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Network Working Group                                       K. WhistlerRequest for Comments: 2482                                       SybaseCategory: Informational                                        G. Adams                                                               Spyglass                                                           January 1999Language Tagging in Unicode Plain TextStatus 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 decisionWhistler & 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 19992.2.4 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 plainWhistler & 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 TAG4.2 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 19994.3 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 validRFC1766 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 Syntaxtag                     :   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 1999language-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 Types5.1 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 inRFC 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 fromRFC 1766   is "ja" (composed of ISO 639 language id) or, alternatively, "ja-JP"   (composed of ISO 639 language id plus ISO 3166 country id).  SinceRFC 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 ofRFC 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 ASCIIWhistler & 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 1999Authors' 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.comWhistler & Adams             Informational                     [Page 13]

RFC 2482         Language Tagging in Unicode Plain Text     January 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Whistler & Adams             Informational                     [Page 14]