Matching of Language Tags
RFC 4647

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,
   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.

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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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