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rfc:rfc2482

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

Full 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]

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