Network Working Group                                   A. Phillips, Ed.
Internet-Draft                                            Quest Software
Obsoletes: 3066 (if approved)                              M. Davis, Ed.
Expires: May 20, 2006                                                IBM
                                                       November 16, 2005

           Matching Tags for the Identification of Languages

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes different mechanisms for comparing, matching,
   and evaluating language tags.  Possible algorithms for language
   negotiation and content selection are described.  This document, in
   combination with RFC 3066bis (replace "3066bis" with the RFC number
   assigned to draft-ietf-ltru-registry-14), replaces RFC 3066, which
   replaced RFC 1766.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Language Range . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Lists of Language Ranges . . . . . . . . . . . . . . . . .  4
     2.2.  Basic Language Range . . . . . . . . . . . . . . . . . . .  4
     2.3.  Extended Language Range  . . . . . . . . . . . . . . . . .  5
   3.  Types of Matching  . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Choosing a Type of Matching  . . . . . . . . . . . . . . .  8
     3.2.  Filtering  . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Filtering with Basic Language Ranges . . . . . . . . . 10
       3.2.2.  Filtering with Extended Language Ranges  . . . . . . . 10
       3.2.3.  Distance Metric Filtering  . . . . . . . . . . . . . . 11
     3.3.  Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Meaning of Language Tags and Ranges  . . . . . . . . . . . 16
     4.2.  Considerations for Private Use Subtags . . . . . . . . . . 17
     4.3.  Length Considerations in Matching  . . . . . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   6.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   8.  Character Set Considerations . . . . . . . . . . . . . . . . . 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
   Intellectual Property and Copyright Statements . . . . . . . . . . 27

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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.

   Information about a user's language preferences commonly needs to be
   identified so that appropriate processing can be applied.  For
   example, the user's language preferences in a 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.

   Given a set of language identifiers, such as those defined in
   [RFC3066bis], various mechanisms can be envisioned for performing
   language negotiation and tag matching.  Applications, protocols, or
   specifications will have varying needs and requirements that will
   affect the choice of a suitable mechanism.  Protocols and
   specifications SHOULD clearly indicate the particular mechanism used
   in selecting or matching language tags.

   This document defines several mechanisms for matching, selecting, or
   filtering content whose natural language is identified using Language
   Tags [RFC3066bis], as well as the syntax (called a "language range")
   associated with each of these mechanisms for specifying the user's
   language preferences.

   This document, in combination with [RFC3066bis] (replace "3066bis"
   globally in this document with the RFC number assigned to
   draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  The Language Range

   Language Tags [RFC3066bis] are used to identify the language of some
   information item or content.  Applications or protocols 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 language ranges in its discussion of the
   Accept-Language header (Section 14.4), which is used for selecting
   content from servers based on the language of that content.

   When selecting content according to its language, it is useful to
   have a mechanism for identifying sets of language tags that share
   specific attributes.  This allows users to select or filter content
   based on specific requirements.  Such an identifier is called a
   "Language Range".

2.1.  Lists of Language Ranges

   When users specify a language preference they often need to specify a
   prioritized list of language ranges in order to best reflect their
   language requirements for the matching operation.  This is especially
   true for speakers of minority languages.  A speaker of Breton in
   France, for example, may specify "be" 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 speaker may wish to
   fallback from Skolt Sami to Northern Sami to Finnish.

   A "Language Priority List" consists of 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.

2.2.  Basic Language Range

   A "Basic Language Range" identifies the set of content whose language
   tags begin with the same sequence of subtags.  A basic language range
   is identified by its 'language-range' tag, by adapting the
   ABNF[RFC4234] from HTTP/1.1 [RFC2616] :

   language-range = language-tag / "*"
   language-tag   = 1*8[alphanum] *["-" 1*8alphanum]
   alphanum       = ALPHA / DIGIT

   That is, a language-range has the same syntax as a language-tag or is
   the single character "*".  Basic Language Ranges imply that there is
   a semantic relationship between language tags that share the same

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   prefix.  While this is often the case, it is not always true and
   users should note that the set of language tags that match a specific
   language-range may not be mutually intelligible.

   Basic language ranges were originally described in [RFC3066] and HTTP
   1.1 [RFC2616] (where they are referred to as simply a "language

   Users SHOULD avoid subtags that add no distinguishing value to a
   language range.  For example, script subtags SHOULD NOT be used to
   form a language range with language subtags which have a matching
   Suppress-Script field in their registry record.  Thus the language
   range "en-Latn" is probably inappropriate in most cases (because the
   vast majority English documents are written in the Latin script and
   thus the 'en' language subtag has a Suppress-Script field for 'Latn'
   in the registry).

   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.

   When working with tags and ranges, note that extensions and most
   private use subtags are generally orthogonal to language tag fallback
   and users SHOULD avoid using these subtags in language ranges, since
   they will often interfere with the selection of available language
   content.  Since these subtags are always at the end of the sequence
   of subtags, they don't normally interfere with the use of prefixes
   for matching in the schemes described below.

   Note that when working with basic language ranges, no attempt is made
   to process the semantics of the tags or ranges in any way.  The
   language tag and language range are compared in a case insensitive
   manner using basic string processing.  Thus the choice of subtags in
   both the language tag and language range may affect the results
   produced as a result.

2.3.  Extended Language Range

   A Basic Language Range does not always provide the most appropriate
   way to specify a user's preferences.  Sometimes it is beneficial to
   define a more granular matching scheme that takes advantage of the
   internal structure of language tags, by allowing the user to specify,
   for example, the value of a specific field in a language tag or to
   indicate which values are of interest in filtering or selecting the

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   In an extended language range, the identifier takes the form of a
   series of subtags which must consist of well-formed subtags or the
   special subtag "*".  For example, the language range "en-*-US"
   specifies a primary language of 'en', followed by any script subtag,
   followed by the region subtag 'US'.

   An extended language range can be represented by the following ABNF:
   extended-language-range  = range ; a range
                 / privateuse              ; private use tag
                 / grandfathered           ; grandfathered registrations

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

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

   extlang       = *2("-" 3ALPHA) ("-" ( 3ALPHA / "*"))
                                          ; reserved for future use
                                          ; wildcard can only appear
                                          ;   at the end

   script        = 4ALPHA                 ; ISO 15924 code
                 / "*"                    ; or wildcard

   region        = 2ALPHA                 ; ISO 3166 code
                 / 3DIGIT                 ; UN M.49 code
                 / "*"                    ; ... or wildcard

   variant       = 5*8alphanum            ; registered variants
                 / (DIGIT 3alphanum)      ;
                 / "*"                    ; ... or wildcard

   extension     = singleton *("-" (2*8alphanum)) [ "-*" ]
                                          ; extension subtags
                                          ; wildcard can only appear
                                          ;   at the end

   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

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   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

   A field not present in the middle of an extended language range MAY
   be treated as if the field contained a "*".  For example, the range
   "en-US" MAY be considered to be equivalent to the range "en-*-US".
   This also means that multiple wildcards can be collapsed (so that
   "en-*-*-US" is equivalent to "en-*-US").

   When working with tags and ranges users SHOULD note the following:

   1.  Private-use and Extension subtags are normally orthogonal to
       language tag fallback.  Implementations or specifications that
       use a lookup (Section 3.3) matching scheme SHOULD ignore
       unrecognized private-use and extension subtags when performing
       language tag fallback.  Since these subtags are always at the end
       of the sequence of subtags, they don't normally interfere with
       the use of prefixes for matching in the schemes described below.

   2.  Applications, specifications, or protocols that choose not to
       interpret one or more private-use or extension subtags SHOULD NOT
       remove or modify these extensions in content that they are
       processing.  When a language tag instance is to be used in a
       specific, known protocol, and is not being passed through to
       other protocols, language tags MAY be filtered to remove subtags
       and extensions that are not supported by that protocol.  Such
       filtering SHOULD be avoided, if possible, since it removes
       information that might be relevant if services on the other end
       of the protocol would make use of that information.

   3.  Some applications of language tags might want or need to consider
       extensions and private-use subtags when matching tags.  If
       extensions and private-use subtags are included in a matching or
       filtering process that utilizes the one of the schemes described
       in this document, then the implementation SHOULD canonicalize the
       language tags and/or ranges before performing the matching.  Note
       that language tag processors that claim to be "well-formed"
       processors as defined in [RFC3066bis] generally fall into this

   There are several matching algorithms or schemes which can be applied
   when matching extended language ranges to language tags.

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3.  Types of Matching

   Matching language ranges to language tags can be done in a number of
   different ways.  This section describes the different types of
   matching scheme, as well as the considerations for choosing between

   There are two basic types of matching scheme: those that produce an
   open-ended set of content (called "filtering") and those that produce
   a single information item for a given request (called "lookup").

   A key difference between these two types of matching scheme is that
   the language range for filtering operations is always the _least_
   specific tag one will accept as a match, while for lookup operations
   the language range is always the _most_ specific tag.

3.1.  Choosing a Type of Matching

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

   Filtering can be used to produce a set of results (such as a
   collection of documents).  For example, if using a search engine, one
   might use filtering to limit the results to documents written in
   French.  It can also be used when deciding whether to perform some
   processing that is language sensitive on some content.  For example,
   a process might cause paragraphs whose language tag matched the
   language range "nl" to be displayed in italics within a document.

   This document describes three types of filtering:

   1.  Basic Filtering (Section 3.2.1) is used to match content using
       basic language rangesSection 2.2.  It is compatible with
       implementations that do not produce extended language ranges.

   2.  Extended Range Filtering (Section 3.2.2) is used to match content
       using extended language rangesSection 2.3.  Newer implementations
       SHOULD use this form of filtering in preference to basic

   3.  Scored Filtering (Section 3.2.3) produces an ordered set of
       content using either basic or extended language ranges.  It
       should be used when the quality of the match within a specific
       language range is important, as when presenting a list of
       documents resulting from a search.

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   Lookup (Section 3.3) is used when each request MUST produce exactly
   one piece of content.  For example, a Web server might use the
   Accept-Language HTTP header to choose which language to return a
   custom 404 page in: since it can return only one page, it must choose
   a single item and it must return some item, even if no content
   matches the language ranges supplied by the user.

   Most types of matching in this document are designed so that
   implementations do not have to examine the values of the subtags
   supplied and, except for scored filtering, they do not need access to
   the Language Subtag Registry nor do they require the use of valid
   subtags in either language tags or language ranges.  This has great
   benefit for speed and simplicity of implementation.

   Implementations might also wish to use semantic information external
   to the langauge tags when performing fallback.  For example, the
   primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal
   Norwegian) might both be usefully matched to the more general subtag
   'no' (Norwegian).  Or an implementation might infer that content
   labeled "zh-CN" is morely likely to match the range "zh-Hans" than
   equivalent content labeled "zh-TW".

3.2.  Filtering

   Filtering is used to select the set of content that matches a given
   prefix.  It is called "filtering" because this set of content may
   contain no items at all or it may return an arbitrary number of
   matching items--as many as match the language range used to specify
   the items, thus filtering out the non-matching content.

   In filtering, the language range represents the _least_ specific tag
   which is an acceptable match.  That is, all of the language tags in
   the set of filtered content will have an equal or greater number of
   subtags than the language range.  For example, if the language range
   is "de-CH", one might see matching content with the tag "de-CH-1996"
   but one will never see a match with the tag "de".

   If the language priority list (see Section 2.1) contains more than
   one range, the content returned is typically ordered in descending
   level of preference.

   Some examples where filtering might be appropriate include:

   o  Applying a style to sections of a document in a particular
      language range.

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

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   o  Selecting all email items written in specific range of languages.

   Filtering can produce either ordered or unordered set of results.
   For example, applying formatting to a document based on the language
   of specific pieces of content does not require the content to be
   ordered.  It is sufficient to know whether a specific piece of
   content matches or does not match.  A search application, on the
   other hand, probably would put the results into a priority order.

   If an ordered set is desired, as described above, then the
   application or protocol needs to determine the relative "quality" of
   the match between different language tags and the language range.

   This measurment is called a "distance metric".  A distance metric
   assigns a numeric value to the comparison of each language tag to a
   language range and represents the 'distance' between the two.  A
   distance of zero means that they are identical, a small distance
   indicates that they are very similar, and a large distance indicated
   that they are very different.  Using a distance metric,
   implementations can, for example, allow users to select a threshold
   distance for a match to be "successful" while filtering or it can use
   the numeric value to order the results.

3.2.1.  Filtering with Basic Language Ranges

   When filtering using a basic language range, the language range
   matches a language tag if it exactly equals the tag, or if it exactly
   equals a prefix of the tag such that the first character following
   the prefix is "-".  (That is, the language-range "de-de" matches the
   language tag "de-DE-1996", but not the language tag "de-Deva".)

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

3.2.2.  Filtering with Extended Language Ranges

   In the Extended Range Matching scheme, each extended language range
   in the language priority list is considered in turn, according to
   priority.  The subtags in each extended language range are compared
   to the corresponding subtags in the language tag being examined.  The
   subtag from the range is considered to match if it exactly matches
   the corresponding subtag in the tag or the range's subtag has the
   value "*" (which matches all subtags, including the empty subtag).
   Extended Range Matching is an extension of basic matching
   (Section 3.2.1): the language range represents the least specific tag

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   which is an acceptable match.

   Private use subtags MAY be specified in the language range and MUST
   NOT be ignored when matching.

   Subtags not specified, including those at the end of the language
   range, are assigned the value "*".  This makes each range into a
   prefix much like that used in basic language range matching.  For
   example, the extended language range "zh-*-CN" matches all of the
   following tags because the unspecified variant field is expanded to







3.2.3.  Distance Metric Filtering

   Both basic and extended language range filtering produce simple
   boolean matches.  Sometimes it may be beneficial to provide an array
   of results with different levels of matching, for example, sorting
   results based on the overall "quality" of the match.  Distance metric
   filtering provides a way to generate these quality values.

   First both the extended language range and the language tags to be
   matched to it must be canonicalized by mapping grandfathered and
   obsolete tags into modern equivalents.

   The language range and the language tags are then transformed into
   quintuples of elements of the form (language, script, country,
   variant, extension).  Any extended language subtags are considered
   part of the language element; private use subtag sequences are
   considered part of the language element if in the initial position in
   the tag and part of the variant element if not.  Language subtags
   'und', 'mul', and the script subtag 'Zyyy' are converted to "*".

   Missing components in the language-tag are set to "*"; thus a "*"
   pattern becomes the quintuple ("*", "*", "*", "*", "*").  Missing
   components in the extended language-range are handled similarly to
   extended range lookup: missing internal subtags are expanded to "*".

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   Missing end subtags are expanded as the empty string.  Thus a pattern
   "en-US" becomes the quintuple ("en","*","US","","").

   Here are some examples of language-tags and their quintuples:

      en-US ("en","*","US","*","*")

      sr-Latn ("sr,"Latn","*","*","*")

      zh-cmn-Hant ("zh-cmn","Hant","*","*","*")

      x-foo ("x-foo","*","*","*","*")

      en-x-foo ("en","*","*","x-foo","*")

      i-default ("i-default","*","*","*","*")

      sl-Latn-IT-roazj ("sl","Latn","IT","rozaj","*")

      zh-r-wadegile ("zh","*","*","*","r-wadegile") // hypothetical

   Each language-range/language-tag pair being compared is assigned a
   distance value, whereby small values indicate better matches and
   large values indicate worse ones.  The distance between the pair is
   the sum of the distances for each of the corresponding elements of
   the quintuple.  If the elements are identical or one is '*', then the
   distance value between them is zero.  Otherwise, it is given by the
   following table:
     256    language mismatch
     128    script mismatch
      32    region mismatch
       4    variant mismatch
       1    extension mismatch

   A value of 0 is a perfect match; 421 is no match at all.  Different
   threshold values might be appropriate for different applications or
   protocols.  Implementations will usually allow users to choose the
   most appropriate selection value, ranking the matched items based on

   Examples of various tag's distances from the range "en-US":

   "fr"             256 (language mismatch, region match)
   "en-GB"          384 (language, region mismatch)
   "en-Latn-US"       0 (all fields match)
   "en-Brai"         32 (region mismatch)
   "en-US-x-foo"      4 (variant mismatch: range is the empty string)
   "en-US-r-wadegile" 1 (extension mismatch: range is the empty string)

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   Implementations or protocols sometimes might wish to use more
   sophisticated weights that depend on the values of the corresponding
   elements.  For example, depending on the domain, an implemenation
   might give a small distance to the difference between the language
   subtag 'no' and the closely related language subtags 'nb' or 'nn'; or
   between the script subtags 'Kata' and 'Hira'; or between the region
   subtags 'US' and 'UM'.

3.3.  Lookup

   Lookup is used to select the single information item that best
   matches the language priority list for a given request.  In lookup,
   each language range in the language priority list represents the
   _most_ specific tag which is an acceptable match; only the closest
   matching item according the user's priority is returned.  For
   example, if the language range is "de-CH", one might expect to
   receive an information item with the tag "de" but never one with the
   tag "de-CH-1996".  Usually if no content matches the request, a
   "default" item 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
   suitable piece of content to insert.  Other examples of lookup might

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

   o  Selection of a graphic containing text for inclusion in a
      particular Web page.

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

   In the Lookup scheme, the language range is progressively truncated
   from the end until a matching piece of content is located.  For
   example, starting with the range "zh-Hant-CN-x-private", the lookup
   would progressively search for content as shown below:

   Range to match: zh-Hant-CN-x-private
   1. zh-Hant-CN-x-private
   2. zh-Hant-CN
   3. zh-Hant
   4. zh
   5. (default content or the empty tag)

   Figure 5: Example of a Lookup Fallback Pattern

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   This scheme allows some flexibility in finding content.  It also
   typically provides better results when data is not available at a
   specific level of tag granularity or is sparsely populated (than if
   the default language for the system or content were used).

   The language range "*" matches any language tag.  In the lookup
   scheme, this language range does not convey enough information to
   determine which content is most appropriate.  If this language range
   is the only one in the language priority list, it matches the default
   content.  If this language range is followed by other language
   ranges, it should be skipped.

   When performing lookup using a language priority list, the
   progressive search MUST proceed to consider each language range
   before finding the default content or empty tag.  The default content
   might be content with no language tag (or with an empty value, as
   with xml:lang in the XML specification), or it might be a particular
   language designated for that bit of content.

   One common way to provide for default content is to allow a specific
   language range to be set as the default for a specific type of
   request.  This language range is then treated as if it were appended
   to the end of the language priority list, rather than after each item
   in the language priority list.

   For example, if a particular user's language priority list were
   "fr-FR; zh-Hant" and the program doing the matching had a default
   language range of "ja-JP", the program would search for content as
   1. fr-FR
   2. fr
   3. zh-Hant // next language
   4. zh
   5. (return default content)
      a. ja-JP
      b. ja
      c. (empty tag or other default content)

   Figure 6: Lookup Using a Language Priority List

   In some cases, the language priority list might 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 are
   supposed to match any value that occurs in that position in a
   language tag.  Since only one item can be returned for any given
   lookup request, the wildcards must be processed in a predictable
   manner (or the same request might produce widely varying results).

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   Thus, for each range in the language priority list, the following
   rules must be applied to produce a basic language range for use in
   the fallback mechanism:

   1.  If the first subtag in the extended language range is a "*" then
       entire range is converted to "*".

   2.  For each subsequent subtag, if the value is a "*" then that
       subtag and its preceeding hyphen are removed.

   For example:

   *-US      becomes  *
   en-*-US   becomes  en-US
   en-Latn-* becomes  en-Latn

   Figure 7: Transformation of Extended Language Ranges

   For the language priority list "*-US; fr-*-FR; zh-Hant", the fallback
   pattern would be:
   1. * (skipped)
   2. fr-FR
   3. fr
   4. zh-Hant
   5. zh
   6. (default content)

   Figure 8: Extended Language Range Fallback Example

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4.  Other Considerations

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

4.1.  Meaning of Language Tags and Ranges

   Selecting content using language ranges requires some understanding
   by users of what they are selecting.  A language tag or range
   identifies a language as spoken (or written, signed or otherwise
   signaled) by human beings for communication of information to other
   human beings.

   If a language tag B contains language tag A as a prefix, then B is
   typically "narrower" or "more specific" than A. For example, "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-Arab"
   (Azerbaijani written using the Arabic script).  A person fluent in
   one script might not be able to read the other, even though the text
   might be otherwise 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.

   Variant subtags in particular seem to represent specific divisions in
   mutual understanding, since they often encode dialects or other
   idiosyncratic variations within a language.

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

   o  For a single information object, the associated language tags
      might be interpreted as the set of languages that are 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.

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   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
      understanding of the document.  Example: MIME multipart/

   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 misapplying
      the Norwegian rules.

4.2.  Considerations for Private Use Subtags

   Private-use subtags require private agreement between the parties
   that intend to use or exchange language tags that use them and great
   caution SHOULD be used in employing them 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.

4.3.  Length Considerations in Matching

   RFC 3066 [RFC3066] did not provide an upper limit on the size of
   language tags or ranges.  RFC 3066 did define the semantics of
   particular subtags in such a way that most language tags or ranges
   consisted of language and region subtags with a combined total length
   of up to six characters.  Larger tags and ranges (in terms of both
   subtags and characters) did exist, however.

   [RFC3066bis] also does not impose a fixed upper limit on the number
   of subtags in a language tag or range (and thus an upper bound on the
   size of either).  The syntax in that document suggests that,
   depending on the specific language or range of languages, more
   subtags (and thus characters) are sometimes necessary as a result.

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   Length considerations and their impact on the selection and
   processing of tags are described in Section 2.1.1 of that document.

   An application or protocol MAY choose to limit the length of the
   language tags or ranges used in matching.  Any such limitation SHOULD
   be clearly documented, and such documentation SHOULD include the
   disposition of any longer tags or ranges (for example, whether an
   error value is generated or the language tag or range is truncated).
   If truncation is permitted it MUST NOT permit a subtag to be divided,
   since this changes the semantics of the subtag being matched and can
   result in false positives or negatives.

   Applications or protocols that restrict storage SHOULD consider the
   impact of tag or range truncation on the resulting matches.  For
   example, removing the "*" from the end of an extended language range
   (see Section 2.3) can greatly modify the set of returned matches.  A
   protocol that allows tags or ranges 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 values in
   substantial ways.

   In practice, most tags do not require additional subtags or
   substantially more characters.  Additional subtags sometimes add
   useful distinguishing information, but extraneous subtags interfere
   with the meaning, understanding, and especially matching of language
   tags.  Since language tags or ranges MAY be truncated by an
   application or protocol that limits storage, when choosing language
   tags or ranges users and applications SHOULD avoid adding subtags
   that add no distinguishing value.  In particular, users and
   implementations SHOULD follow the 'Prefix' and 'Suppress-Script'
   fields in the registry (defined in Section 3.6 of [RFC3066bis]):
   these fields provide guidance on when specific additional subtags
   SHOULD (and SHOULD NOT) be used.

   Implementations MUST support a limit of at least 33 characters.  This
   limit includes at least one subtag of each non-extension, non-private
   use type.  When choosing a buffer limit, a length of at least 42
   characters is strongly RECOMMENDED.

   The practical limit on tags or ranges derived solely from registered
   values is 42 characters.  Implementations MUST be able to handle tags
   and ranges of this length.  Support for tags and ranges of at least
   62 characters in length is RECOMMENDED.  Implementations MAY support
   longer values, including matching extensive sets of private use or
   extension subtags.

   Applications or protocols which have to truncate a tag MUST do so by
   progressively removing subtags along with their preceding "-" from

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   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

   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 9: Example of Tag Truncation

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5.  IANA Considerations

   This document presents no new or existing considerations for IANA.

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6.  Changes

   This is the first version of this document.

   The following changes were put into this document since draft-05:

      Modified the ABNF to match changes in [RFC3066bis] (K.Karlsson)

      Matched the references and reference formats to [RFC3066bis]

      Various edits, additions, and emendations to deal with changes in
      the Last Call of draft-registry as well as cleaning up the text.

      Changed from 'defined' to 'identifies' in Section 4.1.  (M.Gunn)

      Reorganized the text and broke it into sections (M.Duerst)

      Modified occurences of the word "application" to refer to
      "applications or protocols" or otherwise be specific (E. van der

      Removed "Extended Language Range Lookup", merging it with other
      text on lookup to form a single scheme.  (M.Davis)

      Fixed or removed obsolete or dangling references (Ed.)

      Added an introduction to section 4 and added one sentence to make
      it flow better to the start of section 4.1.  (Ed.)

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7.  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
   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.

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8.  Character Set Considerations

   The syntax of language tags and language ranges permit only the
   characters A-Z, a-z, 0-9, and HYPHEN-MINUS (%x2D).  These characters
   are present in most character sets, so presentation of language tags
   should not present any character set issues.

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9.  References

9.1.  Normative References

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

   [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.

              Phillips, A., Ed. and M. Davis, Ed., "Tags for the
              Identification of Languages", October 2005, <http://

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

9.2.  Informative References

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

   [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.

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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 [RFC3066bis], [RFC3066] and [RFC1766], each of
   which is a precursor to 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 by family name)
   contributed to this document:

   Jeremy Carroll, John Cowan, Martin Duerst, Frank Ellermann, Doug
   Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M. Patton, Randy
   Presuhn, Eric van der Poel, 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.

   For this particular document, John Cowan originated the scheme
   described in Section 3.2.3.  Mark Davis originated the scheme
   described in the Section 3.3.

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Authors' Addresses

   Addison Phillips (editor)
   Quest Software

   Email: addison dot phillips at quest dot com

   Mark Davis (editor)

   Email: mark dot davis at ibm dot com

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Copyright Statement

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