 tag values which may be defined. Since only language tags are currently provided with an explicit tag identification character, only language tags are currently affected. The main function of CANCEL TAG is to make possible such operations as blind concatenation of strings in a tagged context without the propagation of inappropriate tag values across the string boundaries. For example, a string tagged with a Japanese language tag can have its tag value "sealed off" with a terminating CANCEL TAG before another string of unknown language value is concatenated to it. This would prevent the string of unknown language from being erroneously marked as being Japanese simply because of a concatenation to a Japanese string. 4.6 Tag Syntax Description An extended BNF (Backus-Naur Form) description of the tags specified in this proposal is found below. Note the following BNF extensions used in this formalism: 1. Semantic constraints are specified by rules in the form of an assertion specified between double braces; the variable $$ denotes the string consisting of all terminal symbols matched by the this non-terminal. Example: {{ Assert ( $$[0] == '?' ); }} Whistler & Adams Informational [Page 7]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 Meaning: The first character of the string matched by this non-terminal must be '?' 2. A number of predicate functions are employed in semantic constraint rules which are not otherwise defined; their name is sufficient for determining their predication. Example: IsRFC1766LanguageIdentifier ( tag-argument ) Meaning: tag-argument is a valid RFC1766 language identifier 3. A lexical expander function, TAG, is employed to denote the tag form of an ASCII character; the argument to this function is either a character or a character set specified by a range or enumeration expression. Example: TAG('-') Meaning: TAG HYPHEN-MINUS Example: TAG([A-Z]) Meaning: TAG LATIN CAPITAL LETTER A ... TAG LATIN CAPITAL LETTER Z 4. A macro is employed to denote terminal symbols that are character literals which can't be directly represented in ASCII. The argument to the macro is the UNICODE (ISO/IEC 10646) character name. Example: '${TAG CANCEL}' Meaning: character literal whose code value is U-000E007F 5. Occurrence indicators used are '+' (one or more) and '*' (zero or more); optional occurrence is indicated by enclosure in '[' and ']'. 4.6.1 Formal Tag Syntax tag : language-tag | cancel-all-tag ; language-tag : language-tag-introducer language-tag-argument ; Whistler & Adams Informational [Page 8]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 language-tag-argument : tag-argument {{ Assert ( IsRFC1766LanguageIdentifier ( $$ ); }} | tag-cancel ; cancel-all-tag : tag-cancel ; tag-argument : tag-character+ ; tag-character : { c : c in TAG( { a : a in printable ASCII characters or SPACE } ) } ; language-tag-introducer : '${TAG LANGUAGE}' ; tag-cancel : '${TAG CANCEL}' ; 5.0 Tag Types 5.1 Language Tags Language tags are of general interest and should have a high degree of interoperability for protocol usage. To this end, a specific LANGUAGE TAG tag identification character is provided. A Plane 14 tag string prefixed by U-000E0001 LANGUAGE TAG is specified to constitute a language tag. Furthermore, the tag values for the language tag are to be spelled out as specified in RFC 1766, making use only of registered tag values or of user-defined language tags starting with the characters "x-". For example, to embed a language tag for Japanese, the Plane 14 characters would be used as follows. The Japanese tag from RFC 1766 is "ja" (composed of ISO 639 language id) or, alternatively, "ja-JP" (composed of ISO 639 language id plus ISO 3166 country id). Since RFC 1766 specifies that language tags are not case significant, it is recommended that for language tags, the entire tag be lowercased before conversion to Plane 14 tag characters. (This would not be required for Unicode conformance, but should be followed as general practice by protocols making use of RFC 1766 language tags, to simplify and speed up the processing for operations which need to identify or ignore language tags embedded in text.) Lowercasing, Whistler & Adams Informational [Page 9]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 rather than uppercasing, is recommended because it follows the majority practice of expressing language tag values in lowercase letters. Thus the entire language tag (in its longer form) would be converted to Plane 14 tag characters as follows: U-000E0001 U-000E006A U-000E0061 U-000E002D U-000E006A U-000E0070 The language tag (in its shorter, "ja" form) could be expressed as follows: U-000E0001 U-000E006A U-000E0061 The value of this string is then expressed in whichever encoding form (UCS-4, UTF-16, UTF-8) is required and embedded in text at the relevant point. 5.2 Additional Tags Additional tag identification characters might be defined in the future. An example would be a CHARACTER SET SOURCE TAG, or a GENERIC TAG for private definition of tags. In each case, when a specific tag identification character is encoded, a corresponding reference standard for the values of the tags associated with the identifier should be designated, so that interoperating parties which make use of the tags will know how to interpret the values the tags may take. 6.0 Display Issues All characters in the tag character block are considered to have no visible rendering in normal text. A process which interprets tags may choose to modify the rendering of text based on the tag values (as for example, changing font to preferred style for rendering Chinese versus Japanese). The tag characters themselves have no display; they may be considered similar to a U+200B ZERO WIDTH SPACE in that regard. The tag characters also do not affect breaking, joining, or any other format or layout properties, except insofar as the process interpreting the tag chooses to impose such behavior based on the tag value. For debugging or other operations which must render the tags themselves visible, it is advisable that the tag characters be rendered using the corresponding ASCII character glyphs (perhaps modified systematically to differentiate them from normal ASCII Whistler & Adams Informational [Page 10]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 characters). But, as noted below, the tag character values are chosen so that even without display support, the tag characters will be interpretable in most debuggers. 7.0 Unicode Conformance Issues The basic rules for Unicode conformance for the tag characters are exactly the same as for any other Unicode characters. A conformant process is not required to interpret the tag characters. If it does not interpret tag characters, it should leave their values undisturbed and do whatever it does with any other uninterpreted characters. If it does interpret them, it should interpret them according to the standard, i.e. as spelled-out tags. So for a non-TagAware Unicode application, any language tag characters (or any other kind of tag expressed with Plane 14 tag characters) encountered would be handled exactly as for uninterpreted Tibetan from the BMP, uninterpreted Linear B from Plane 1, or uninterpreted Egyptian hieroglyphics from private use space in Plane 15. A TagAware but TagPhobic Unicode application can recognize the tag character range in Plane 14 and choose to deliberately strip them out completely to produce plain text with no tags. The presence of a correctly formed tag cannot be taken as a guarantee that the data so tagged is correctly tagged. For example, nothing prevents an application from erroneously labelling French data as Spanish, or from labelling JIS-derived data as Japanese, even if it contains Greek or Cyrillic characters. 7.1 Note on Encoding Language Tags The fact that this proposal for encoding tag characters in Unicode includes a mechanism for specifying language tag values does not mean that Unicode is departing from one of its basic encoding principles: Unicode encodes scripts, not languages. This is still true of the Unicode encoding (and ISO/IEC 10646), even in the presence of a mechanism for specifying language tags in plain text. There is nothing obligatory about the use of Plane 14 tags, whether for language tags or any other kind of tags. Language tagging in no way impacts current encoded characters or the encoding of future scripts. Whistler & Adams Informational [Page 11]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 It is fully anticipated that implementations of Unicode which already make use of out-of-band mechanisms for language tagging or "heavy- weight" in-band mechanisms such as HTML will continue to do exactly what they are doing and will ignore Plane 14 tag characters completely. 8.0 Security Considerations There are no known security issues raised by this document. References [ISO10646] ISO/IEC 10646-1:1993 International Organization for Standardization. "Information Technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane", Geneva, 1993. [RFC1766] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995. [RFC2070] Yergeau, F., Nicol, G. Adams, G. and M. Duerst, "Internationalization of the Hypertext Markup Language", RFC 2070, January 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2130] Weider, C. Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin, M. and P. Svanberg, "The Report of the IAB Character Set Workshop held 29 February - 1 March, 1996", RFC 2130, April 1997. [UNICODE] The Unicode Standard, Version 2.0, The Unicode Consortium, Addison-Wesley, July 1996. Acknowledgements The following people also contributed to this document, directly or indirectly: Chris Newman, Mark Crispin, Rick McGowan, Joe Becker, John Jenkins, and Asmus Freytag. This document also was reviewed by the Unicode Technical Committee, and the authors wish to thank all of the UTC representatives for their input. The authors are, of course, responsible for any errors or omissions which may remain in the text. Whistler & Adams Informational [Page 12]
RFC 2482 Language Tagging in Unicode Plain Text January 1999 Authors' Addresses Ken Whistler Sybase, Inc. 6475 Christie Ave. Emeryville, CA 94608-1050 Phone: +1 510 922 3611 EMail: kenw@sybase.com Glenn Adams Spyglass, Inc. One Cambridge Center Cambridge, MA 02142 Phone: +1 617 679 4652 EMail: glenn@spyglass.com Whistler & Adams Informational [Page 13]
RFC 2482 Language Tagging in Unicode Plain Text January 1999
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Whistler & Adams Informational [Page 14]
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