xref: /third_party/tex-hyphen/data/language-codes/bcp47.txt

[Note that this file is a concatenation of more than one RFC.]



Network Working Group                                   A. Phillips, Ed.
Request for Comments: 4646                                   Yahoo! Inc.
BCP: 47                                                    M. Davis, Ed.
Obsoletes: 3066                                                   Google
Category: Best Current Practice                           September 2006


                     Tags for Identifying Languages

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes the structure, content, construction, and
   semantics of language tags for use in cases where it is desirable to
   indicate the language used in an information object.  It also
   describes how to register values for use in language tags and the
   creation of user-defined extensions for private interchange.  This
   document, in combination with RFC 4647, replaces RFC 3066, which
   replaced RFC 1766.























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Table of Contents

   1. Introduction ....................................................3
   2. The Language Tag ................................................4
      2.1. Syntax .....................................................4
      2.2. Language Subtag Sources and Interpretation .................7
           2.2.1. Primary Language Subtag .............................8
           2.2.2. Extended Language Subtags ..........................10
           2.2.3. Script Subtag ......................................11
           2.2.4. Region Subtag ......................................11
           2.2.5. Variant Subtags ....................................13
           2.2.6. Extension Subtags ..................................14
           2.2.7. Private Use Subtags ................................16
           2.2.8. Preexisting RFC 3066 Registrations .................16
           2.2.9. Classes of Conformance .............................17
   3. Registry Format and Maintenance ................................18
      3.1. Format of the IANA Language Subtag Registry ...............18
      3.2. Language Subtag Reviewer ..................................24
      3.3. Maintenance of the Registry ...............................24
      3.4. Stability of IANA Registry Entries ........................25
      3.5. Registration Procedure for Subtags ........................29
      3.6. Possibilities for Registration ............................32
      3.7. Extensions and Extensions Registry ........................34
      3.8. Initialization of the Registries ..........................37
   4. Formation and Processing of Language Tags ......................38
      4.1. Choice of Language Tag ....................................38
      4.2. Meaning of the Language Tag ...............................40
      4.3. Length Considerations .....................................41
           4.3.1. Working with Limited Buffer Sizes ..................42
           4.3.2. Truncation of Language Tags ........................43
      4.4. Canonicalization of Language Tags .........................44
      4.5. Considerations for Private Use Subtags ....................45
   5. IANA Considerations ............................................46
      5.1. Language Subtag Registry ..................................46
      5.2. Extensions Registry .......................................47
   6. Security Considerations ........................................48
   7. Character Set Considerations ...................................48
   8. Changes from RFC 3066 ..........................................49
   9. References .....................................................52
      9.1. Normative References ......................................52
      9.2. Informative References ....................................53
   Appendix A. Acknowledgements ......................................55
   Appendix B. Examples of Language Tags (Informative) ...............56








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1.  Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   A user's language preferences often need to be identified so that
   appropriate processing can be applied.  For example, the user's
   language preferences in a Web browser can be used to select Web pages
   appropriately.  Language preferences can also be used to select among
   tools (such as dictionaries) to assist in the processing or
   understanding of content in different languages.

   In addition, knowledge about the particular language used by some
   piece of information content might be useful or even required by some
   types of processing; for example, spell-checking, computer-
   synthesized speech, Braille transcription, or high-quality print
   renderings.

   One means of indicating the language used is by labeling the
   information content with an identifier or "tag".  These tags can be
   used to specify user preferences when selecting information content,
   or for labeling additional attributes of content and associated
   resources.

   Tags can also be used to indicate additional language attributes of
   content.  For example, indicating specific information about the
   dialect, writing system, or orthography used in a document or
   resource may enable the user to obtain information in a form that
   they can understand, or it can be important in processing or
   rendering the given content into an appropriate form or style.

   This document specifies a particular identifier mechanism (the
   language tag) and a registration function for values to be used to
   form tags.  It also defines a mechanism for private use values and
   future extension.

   This document, in combination with [RFC4647], replaces [RFC3066],
   which replaced [RFC1766].  For a list of changes in this document,
   see Section 8.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].







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2.  The Language Tag

   Language tags are used to help identify languages, whether spoken,
   written, signed, or otherwise signaled, for the purpose of
   communication.  This includes constructed and artificial languages,
   but excludes languages not intended primarily for human
   communication, such as programming languages.

2.1.  Syntax

   The language tag is composed of one or more parts, known as
   "subtags".  Each subtag consists of a sequence of alphanumeric
   characters.  Subtags are distinguished and separated from one another
   by a hyphen ("-", ABNF [RFC4234] %x2D).  A language tag consists of a
   "primary language" subtag and a (possibly empty) series of subsequent
   subtags, each of which refines or narrows the range of languages
   identified by the overall tag.

   Usually, each type of subtag is distinguished by length, position in
   the tag, and content: subtags can be recognized solely by these
   features.  The only exception to this is a fixed list of
   grandfathered tags registered under RFC 3066 [RFC3066].  This makes
   it possible to construct a parser that can extract and assign some
   semantic information to the subtags, even if the specific subtag
   values are not recognized.  Thus, a parser need not have an up-to-
   date copy (or any copy at all) of the subtag registry to perform most
   searching and matching operations.
























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   The syntax of the language tag in ABNF [RFC4234] is:

   Language-Tag  = langtag
                 / privateuse             ; private use tag
                 / grandfathered          ; grandfathered registrations

   langtag       = (language
                    ["-" script]
                    ["-" region]
                    *("-" variant)
                    *("-" extension)
                    ["-" privateuse])

   language      = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code
                 / 4ALPHA                 ; reserved for future use
                 / 5*8ALPHA               ; registered language subtag

   extlang       = *3("-" 3ALPHA)         ; reserved for future use

   script        = 4ALPHA                 ; ISO 15924 code

   region        = 2ALPHA                 ; ISO 3166 code
                 / 3DIGIT                 ; UN M.49 code

   variant       = 5*8alphanum            ; registered variants
                 / (DIGIT 3alphanum)

   extension     = singleton 1*("-" (2*8alphanum))

   singleton     = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT
                 ; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
                 ; Single letters: x/X is reserved for private use

   privateuse    = ("x"/"X") 1*("-" (1*8alphanum))

   grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum))
                   ; grandfathered registration
                   ; Note: i is the only singleton
                   ; that starts a grandfathered tag

   alphanum      = (ALPHA / DIGIT)       ; letters and numbers

                        Figure 1: Language Tag ABNF

   Note: There is a subtlety in the ABNF for 'variant': variants
   starting with a digit MAY be four characters long, while those
   starting with a letter MUST be at least five characters long.




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   All subtags have a maximum length of eight characters and whitespace
   is not permitted in a language tag.  For examples of language tags,
   see Appendix B.

   Note that although [RFC4234] refers to octets, the language tags
   described in this document are sequences of characters from the
   US-ASCII [ISO646] repertoire.  Language tags MAY be used in documents
   and applications that use other encodings, so long as these encompass
   the US-ASCII repertoire.  An example of this would be an XML document
   that uses the UTF-16LE [RFC2781] encoding of [Unicode].

   The tags and their subtags, including private use and extensions, are
   to be treated as case insensitive: there exist conventions for the
   capitalization of some of the subtags, but these MUST NOT be taken to
   carry meaning.

   For example:

   o  [ISO639-1] recommends that language codes be written in lowercase
      ('mn' Mongolian).

   o  [ISO3166-1] recommends that country codes be capitalized ('MN'
      Mongolia).

   o  [ISO15924] recommends that script codes use lowercase with the
      initial letter capitalized ('Cyrl' Cyrillic).

   However, in the tags defined by this document, the uppercase US-ASCII
   letters in the range 'A' through 'Z' are considered equivalent and
   mapped directly to their US-ASCII lowercase equivalents in the range
   'a' through 'z'.  Thus, the tag "mn-Cyrl-MN" is not distinct from
   "MN-cYRL-mn" or "mN-cYrL-Mn" (or any other combination), and each of
   these variations conveys the same meaning: Mongolian written in the
   Cyrillic script as used in Mongolia.

   Although case distinctions do not carry meaning in language tags,
   consistent formatting and presentation of the tags will aid users.
   The format of the tags and subtags in the registry is RECOMMENDED.
   In this format, all non-initial two-letter subtags are uppercase, all
   non-initial four-letter subtags are titlecase, and all other subtags
   are lowercase.










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2.2.  Language Subtag Sources and Interpretation

   The namespace of language tags and their subtags is administered by
   the Internet Assigned Numbers Authority (IANA) [RFC2860] according to
   the rules in Section 5 of this document.  The Language Subtag
   Registry maintained by IANA is the source for valid subtags: other
   standards referenced in this section provide the source material for
   that registry.

   Terminology in this section:

   o  Tag or tags refers to a complete language tag, such as
      "fr-Latn-CA".  Examples of tags in this document are enclosed in
      double-quotes ("en-US").

   o  Subtag refers to a specific section of a tag, delimited by hyphen,
      such as the subtag 'Latn' in "fr-Latn-CA".  Examples of subtags in
      this document are enclosed in single quotes ('Latn').

   o  Code or codes refers to values defined in external standards (and
      that are used as subtags in this document).  For example, 'Latn'
      is an [ISO15924] script code that was used to define the 'Latn'
      script subtag for use in a language tag.  Examples of codes in
      this document are enclosed in single quotes ('en', 'Latn').

   The definitions in this section apply to the various subtags within
   the language tags defined by this document, excepting those
   "grandfathered" tags defined in Section 2.2.8.

   Language tags are designed so that each subtag type has unique length
   and content restrictions.  These make identification of the subtag's
   type possible, even if the content of the subtag itself is
   unrecognized.  This allows tags to be parsed and processed without
   reference to the latest version of the underlying standards or the
   IANA registry and makes the associated exception handling when
   parsing tags simpler.

   Subtags in the IANA registry that do not come from an underlying
   standard can only appear in specific positions in a tag.
   Specifically, they can only occur as primary language subtags or as
   variant subtags.

   Note that sequences of private use and extension subtags MUST occur
   at the end of the sequence of subtags and MUST NOT be interspersed
   with subtags defined elsewhere in this document.

   Single-letter and single-digit subtags are reserved for current or
   future use.  These include the following current uses:



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   o  The single-letter subtag 'x' is reserved to introduce a sequence
      of private use subtags.  The interpretation of any private use
      subtags is defined solely by private agreement and is not defined
      by the rules in this section or in any standard or registry
      defined in this document.

   o  All other single-letter subtags are reserved to introduce
      standardized extension subtag sequences as described in
      Section 3.7.

   The single-letter subtag 'i' is used by some grandfathered tags, such
   as "i-enochian", where it always appears in the first position and
   cannot be confused with an extension.

2.2.1.  Primary Language Subtag

   The primary language subtag is the first subtag in a language tag
   (with the exception of private use and certain grandfathered tags)
   and cannot be omitted.  The following rules apply to the primary
   language subtag:

   1.  All two-character language subtags were defined in the IANA
       registry according to the assignments found in the standard ISO
       639 Part 1, "ISO 639-1:2002, Codes for the representation of
       names of languages -- Part 1: Alpha-2 code" [ISO639-1], or using
       assignments subsequently made by the ISO 639 Part 1 maintenance
       agency or governing standardization bodies.

   2.  All three-character language subtags were defined in the IANA
       registry according to the assignments found in ISO 639 Part 2,
       "ISO 639-2:1998 - Codes for the representation of names of
       languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2], or
       assignments subsequently made by the ISO 639 Part 2 maintenance
       agency or governing standardization bodies.

   3.  The subtags in the range 'qaa' through 'qtz' are reserved for
       private use in language tags.  These subtags correspond to codes
       reserved by ISO 639-2 for private use.  These codes MAY be used
       for non-registered primary language subtags (instead of using
       private use subtags following 'x-').  Please refer to Section 4.5
       for more information on private use subtags.

   4.  All four-character language subtags are reserved for possible
       future standardization.

   5.  All language subtags of 5 to 8 characters in length in the IANA
       registry were defined via the registration process in Section 3.5
       and MAY be used to form the primary language subtag.  At the time



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       this document was created, there were no examples of this kind of
       subtag and future registrations of this type will be discouraged:
       primary languages are strongly RECOMMENDED for registration with
       ISO 639, and proposals rejected by ISO 639/RA will be closely
       scrutinized before they are registered with IANA.

   6.  The single-character subtag 'x' as the primary subtag indicates
       that the language tag consists solely of subtags whose meaning is
       defined by private agreement.  For example, in the tag "x-fr-CH",
       the subtags 'fr' and 'CH' SHOULD NOT be taken to represent the
       French language or the country of Switzerland (or any other value
       in the IANA registry) unless there is a private agreement in
       place to do so.  See Section 4.5.

   7.  The single-character subtag 'i' is used by some grandfathered
       tags (see Section 2.2.8) such as "i-klingon" and "i-bnn".  (Other
       grandfathered tags have a primary language subtag in their first
       position.)

   8.  Other values MUST NOT be assigned to the primary subtag except by
       revision or update of this document.

   Note: For languages that have both an ISO 639-1 two-character code
   and an ISO 639-2 three-character code, only the ISO 639-1 two-
   character code is defined in the IANA registry.

   Note: For languages that have no ISO 639-1 two-character code and for
   which the ISO 639-2/T (Terminology) code and the ISO 639-2/B
   (Bibliographic) codes differ, only the Terminology code is defined in
   the IANA registry.  At the time this document was created, all
   languages that had both kinds of three-character code were also
   assigned a two-character code; it is not expected that future
   assignments of this nature will occur.

   Note: To avoid problems with versioning and subtag choice as
   experienced during the transition between RFC 1766 and RFC 3066, as
   well as the canonical nature of subtags defined by this document, the
   ISO 639 Registration Authority Joint Advisory Committee (ISO 639/
   RA-JAC) has included the following statement in [iso639.prin]:

   "A language code already in ISO 639-2 at the point of freezing ISO
   639-1 shall not later be added to ISO 639-1.  This is to ensure
   consistency in usage over time, since users are directed in Internet
   applications to employ the alpha-3 code when an alpha-2 code for that
   language is not available."






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   In order to avoid instability in the canonical form of tags, if a
   two-character code is added to ISO 639-1 for a language for which a
   three-character code was already included in ISO 639-2, the two-
   character code MUST NOT be registered.  See Section 3.4.

   For example, if some content were tagged with 'haw' (Hawaiian), which
   currently has no two-character code, the tag would not be invalidated
   if ISO 639-1 were to assign a two-character code to the Hawaiian
   language at a later date.

   For example, one of the grandfathered IANA registrations is
   "i-enochian".  The subtag 'enochian' could be registered in the IANA
   registry as a primary language subtag (assuming that ISO 639 does not
   register this language first), making tags such as "enochian-AQ" and
   "enochian-Latn" valid.

2.2.2.  Extended Language Subtags

   The following rules apply to the extended language subtags:

   1.  Three-letter subtags immediately following the primary subtag are
       reserved for future standardization, anticipating work that is
       currently under way on ISO 639.

   2.  Extended language subtags MUST follow the primary subtag and
       precede any other subtags.

   3.  There MAY be up to three extended language subtags.

   4.  Extended language subtags MUST NOT be registered or used to form
       language tags.  Their syntax is described here so that
       implementations can be compatible with any future revision of
       this document that does provide for their registration.

   Extended language subtag records, once they appear in the registry,
   MUST include exactly one 'Prefix' field indicating an appropriate
   language subtag or sequence of subtags that MUST always appear as a
   prefix to the extended language subtag.

   Example: In a future revision or update of this document, the tag
   "zh-gan" (registered under RFC 3066) might become a valid non-
   grandfathered (that is, redundant) tag in which the subtag 'gan'
   might represent the Chinese dialect 'Gan'.








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2.2.3.  Script Subtag

   Script subtags are used to indicate the script or writing system
   variations that distinguish the written forms of a language or its
   dialects.  The following rules apply to the script subtags:

   1.  All four-character subtags were defined according to
       [ISO15924]--"Codes for the representation of names of scripts":
       alpha-4 script codes, or subsequently assigned by the ISO 15924
       maintenance agency or governing standardization bodies, denoting
       the script or writing system used in conjunction with this
       language.

   2.  Script subtags MUST immediately follow the primary language
       subtag and all extended language subtags and MUST occur before
       any other type of subtag described below.

   3.  The script subtags 'Qaaa' through 'Qabx' are reserved for private
       use in language tags.  These subtags correspond to codes reserved
       by ISO 15924 for private use.  These codes MAY be used for non-
       registered script values.  Please refer to Section 4.5 for more
       information on private use subtags.

   4.  Script subtags MUST NOT be registered using the process in
       Section 3.5 of this document.  Variant subtags MAY be considered
       for registration for that purpose.

   5.  There MUST be at most one script subtag in a language tag, and
       the script subtag SHOULD be omitted when it adds no
       distinguishing value to the tag or when the primary language
       subtag's record includes a Suppress-Script field listing the
       applicable script subtag.

   Example: "sr-Latn" represents Serbian written using the Latin script.

2.2.4.  Region Subtag

   Region subtags are used to indicate linguistic variations associated
   with or appropriate to a specific country, territory, or region.
   Typically, a region subtag is used to indicate regional dialects or
   usage, or region-specific spelling conventions.  A region subtag can
   also be used to indicate that content is expressed in a way that is
   appropriate for use throughout a region, for instance, Spanish
   content tailored to be useful throughout Latin America.







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   The following rules apply to the region subtags:

   1.  Region subtags MUST follow any language, extended language, or
       script subtags and MUST precede all other subtags.

   2.  All two-character subtags following the primary subtag were
       defined in the IANA registry according to the assignments found
       in [ISO3166-1] ("Codes for the representation of names of
       countries and their subdivisions -- Part 1: Country codes") using
       the list of alpha-2 country codes, or using assignments
       subsequently made by the ISO 3166 maintenance agency or governing
       standardization bodies.

   3.  All three-character subtags consisting of digit (numeric)
       characters following the primary subtag were defined in the IANA
       registry according to the assignments found in UN Standard
       Country or Area Codes for Statistical Use [UN_M.49] or
       assignments subsequently made by the governing standards body.
       Note that not all of the UN M.49 codes are defined in the IANA
       registry.  The following rules define which codes are entered
       into the registry as valid subtags:

       A.  UN numeric codes assigned to 'macro-geographical
           (continental)' or sub-regions MUST be registered in the
           registry.  These codes are not associated with an assigned
           ISO 3166 alpha-2 code and represent supra-national areas,
           usually covering more than one nation, state, province, or
           territory.

       B.  UN numeric codes for 'economic groupings' or 'other
           groupings' MUST NOT be registered in the IANA registry and
           MUST NOT be used to form language tags.

       C.  UN numeric codes for countries or areas with ambiguous ISO
           3166 alpha-2 codes, when entered into the registry, MUST be
           defined according to the rules in Section 3.4 and MUST be
           used to form language tags that represent the country or
           region for which they are defined.

       D.  UN numeric codes for countries or areas for which there is an
           associated ISO 3166 alpha-2 code in the registry MUST NOT be
           entered into the registry and MUST NOT be used to form
           language tags.  Note that the ISO 3166-based subtag in the
           registry MUST actually be associated with the UN M.49 code in
           question.






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       E.  UN numeric codes and ISO 3166 alpha-2 codes for countries or
           areas listed as eligible for registration in [RFC4645] but
           not presently registered MAY be entered into the IANA
           registry via the process described in Section 3.5.  Once
           registered, these codes MAY be used to form language tags.

       F.  All other UN numeric codes for countries or areas that do not
           have an associated ISO 3166 alpha-2 code MUST NOT be entered
           into the registry and MUST NOT be used to form language tags.
           For more information about these codes, see Section 3.4.

   4.  Note: The alphanumeric codes in Appendix X of the UN document
       MUST NOT be entered into the registry and MUST NOT be used to
       form language tags.  (At the time this document was created,
       these values matched the ISO 3166 alpha-2 codes.)

   5.  There MUST be at most one region subtag in a language tag and the
       region subtag MAY be omitted, as when it adds no distinguishing
       value to the tag.

   6.  The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ' are
       reserved for private use in language tags.  These subtags
       correspond to codes reserved by ISO 3166 for private use.  These
       codes MAY be used for private use region subtags (instead of
       using a private use subtag sequence).  Please refer to
       Section 4.5 for more information on private use subtags.

   "de-CH" represents German ('de') as used in Switzerland ('CH').

   "sr-Latn-CS" represents Serbian ('sr') written using Latin script
   ('Latn') as used in Serbia and Montenegro ('CS').

   "es-419" represents Spanish ('es') appropriate to the UN-defined
   Latin America and Caribbean region ('419').

2.2.5.  Variant Subtags

   Variant subtags are used to indicate additional, well-recognized
   variations that define a language or its dialects that are not
   covered by other available subtags.  The following rules apply to the
   variant subtags:

   1.  Variant subtags are not associated with any external standard.
       Variant subtags and their meanings are defined by the
       registration process defined in Section 3.5.

   2.  Variant subtags MUST follow all of the other defined subtags, but
       precede any extension or private use subtag sequences.



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   3.  More than one variant MAY be used to form the language tag.

   4.  Variant subtags MUST be registered with IANA according to the
       rules in Section 3.5 of this document before being used to form
       language tags.  In order to distinguish variants from other types
       of subtags, registrations MUST meet the following length and
       content restrictions:

       1.  Variant subtags that begin with a letter (a-z, A-Z) MUST be
           at least five characters long.

       2.  Variant subtags that begin with a digit (0-9) MUST be at
           least four characters long.

   Variant subtag records in the language subtag registry MAY include
   one or more 'Prefix' fields, which indicate the language tag or tags
   that would make a suitable prefix (with other subtags, as
   appropriate) in forming a language tag with the variant.  For
   example, the subtag 'nedis' has a Prefix of "sl", making it suitable
   to form language tags such as "sl-nedis" and "sl-IT-nedis", but not
   suitable for use in a tag such as "zh-nedis" or "it-IT-nedis".

   "sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.

   "de-CH-1996" represents German as used in Switzerland and as written
   using the spelling reform beginning in the year 1996 C.E.

   Most variants that share a prefix are mutually exclusive.  For
   example, the German orthographic variations '1996' and '1901' SHOULD
   NOT be used in the same tag, as they represent the dates of different
   spelling reforms.  A variant that can meaningfully be used in
   combination with another variant SHOULD include a 'Prefix' field in
   its registry record that lists that other variant.  For example, if
   another German variant 'example' were created that made sense to use
   with '1996', then 'example' should include two Prefix fields: "de"
   and "de-1996".

2.2.6.  Extension Subtags

   Extensions provide a mechanism for extending language tags for use in
   various applications.  See Section 3.7.  The following rules apply to
   extensions:

   1.   Extension subtags are separated from the other subtags defined
        in this document by a single-character subtag ("singleton").
        The singleton MUST be one allocated to a registration authority
        via the mechanism described in Section 3.7 and MUST NOT be the
        letter 'x', which is reserved for private use subtag sequences.



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   2.   Note: Private use subtag sequences starting with the singleton
        subtag 'x' are described in Section 2.2.7 below.

   3.   An extension MUST follow at least a primary language subtag.
        That is, a language tag cannot begin with an extension.
        Extensions extend language tags, they do not override or replace
        them.  For example, "a-value" is not a well-formed language tag,
        while "de-a-value" is.

   4.   Each singleton subtag MUST appear at most one time in each tag
        (other than as a private use subtag).  That is, singleton
        subtags MUST NOT be repeated.  For example, the tag
        "en-a-bbb-a-ccc" is invalid because the subtag 'a' appears
        twice.  Note that the tag "en-a-bbb-x-a-ccc" is valid because
        the second appearance of the singleton 'a' is in a private use
        sequence.

   5.   Extension subtags MUST meet all of the requirements for the
        content and format of subtags defined in this document.

   6.   Extension subtags MUST meet whatever requirements are set by the
        document that defines their singleton prefix and whatever
        requirements are provided by the maintaining authority.

   7.   Each extension subtag MUST be from two to eight characters long
        and consist solely of letters or digits, with each subtag
        separated by a single '-'.

   8.   Each singleton MUST be followed by at least one extension
        subtag.  For example, the tag "tlh-a-b-foo" is invalid because
        the first singleton 'a' is followed immediately by another
        singleton 'b'.

   9.   Extension subtags MUST follow all language, extended language,
        script, region, and variant subtags in a tag.

   10.  All subtags following the singleton and before another singleton
        are part of the extension.  Example: In the tag "fr-a-Latn", the
        subtag 'Latn' does not represent the script subtag 'Latn'
        defined in the IANA Language Subtag Registry.  Its meaning is
        defined by the extension 'a'.

   11.  In the event that more than one extension appears in a single
        tag, the tag SHOULD be canonicalized as described in
        Section 4.4.






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   For example, if the prefix singleton 'r' and the shown subtags were
   defined, then the following tag would be a valid example:
   "en-Latn-GB-boont-r-extended-sequence-x-private".

2.2.7.  Private Use Subtags

   Private use subtags are used to indicate distinctions in language
   important in a given context by private agreement.  The following
   rules apply to private use subtags:

   1.  Private use subtags are separated from the other subtags defined
       in this document by the reserved single-character subtag 'x'.

   2.  Private use subtags MUST conform to the format and content
       constraints defined in the ABNF for all subtags.

   3.  Private use subtags MUST follow all language, extended language,
       script, region, variant, and extension subtags in the tag.
       Another way of saying this is that all subtags following the
       singleton 'x' MUST be considered private use.  Example: The
       subtag 'US' in the tag "en-x-US" is a private use subtag.

   4.  A tag MAY consist entirely of private use subtags.

   5.  No source is defined for private use subtags.  Use of private use
       subtags is by private agreement only.

   6.  Private use subtags are NOT RECOMMENDED where alternatives exist
       or for general interchange.  See Section 4.5 for more information
       on private use subtag choice.

   For example: Users who wished to utilize codes from the Ethnologue
   publication of SIL International for language identification might
   agree to exchange tags such as "az-Arab-x-AZE-derbend".  This example
   contains two private use subtags.  The first is 'AZE' and the second
   is 'derbend'.

2.2.8.  Preexisting RFC 3066 Registrations

   Existing IANA-registered language tags from RFC 1766 and/or RFC 3066
   maintain their validity.  These tags will be maintained in the
   registry in records of either the "grandfathered" or "redundant"
   type.  Grandfathered tags contain one or more subtags that are not
   defined in the Language Subtag Registry (see Section 3).  Redundant
   tags consist entirely of subtags defined above and whose independent
   registration is superseded by this document.  For more information,
   see Section 3.8.




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   It is important to note that all language tags formed under the
   guidelines in this document were either legal, well-formed tags or
   could have been registered under RFC 3066.

2.2.9.  Classes of Conformance

   Implementations sometimes need to describe their capabilities with
   regard to the rules and practices described in this document.  There
   are two classes of conforming implementations described by this
   document: "well-formed" processors and "validating" processors.
   Claims of conformance SHOULD explicitly reference one of these
   definitions.

   An implementation that claims to check for well-formed language tags
   MUST:

   o  Check that the tag and all of its subtags, including extension and
      private use subtags, conform to the ABNF or that the tag is on the
      list of grandfathered tags.

   o  Check that singleton subtags that identify extensions do not
      repeat.  For example, the tag "en-a-xx-b-yy-a-zz" is not well-
      formed.

   Well-formed processors are strongly encouraged to implement the
   canonicalization rules contained in Section 4.4.

   An implementation that claims to be validating MUST:

   o  Check that the tag is well-formed.

   o  Specify the particular registry date for which the implementation
      performs validation of subtags.

   o  Check that either the tag is a grandfathered tag, or that all
      language, script, region, and variant subtags consist of valid
      codes for use in language tags according to the IANA registry as
      of the particular date specified by the implementation.

   o  Specify which, if any, extension RFCs as defined in Section 3.7
      are supported, including version, revision, and date.

   o  For any such extensions supported, check that all subtags used in
      that extension are valid.

   o  For variant and extended language subtags, if the registry
      contains one or more 'Prefix' fields for that subtag, check that
      the tag matches at least one prefix.  The tag matches if all the



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      subtags in the 'Prefix' also appear in the tag.  For example, the
      prefix "es-CO" matches the tag "es-Latn-CO-x-private" because both
      the 'es' language subtag and 'CO' region subtag appear in the tag.

3.  Registry Format and Maintenance

   This section defines the Language Subtag Registry and the maintenance
   and update procedures associated with it, as well as a registry for
   extensions to language tags (Section 3.7).

   The Language Subtag Registry contains a comprehensive list of all of
   the subtags valid in language tags.  This allows implementers a
   straightforward and reliable way to validate language tags.  The
   Language Subtag Registry will be maintained so that, except for
   extension subtags, it is possible to validate all of the subtags that
   appear in a language tag under the provisions of this document or its
   revisions or successors.  In addition, the meaning of the various
   subtags will be unambiguous and stable over time.  (The meaning of
   private use subtags, of course, is not defined by the IANA registry.)

3.1.  Format of the IANA Language Subtag Registry

   The IANA Language Subtag Registry ("the registry") consists of a text
   file that is machine readable in the format described in this
   section, plus copies of the registration forms approved in accordance
   with the process described in Section 3.5.  The existing registration
   forms for grandfathered and redundant tags taken from RFC 3066 will
   be maintained as part of the obsolete RFC 3066 registry.  The
   remaining set of initial subtags will not have registration forms
   created for them.

   The registry is in the text format described below.  This format was
   based on the record-jar format described in [record-jar].

   Each line of text is limited to 72 characters, including all
   whitespace.  Records are separated by lines containing only the
   sequence "%%" (%x25.25).

   Each field can be viewed as a single, logical line of ASCII
   characters, comprising a field-name and a field-body separated by a
   COLON character (%x3A).  For convenience, the field-body portion of
   this conceptual entity can be split into a multiple-line
   representation; this is called "folding".  The format of the registry
   is described by the following ABNF (per [RFC4234]):







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   registry   = record *("%%" CRLF record)
   record     = 1*( field-name *SP ":" *SP field-body CRLF )
   field-name = (ALPHA / DIGIT) [*(ALPHA / DIGIT / "-") (ALPHA / DIGIT)]
   field-body = *(ASCCHAR/LWSP)
   ASCCHAR    = %x21-25 / %x27-7E / UNICHAR ; Note: AMPERSAND is %x26
   UNICHAR    = "&#x" 2*6HEXDIG ";"

                      Figure 2: Registry Format ABNF

   The sequence '..' (%x2E.2E) in a field-body denotes a range of
   values.  Such a range represents all subtags of the same length that
   are in alphabetic or numeric order within that range, including the
   values explicitly mentioned.  For example 'a..c' denotes the values
   'a', 'b', and 'c' and '11..13' denotes the values '11', '12', and
   '13'.

   Characters from outside the US-ASCII [ISO646] repertoire, as well as
   the AMPERSAND character ("&", %x26) when it occurs in a field-body,
   are represented by a "Numeric Character Reference" using hexadecimal
   notation in the style used by [XML10] (see
   <http://www.w3.org/TR/REC-xml/#dt-charref>).  This consists of the
   sequence "&#x" (%x26.23.78) followed by a hexadecimal representation
   of the character's code point in [ISO10646] followed by a closing
   semicolon (%x3B).  For example, the EURO SIGN, U+20AC, would be
   represented by the sequence "&#x20AC;".  Note that the hexadecimal
   notation MAY have between two and six digits.

   All fields whose field-body contains a date value use the "full-date"
   format specified in [RFC3339].  For example: "2004-06-28" represents
   June 28, 2004, in the Gregorian calendar.

   The first record in the file contains the single field whose field-
   name is "File-Date" (see Figure 3).  The field-body of this record
   contains the last modification date of this copy of the registry,
   making it possible to compare different versions of the registry.
   The registry on the IANA website is the most current.  Versions with
   an older date than that one are not up-to-date.

   File-Date: 2004-06-28
   %%

                 Figure 3: Example of the File-Date Record

   Subsequent records represent subtags in the registry.  Each of the
   fields in each record MUST occur no more than once, unless otherwise
   noted below.  Each record MUST contain the following fields:





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   o  'Type'

      *  Type's field-value MUST consist of one of the following
         strings: "language", "extlang", "script", "region", "variant",
         "grandfathered", and "redundant" and denotes the type of tag or
         subtag.

   o  Either 'Subtag' or 'Tag'

      *  Subtag's field-value contains the subtag being defined.  This
         field MUST only appear in records of whose 'Type' has one of
         these values: "language", "extlang", "script", "region", or
         "variant".

      *  Tag's field-value contains a complete language tag.  This field
         MUST only appear in records whose 'Type' has one of these
         values: "grandfathered" or "redundant".  Note that the field-
         value will always follow the 'grandfathered' production in the
         ABNF in Section 2.1

   o  Description

      *  Description's field-value contains a non-normative description
         of the subtag or tag.

   o  Added

      *  Added's field-value contains the date the record was added to
         the registry.

   The 'Subtag' or 'Tag' field MUST use lowercase letters to form the
   subtag or tag, with two exceptions.  Subtags whose 'Type' field is
   'script' (in other words, subtags defined by ISO 15924) MUST use
   titlecase.  Subtags whose 'Type' field is 'region' (in other words,
   subtags defined by ISO 3166) MUST use uppercase.  These exceptions
   mirror the use of case in the underlying standards.

   The field 'Description' MAY appear more than one time and contains a
   description of the tag or subtag in the record.  At least one of the
   'Description' fields MUST be written or transcribed into the Latin
   script; the same or additional fields MAY also include a description
   in a non-Latin script.  The 'Description' field is used for
   identification purposes and SHOULD NOT be taken to represent the
   actual native name of the language or variation or to be in any
   particular language.  Most descriptions are taken directly from
   source standards such as ISO 639 or ISO 3166.





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   Note: Descriptions in registry entries that correspond to ISO 639,
   ISO 15924, ISO 3166, or UN M.49 codes are intended only to indicate
   the meaning of that identifier as defined in the source standard at
   the time it was added to the registry.  The description does not
   replace the content of the source standard itself.  The descriptions
   are not intended to be the English localized names for the subtags.
   Localization or translation of language tag and subtag descriptions
   is out of scope of this document.

   Each record MAY also contain the following fields:

   o  Preferred-Value

      *  For fields of type 'language', 'extlang', 'script', 'region',
         and 'variant', 'Preferred-Value' contains the subtag of the
         same 'Type' that is preferred for forming the language tag.

      *  For fields of type 'grandfathered' and 'redundant', a canonical
         mapping to a complete language tag.

   o  Deprecated

      *  Deprecated's field-value contains the date the record was
         deprecated.

   o  Prefix

      *  Prefix's field-value contains a language tag with which this
         subtag MAY be used to form a new language tag, perhaps with
         other subtags as well.  This field MUST only appear in records
         whose 'Type' field-value is 'variant' or 'extlang'.  For
         example, the 'Prefix' for the variant 'nedis' is 'sl', meaning
         that the tags "sl-nedis" and "sl-IT-nedis" might be appropriate
         while the tag "is-nedis" is not.

   o  Comments

      *  Comments contains additional information about the subtag, as
         deemed appropriate for understanding the registry and
         implementing language tags using the subtag or tag.

   o  Suppress-Script

      *  Suppress-Script contains a script subtag that SHOULD NOT be
         used to form language tags with the associated primary language
         subtag.  This field MUST only appear in records whose 'Type'
         field-value is 'language'.  See Section 4.1.




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   The field 'Deprecated' MAY be added to any record via the maintenance
   process described in Section 3.3 or via the registration process
   described in Section 3.5.  Usually, the addition of a 'Deprecated'
   field is due to the action of one of the standards bodies, such as
   ISO 3166, withdrawing a code.  In some historical cases, it might not
   have been possible to reconstruct the original deprecation date.  For
   these cases, an approximate date appears in the registry.  Although
   valid in language tags, subtags and tags with a 'Deprecated' field
   are deprecated and validating processors SHOULD NOT generate these
   subtags.  Note that a record that contains a 'Deprecated' field and
   no corresponding 'Preferred-Value' field has no replacement mapping.

   The field 'Preferred-Value' contains a mapping between the record in
   which it appears and another tag or subtag.  The value in this field
   is STRONGLY RECOMMENDED as the best choice to represent the value of
   this record when selecting a language tag.  These values form three
   groups:

   1.  ISO 639 language codes that were later withdrawn in favor of
       other codes.  These values are mostly a historical curiosity.

   2.  ISO 3166 region codes that have been withdrawn in favor of a new
       code.  This sometimes happens when a country changes its name or
       administration in such a way that warrants a new region code.

   3.  Tags grandfathered from RFC 3066.  In many cases, these tags have
       become obsolete because the values they represent were later
       encoded by ISO 639.

   Records that contain a 'Preferred-Value' field MUST also have a
   'Deprecated' field.  This field contains a date of deprecation.
   Thus, a language tag processor can use the registry to construct the
   valid, non-deprecated set of subtags for a given date.  In addition,
   for any given tag, a processor can construct the set of valid
   language tags that correspond to that tag for all dates up to the
   date of the registry.  The ability to do these mappings MAY be
   beneficial to applications that are matching, selecting, for
   filtering content based on its language tags.

   Note that 'Preferred-Value' mappings in records of type 'region'
   sometimes do not represent exactly the same meaning as the original
   value.  There are many reasons for a country code to be changed, and
   the effect this has on the formation of language tags will depend on
   the nature of the change in question.

   In particular, the 'Preferred-Value' field does not imply retagging
   content that uses the affected subtag.




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   The field 'Preferred-Value' MUST NOT be modified once created in the
   registry.  The field MAY be added to records of type "grandfathered"
   and "region" according to the rules in Section 3.3.  Otherwise the
   field MUST NOT be added to any record already in the registry.

   The 'Preferred-Value' field in records of type "grandfathered" and
   "redundant" contains whole language tags that are strongly
   RECOMMENDED for use in place of the record's value.  In many cases,
   the mappings were created by deprecation of the tags during the
   period before this document was adopted.  For example, the tag
   "no-nyn" was deprecated in favor of the ISO 639-1-defined language
   code 'nn'.

   Records of type 'variant' MAY have more than one field of type
   'Prefix'.  Additional fields of this type MAY be added to a 'variant'
   record via the registration process.

   Records of type 'extlang' MUST have _exactly_ one 'Prefix' field.

   The field-value of the 'Prefix' field consists of a language tag
   whose subtags are appropriate to use with this subtag.  For example,
   the variant subtag '1996' has a 'Prefix' field of "de".  This means
   that tags starting with the sequence "de-" are appropriate with this
   subtag, so "de-Latg-1996" and "de-CH-1996" are both acceptable, while
   the tag "fr-1996" is an inappropriate choice.

   The field of type 'Prefix' MUST NOT be removed from any record.  The
   field-value for this type of field MUST NOT be modified.

   The field 'Comments' MAY appear more than once per record.  This
   field MAY be inserted or changed via the registration process and no
   guarantee of stability is provided.  The content of this field is not
   restricted, except by the need to register the information, the
   suitability of the request, and by reasonable practical size
   limitations.

   The field 'Suppress-Script' MUST only appear in records whose 'Type'
   field-value is 'language'.  This field MUST NOT appear more than one
   time in a record.  This field indicates a script used to write the
   overwhelming majority of documents for the given language and that
   therefore adds no distinguishing information to a language tag.  It
   helps ensure greater compatibility between the language tags
   generated according to the rules in this document and language tags
   and tag processors or consumers based on RFC 3066.  For example,
   virtually all Icelandic documents are written in the Latin script,
   making the subtag 'Latn' redundant in the tag "is-Latn".





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3.2.  Language Subtag Reviewer

   The Language Subtag Reviewer is appointed by the IESG for an
   indefinite term, subject to removal or replacement at the IESG's
   discretion.  The Language Subtag Reviewer moderates the ietf-
   languages mailing list, responds to requests for registration, and
   performs the other registry maintenance duties described in
   Section 3.3.  Only the Language Subtag Reviewer is permitted to
   request IANA to change, update, or add records to the Language Subtag
   Registry.

   The performance or decisions of the Language Subtag Reviewer MAY be
   appealed to the IESG under the same rules as other IETF decisions
   (see [RFC2026]).  The IESG can reverse or overturn the decision of
   the Language Subtag Reviewer, provide guidance, or take other
   appropriate actions.

3.3.  Maintenance of the Registry

   Maintenance of the registry requires that as codes are assigned or
   withdrawn by ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language
   Subtag Reviewer MUST evaluate each change, determine whether it
   conflicts with existing registry entries, and submit the information
   to IANA for inclusion in the registry.  If a change takes place and
   the Language Subtag Reviewer does not do this in a timely manner,
   then any interested party MAY use the procedure in Section 3.5 to
   register the appropriate update.

   Note: The redundant and grandfathered entries together are the
   complete list of tags registered under [RFC3066].  The redundant tags
   are those that can now be formed using the subtags defined in the
   registry together with the rules of Section 2.2.  The grandfathered
   entries include those that can never be legal under those same
   provisions.

   The set of redundant and grandfathered tags is permanent and stable:
   new entries in this section MUST NOT be added and existing entries
   MUST NOT be removed.  Records of type 'grandfathered' MAY have their
   type converted to 'redundant'; see item 12 in Section 3.6 for more
   information.  The decision-making process about which tags were
   initially grandfathered and which were made redundant is described in
   [RFC4645].

   RFC 3066 tags that were deprecated prior to the adoption of this
   document are part of the list of grandfathered tags, and their
   component subtags were not included as registered variants (although
   they remain eligible for registration).  For example, the tag
   "art-lojban" was deprecated in favor of the language subtag 'jbo'.



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   The Language Subtag Reviewer MUST ensure that new subtags meet the
   requirements in Section 4.1 or submit an appropriate alternate subtag
   as described in that section.  When either a change or addition to
   the registry is needed, the Language Subtag Reviewer MUST prepare the
   complete record, including all fields, and forward it to IANA for
   insertion into the registry.  Each record being modified or inserted
   MUST be forwarded in a separate message.

   If a record represents a new subtag that does not currently exist in
   the registry, then the message's subject line MUST include the word
   "INSERT".  If the record represents a change to an existing subtag,
   then the subject line of the message MUST include the word "MODIFY".
   The message MUST contain both the record for the subtag being
   inserted or modified and the new File-Date record.  Here is an
   example of what the body of the message might contain:

   LANGUAGE SUBTAG MODIFICATION
   File-Date: 2005-01-02
   %%
   Type: variant
   Subtag: nedis
   Description: Natisone dialect
   Description: Nadiza dialect
   Added: 2003-10-09
   Prefix: sl
   Comments: This is a comment shown
     as an example.
   %%

         Figure 4: Example of a Language Subtag Modification Form

   Whenever an entry is created or modified in the registry, the
   'File-Date' record at the start of the registry is updated to reflect
   the most recent modification date in the [RFC3339] "full-date"
   format.

   Before forwarding a new registration to IANA, the Language Subtag
   Reviewer MUST ensure that values in the 'Subtag' field match case
   according to the description in Section 3.1.

3.4.  Stability of IANA Registry Entries

   The stability of entries and their meaning in the registry is
   critical to the long-term stability of language tags.  The rules in
   this section guarantee that a specific language tag's meaning is
   stable over time and will not change.





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   These rules specifically deal with how changes to codes (including
   withdrawal and deprecation of codes) maintained by ISO 639, ISO
   15924, ISO 3166, and UN M.49 are reflected in the IANA Language
   Subtag Registry.  Assignments to the IANA Language Subtag Registry
   MUST follow the following stability rules:

   1.   Values in the fields 'Type', 'Subtag', 'Tag', 'Added',
        'Deprecated' and 'Preferred-Value' MUST NOT be changed and are
        guaranteed to be stable over time.

   2.   Values in the 'Description' field MUST NOT be changed in a way
        that would invalidate previously-existing tags.  They MAY be
        broadened somewhat in scope, changed to add information, or
        adapted to the most common modern usage.  For example, countries
        occasionally change their official names; a historical example
        of this would be "Upper Volta" changing to "Burkina Faso".

   3.   Values in the field 'Prefix' MAY be added to records of type
        'variant' via the registration process.

   4.   Values in the field 'Prefix' MAY be modified, so long as the
        modifications broaden the set of prefixes.  That is, a prefix
        MAY be replaced by one of its own prefixes.  For example, the
        prefix "en-US" could be replaced by "en", but not by the
        prefixes "en-Latn", "fr", or "en-US-boont".  If one of those
        prefixes were needed, a new Prefix SHOULD be registered.

   5.   Values in the field 'Prefix' MUST NOT be removed.

   6.   The field 'Comments' MAY be added, changed, modified, or removed
        via the registration process or any of the processes or
        considerations described in this section.

   7.   The field 'Suppress-Script' MAY be added or removed via the
        registration process.

   8.   Codes assigned by ISO 639, ISO 15924, and ISO 3166 that do not
        conflict with existing subtags of the associated type and whose
        meaning is not the same as an existing subtag of the same type
        are entered into the IANA registry as new records.

   9.   Codes assigned by ISO 639, ISO 15924, or ISO 3166 that are
        withdrawn by their respective maintenance or registration
        authority remain valid in language tags.  A 'Deprecated' field
        containing the date of withdrawal is added to the record.  If a
        new record of the same type is added that represents a





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        replacement value, then a 'Preferred-Value' field MAY also be
        added.  The registration process MAY be used to add comments
        about the withdrawal of the code by the respective standard.

        Example
           The region code 'TL' was assigned to the country 'Timor-
           Leste', replacing the code 'TP' (which was assigned to 'East
           Timor' when it was under administration by Portugal).  The
           subtag 'TP' remains valid in language tags, but its record
           contains the a 'Preferred-Value' of 'TL' and its field
           'Deprecated' contains the date the new code was assigned
           ('2004-07-06').

   10.  Codes assigned by ISO 639, ISO 15924, or ISO 3166 that conflict
        with existing subtags of the associated type, including subtags
        that are deprecated, MUST NOT be entered into the registry.  The
        following additional considerations apply to subtag values that
        are reassigned:

        A.  For ISO 639 codes, if the newly assigned code's meaning is
            not represented by a subtag in the IANA registry, the
            Language Subtag Reviewer, as described in Section 3.5, SHALL
            prepare a proposal for entering in the IANA registry as soon
            as practical a registered language subtag as an alternate
            value for the new code.  The form of the registered language
            subtag will be at the discretion of the Language Subtag
            Reviewer and MUST conform to other restrictions on language
            subtags in this document.

        B.  For all subtags whose meaning is derived from an external
            standard (i.e., ISO 639, ISO 15924, ISO 3166, or UN M.49),
            if a new meaning is assigned to an existing code and the new
            meaning broadens the meaning of that code, then the meaning
            for the associated subtag MAY be changed to match.  The
            meaning of a subtag MUST NOT be narrowed, however, as this
            can result in an unknown proportion of the existing uses of
            a subtag becoming invalid.  Note: ISO 639 maintenance
            agency/registration authority (MA/RA) has adopted a similar
            stability policy.

        C.  For ISO 15924 codes, if the newly assigned code's meaning is
            not represented by a subtag in the IANA registry, the
            Language Subtag Reviewer, as described in Section 3.5, SHALL
            prepare a proposal for entering in the IANA registry as soon
            as practical a registered variant subtag as an alternate
            value for the new code.  The form of the registered variant





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            subtag will be at the discretion of the Language Subtag
            Reviewer and MUST conform to other restrictions on variant
            subtags in this document.

        D.  For ISO 3166 codes, if the newly assigned code's meaning is
            associated with the same UN M.49 code as another 'region'
            subtag, then the existing region subtag remains as the
            preferred value for that region and no new entry is created.
            A comment MAY be added to the existing region subtag
            indicating the relationship to the new ISO 3166 code.

        E.  For ISO 3166 codes, if the newly assigned code's meaning is
            associated with a UN M.49 code that is not represented by an
            existing region subtag, then the Language Subtag Reviewer,
            as described in Section 3.5, SHALL prepare a proposal for
            entering the appropriate UN M.49 country code as an entry in
            the IANA registry.

        F.  For ISO 3166 codes, if there is no associated UN numeric
            code, then the Language Subtag Reviewer SHALL petition the
            UN to create one.  If there is no response from the UN
            within ninety days of the request being sent, the Language
            Subtag Reviewer SHALL prepare a proposal for entering in the
            IANA registry as soon as practical a registered variant
            subtag as an alternate value for the new code.  The form of
            the registered variant subtag will be at the discretion of
            the Language Subtag Reviewer and MUST conform to other
            restrictions on variant subtags in this document.  This
            situation is very unlikely to ever occur.

   11.  UN M.49 has codes for both countries and areas (such as '276'
        for Germany) and geographical regions and sub-regions (such as
        '150' for Europe).  UN M.49 country or area codes for which
        there is no corresponding ISO 3166 code SHOULD NOT be
        registered, except as a surrogate for an ISO 3166 code that is
        blocked from registration by an existing subtag.  If such a code
        becomes necessary, then the registration authority for ISO 3166
        SHOULD first be petitioned to assign a code to the region.  If
        the petition for a code assignment by ISO 3166 is refused or not
        acted on in a timely manner, the registration process described
        in Section 3.5 MAY then be used to register the corresponding UN
        M.49 code.  At the time this document was written, there were
        only four such codes: 830 (Channel Islands), 831 (Guernsey), 832
        (Jersey), and 833 (Isle of Man).  This way, UN M.49 codes remain
        available as the value of last resort in cases where ISO 3166
        reassigns a deprecated value in the registry.





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   12.  Stability provisions apply to grandfathered tags with this
        exception: should all of the subtags in a grandfathered tag
        become valid subtags in the IANA registry, then the field 'Type'
        in that record is changed from 'grandfathered' to 'redundant'.
        Note that this will not affect language tags that match the
        grandfathered tag, since these tags will now match valid
        generative subtag sequences.  For example, if the subtag 'gan'
        in the language tag "zh-gan" were to be registered as an
        extended language subtag, then the grandfathered tag "zh-gan"
        would be deprecated (but existing content or implementations
        that use "zh-gan" would remain valid).

3.5.  Registration Procedure for Subtags

   The procedure given here MUST be used by anyone who wants to use a
   subtag not currently in the IANA Language Subtag Registry.

   Only subtags of type 'language' and 'variant' will be considered for
   independent registration of new subtags.  Handling of subtags needed
   for stability and subtags necessary to keep the registry synchronized
   with ISO 639, ISO 15924, ISO 3166, and UN M.49 within the limits
   defined by this document are described in Section 3.3.  Stability
   provisions are described in Section 3.4.

   This procedure MAY also be used to register or alter the information
   for the 'Description', 'Comments', 'Deprecated', or 'Prefix' fields
   in a subtag's record as described in Section 3.4.  Changes to all
   other fields in the IANA registry are NOT permitted.

   Registering a new subtag or requesting modifications to an existing
   tag or subtag starts with the requester filling out the registration
   form reproduced below.  Note that each response is not limited in
   size so that the request can adequately describe the registration.
   The fields in the "Record Requested" section SHOULD follow the
   requirements in Section 3.1.
















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   LANGUAGE SUBTAG REGISTRATION FORM
   1. Name of requester:
   2. E-mail address of requester:
   3. Record Requested:

      Type:
      Subtag:
      Description:
      Prefix:
      Preferred-Value:
      Deprecated:
      Suppress-Script:
      Comments:

   4. Intended meaning of the subtag:
   5. Reference to published description
      of the language (book or article):
   6. Any other relevant information:

              Figure 5: The Language Subtag Registration Form

   The subtag registration form MUST be sent to
   <ietf-languages@iana.org> for a two-week review period before it can
   be submitted to IANA.  (This is an open list and can be joined by
   sending a request to <ietf-languages-request@iana.org>.)

   Variant subtags are usually registered for use with a particular
   range of language tags.  For example, the subtag 'rozaj' is intended
   for use with language tags that start with the primary language
   subtag "sl", since Resian is a dialect of Slovenian.  Thus, the
   subtag 'rozaj' would be appropriate in tags such as "sl-Latn-rozaj"
   or "sl-IT-rozaj".  This information is stored in the 'Prefix' field
   in the registry.  Variant registration requests SHOULD include at
   least one 'Prefix' field in the registration form.

   Extended language subtags are reserved for future standardization.
   These subtags will be REQUIRED to include exactly one 'Prefix' field
   once they are allowed for registration.

   The 'Prefix' field for a given registered subtag exists in the IANA
   registry as a guide to usage.  Additional prefixes MAY be added by
   filing an additional registration form.  In that form, the "Any other
   relevant information:" field MUST indicate that it is the addition of
   a prefix.

   Requests to add a prefix to a variant subtag that imply a different
   semantic meaning will probably be rejected.  For example, a request
   to add the prefix "de" to the subtag 'nedis' so that the tag



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   "de-nedis" represented some German dialect would be rejected.  The
   'nedis' subtag represents a particular Slovenian dialect and the
   additional registration would change the semantic meaning assigned to
   the subtag.  A separate subtag SHOULD be proposed instead.

   The 'Description' field MUST contain a description of the tag being
   registered written or transcribed into the Latin script; it MAY also
   include a description in a non-Latin script.  Non-ASCII characters
   MUST be escaped using the syntax described in Section 3.1.  The
   'Description' field is used for identification purposes and doesn't
   necessarily represent the actual native name of the language or
   variation or to be in any particular language.

   While the 'Description' field itself is not guaranteed to be stable
   and errata corrections MAY be undertaken from time to time, attempts
   to provide translations or transcriptions of entries in the registry
   itself will probably be frowned upon by the community or rejected
   outright, as changes of this nature have an impact on the provisions
   in Section 3.4.

   When the two-week period has passed, the Language Subtag Reviewer
   either forwards the record to be inserted or modified to
   iana@iana.org according to the procedure described in Section 3.3, or
   rejects the request because of significant objections raised on the
   list or due to problems with constraints in this document (which MUST
   be explicitly cited).  The Language Subtag Reviewer MAY also extend
   the review period in two-week increments to permit further
   discussion.  The Language Subtag Reviewer MUST indicate on the list
   whether the registration has been accepted, rejected, or extended
   following each two-week period.

   Note that the Language Subtag Reviewer MAY raise objections on the
   list if he or she so desires.  The important thing is that the
   objection MUST be made publicly.

   The applicant is free to modify a rejected application with
   additional information and submit it again; this restarts the two-
   week comment period.

   Decisions made by the Language Subtag Reviewer MAY be appealed to the
   IESG [RFC2028] under the same rules as other IETF decisions
   [RFC2026].

   All approved registration forms are available online in the directory
   http://www.iana.org/numbers.html under "languages".






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   Updates or changes to existing records follow the same procedure as
   new registrations.  The Language Subtag Reviewer decides whether
   there is consensus to update the registration following the two-week
   review period; normally, objections by the original registrant will
   carry extra weight in forming such a consensus.

   Registrations are permanent and stable.  Once registered, subtags
   will not be removed from the registry and will remain a valid way in
   which to specify a specific language or variant.

   Note: The purpose of the "Description" in the registration form is to
   aid people trying to verify whether a language is registered or what
   language or language variation a particular subtag refers to.  In
   most cases, reference to an authoritative grammar or dictionary of
   that language will be useful; in cases where no such work exists,
   other well-known works describing that language or in that language
   MAY be appropriate.  The Language Subtag Reviewer decides what
   constitutes "good enough" reference material.  This requirement is
   not intended to exclude particular languages or dialects due to the
   size of the speaker population or lack of a standardized orthography.
   Minority languages will be considered equally on their own merits.

3.6.  Possibilities for Registration

   Possibilities for registration of subtags or information about
   subtags include:

   o  Primary language subtags for languages not listed in ISO 639 that
      are not variants of any listed or registered language MAY be
      registered.  At the time this document was created, there were no
      examples of this form of subtag.  Before attempting to register a
      language subtag, there MUST be an attempt to register the language
      with ISO 639.  Subtags MUST NOT be registered for codes that exist
      in ISO 639-1 or ISO 639-2, that are under consideration by the ISO
      639 maintenance or registration authorities, or that have never
      been attempted for registration with those authorities.  If ISO
      639 has previously rejected a language for registration, it is
      reasonable to assume that there must be additional, very
      compelling evidence of need before it will be registered in the
      IANA registry (to the extent that it is very unlikely that any
      subtags will be registered of this type).

   o  Dialect or other divisions or variations within a language, its
      orthography, writing system, regional or historical usage,
      transliteration or other transformation, or distinguishing
      variation MAY be registered as variant subtags.  An example is the
      'rozaj' subtag (the Resian dialect of Slovenian).




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   o  The addition or maintenance of fields (generally of an
      informational nature) in Tag or Subtag records as described in
      Section 3.1 and subject to the stability provisions in
      Section 3.4.  This includes descriptions, comments, deprecation
      and preferred values for obsolete or withdrawn codes, or the
      addition of script or extlang information to primary language
      subtags.

   o  The addition of records and related field value changes necessary
      to reflect assignments made by ISO 639, ISO 15924, ISO 3166, and
      UN M.49 as described in Section 3.4.

   Subtags proposed for registration that would cause all or part of a
   grandfathered tag to become redundant but whose meaning conflicts
   with or alters the meaning of the grandfathered tag MUST be rejected.

   This document leaves the decision on what subtags or changes to
   subtags are appropriate (or not) to the registration process
   described in Section 3.5.

   Note: four-character primary language subtags are reserved to allow
   for the possibility of alpha4 codes in some future addition to the
   ISO 639 family of standards.

   ISO 639 defines a maintenance agency for additions to and changes in
   the list of languages in ISO 639.  This agency is:

   International Information Centre for Terminology (Infoterm)
   Aichholzgasse 6/12, AT-1120
   Wien, Austria
   Phone: +43 1 26 75 35 Ext. 312 Fax: +43 1 216 32 72

   ISO 639-2 defines a maintenance agency for additions to and changes
   in the list of languages in ISO 639-2.  This agency is:

   Library of Congress
   Network Development and MARC Standards Office
   Washington, D.C. 20540 USA
   Phone: +1 202 707 6237 Fax: +1 202 707 0115
   URL: http://www.loc.gov/standards/iso639-2











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   The maintenance agency for ISO 3166 (country codes) is:

   ISO 3166 Maintenance Agency
   c/o International Organization for Standardization
   Case postale 56
   CH-1211 Geneva 20 Switzerland
   Phone: +41 22 749 72 33 Fax: +41 22 749 73 49
   URL: http://www.iso.org/iso/en/prods-services/iso3166ma/index.html

   The registration authority for ISO 15924 (script codes) is:

   Unicode Consortium Box 391476
   Mountain View, CA 94039-1476, USA
   URL: http://www.unicode.org/iso15924

   The Statistics Division of the United Nations Secretariat maintains
   the Standard Country or Area Codes for Statistical Use and can be
   reached at:

   Statistical Services Branch
   Statistics Division
   United Nations, Room DC2-1620
   New York, NY 10017, USA

   Fax: +1-212-963-0623
   E-mail: statistics@un.org
   URL: http://unstats.un.org/unsd/methods/m49/m49alpha.htm

3.7.  Extensions and Extensions Registry

   Extension subtags are those introduced by single-character subtags
   ("singletons") other than 'x'.  They are reserved for the generation
   of identifiers that contain a language component and are compatible
   with applications that understand language tags.

   The structure and form of extensions are defined by this document so
   that implementations can be created that are forward compatible with
   applications that might be created using singletons in the future.
   In addition, defining a mechanism for maintaining singletons will
   lend stability to this document by reducing the likely need for
   future revisions or updates.

   Single-character subtags are assigned by IANA using the "IETF
   Consensus" policy defined by [RFC2434].  This policy requires the
   development of an RFC, which SHALL define the name, purpose,
   processes, and procedures for maintaining the subtags.  The
   maintaining or registering authority, including name, contact email,




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   discussion list email, and URL location of the registry, MUST be
   indicated clearly in the RFC.  The RFC MUST specify or include each
   of the following:

   o  The specification MUST reference the specific version or revision
      of this document that governs its creation and MUST reference this
      section of this document.

   o  The specification and all subtags defined by the specification
      MUST follow the ABNF and other rules for the formation of tags and
      subtags as defined in this document.  In particular, it MUST
      specify that case is not significant and that subtags MUST NOT
      exceed eight characters in length.

   o  The specification MUST specify a canonical representation.

   o  The specification of valid subtags MUST be available over the
      Internet and at no cost.

   o  The specification MUST be in the public domain or available via a
      royalty-free license acceptable to the IETF and specified in the
      RFC.

   o  The specification MUST be versioned, and each version of the
      specification MUST be numbered, dated, and stable.

   o  The specification MUST be stable.  That is, extension subtags,
      once defined by a specification, MUST NOT be retracted or change
      in meaning in any substantial way.

   o  The specification MUST include in a separate section the
      registration form reproduced in this section (below) to be used in
      registering the extension upon publication as an RFC.

   o  IANA MUST be informed of changes to the contact information and
      URL for the specification.

   IANA will maintain a registry of allocated single-character
   (singleton) subtags.  This registry MUST use the record-jar format
   described by the ABNF in Section 3.1.  Upon publication of an
   extension as an RFC, the maintaining authority defined in the RFC
   MUST forward this registration form to iesg@ietf.org, who MUST
   forward the request to iana@iana.org.  The maintaining authority of
   the extension MUST maintain the accuracy of the record by sending an
   updated full copy of the record to iana@iana.org with the subject
   line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes.  Only
   the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY
   be modified in these updates.



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   Failure to maintain this record, maintain the corresponding registry,
   or meet other conditions imposed by this section of this document MAY
   be appealed to the IESG [RFC2028] under the same rules as other IETF
   decisions (see [RFC2026]) and MAY result in the authority to maintain
   the extension being withdrawn or reassigned by the IESG.

   %%
   Identifier:
   Description:
   Comments:
   Added:
   RFC:
   Authority:
   Contact_Email:
   Mailing_List:
   URL:
   %%

    Figure 6: Format of Records in the Language Tag Extensions Registry

   'Identifier' contains the single-character subtag (singleton)
   assigned to the extension.  The Internet-Draft submitted to define
   the extension SHOULD specify which letter or digit to use, although
   the IESG MAY change the assignment when approving the RFC.

   'Description' contains the name and description of the extension.

   'Comments' is an OPTIONAL field and MAY contain a broader description
   of the extension.

   'Added' contains the date the RFC was published in the "full-date"
   format specified in [RFC3339].  For example: 2004-06-28 represents
   June 28, 2004, in the Gregorian calendar.

   'RFC' contains the RFC number assigned to the extension.

   'Authority' contains the name of the maintaining authority for the
   extension.

   'Contact_Email' contains the email address used to contact the
   maintaining authority.

   'Mailing_List' contains the URL or subscription email address of the
   mailing list used by the maintaining authority.

   'URL' contains the URL of the registry for this extension.





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   The determination of whether an Internet-Draft meets the above
   conditions and the decision to grant or withhold such authority rests
   solely with the IESG and is subject to the normal review and appeals
   process associated with the RFC process.

   Extension authors are strongly cautioned that many (including most
   well-formed) processors will be unaware of any special relationships
   or meaning inherent in the order of extension subtags.  Extension
   authors SHOULD avoid subtag relationships or canonicalization
   mechanisms that interfere with matching or with length restrictions
   that sometimes exist in common protocols where the extension is used.
   In particular, applications MAY truncate the subtags in doing
   matching or in fitting into limited lengths, so it is RECOMMENDED
   that the most significant information be in the most significant
   (left-most) subtags and that the specification gracefully handle
   truncated subtags.

   When a language tag is to be used in a specific, known, protocol, it
   is RECOMMENDED that the language tag not contain extensions not
   supported by that protocol.  In addition, note that some protocols
   MAY impose upper limits on the length of the strings used to store or
   transport the language tag.

3.8.  Initialization of the Registries

   Upon adoption of this document, an initial version of the Language
   Subtag Registry containing the various subtags initially valid in a
   language tag is necessary.  This collection of subtags, along with a
   description of the process used to create it, is described by
   [RFC4645].  IANA SHALL publish the initial version of the registry
   described by this document from the content of [RFC4645].  Once
   published by IANA, the maintenance procedures, rules, and
   registration processes described in this document will be available
   for new registrations or updates.

   Registrations that are in process under the rules defined in
   [RFC3066] when this document is adopted MAY be completed under the
   former rules, at the discretion of the Language Tag Reviewer (as
   described in [RFC3066]).  Until the IESG officially appoints a
   Language Subtag Reviewer, the existing Language Tag Reviewer SHALL
   serve as the Language Subtag Reviewer.

   Any new registrations submitted using the RFC 3066 forms or format
   after the adoption of this document and publication of the registry
   by IANA MUST be rejected.






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   An initial version of the Language Tag Extensions Registry described
   in Section 3.7 is also needed.  The Language Tag Extensions Registry
   SHALL be initialized with a single record containing a single field
   of type "File-Date" as a placeholder for future assignments.

4.  Formation and Processing of Language Tags

   This section addresses how to use the information in the registry
   with the tag syntax to choose, form, and process language tags.

4.1.  Choice of Language Tag

   One is sometimes faced with the choice between several possible tags
   for the same body of text.

   Interoperability is best served when all users use the same language
   tag in order to represent the same language.  If an application has
   requirements that make the rules here inapplicable, then that
   application risks damaging interoperability.  It is strongly
   RECOMMENDED that users not define their own rules for language tag
   choice.

   Subtags SHOULD only be used where they add useful distinguishing
   information; extraneous subtags interfere with the meaning,
   understanding, and processing of language tags.  In particular, users
   and implementations SHOULD follow the 'Prefix' and 'Suppress-Script'
   fields in the registry (defined in Section 3.1): these fields provide
   guidance on when specific additional subtags SHOULD (and SHOULD NOT)
   be used in a language tag.

   Of particular note, many applications can benefit from the use of
   script subtags in language tags, as long as the use is consistent for
   a given context.  Script subtags were not formally defined in RFC
   3066 and their use can affect matching and subtag identification by
   implementations of RFC 3066, as these subtags appear between the
   primary language and region subtags.  For example, if a user requests
   content in an implementation of Section 2.5 of [RFC3066] using the
   language range "en-US", content labeled "en-Latn-US" will not match
   the request.  Therefore, it is important to know when script subtags
   will customarily be used and when they ought not be used.  In the
   registry, the Suppress-Script field helps ensure greater
   compatibility between the language tags generated according to the
   rules in this document and language tags and tag processors or
   consumers based on RFC 3066 by defining when users SHOULD NOT include
   a script subtag with a particular primary language subtag.






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   Extended language subtags (type 'extlang' in the registry; see
   Section 3.1) also appear between the primary language and region
   subtags and are reserved for future standardization.  Applications
   might benefit from their judicious use in forming language tags in
   the future.  Similar recommendations are expected to apply to their
   use as apply to script subtags.

   Standards, protocols, and applications that reference this document
   normatively but apply different rules to the ones given in this
   section MUST specify how the procedure varies from the one given
   here.

   The choice of subtags used to form a language tag SHOULD be guided by
   the following rules:

   1.  Use as precise a tag as possible, but no more specific than is
       justified.  Avoid using subtags that are not important for
       distinguishing content in an application.

       *  For example, 'de' might suffice for tagging an email written
          in German, while "de-CH-1996" is probably unnecessarily
          precise for such a task.

   2.  The script subtag SHOULD NOT be used to form language tags unless
       the script adds some distinguishing information to the tag.  The
       field 'Suppress-Script' in the primary language record in the
       registry indicates which script subtags do not add distinguishing
       information for most applications.

       *  For example, the subtag 'Latn' should not be used with the
          primary language 'en' because nearly all English documents are
          written in the Latin script and it adds no distinguishing
          information.  However, if a document were written in English
          mixing Latin script with another script such as Braille
          ('Brai'), then it might be appropriate to choose to indicate
          both scripts to aid in content selection, such as the
          application of a style sheet.

   3.  If a tag or subtag has a 'Preferred-Value' field in its registry
       entry, then the value of that field SHOULD be used to form the
       language tag in preference to the tag or subtag in which the
       preferred value appears.

       *  For example, use 'he' for Hebrew in preference to 'iw'.







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   4.  The 'und' (Undetermined) primary language subtag SHOULD NOT be
       used to label content, even if the language is unknown.  Omitting
       the language tag altogether is preferred to using a tag with a
       primary language subtag of 'und'.  The 'und' subtag MAY be useful
       for protocols that require a language tag to be provided.  The
       'und' subtag MAY also be useful when matching language tags in
       certain situations.

   5.  The 'mul' (Multiple) primary language subtag SHOULD NOT be used
       whenever the protocol allows the separate tags for multiple
       languages, as is the case for the Content-Language header in
       HTTP.  The 'mul' subtag conveys little useful information:
       content in multiple languages SHOULD individually tag the
       languages where they appear or otherwise indicate the actual
       language in preference to the 'mul' subtag.

   6.  The same variant subtag SHOULD NOT be used more than once within
       a language tag.

       *  For example, do not use "de-DE-1901-1901".

   To ensure consistent backward compatibility, this document contains
   several provisions to account for potential instability in the
   standards used to define the subtags that make up language tags.
   These provisions mean that no language tag created under the rules in
   this document will become obsolete.

4.2.  Meaning of the Language Tag

   The relationship between the tag and the information it relates to is
   defined by the context in which the tag appears.  Accordingly, this
   section gives only possible examples of its usage.

   o  For a single information object, the associated language tags
      might be interpreted as the set of languages that is necessary for
      a complete comprehension of the complete object.  Example: Plain
      text documents.

   o  For an aggregation of information objects, the associated language
      tags could be taken as the set of languages used inside components
      of that aggregation.  Examples: Document stores and libraries.

   o  For information objects whose purpose is to provide alternatives,
      the associated language tags could be regarded as a hint that the
      content is provided in several languages and that one has to
      inspect each of the alternatives in order to find its language or
      languages.  In this case, the presence of multiple tags might not
      mean that one needs to be multi-lingual to get complete



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      understanding of the document.  Example: MIME multipart/
      alternative.

   o  In markup languages, such as HTML and XML, language information
      can be added to each part of the document identified by the markup
      structure (including the whole document itself).  For example, one
      could write <span lang="fr">C'est la vie.</span> inside a
      Norwegian document; the Norwegian-speaking user could then access
      a French-Norwegian dictionary to find out what the marked section
      meant.  If the user were listening to that document through a
      speech synthesis interface, this formation could be used to signal
      the synthesizer to appropriately apply French text-to-speech
      pronunciation rules to that span of text, instead of applying the
      inappropriate Norwegian rules.

   Language tags are related when they contain a similar sequence of
   subtags.  For example, if a language tag B contains language tag A as
   a prefix, then B is typically "narrower" or "more specific" than A.
   Thus, "zh-Hant-TW" is more specific than "zh-Hant".

   This relationship is not guaranteed in all cases: specifically,
   languages that begin with the same sequence of subtags are NOT
   guaranteed to be mutually intelligible, although they might be.  For
   example, the tag "az" shares a prefix with both "az-Latn"
   (Azerbaijani written using the Latin script) and "az-Cyrl"
   (Azerbaijani written using the Cyrillic script).  A person fluent in
   one script might not be able to read the other, even though the text
   might be identical.  Content tagged as "az" most probably is written
   in just one script and thus might not be intelligible to a reader
   familiar with the other script.

4.3.  Length Considerations

   [RFC3066] did not provide an upper limit on the size of language
   tags.  While RFC 3066 did define the semantics of particular subtags
   in such a way that most language tags consisted of language and
   region subtags with a combined total length of up to six characters,
   larger registered tags were not only possible but were actually
   registered.

   Neither the language tag syntax nor other requirements in this
   document impose a fixed upper limit on the number of subtags in a
   language tag (and thus an upper bound on the size of a tag).  The
   language tag syntax suggests that, depending on the specific
   language, more subtags (and thus a longer tag) are sometimes
   necessary to completely identify the language for certain
   applications; thus, it is possible to envision long or complex subtag
   sequences.



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4.3.1.  Working with Limited Buffer Sizes

   Some applications and protocols are forced to allocate fixed buffer
   sizes or otherwise limit the length of a language tag.  A conformant
   implementation or specification MAY refuse to support the storage of
   language tags that exceed a specified length.  Any such limitation
   SHOULD be clearly documented, and such documentation SHOULD include
   what happens to longer tags (for example, whether an error value is
   generated or the language tag is truncated).  A protocol that allows
   tags to be truncated at an arbitrary limit, without giving any
   indication of what that limit is, has the potential for causing harm
   by changing the meaning of tags in substantial ways.

   In practice, most language tags do not require more than a few
   subtags and will not approach reasonably sized buffer limitations;
   see Section 4.1.

   Some specifications or protocols have limits on tag length but do not
   have a fixed length limitation.  For example, [RFC2231] has no
   explicit length limitation: the length available for the language tag
   is constrained by the length of other header components (such as the
   charset's name) coupled with the 76-character limit in [RFC2047].
   Thus, the "limit" might be 50 or more characters, but it could
   potentially be quite small.

   The considerations for assigning a buffer limit are:

      Implementations SHOULD NOT truncate language tags unless the
      meaning of the tag is purposefully being changed, or unless the
      tag does not fit into a limited buffer size specified by a
      protocol for storage or transmission.

      Implementations SHOULD warn the user when a tag is truncated since
      truncation changes the semantic meaning of the tag.

      Implementations of protocols or specifications that are space
      constrained but do not have a fixed limit SHOULD use the longest
      possible tag in preference to truncation.

      Protocols or specifications that specify limited buffer sizes for
      language tags MUST allow for language tags of up to 33 characters.

      Protocols or specifications that specify limited buffer sizes for
      language tags SHOULD allow for language tags of at least 42
      characters.






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   The following illustration shows how the 42-character recommendation
   was derived.  The combination of language and extended language
   subtags was chosen for future compatibility.  At up to 15 characters,
   this combination is longer than the longest possible primary language
   subtag (8 characters):

   language      =  3 (ISO 639-2; ISO 639-1 requires 2)
   extlang1      =  4 (each subsequent subtag includes '-')
   extlang2      =  4 (unlikely: needs prefix="language-extlang1")
   extlang3      =  4 (extremely unlikely)
   script        =  5 (if not suppressed: see Section 4.1)
   region        =  4 (UN M.49; ISO 3166 requires 3)
   variant1      =  9 (MUST have language as a prefix)
   variant2      =  9 (MUST have language-variant1 as a prefix)

   total         = 42 characters

              Figure 7: Derivation of the Limit on Tag Length

4.3.2.  Truncation of Language Tags

   Truncation of a language tag alters the meaning of the tag, and thus
   SHOULD be avoided.  However, truncation of language tags is sometimes
   necessary due to limited buffer sizes.  Such truncation MUST NOT
   permit a subtag to be chopped off in the middle or the formation of
   invalid tags (for example, one ending with the "-" character).

   This means that applications or protocols that truncate tags MUST do
   so by progressively removing subtags along with their preceding "-"
   from the right side of the language tag until the tag is short enough
   for the given buffer.  If the resulting tag ends with a single-
   character subtag, that subtag and its preceding "-" MUST also be
   removed.  For example:

   Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
   1. zh-Latn-CN-variant1-a-extend1-x-wadegile
   2. zh-Latn-CN-variant1-a-extend1
   3. zh-Latn-CN-variant1
   4. zh-Latn-CN
   5. zh-Latn
   6. zh

                    Figure 8: Example of Tag Truncation








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4.4.  Canonicalization of Language Tags

   Since a particular language tag is sometimes used by many processes,
   language tags SHOULD always be created or generated in a canonical
   form.

   A language tag is in canonical form when:

   1.  The tag is well-formed according the rules in Section 2.1 and
       Section 2.2.

   2.  Subtags of type 'Region' that have a Preferred-Value mapping in
       the IANA registry (see Section 3.1) SHOULD be replaced with their
       mapped value.  Note: In rare cases, the mapped value will also
       have a Preferred-Value.

   3.  Redundant or grandfathered tags that have a Preferred-Value
       mapping in the IANA registry (see Section 3.1) MUST be replaced
       with their mapped value.  These items either are deprecated
       mappings created before the adoption of this document (such as
       the mapping of "no-nyn" to "nn" or "i-klingon" to "tlh") or are
       the result of later registrations or additions to this document
       (for example, "zh-guoyu" might be mapped to a language-extlang
       combination such as "zh-cmn" by some future update of this
       document).

   4.  Other subtags that have a Preferred-Value mapping in the IANA
       registry (see Section 3.1) MUST be replaced with their mapped
       value.  These items consist entirely of clerical corrections to
       ISO 639-1 in which the deprecated subtags have been maintained
       for compatibility purposes.

   5.  If more than one extension subtag sequence exists, the extension
       sequences are ordered into case-insensitive ASCII order by
       singleton subtag.

   Example: The language tag "en-A-aaa-B-ccc-bbb-x-xyz" is in canonical
   form, while "en-B-ccc-bbb-A-aaa-X-xyz" is well-formed but not in
   canonical form.

   Example: The language tag "en-BU" (English as used in Burma) is not
   canonical because the 'BU' subtag has a canonical mapping to 'MM'
   (Myanmar), although the tag "en-BU" maintains its validity.

   Canonicalization of language tags does not imply anything about the
   use of upper or lowercase letters when processing or comparing
   subtags (and as described in Section 2.1).  All comparisons MUST be
   performed in a case-insensitive manner.



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   When performing canonicalization of language tags, processors MAY
   regularize the case of the subtags (that is, this process is
   OPTIONAL), following the case used in the registry.  Note that this
   corresponds to the following casing rules: uppercase all non-initial
   two-letter subtags; titlecase all non-initial four-letter subtags;
   lowercase everything else.

   Note: Case folding of ASCII letters in certain locales, unless
   carefully handled, sometimes produces non-ASCII character values.
   The Unicode Character Database file "SpecialCasing.txt" defines the
   specific cases that are known to cause problems with this.  In
   particular, the letter 'i' (U+0069) in Turkish and Azerbaijani is
   uppercased to U+0130 (LATIN CAPITAL LETTER I WITH DOT ABOVE).
   Implementers SHOULD specify a locale-neutral casing operation to
   ensure that case folding of subtags does not produce this value,
   which is illegal in language tags.  For example, if one were to
   uppercase the region subtag 'in' using Turkish locale rules, the
   sequence U+0130 U+004E would result instead of the expected 'IN'.

   Note: if the field 'Deprecated' appears in a registry record without
   an accompanying 'Preferred-Value' field, then that tag or subtag is
   deprecated without a replacement.  Validating processors SHOULD NOT
   generate tags that include these values, although the values are
   canonical when they appear in a language tag.

   An extension MUST define any relationships that exist between the
   various subtags in the extension and thus MAY define an alternate
   canonicalization scheme for the extension's subtags.  Extensions MAY
   define how the order of the extension's subtags are interpreted.  For
   example, an extension could define that its subtags are in canonical
   order when the subtags are placed into ASCII order: that is,
   "en-a-aaa-bbb-ccc" instead of "en-a-ccc-bbb-aaa".  Another extension
   might define that the order of the subtags influences their semantic
   meaning (so that "en-b-ccc-bbb-aaa" has a different value from
   "en-b-aaa-bbb-ccc").  However, extension specifications SHOULD be
   designed so that they are tolerant of the typical processes described
   in Section 3.7.

4.5.  Considerations for Private Use Subtags

   Private use subtags, like all other subtags, MUST conform to the
   format and content constraints in the ABNF.  Private use subtags have
   no meaning outside the private agreement between the parties that
   intend to use or exchange language tags that employ them.  The same
   subtags MAY be used with a different meaning under a separate private
   agreement.  They SHOULD NOT be used where alternatives exist and
   SHOULD NOT be used in content or protocols intended for general use.




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   Private use subtags are simply useless for information exchange
   without prior arrangement.  The value and semantic meaning of private
   use tags and of the subtags used within such a language tag are not
   defined by this document.

   Subtags defined in the IANA registry as having a specific private use
   meaning convey more information that a purely private use tag
   prefixed by the singleton subtag 'x'.  For applications, this
   additional information MAY be useful.

   For example, the region subtags 'AA', 'ZZ', and in the ranges
   'QM'-'QZ' and 'XA'-'XZ' (derived from ISO 3166 private use codes) MAY
   be used to form a language tag.  A tag such as "zh-Hans-XQ" conveys a
   great deal of public, interchangeable information about the language
   material (that it is Chinese in the simplified Chinese script and is
   suitable for some geographic region 'XQ').  While the precise
   geographic region is not known outside of private agreement, the tag
   conveys far more information than an opaque tag such as "x-someLang",
   which contains no information about the language subtag or script
   subtag outside of the private agreement.

   However, in some cases content tagged with private use subtags MAY
   interact with other systems in a different and possibly unsuitable
   manner compared to tags that use opaque, privately defined subtags,
   so the choice of the best approach sometimes depends on the
   particular domain in question.

5.  IANA Considerations

   This section deals with the processes and requirements necessary for
   IANA to undertake to maintain the subtag and extension registries as
   defined by this document and in accordance with the requirements of
   [RFC2434].

   The impact on the IANA maintainers of the two registries defined by
   this document will be a small increase in the frequency of new
   entries or updates.

5.1.  Language Subtag Registry

   Upon adoption of this document, the registry will be initialized by a
   companion document: [RFC4645].  The criteria and process for
   selecting the initial set of records are described in that document.
   The initial set of records represents no impact on IANA, since the
   work to create it will be performed externally.






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   The new registry MUST be listed under "Language Tags" at
   <http://www.iana.org/numbers.html>, replacing the existing
   registrations defined by [RFC3066].  The existing set of registration
   forms and RFC 3066 registrations MUST be relabeled as "Language Tags
   (Obsolete)" and maintained (but not added to or modified).

   Future work on the Language Subtag Registry SHALL be limited to
   inserting or replacing whole records preformatted for IANA by the
   Language Subtag Reviewer as described in Section 3.3 of this document
   and archiving the forwarded registration form.

   Each record MUST be sent to iana@iana.org with a subject line
   indicating whether the enclosed record is an insertion of a new
   record (indicated by the word "INSERT" in the subject line) or a
   replacement of an existing record (indicated by the word "MODIFY" in
   the subject line).  Records MUST NOT be deleted from the registry.
   IANA MUST place any inserted or modified records into the appropriate
   section of the language subtag registry, grouping the records by
   their 'Type' field.  Inserted records MAY be placed anywhere in the
   appropriate section; there is no guarantee of the order of the
   records beyond grouping them together by 'Type'.  Modified records
   MUST overwrite the record they replace.

   Included in any request to insert or modify records MUST be a new
   File-Date record.  This record MUST be placed first in the registry.
   In the event that the File-Date record present in the registry has a
   later date than the record being inserted or modified, the existing
   record MUST be preserved.

5.2.  Extensions Registry

   The Language Tag Extensions Registry will also be generated and sent
   to IANA as described in Section 3.7.  This registry can contain at
   most 35 records, and thus changes to this registry are expected to be
   very infrequent.

   Future work by IANA on the Language Tag Extensions Registry is
   limited to two cases.  First, the IESG MAY request that new records
   be inserted into this registry from time to time.  These requests
   MUST include the record to insert in the exact format described in
   Section 3.7.  In addition, there MAY be occasional requests from the
   maintaining authority for a specific extension to update the contact
   information or URLs in the record.  These requests MUST include the
   complete, updated record.  IANA is not responsible for validating the
   information provided, only that it is properly formatted.  It should
   reasonably be seen to come from the maintaining authority named in
   the record present in the registry.




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6.  Security Considerations

   Language tags used in content negotiation, like any other information
   exchanged on the Internet, might be a source of concern because they
   might be used to infer the nationality of the sender, and thus
   identify potential targets for surveillance.

   This is a special case of the general problem that anything sent is
   visible to the receiving party and possibly to third parties as well.
   It is useful to be aware that such concerns can exist in some cases.

   The evaluation of the exact magnitude of the threat, and any possible
   countermeasures, is left to each application protocol (see BCP 72
   [RFC3552] for best current practice guidance on security threats and
   defenses).

   The language tag associated with a particular information item is of
   no consequence whatsoever in determining whether that content might
   contain possible homographs.  The fact that a text is tagged as being
   in one language or using a particular script subtag provides no
   assurance whatsoever that it does not contain characters from scripts
   other than the one(s) associated with or specified by that language
   tag.

   Since there is no limit to the number of variant, private use, and
   extension subtags, and consequently no limit on the possible length
   of a tag, implementations need to guard against buffer overflow
   attacks.  See Section 4.3 for details on language tag truncation,
   which can occur as a consequence of defenses against buffer overflow.

   Although the specification of valid subtags for an extension (see
   Section 3.7) MUST be available over the Internet, implementations
   SHOULD NOT mechanically depend on it being always accessible, to
   prevent denial-of-service attacks.

7.  Character Set Considerations

   The syntax in this document requires that language tags use only the
   characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in most
   character sets, so the composition of language tags should not have
   any character set issues.

   Rendering of characters based on the content of a language tag is not
   addressed in this memo.  Historically, some languages have relied on
   the use of specific character sets or other information in order to
   infer how a specific character should be rendered (notably this
   applies to language- and culture-specific variations of Han
   ideographs as used in Japanese, Chinese, and Korean).  When language



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   tags are applied to spans of text, rendering engines sometimes use
   that information in deciding which font to use in the absence of
   other information, particularly where languages with distinct writing
   traditions use the same characters.

8.  Changes from RFC 3066

   The main goals for this revision of language tags were the following:

   *Compatibility.* All RFC 3066 language tags (including those in the
   IANA registry) remain valid in this specification.  The changes in
   this document represent additional constraints on language tags.
   That is, in no case is the syntax more permissive and processors
   based on the ABNF and other provisions of RFC 3066 (such as those
   described in [XMLSchema]) will be able to process the tags described
   by this document.  In addition, this document defines language tags
   in such as way as to ensure future compatibility.

   *Stability.* Because of changes in the past in the underlying ISO
   standards, a valid RFC 3066 language tag could become invalid or have
   its meaning change.  This has the potential of invalidating content
   that may have an extensive shelf-life.  In this specification, once a
   language tag is valid, it remains valid forever.

   *Validity.* The structure of language tags defined by this document
   makes it possible to determine if a particular tag is well-formed
   without regard for the actual content or "meaning" of the tag as a
   whole.  This is important because the registry grows and underlying
   standards change over time.  In addition, it must be possible to
   determine if a tag is valid (or not) for a given point in time in
   order to provide reproducible, testable results.  This process must
   not be error-prone; otherwise implementations might give different
   results.  By having an authoritative registry with specific
   versioning information, the validity of language tags at any point in
   time can be precisely determined (instead of interpolating values
   from many separate sources).

   *Utility.* It is sometimes important to be able to differentiate
   between written forms of a language -- for many implementations this
   is more important than distinguishing between the spoken variants of
   a language.  Languages are written in a wide variety of different
   scripts, so this document provides for the generative use of ISO
   15924 script codes.  Like the generative use of ISO language and
   country codes in RFC 3066, this allows combinations to be produced
   without resorting to the registration process.  The addition of UN
   M.49 codes provides for the generation of language tags with regional
   scope, which is also required by some applications.




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   The recast of the registry from containing whole language tags to
   subtags is a key part of this.  An important feature of RFC 3066 was
   that it allowed generative use of subtags.  This allows people to
   meaningfully use generated tags, without the delays in registering
   whole tags or the need to register all of the combinations that might
   be useful.

   The choice of placing the extended language and script subtags
   between the primary language and region subtags was widely debated.
   This design was chosen because the prevalent matching and content
   negotiation schemes rely on the subtags being arranged in order of
   increasing specificity.  That is, the subtags that mark a greater
   barrier to mutual intelligibility appear left-most in a tag.  For
   example, when selecting content written in Azerbaijani, the script
   (Arabic, Cyrillic, or Latin) represents a greater barrier to
   understanding than any regional variations (those associated with
   Azerbaijan or Iran, for example).  Individuals who prefer documents
   in a particular script, but can deal with the minor regional
   differences, can therefore select appropriate content.  Applications
   that do not deal with written content will continue to omit these
   subtags.

   *Extensibility.* Because of the widespread use of language tags, it
   is disruptive to have periodic revisions of the core specification,
   even in the face of demonstrated need.  The extension mechanism
   provides for a way for independent RFCs to define extensions to
   language tags.  These extensions have a very constrained, well-
   defined structure that prevents extensions from interfering with
   implementations of language tags defined in this document.

   The document also anticipates features of ISO 639-3 with the addition
   of the extended language subtags, as well as the possibility of other
   ISO 639 parts becoming useful for the formation of language tags in
   the future.

   The use and definition of private use tags have also been modified,
   to allow people to use private use subtags to extend or modify
   defined tags and to move as much information as possible out of
   private use and into the regular structure.

   The goal for each of these modifications is to reduce or eliminate
   the need for future revisions of this document.









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   The specific changes in this document to meet these goals are:

   o  Defines the ABNF and rules for subtags so that the category of all
      subtags can be determined without reference to the registry.

   o  Adds the concept of well-formed vs. validating processors,
      defining the rules by which an implementation can claim to be one
      or the other.

   o  Replaces the IANA language tag registry with a language subtag
      registry that provides a complete list of valid subtags in the
      IANA registry.  This allows for robust implementation and ease of
      maintenance.  The language subtag registry becomes the canonical
      source for forming language tags.

   o  Provides a process that guarantees stability of language tags, by
      handling reuse of values by ISO 639, ISO 15924, and ISO 3166 in
      the event that they register a previously used value for a new
      purpose.

   o  Allows ISO 15924 script code subtags and allows them to be used
      generatively.  Defines a method for indicating in the registry
      when script subtags are necessary for a given language tag.

   o  Adds the concept of a variant subtag and allows variants to be
      used generatively.

   o  Adds the ability to use a class of UN M.49 tags for supra-national
      regions and to resolve conflicts in the assignment of ISO 3166
      codes.

   o  Defines the private use tags in ISO 639, ISO 15924, and ISO 3166
      as the mechanism for creating private use language, script, and
      region subtags, respectively.

   o  Adds a well-defined extension mechanism.

   o  Defines an extended language subtag, possibly for use with certain
      anticipated features of ISO 639-3.












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9.  References

9.1.  Normative References

   [ISO10646]     International Organization for Standardization,
                  "ISO/IEC 10646:2003. Information technology --
                  Universal Multiple-Octet Coded Character Set (UCS)",
                  2003.

   [ISO15924]     International Organization for Standardization, "ISO
                  15924:2004. Information and documentation -- Codes for
                  the representation of names of scripts", January 2004.

   [ISO3166-1]    International Organization for Standardization, "ISO
                  3166-1:1997. Codes for the representation of names of
                  countries and their subdivisions -- Part 1: Country
                  codes", 1997.

   [ISO639-1]     International Organization for Standardization, "ISO
                  639-1:2002. Codes for the representation of names of
                  languages -- Part 1: Alpha-2 code", 2002.

   [ISO639-2]     International Organization for Standardization, "ISO
                  639-2:1998. Codes for the representation of names of
                  languages -- Part 2: Alpha-3 code, first edition",
                  1998.

   [ISO646]       International Organization for Standardization,
                  "ISO/IEC 646:1991, Information technology -- ISO 7-bit
                  coded character set for information interchange.",
                  1991.

   [RFC2026]      Bradner, S., "The Internet Standards Process --
                  Revision 3", BCP 9, RFC 2026, October 1996.

   [RFC2028]      Hovey, R. and S. Bradner, "The Organizations Involved
                  in the IETF Standards Process", BCP 11, RFC 2028,
                  October 1996.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26,
                  RFC 2434, October 1998.






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   [RFC2860]      Carpenter, B., Baker, F., and M. Roberts, "Memorandum
                  of Understanding Concerning the Technical Work of the
                  Internet Assigned Numbers Authority", RFC 2860,
                  June 2000.

   [RFC3339]      Klyne, G., Ed. and C. Newman, "Date and Time on the
                  Internet: Timestamps", RFC 3339, July 2002.

   [RFC4234]      Crocker, D., Ed. and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", RFC 4234, October 2005.

   [UN_M.49]      Statistics Division, United Nations, "Standard Country
                  or Area Codes for Statistical Use", UN Standard
                  Country or Area Codes for Statistical Use, Revision 4
                  (United Nations publication, Sales No. 98.XVII.9,
                  June 1999.

9.2.  Informative References

   [RFC1766]      Alvestrand, H., "Tags for the Identification of
                  Languages", RFC 1766, March 1995.

   [RFC2047]      Moore, K., "MIME (Multipurpose Internet Mail
                  Extensions) Part Three: Message Header Extensions for
                  Non-ASCII Text", RFC 2047, November 1996.

   [RFC2231]      Freed, N. and K. Moore, "MIME Parameter Value and
                  Encoded Word Extensions: Character Sets, Languages,
                  and Continuations", RFC 2231, November 1997.

   [RFC2781]      Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                  ISO 10646", RFC 2781, February 2000.

   [RFC3066]      Alvestrand, H., "Tags for the Identification of
                  Languages", BCP 47, RFC 3066, January 2001.

   [RFC3552]      Rescorla, E. and B. Korver, "Guidelines for Writing
                  RFC Text on Security Considerations", BCP 72,
                  RFC 3552, July 2003.

   [RFC4645]      Ewell, D., Ed., "Initial Language Subtag Registry",
                  RFC 4645, September 2006.

   [RFC4647]      Phillips, A., Ed. and M. Davis, Ed., "Matching of
                  Language Tags", BCP 47, RFC 4647, September 2006.






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   [Unicode]      Unicode Consortium, "The Unicode Standard, Version
                  5.0", Boston, MA, Addison-Wesley, 2007. ISBN 0-321-
                  48091-0.

   [XML10]        Bray (et al), T., "Extensible Markup Language (XML)
                  1.0", 02 2004.

   [XMLSchema]    Biron, P., Ed. and A. Malhotra, Ed., "XML Schema Part
                  2: Datatypes Second Edition", 10 2004, <
                  http://www.w3.org/TR/xmlschema-2/>.

   [iso639.prin]  ISO 639 Joint Advisory Committee, "ISO 639 Joint
                  Advisory Committee:  Working principles for ISO 639
                  maintenance", March 2000, <http://www.loc.gov/
                  standards/iso639-2/iso639jac_n3r.html>.

   [record-jar]   Raymond, E., "The Art of Unix Programming", 2003,
                  <urn:isbn:0-13-142901-9>.

































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Appendix A.  Acknowledgements

   Any list of contributors is bound to be incomplete; please regard the
   following as only a selection from the group of people who have
   contributed to make this document what it is today.

   The contributors to RFC 3066 and RFC 1766, the precursors of this
   document, made enormous contributions directly or indirectly to this
   document and are generally responsible for the success of language
   tags.

   The following people (in alphabetical order) contributed to this
   document or to RFCs 1766 and 3066:

   Glenn Adams, Harald Tveit Alvestrand, Tim Berners-Lee, Marc Blanchet,
   Nathaniel Borenstein, Karen Broome, Eric Brunner, Sean M. Burke, M.T.
   Carrasco Benitez, Jeremy Carroll, John Clews, Jim Conklin, Peter
   Constable, John Cowan, Mark Crispin, Dave Crocker, Elwyn Davies,
   Martin Duerst, Frank Ellerman, Michael Everson, Doug Ewell, Ned
   Freed, Tim Goodwin, Dirk-Willem van Gulik, Marion Gunn, Joel Halpren,
   Elliotte Rusty Harold, Paul Hoffman, Scott Hollenbeck, Richard
   Ishida, Olle Jarnefors, Kent Karlsson, John Klensin, Erkki
   Kolehmainen, Alain LaBonte, Eric Mader, Ira McDonald, Keith Moore,
   Chris Newman, Masataka Ohta, Dylan Pierce, Randy Presuhn, George
   Rhoten, Felix Sasaki, Markus Scherer, Keld Jorn Simonsen, Thierry
   Sourbier, Otto Stolz, Tex Texin, Andrea Vine, Rhys Weatherley, Misha
   Wolf, Francois Yergeau and many, many others.

   Very special thanks must go to Harald Tveit Alvestrand, who
   originated RFCs 1766 and 3066, and without whom this document would
   not have been possible.  Special thanks must go to Michael Everson,
   who has served as Language Tag Reviewer for almost the complete
   period since the publication of RFC 1766.  Special thanks to Doug
   Ewell, for his production of the first complete subtag registry, and
   his work in producing a test parser for verifying language tags.
















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Appendix B.  Examples of Language Tags (Informative)

   Simple language subtag:

      de (German)

      fr (French)

      ja (Japanese)

      i-enochian (example of a grandfathered tag)

   Language subtag plus Script subtag:

      zh-Hant (Chinese written using the Traditional Chinese script)

      zh-Hans (Chinese written using the Simplified Chinese script)

      sr-Cyrl (Serbian written using the Cyrillic script)

      sr-Latn (Serbian written using the Latin script)

   Language-Script-Region:

      zh-Hans-CN (Chinese written using the Simplified script as used in
      mainland China)

      sr-Latn-CS (Serbian written using the Latin script as used in
      Serbia and Montenegro)

   Language-Variant:

      sl-rozaj (Resian dialect of Slovenian

      sl-nedis (Nadiza dialect of Slovenian)

   Language-Region-Variant:

      de-CH-1901 (German as used in Switzerland using the 1901 variant
      [orthography])

      sl-IT-nedis (Slovenian as used in Italy, Nadiza dialect)









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   Language-Script-Region-Variant:

      sl-Latn-IT-nedis (Nadiza dialect of Slovenian written using the
      Latin script as used in Italy.  Note that this tag is NOT
      RECOMMENDED because subtag 'sl' has a Suppress-Script value of
      'Latn')

   Language-Region:

      de-DE (German for Germany)

      en-US (English as used in the United States)

      es-419 (Spanish appropriate for the Latin America and Caribbean
      region using the UN region code)

   Private use subtags:

      de-CH-x-phonebk

      az-Arab-x-AZE-derbend

   Extended language subtags (examples ONLY: extended languages MUST be
   defined by revision or update to this document):

      zh-min

      zh-min-nan-Hant-CN

   Private use registry values:

      x-whatever (private use using the singleton 'x')

      qaa-Qaaa-QM-x-southern (all private tags)

      de-Qaaa (German, with a private script)

      sr-Latn-QM (Serbian, Latin-script, private region)

      sr-Qaaa-CS (Serbian, private script, for Serbia and Montenegro)

   Tags that use extensions (examples ONLY: extensions MUST be defined
   by revision or update to this document or by RFC):

      en-US-u-islamCal

      zh-CN-a-myExt-x-private




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      en-a-myExt-b-another

   Some Invalid Tags:

      de-419-DE (two region tags)

      a-DE (use of a single-character subtag in primary position; note
      that there are a few grandfathered tags that start with "i-" that
      are valid)

      ar-a-aaa-b-bbb-a-ccc (two extensions with same single-letter
      prefix)

Authors' Addresses

   Addison Phillips (Editor)
   Yahoo! Inc.

   EMail: addison@inter-locale.com


   Mark Davis (Editor)
   Google

   EMail: mark.davis@macchiato.com or mark.davis@google.com


























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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







Phillips & Davis         Best Current Practice                 [Page 59]

========================================================================






Network Working Group                                   A. Phillips, Ed.
Request for Comments: 4647                                   Yahoo! Inc.
BCP: 47                                                    M. Davis, Ed.
Obsoletes: 3066                                                   Google
Category: Best Current Practice                           September 2006


                       Matching of Language Tags

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a syntax, called a "language-range", for
   specifying items in a user's list of language preferences.  It also
   describes different mechanisms for comparing and matching these to
   language tags.  Two kinds of matching mechanisms, filtering and
   lookup, are defined.  Filtering produces a (potentially empty) set of
   language tags, whereas lookup produces a single language tag.
   Possible applications include language negotiation or content
   selection.  This document, in combination with RFC 4646, replaces RFC
   3066, which replaced RFC 1766.





















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Table of Contents

   1. Introduction ....................................................3
   2. The Language Range ..............................................3
      2.1. Basic Language Range .......................................4
      2.2. Extended Language Range ....................................4
      2.3. The Language Priority List .................................5
   3. Types of Matching ...............................................6
      3.1. Choosing a Matching Scheme .................................6
      3.2. Implementation Considerations ..............................7
      3.3. Filtering ..................................................8
           3.3.1. Basic Filtering .....................................9
           3.3.2. Extended Filtering .................................10
      3.4. Lookup ....................................................12
           3.4.1. Default Values .....................................14
   4. Other Considerations ...........................................15
      4.1. Choosing Language Ranges ..................................15
      4.2. Meaning of Language Tags and Ranges .......................16
      4.3. Considerations for Private-Use Subtags ....................17
      4.4. Length Considerations for Language Ranges .................17
   5. Security Considerations ........................................17
   6. Character Set Considerations ...................................17
   7. References .....................................................18
      7.1. Normative References ......................................18
      7.2. Informative References ....................................18
   Appendix A. Acknowledgements ......................................19

























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1.  Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   Applications, protocols, or specifications that use language
   identifiers, such as the language tags defined in [RFC4646],
   sometimes need to match language tags to a user's language
   preferences.

   This document defines a syntax (called a language range (Section 2))
   for specifying items in the user's list of language preferences
   (called a language priority list (Section 2.3)), as well as several
   schemes for selecting or filtering sets of language tags by comparing
   the language tags to the user's preferences.  Applications,
   protocols, or specifications will have varying needs and requirements
   that affect the choice of a suitable matching scheme.

   This document describes how to indicate a user's preferences using
   language ranges, three schemes for matching these ranges to a set of
   language tags, and the various practical considerations that apply to
   implementing and using these schemes.

   This document, in combination with [RFC4646], replaces [RFC3066],
   which replaced [RFC1766].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  The Language Range

   Language tags [RFC4646] are used to help identify languages, whether
   spoken, written, signed, or otherwise signaled, for the purpose of
   communication.  Applications, protocols, or specifications that use
   language tags are often faced with the problem of identifying sets of
   content that share certain language attributes.  For example,
   HTTP/1.1 [RFC2616] describes one such mechanism in its discussion of
   the Accept-Language header (Section 14.4), which is used when
   selecting content from servers based on the language of that content.

   It is, thus, useful to have a mechanism for identifying sets of
   language tags that share specific attributes.  This allows users to
   select or filter the language tags based on specific requirements.
   Such an identifier is called a "language range".





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   There are different types of language range, whose specific
   attributes vary according to their application.  Language ranges are
   similar to language tags: they consist of a sequence of subtags
   separated by hyphens.  In a language range, each subtag MUST either
   be a sequence of ASCII alphanumeric characters or the single
   character '*' (%x2A, ASTERISK).  The character '*' is a "wildcard"
   that matches any sequence of subtags.  The meaning and uses of
   wildcards vary according to the type of language range.

   Language tags and thus language ranges are to be treated as case-
   insensitive: there exist conventions for the capitalization of some
   of the subtags, but these MUST NOT be taken to carry meaning.
   Matching of language tags to language ranges MUST be done in a case-
   insensitive manner.

2.1.  Basic Language Range

   A "basic language range" has the same syntax as an [RFC3066] language
   tag or is the single character "*".  The basic language range was
   originally described by HTTP/1.1 [RFC2616] and later [RFC3066].  It
   is defined by the following ABNF [RFC4234]:

   language-range   = (1*8ALPHA *("-" 1*8alphanum)) / "*"
   alphanum         = ALPHA / DIGIT

   A basic language range differs from the language tags defined in
   [RFC4646] only in that there is no requirement that it be "well-
   formed" or be validated against the IANA Language Subtag Registry.
   Such ill-formed ranges will probably not match anything.  Note that
   the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the
   use of digits anywhere in the 'language-range' (see [RFC2616errata]).

2.2.  Extended Language Range

   Occasionally, users will wish to select a set of language tags based
   on the presence of specific subtags.  An "extended language range"
   describes a user's language preference as an ordered sequence of
   subtags.  For example, a user might wish to select all language tags
   that contain the region subtag 'CH' (Switzerland).  Extended language
   ranges are useful for specifying a particular sequence of subtags
   that appear in the set of matching tags without having to specify all
   of the intervening subtags.

   An extended language range can be represented by the following ABNF:

   extended-language-range = (1*8ALPHA / "*")
                             *("-" (1*8alphanum / "*"))




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   The wildcard subtag '*' can occur in any position in the extended
   language range, where it matches any sequence of subtags that might
   occur in that position in a language tag.  However, wildcards outside
   the first position are ignored by Extended Filtering (see Section
   3.2.2).  The use or absence of one or more wildcards cannot be taken
   to imply that a certain number of subtags will appear in the matching
   set of language tags.

2.3.  The Language Priority List

   A user's language preferences will often need to specify more than
   one language range, and thus users often need to specify a
   prioritized list of language ranges in order to best reflect their
   language preferences.  This is especially true for speakers of
   minority languages.  A speaker of Breton in France, for example, can
   specify "br" followed by "fr", meaning that if Breton is available,
   it is preferred, but otherwise French is the best alternative.  It
   can get more complex: a different user might want to fall back from
   Skolt Sami to Northern Sami to Finnish.

   A "language priority list" is a prioritized or weighted list of
   language ranges.  One well-known example of such a list is the
   "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
   14.4) and RFC 3282 [RFC3282].

   The various matching operations described in this document include
   considerations for using a language priority list.  This document
   does not define the syntax for a language priority list; defining
   such a syntax is the responsibility of the protocol, application, or
   specification that uses it.  When given as examples in this document,
   language priority lists will be shown as a quoted sequence of ranges
   separated by commas, like this: "en, fr, zh-Hant" (which is read
   "English before French before Chinese as written in the Traditional
   script").

   A simple list of ranges is considered to be in descending order of
   priority.  Other language priority lists provide "quality weights"
   for the language ranges in order to specify the relative priority of
   the user's language preferences.  An example of this is the use of
   "q" values in the syntax of the "Accept-Language" header (defined in
   [RFC2616], Section 14.4, and [RFC3282]).










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3.  Types of Matching

   Matching language ranges to language tags can be done in many
   different ways.  This section describes three such matching schemes,
   as well as the considerations for choosing between them.  Protocols
   and specifications requiring conformance to this specification MUST
   clearly indicate the particular mechanism used in selecting or
   matching language tags.

   There are two types of matching scheme in this document.  A matching
   scheme that produces zero or more matching language tags is called
   "filtering".  A matching scheme that produces exactly one match for a
   given request is called "lookup".

3.1.  Choosing a Matching Scheme

   Applications, protocols, and specifications are faced with the
   decision of what type of matching to use.  Sometimes, different
   styles of matching are suited to different kinds of processing within
   a particular application or protocol.

   This document describes three matching schemes:

   1.  Basic Filtering (Section 3.3.1) matches a language priority list
       consisting of basic language ranges (Section 2.1) to sets of
       language tags.

   2.  Extended Filtering (Section 3.3.2) matches a language priority
       list consisting of extended language ranges (Section 2.2) to sets
       of language tags.

   3.  Lookup (Section 3.4) matches a language priority list consisting
       of basic language ranges to sets of language tags to find the one
       exact language tag that best matches the range.

   Filtering can be used to produce a set of results (such as a
   collection of documents) by comparing the user's preferences to a set
   of language tags.  For example, when performing a search, filtering
   can be used to limit the results to items tagged as being in the
   French language.  Filtering can also be used when deciding whether to
   perform a language-sensitive process on some content.  For example, a
   process might cause paragraphs whose language tag matched the
   language range "nl" (Dutch) to be displayed in italics within a
   document.

   Lookup produces the single result that best matches the user's
   preferences from the list of available tags, so it is useful in cases
   in which a single item is required (and for which only a single item



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   can be returned).  For example, if a process were to insert a human-
   readable error message into a protocol header, it might select the
   text based on the user's language priority list.  Since the process
   can return only one item, it is forced to choose a single item and it
   has to return some item, even if none of the content's language tags
   match the language priority list supplied by the user.

3.2.  Implementation Considerations

   Language tag matching is a tool, and does not by itself specify a
   complete procedure for the use of language tags.  Such procedures are
   intimately tied to the application protocol in which they occur.
   When specifying a protocol operation using matching, the protocol
   MUST specify:

   o  Which type(s) of language tag matching it uses

   o  Whether the operation returns a single result (lookup) or a
      possibly empty set of results (filtering)

   o  For lookup, what the default item is (or the sequence of
      operations or configuration information used to determine the
      default) when no matching tag is found.  For instance, a protocol
      might define the result as failure of the operation, an empty
      value, returning some protocol defined or implementation defined
      default, or returning i-default [RFC2277].

   Applications, protocols, and specifications are not required to
   validate or understand any of the semantics of the language tags or
   ranges or of the subtags in them, nor do they require access to the
   IANA Language Subtag Registry (see Section 3 in [RFC4646]).  This
   simplifies implementation.

   However, designers of applications, protocols, or specifications are
   encouraged to use the information from the IANA Language Subtag
   Registry to support canonicalizing language tags and ranges in order
   to map grandfathered and obsolete tags or subtags into modern
   equivalents.

   Applications, protocols, or specifications that canonicalize ranges
   MUST either perform matching operations with both the canonical and
   original (unmodified) form of the range or MUST also canonicalize
   each tag for the purposes of comparison.








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   Note that canonicalizing language ranges makes certain operations
   impossible.  For example, an implementation that canonicalizes the
   language range "art-lojban" (artificial language, lojban variant) to
   use the more modern "jbo" (Lojban) cannot be used to select just the
   items with the older tag.

   Applications, protocols, or specifications that use basic ranges
   might sometimes receive extended language ranges instead.  An
   application, protocol, or specification MUST choose to a) map
   extended language ranges to basic ranges using the algorithm below,
   b) reject any extended language ranges in the language priority list
   that are not valid basic language ranges, or c) treat each extended
   language range as if it were a basic language range, which will have
   the same result as ignoring them, since these ranges will not match
   any valid language tags.

   An extended language range is mapped to a basic language range as
   follows: if the first subtag is a '*' then the entire range is
   treated as "*", otherwise each wildcard subtag is removed.  For
   example, the extended language range "en-*-US" maps to "en-US"
   (English, United States).

   Applications, protocols, or specifications, in addressing their
   particular requirements, can offer pre-processing or configuration
   options.  For example, an implementation could allow a user to
   associate or map a particular language range to a different value.
   Such a user might wish to associate the language range subtags 'nn'
   (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian) with the more general
   subtag 'no' (Norwegian).  Or perhaps a user would want to associate
   requests for the range "zh-Hans" (Chinese as written in the
   Simplified script) with content bearing the language tag "zh-CN"
   (Chinese as used in China, where the Simplified script is
   predominant).  Documentation on how the ranges or tags are altered,
   prioritized, or compared in the subsequent match in such an
   implementation will assist users in making these types of
   configuration choices.

3.3.  Filtering

   Filtering is used to select the set of language tags that matches a
   given language priority list.  It is called "filtering" because this
   set might contain no items at all or it might return an arbitrarily
   large number of matching items: as many items as match the language
   priority list, thus "filtering out" the non-matching items.

   In filtering, each language range represents the least specific
   language tag (that is, the language tag with fewest number of
   subtags) that is an acceptable match.  All of the language tags in



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   the matching set of tags will have an equal or greater number of
   subtags than the language range.  Every non-wildcard subtag in the
   language range will appear in every one of the matching language
   tags.  For example, if the language priority list consists of the
   range "de-CH" (German as used in Switzerland), one might see tags
   such as "de-CH-1996" (German as used in Switzerland, orthography of
   1996) but one will never see a tag such as "de" (because the 'CH'
   subtag is missing).

   If the language priority list (see Section 2.3) contains more than
   one range, the content returned is typically ordered in descending
   level of preference, but it MAY be unordered, according to the needs
   of the application or protocol.

   Some examples of applications where filtering might be appropriate
   include:

   o  Applying a style to sections of a document in a particular set of
      languages.

   o  Displaying the set of documents containing a particular set of
      keywords written in a specific set of languages.

   o  Selecting all email items written in a specific set of languages.

   o  Selecting audio files spoken in a particular language.

   Filtering seems to imply that there is a semantic relationship
   between language tags that share the same prefix.  While this is
   often the case, it is not always true: the language tags that match a
   specific language range do not necessarily represent mutually
   intelligible languages.

3.3.1.  Basic Filtering

   Basic filtering compares basic language ranges to language tags.
   Each basic language range in the language priority list is considered
   in turn, according to priority.  A language range matches a
   particular language tag if, in a case-insensitive comparison, it
   exactly equals the tag, or if it exactly equals a prefix of the tag
   such that the first character following the prefix is "-".  For
   example, the language-range "de-de" (German as used in Germany)
   matches the language tag "de-DE-1996" (German as used in Germany,
   orthography of 1996), but not the language tags "de-Deva" (German as
   written in the Devanagari script) or "de-Latn-DE" (German, Latin
   script, as used in Germany).





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   The special range "*" in a language priority list matches any tag.  A
   protocol that uses language ranges MAY specify additional rules about
   the semantics of "*"; for instance, HTTP/1.1 [RFC2616] specifies that
   the range "*" matches only languages not matched by any other range
   within an "Accept-Language" header.

   Basic filtering is identical to the type of matching described in
   [RFC3066], Section 2.5 (Language-range).

3.3.2.  Extended Filtering

   Extended filtering compares extended language ranges to language
   tags.  Each extended language range in the language priority list is
   considered in turn, according to priority.  A language range matches
   a particular language tag if each respective list of subtags matches.
   To determine a match:

   1.  Split both the extended language range and the language tag being
       compared into a list of subtags by dividing on the hyphen (%x2D)
       character.  Two subtags match if either they are the same when
       compared case-insensitively or the language range's subtag is the
       wildcard '*'.

   2.  Begin with the first subtag in each list.  If the first subtag in
       the range does not match the first subtag in the tag, the overall
       match fails.  Otherwise, move to the next subtag in both the
       range and the tag.

   3.  While there are more subtags left in the language range's list:

       A.  If the subtag currently being examined in the range is the
           wildcard ('*'), move to the next subtag in the range and
           continue with the loop.

       B.  Else, if there are no more subtags in the language tag's
           list, the match fails.

       C.  Else, if the current subtag in the range's list matches the
           current subtag in the language tag's list, move to the next
           subtag in both lists and continue with the loop.

       D.  Else, if the language tag's subtag is a "singleton" (a single
           letter or digit, which includes the private-use subtag 'x')
           the match fails.

       E.  Else, move to the next subtag in the language tag's list and
           continue with the loop.




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   4.  When the language range's list has no more subtags, the match
       succeeds.

   Subtags not specified, including those at the end of the language
   range, are thus treated as if assigned the wildcard value '*'.  Much
   like basic filtering, extended filtering selects content with
   arbitrarily long tags that share the same initial subtags as the
   language range.  In addition, extended filtering selects language
   tags that contain any intermediate subtags not specified in the
   language range.  For example, the extended language range "de-*-DE"
   (or its synonym "de-DE") matches all of the following tags:

      de-DE (German, as used in Germany)

      de-de (German, as used in Germany)

      de-Latn-DE (Latin script)

      de-Latf-DE (Fraktur variant of Latin script)

      de-DE-x-goethe (private-use subtag)

      de-Latn-DE-1996 (orthography of 1996)

      de-Deva-DE (Devanagari script)

   The same range does not match any of the following tags for the
   reasons shown:

      de (missing 'DE')

      de-x-DE (singleton 'x' occurs before 'DE')

      de-Deva ('Deva' not equal to 'DE')

   Note: [RFC4646] defines each type of subtag (language, script,
   region, and so forth) according to position, size, and content.  This
   means that subtags in a language range can only match specific types
   of subtags in a language tag.  For example, a subtag such as 'Latn'
   is always a script subtag (unless it follows a singleton) while a
   subtag such as 'nedis' can only match the equivalent variant subtag.
   Two-letter subtags in the initial position have a different type
   (language) than two-letter subtags in later positions (region).  This
   is the reason why a wildcard in the extended language range is
   significant in the first position but is ignored in all other
   positions.





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3.4.  Lookup

   Lookup is used to select the single language tag that best matches
   the language priority list for a given request.  When performing
   lookup, each language range in the language priority list is
   considered in turn, according to priority.  By contrast with
   filtering, each language range represents the most specific tag that
   is an acceptable match.  The first matching tag found, according to
   the user's priority, is considered the closest match and is the item
   returned.  For example, if the language range is "de-ch", a lookup
   operation can produce content with the tags "de" or "de-CH" but never
   content with the tag "de-CH-1996".  If no language tag matches the
   request, the "default" value is returned.

   For example, if an application inserts some dynamic content into a
   document, returning an empty string if there is no exact match is not
   an option.  Instead, the application "falls back" until it finds a
   matching language tag associated with a suitable piece of content to
   insert.  Some applications of lookup include:

   o  Selection of a template containing the text for an automated email
      response.

   o  Selection of an item containing some text for inclusion in a
      particular Web page.

   o  Selection of a string of text for inclusion in an error log.

   o  Selection of an audio file to play as a prompt in a phone system.

   In the lookup scheme, the language range is progressively truncated
   from the end until a matching language tag is located.  Single letter
   or digit subtags (including both the letter 'x', which introduces
   private-use sequences, and the subtags that introduce extensions) are
   removed at the same time as their closest trailing subtag.  For
   example, starting with the range "zh-Hant-CN-x-private1-private2"
   (Chinese, Traditional script, China, two private-use tags) the lookup
   progressively searches for content as shown below:

   Example of a Lookup Fallback Pattern

   Range to match: zh-Hant-CN-x-private1-private2
   1. zh-Hant-CN-x-private1-private2
   2. zh-Hant-CN-x-private1
   3. zh-Hant-CN
   4. zh-Hant
   5. zh
   6. (default)



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   This fallback behavior allows some flexibility in finding a match.
   Without fallback, the default content would be returned immediately
   if exactly matching content is unavailable.  With fallback, a result
   more closely matching the user request can be provided.

   Extensions and unrecognized private-use subtags might be unrelated to
   a particular application of lookup.  Since these subtags come at the
   end of the subtag sequence, they are removed first during the
   fallback process and usually pose no barrier to interoperability.
   However, an implementation MAY remove these from ranges prior to
   performing the lookup (provided the implementation also removes them
   from the tags being compared).  Such modification is internal to the
   implementation and applications, protocols, or specifications SHOULD
   NOT remove or modify subtags in content that they return or forward,
   because this removes information that can be used elsewhere.

   The special language range "*" matches any language tag.  In the
   lookup scheme, this range does not convey enough information by
   itself to determine which language tag is most appropriate, since it
   matches everything.  If the language range "*" is followed by other
   language ranges, it is skipped.  If the language range "*" is the
   only one in the language priority list or if no other language range
   follows, the default value is computed and returned.

   In some cases, the language priority list can contain one or more
   extended language ranges (as, for example, when the same language
   priority list is used as input for both lookup and filtering
   operations).  Wildcard values in an extended language range normally
   match any value that can occur in that position in a language tag.
   Since only one item can be returned for any given lookup request,
   wildcards in a language range have to be processed in a consistent
   manner or the same request will produce widely varying results.
   Applications, protocols, or specifications that accept extended
   language ranges MUST define which item is returned when more than one
   item matches the extended language range.

   For example, an implementation could map the extended language ranges
   to basic ranges.  Another possibility would be for an implementation
   to return the matching tag that is first in ASCII-order.  If the
   language range were "*-CH" ('CH' represents Switzerland) and the set
   of tags included "de-CH" (German as used in Switzerland), "fr-CH"
   (French, Switzerland), and "it-CH" (Italian, Switzerland), then the
   tag "de-CH" would be returned.








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3.4.1.  Default Values

   Each application, protocol, or specification that uses lookup MUST
   define the defaulting behavior when no tag matches the language
   priority list.  What this action consists of strongly depends on how
   lookup is being applied.  Some examples of defaulting behavior
   include:

   o  return an item with no language tag or an item of a non-linguistic
      nature, such as an image or sound

   o  return a null string as the language tag value, in cases where the
      protocol permits the empty value (see, for example, "xml:lang" in
      [XML10])

   o  return a particular language tag designated for the operation

   o  return the language tag "i-default" (see [RFC2277])

   o  return an error condition or error message

   o  return a list of available languages for the user to select from

   When performing lookup using a language priority list, the
   progressive search MUST process each language range in the list
   before seeking or calculating the default.

   The default value MAY be calculated or include additional searching
   or matching.  Applications, protocols, or specifications can specify
   different ways in which users can specify or override the defaults.

   One common way to provide for a default is to allow a specific
   language range to be set as the default for a specific type of
   request.  If this approach is chosen, this language range MUST be
   treated as if it were appended to the end of the language priority
   list as a whole, rather than after each item in the language priority
   list.  The application, protocol, or specification MUST also define
   the defaulting behavior if that search fails to find a matching tag
   or item.

   For example, if a particular user's language priority list is "fr-FR,
   zh-Hant" (French as used in France followed by Chinese as written in
   the Traditional script) and the program doing the matching had a
   default language range of "ja-JP" (Japanese as used in Japan), then
   the program searches as follows:






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   1. fr-FR
   2. fr
   3. zh-Hant // next language
   4. zh
   5. ja-JP   // now searching for the default content
   6. ja
   7. (implementation defined default)

4.  Other Considerations

   When working with language ranges and matching schemes, there are
   some additional points that can influence the choice of either.

4.1.  Choosing Language Ranges

   Users indicate their language preferences via the choice of a
   language range or the list of language ranges in a language priority
   list.  The type of matching affects what the best choice is for a
   user.

   Most matching schemes make no attempt to process the semantic meaning
   of the subtags.  The language range is compared, in a case-
   insensitive manner, to each language tag being matched, using basic
   string processing.  Users SHOULD select language ranges that are
   well-formed, valid language tags according to [RFC4646] (substituting
   wildcards as appropriate in extended language ranges).

   Applications are encouraged to canonicalize language tags and ranges
   by using the Preferred-Value from the IANA Language Subtag Registry
   for tags or subtags that have been deprecated.  If the user is
   working with content that might use the older form, the user might
   want to include both the new and old forms in a language priority
   list.  For example, the tag "art-lojban" is deprecated.  The subtag
   'jbo' is supposed to be used instead, so the user might use it to
   form the language range.  Or the user might include both in a
   language priority list: "jbo, art-lojban".

   Users SHOULD avoid subtags that add no distinguishing value to a
   language range.  When filtering, the fewer the number of subtags that
   appear in the language range, the more content the range will
   probably match, while in lookup unnecessary subtags can cause
   "better", more-specific content to be skipped in favor of less
   specific content.  For example, the range "de-Latn-DE" returns
   content tagged "de" instead of content tagged "de-DE", even though
   the latter is probably a better match.






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   Whether a subtag adds distinguishing value can depend on the context
   of the request.  For example, a user who reads both Simplified and
   Traditional Chinese, but who prefers Simplified, might use the range
   "zh" for filtering (matching all items that user can read) but
   "zh-Hans" for lookup (making sure that user gets the preferred form
   if it's available, but the fallback to "zh" will still work).  On the
   other hand, content in this case ought to be labeled as "zh-Hans" (or
   "zh-Hant" if that applies) for filtering, while for lookup, if there
   is either "zh-Hans" content or "zh-Hant" content, one of them (the
   one considered 'default') also ought to be made available with the
   simple "zh".  Note that the user can create a language priority list
   "zh-Hans, zh" that delivers the best possible results for both
   schemes.  If the user cannot be sure which scheme is being used (or
   if more than one might be applied to a given request), the user
   SHOULD specify the most specific (largest number of subtags) range
   first and then supply shorter prefixes later in the list to ensure
   that filtering returns a complete set of tags.

   Many languages are written predominantly in a single script.  This is
   usually recorded in the Suppress-Script field in that language
   subtag's registry entry.  For these languages, script subtags SHOULD
   NOT be used to form a language range.  Thus, the language range
   "en-Latn" is inappropriate in most cases (because the vast majority
   of English documents are written in the Latin script and thus the
   'en' language subtag has a Suppress-Script field for 'Latn' in the
   registry).

   When working with tags and ranges, note that extensions and most
   private-use subtags are orthogonal to language tag matching, in that
   they specify additional attributes of the text not related to the
   goals of most matching schemes.  Users SHOULD avoid using these
   subtags in language ranges, since they interfere with the selection
   of available content.  When used in language tags (as opposed to
   ranges), these subtags normally do not interfere with filtering
   (Section 3), since they appear at the end of the tag and will match
   all prefixes.  Lookup (Section 3.4) implementations are advised to
   ignore unrecognized private-use and extension subtags when performing
   language tag fallback.

4.2.  Meaning of Language Tags and Ranges

   Selecting language tags using language ranges requires some
   understanding by users of what they are selecting.  The meanings of
   the various subtags in a language range are identical to their
   meanings in a language tag (see Section 4.2 in [RFC4646]), with the
   addition that the wildcard "*" represents any matching sequence of
   values.




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4.3.  Considerations for Private-Use Subtags

   Private agreement is necessary between the parties that intend to use
   or exchange language tags that contain private-use subtags.  Great
   caution SHOULD be used in employing private-use subtags in content or
   protocols intended for general use.  Private-use subtags are simply
   useless for information exchange without prior arrangement.

   The value and semantic meaning of private-use tags and of the subtags
   used within such a language tag are not defined.  Matching private-
   use tags using language ranges or extended language ranges can result
   in unpredictable content being returned.

4.4.  Length Considerations for Language Ranges

   Language ranges are very similar to language tags in terms of content
   and usage.  The same types of restrictions on length that can be
   applied to language tags can also be applied to language ranges.  See
   [RFC4646] Section 4.3 (Length Considerations).

5.  Security Considerations

   Language ranges used in content negotiation might be used to infer
   the nationality of the sender, and thus identify potential targets
   for surveillance.  In addition, unique or highly unusual language
   ranges or combinations of language ranges might be used to track a
   specific individual's activities.

   This is a special case of the general problem that anything you send
   is visible to the receiving party.  It is useful to be aware that
   such concerns can exist in some cases.

   The evaluation of the exact magnitude of the threat, and any possible
   countermeasures, is left to each application or protocol.

6.  Character Set Considerations

   Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
   MINUS (%x2D).  Language ranges also use the character ASTERISK
   (%x2A).  These characters are present in most character sets, so
   presentation or exchange of language tags or ranges should not be
   constrained by character set issues.









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7.  References

7.1.  Normative References

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277]       Alvestrand, H., "IETF Policy on Character Sets and
                   Languages", BCP 18, RFC 2277, January 1998.

   [RFC4234]       Crocker, D., Ed. and P. Overell, "Augmented BNF for
                   Syntax Specifications: ABNF", RFC 4234, October 2005.

   [RFC4646]       Phillips, A., Ed., and M. Davis, Ed., "Tags for
                   Identifying Languages", BCP 47, RFC 4646, September
                   2006.

7.2.  Informative References

   [RFC1766]       Alvestrand, H., "Tags for the Identification of
                   Languages", RFC 1766, March 1995.

   [RFC2616]       Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P., and T. Berners-Lee,
                   "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                   June 1999.

   [RFC2616errata] IETF, "HTTP/1.1 Specification Errata", October 2004,
                   <http://purl.org/NET/http-errata>.

   [RFC3066]       Alvestrand, H., "Tags for the Identification of
                   Languages", BCP 47, RFC 3066, January 2001.

   [RFC3282]       Alvestrand, H., "Content Language Headers", RFC 3282,
                   May 2002.

   [XML10]         Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
                   and F. Yergeau, "Extensible Markup Language (XML) 1.0
                   (Third Edition)", World Wide Web Consortium
                   Recommendation, February 2004,
                   <http://www.w3.org/TR/REC-xml>.










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Appendix A.  Acknowledgements

   Any list of contributors is bound to be incomplete; please regard the
   following as only a selection from the group of people who have
   contributed to make this document what it is today.

   The contributors to [RFC1766] and [RFC3066], each of which was a
   precursor to this document, contributed greatly to the development of
   language tag matching, and, in particular, the basic language range
   and the basic matching scheme.  This document was originally part of
   [RFC4646], but was split off before that document's completion.
   Thus, directly or indirectly, those acknowledged in [RFC4646] also
   had a hand in the development of this document, and work done prior
   to the split is acknowledged in that document.

   The following people (in alphabetical order by family name)
   contributed to this document:

   Harald Alvestrand, Stephane Bortzmeyer, Jeremy Carroll, Peter
   Constable, John Cowan, Mark Crispin, Martin Duerst, Frank Ellermann,
   Doug Ewell, Debbie Garside, Marion Gunn, Jon Hanna, Kent Karlsson,
   Erkki Kolehmainen, Jukka Korpela, Ira McDonald, M. Patton, Randy
   Presuhn, Eric van der Poel, Markus Scherer, Misha Wolf, and many,
   many others.

   Very special thanks must go to Harald Tveit Alvestrand, who
   originated RFCs 1766 and 3066, and without whom this document would
   not have been possible.

Authors' Addresses

   Addison Phillips (Editor)
   Yahoo! Inc.

   EMail: addison@inter-locale.com


   Mark Davis (Editor)
   Google

   EMail: mark.davis@macchiato.com or mark.davis@google.com










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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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