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Versions: 00 01 02 03 04 05 06 07 08 RFC 6570

Network Working Group                                        J. Gregorio
Internet-Draft                                                    Google
Intended status: Standards Track                        R. Fielding, Ed.
Expires: January 12, 2012                                          Adobe
                                                               M. Hadley
                                                                  Oracle
                                                           M. Nottingham
                                                              D. Orchard
                                                            Jul 11, 2011


                              URI Template
                     draft-gregorio-uritemplate-05

Abstract

   A URI Template is a compact sequence of characters for describing a
   range of Uniform Resource Identifiers through variable expansion.
   This specification defines the URI Template syntax and the process
   for expanding a URI Template into a URI reference, along with
   guidelines for the use of URI Templates on the Internet.

Editorial Note (to be removed by RFC Editor)

   To provide feedback on this Internet-Draft, join the W3C URI mailing
   list (http://lists.w3.org/Archives/Public/uri/) [1].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 12, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.






























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Levels and Expression Types  . . . . . . . . . . . . . . .  6
     1.3.  Design Considerations  . . . . . . . . . . . . . . . . . . 10
     1.4.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . 11
     1.5.  Notational Conventions . . . . . . . . . . . . . . . . . . 11
     1.6.  Character Encoding and Unicode Normalization . . . . . . . 12
   2.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.1.  Literals . . . . . . . . . . . . . . . . . . . . . . . . . 14
     2.2.  Expressions  . . . . . . . . . . . . . . . . . . . . . . . 14
     2.3.  Variables  . . . . . . . . . . . . . . . . . . . . . . . . 15
     2.4.  Value Modifiers  . . . . . . . . . . . . . . . . . . . . . 16
       2.4.1.  Prefix Values  . . . . . . . . . . . . . . . . . . . . 16
       2.4.2.  Component Values . . . . . . . . . . . . . . . . . . . 17
     2.5.  Value Defaults . . . . . . . . . . . . . . . . . . . . . . 17
   3.  Expansion  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     3.1.  Literal Expansion  . . . . . . . . . . . . . . . . . . . . 19
     3.2.  Expression Expansion . . . . . . . . . . . . . . . . . . . 20
       3.2.1.  Undefined Variable Expansion . . . . . . . . . . . . . 20
       3.2.2.  Simple String Expansion: {var} . . . . . . . . . . . . 20
       3.2.3.  Reserved expansion: {+var} . . . . . . . . . . . . . . 21
       3.2.4.  Label expansion with dot-prefix: {.var}  . . . . . . . 22
       3.2.5.  Path segment expansion: {/var} . . . . . . . . . . . . 23
       3.2.6.  Path-style parameter expansion: {;var} . . . . . . . . 25
       3.2.7.  Form-style query expansion: {?var} . . . . . . . . . . 25
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Example URI Template Parser . . . . . . . . . . . . . 26
   Appendix B.  Revision History (to be removed by RFC Editor)  . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27

















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1.  Introduction

1.1.  Overview

   A Uniform Resource Identifier (URI) [RFC3986] is often used to
   identify a specific resource within a common space of similar
   resources.  For example, personal web spaces are often delegated
   using a common pattern, such as

     http://example.com/~fred/
     http://example.com/~mark/

   or a set of dictionary entries might be grouped in a hierarchy by the
   first letter of the term, as in

     http://example.com/dictionary/c/cat
     http://example.com/dictionary/d/dog

   or a service interface might be invoked with various user input in a
   common pattern, as in

     http://example.com/search?q=cat&lang=en
     http://example.com/search?q=dog&lang=fr

   URI Templates provide a mechanism for abstracting a space of resource
   identifiers such that the variable parts can be easily identified and
   described.  URI templates can have many uses, including discovery of
   available services, configuring resource mappings, defining computed
   links, specifying interfaces, and other forms of programmatic
   interaction with resources.  For example, the above resources could
   be described by the following URI templates:

     http://example.com/~{username}/
     http://example.com/dictionary/{term:1}/{term}
     http://example.com/search{?q,lang}

   We define the following terms:
   o  expression - The text between '{' and '}', including the enclosing
      braces, as defined in Section 2.
   o  expansion - The string result obtained from a template expression
      after processing it according to its expression type, list of
      variable names, and value modifiers, as defined in Section 3.
   o  template processor - A program or library that, given a URI
      Template and a set of variables with values, transforms the
      template string into a URI-reference by parsing the template for
      expressions and substituting each one with its corresponding
      expansion.




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   A URI Template provides both a structural description of a URI space
   and, when variable values are provided, a simple instruction on how
   to construct a URI corresponding to those values.  A URI Template is
   transformed into a URI-reference by replacing each delimited
   expression with its value as defined by the expression type and the
   values of variables named within the expression.  The expression
   types range from simple string expansion to multiple key=value lists.
   The expansions are based on the URI generic syntax, allowing an
   implementation to process any URI Template without knowing the
   scheme-specific requirements of every possible resulting URI.

   For example, the following URI Template includes a form-style
   parameter expression, as indicated by the "?" operator appearing
   before the variable names.

     http://www.example.com/foo{?query,number}

   Each template expression describes, in a machine-readable manner, how
   a URI is to be constructed.  In this example, the expansion process
   for templates beginning with the question-mark ("?") operator follows
   the same pattern as form-style interfaces on the World Wide Web.

     http://www.example.com/foo{?query,number}
                               \_____________/
                                  |
                                  |
             For each defined variable in [ 'query', 'number' ],
             substitute "?" if it is the first substitution or "&"
             thereafter, followed by the variable name, '=', and the
             variable's value.

   If the variables have the values

     query  := "mycelium"
     number := 100

   then the expansion of the above URI Template is

     http://www.example.com/foo?query=mycelium&number=100

   Alternatively, if 'query' is undefined, then the expansion would be

     http://www.example.com/foo?number=100

   or if both variables are undefined, then it would be

     http://www.example.com/foo




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   A URI Template may be provided in absolute form, as in the examples
   above, or in relative form.  A template MUST be expanded before the
   resulting reference can be resolved from relative to absolute form.

   Although the URI syntax is used for the result, the template string
   is allowed to contain the broader set of characters that can be found
   in IRI references [RFC3987].  A URI Template is therefore also an IRI
   template, and the result of template processing can be rendered as an
   IRI by transforming each of the pct-encoded sequences to their
   corresponding Unicode character if that character is not in the
   reserved set.

1.2.  Levels and Expression Types

   URI Templates are similar to a macro language with a fixed set of
   macro definitions: the expression type determines the expansion
   process.  The default expression type is simple string expansion,
   wherein a single named variable is replaced by its value as a string
   after UTF-8 encoding the characters and then pct-encoding any octets
   that are not in the unreserved set.

   Since most template processors implemented prior to this
   specification have only implemented the default expression type, we
   refer to these as Level 1 templates.

   .-----------------------------------------------------------------.
   | Level 1 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             empty := ""                                         |
   |             undef := null                                       |
   |                                                                 |
   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | Simple string expansion                       (Sec 3.2.2) |
   |     |                                                           |
   |     |    {var}                 value                            |
   |     |    {hello}               Hello%20World%21                 |
   |     |    O{empty}X             OX                               |
   |     |    O{undef}X             OX                               |
   `-----------------------------------------------------------------'

   Level 2 templates add the ability to define a default string, which
   is substituted if the variable is undefined, and the plus ("+")
   operator for expansion values that are allowed to include reserved
   characters.



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   .-----------------------------------------------------------------.
   | Level 2 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             empty := ""                                         |
   |             undef := null                                       |
   |             path  := "/foo/bar"                                 |
   |                                                                 |
   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | String expansion with defaults                (Sec 3.2.2) |
   |     |                                                           |
   |     |    {var|default}         value                            |
   |     |    O{empty|default}X     OX                               |
   |     |    O{undef|default}X     OdefaultX                        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  +  | Reserved expansion with defaults              (Sec 3.2.3) |
   |     |                                                           |
   |     |    {+var}                value                            |
   |     |    {+hello}              Hello%20World!                   |
   |     |    {+path}/here          /foo/bar/here                    |
   |     |    here?ref={+path}      here?ref=/foo/bar                |
   |     |    up{+path}{x}/here     up/foo/bar1024/here              |
   |     |    up{+empty|/1}/here    up/here                          |
   |     |    up{+undef|/1}/here    up/1/here                        |
   |     |                                                           |
   `-----------------------------------------------------------------'

   Level 3 templates add more complex operators for lists of comma-
   separated values, dot-prefixed labels, slash-prefixed path segments,
   semicolon-prefixed path parameters, and the forms-style construction
   of a query syntax consisting of key=value pairs that are separated by
   an ampersand character.

   .-----------------------------------------------------------------.
   | Level 3 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             empty := ""                                         |
   |             undef := null                                       |
   |             path  := "/foo/bar"                                 |
   |             x     := "1024"                                     |
   |             y     := "768"                                      |
   |                                                                 |



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   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | String expansion with multiple variables      (Sec 3.2.2) |
   |     |                                                           |
   |     |    {x,y}                 1024,768                         |
   |     |    {x,hello,y}           1024,Hello%20World%21,768        |
   |     |    ?{x,empty}            ?1024,                           |
   |     |    ?{x,undef}            ?1024                            |
   |     |    ?{undef,y}            ?768                             |
   |     |    ?{x,undef|0}          ?1024,0                          |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  +  | Reserved expansion with multiple variables    (Sec 3.2.3) |
   |     |                                                           |
   |     |    {+x,hello,y}          1024,Hello%20World!,768          |
   |     |    {+path,x}/here        /foo/bar/1024/here               |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  .  | Label expansion, dot-prefixed                 (Sec 3.2.4) |
   |     |                                                           |
   |     |    X{.var}               X.value                          |
   |     |    X{.empty}             X.                               |
   |     |    X{.undef}             X                                |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  /  | Path segments, slash-prefixed                 (Sec 3.2.5) |
   |     |                                                           |
   |     |    {/var}                /value                           |
   |     |    {/var,empty}          /value/                          |
   |     |    {/var,undef}          /value                           |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ;  | Path-style parameters, semicolon-prefixed     (Sec 3.2.6) |
   |     |                                                           |
   |     |    {;x,y}                ;x=1024;y=768                    |
   |     |    {;x,y,empty}          ;x=1024;y=768;empty              |
   |     |    {;x,y,undef}          ;x=1024;y=768                    |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ?  | Form-style query, ampersand-separated         (Sec 3.2.7) |
   |     |                                                           |
   |     |    {?x,y}                ?x=1024&y=768                    |
   |     |    {?x,y,empty}          ?x=1024&y=768&empty=             |
   |     |    {?x,y,undef}          ?x=1024&y=768                    |
   |     |                                                           |
   `-----------------------------------------------------------------'




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   Finally, Level 4 templates add the ability to specify value modifiers
   as a suffix to the variable name.  The prefix modifier (":")
   indicates that only a limited number of characters from the beginning
   of the value are used by the expansion.  The explode ("*") modifier
   tells the expansion process to treat the value as a multivalued
   structure --- a list of values or key=value pairs -- rather than as a
   single string.

   .-----------------------------------------------------------------.
   | Level 4 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             path  := "/foo/bar"                                 |
   |             list  := [ "red", "green", "blue" ]                 |
   |             keys  := [("semi", ";"), ("dot", ".")]              |
   |                                                                 |
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | String expansion with value modifiers         (Sec 3.2.2) |
   |     |                                                           |
   |     |    {var:3}               val                              |
   |     |    {var:30}              value                            |
   |     |    {list}                red,green,blue                   |
   |     |    {list*}               red,green,blue                   |
   |     |    {keys}                semi,%3B,dot,.,comma,%2C         |
   |     |    {keys*}               semi=%3B,dot=.,comma=%2C         |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  +  | Reserved expansion with value modifiers       (Sec 3.2.3) |
   |     |                                                           |
   |     |    {+path:6}/here        /foo/b/here                      |
   |     |    {+list}               red,green,blue                   |
   |     |    {+list*}              red,green,blue                   |
   |     |    {+keys}               semi,;,dot,.,comma,,             |
   |     |    {+keys*}              semi=;,dot=.,comma=,             |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  .  | Label expansion, dot-prefixed                 (Sec 3.2.4) |
   |     |                                                           |
   |     |    X{.var:3}             X.val                            |
   |     |    X{.list}              X.red,green,blue                 |
   |     |    X{.list*}             X.red.green.blue                 |
   |     |    X{.keys}              X.semi,%3B,dot,.,comma,%2C       |
   |     |    X{.keys*}             X.semi=%3B.dot=..comma=%2C       |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  /  | Path segments, slash-prefixed                 (Sec 3.2.5) |



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   |     |                                                           |
   |     |    {/var:1,var}          /v/value                         |
   |     |    {/list}               /red,green,blue                  |
   |     |    {/list*}              /red/green/blue                  |
   |     |    {/list*,path:4}       /red/green/blue/%2Ffoo           |
   |     |    {/keys}               /semi,%3B,dot,.,comma,%2C        |
   |     |    {/keys*}              /semi=%3B/dot=./comma=%2C        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ;  | Path-style parameters, semicolon-prefixed     (Sec 3.2.6) |
   |     |                                                           |
   |     |    {;hello:5}            ;hello=Hello                     |
   |     |    {;list}               ;red,green,blue                  |
   |     |    {;list*}              ;red;green;blue                  |
   |     |    {;keys}               ;semi,%3B,dot,.,comma,%2C        |
   |     |    {;keys*}              ;semi=%3B;dot=.;comma=%2C        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ?  | Form-style query, ampersand-separated         (Sec 3.2.7) |
   |     |                                                           |
   |     |    {?var:3}              ?var=val                         |
   |     |    {?list}               ?list=red,green,blue             |
   |     |    {?list*}              ?list=red&list=green&list=blue   |
   |     |    {?keys}               ?keys=semi,%3B,dot,.,comma,%2C   |
   |     |    {?keys*}              ?semi=%3B&dot=.&comma=%2C        |
   |     |                                                           |
   `-----------------------------------------------------------------'

1.3.  Design Considerations

   Mechanisms similar to URI Templates have been defined within several
   specifications, including WSDL, WADL and OpenSearch.  This
   specification extends and formally defines the syntax so that URI
   Templates can be used consistently across multiple Internet
   applications and within Internet message fields, while at the same
   time retaining compatibility with those earlier definitions.

   The URI Template syntax has been designed to carefully balance the
   need for a powerful expansion mechanism with the need for ease of
   implementation.  The syntax is designed to be trivial to parse while
   at the same time providing enough flexibility to express many common
   template scenarios.  Implementations are able to parse the template
   and perform the expansions in a single pass.

   Templates are simple and readable when used with common examples
   because the single-character operators match the URI generic syntax
   delimiters.  The operator's associated delimiter (";", "?", "/", and
   ".") is omitted when none of the listed variables are defined.



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   Likewise, the expansion process for ";" (path-style parameters) will
   omit the "=" when the variable value is empty, whereas the process
   for "?" (form-style parameters) will not omit the "=" when the value
   is empty.  Multiple variables and list values have their values
   joined with "," if there is no predefined joining mechanism for the
   operator.  Only one operator, plus ("+"), will substitute unencoded
   reserved characters found inside the variable values; the other
   operators will pct-encode reserved characters found in the variable
   values prior to expansion.

   The most common cases for URI spaces can be described with Level 1
   template expressions.  If we were only concerned with URI generation,
   then the template syntax could be limited to just simple variable
   expansion, since more complex forms could be generated by changing
   the variable values.  However, URI Templates have the additional goal
   of describing the layout of identifiers in terms of preexisting data
   values.  The template syntax therefore includes operators that
   reflect how resource identifiers are commonly allocated.  Likewise,
   since prefix substrings are often used to partition large spaces of
   resources, modifiers on variable values provide a way to specify both
   the substring and the full value string with a single variable name.

1.4.  Limitations

   Since a URI Template describes a superset of the identifiers, there
   is no implication that every possible expansion for each delimited
   variable expression corresponds to a URI of an existing resource.
   Our expectation is that an application constructing URIs according to
   the template will be provided with an appropriate set of values for
   the variables being substituted and will be able to cope with any
   errors that might occur when the resulting URI is used for name
   resolution or access.

   URI Templates are not URIs: they do not identify an abstract or
   physical resource, they are not parsed as URIs, and should not be
   used in places where a URI would be expected unless the template
   expressions will be expanded by a template processor prior to use.
   Distinct field, element, or attribute names should be used to
   differentiate protocol elements that carry a URI Template from those
   that expect a URI reference.

1.5.  Notational Conventions

   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].

   This specification uses the Augmented Backus-Naur Form (ABNF)



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   notation of [RFC5234].  The following ABNF rules are imported from
   the normative references [RFC5234], [RFC3986], and [RFC3987].

     ALPHA          =  %x41-5A / %x61-7A   ; A-Z / a-z
     DIGIT          =  %x30-39             ; 0-9
     HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

     pct-encoded    =  "%" HEXDIG HEXDIG
     unreserved     =  ALPHA / DIGIT / "-" / "." / "_" / "~"
     reserved       =  gen-delims / sub-delims
     gen-delims     =  ":" / "/" / "?" / "#" / "[" / "]" / "@"
     sub-delims     =  "!" / "$" / "&" / "'" / "(" / ")"
                    /  "*" / "+" / "," / ";" / "="

     ucschar        =  %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
                    /  %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
                    /  %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
                    /  %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
                    /  %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
                    /  %xD0000-DFFFD / %xE1000-EFFFD

     iprivate       =  %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD

1.6.  Character Encoding and Unicode Normalization

   This specification uses the terms "character" and "coded character
   set" in accordance with the definitions provided in [RFC2978], and
   "character encoding" in place of what [RFC2978] refers to as a
   "charset".

   The ABNF notation defines its terminal values to be non-negative
   integers (codepoints) that are a superset of the US-ASCII coded
   character set [ASCII].  This specification defines terminal values as
   codepoints within the Unicode coded character set [UNIV4].

   In spite of the syntax and template expansion process being defined
   in terms of Unicode codepoints, it should be understood that
   templates occur in practice as a sequence of characters in whatever
   form or encoding is suitable for the context in which they occur,
   whether that be octets embedded in a network protocol element or
   paint applied to the side of a bus.  This specification does not
   mandate any particular character encoding for mapping between URI
   Template characters and the octets used to store or transmit those
   characters.  When a URI Template appears in a protocol element, the
   character encoding is defined by that protocol; without such a
   definition, a URI Template is assumed to be in the same character
   encoding as the surrounding text.  It is only during the process of
   template expansion that a string of characters in a URI Template MUST



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   be transformed into its corresponding sequence of normalized Unicode
   codepoints.

   The Unicode Standard [UNIV4] defines various equivalences between
   sequences of characters for various purposes.  Unicode Standard Annex
   #15 [UTR15] defines various Normalization Forms for these
   equivalences, in particular Normalization Form KC (NFKC:
   Compatibility Decomposition followed by Canonical Composition).  The
   normalization form determines how to consistently encode the
   equivalent strings.

   In theory, all URI processing implementations, including template
   processors, should use the same normalization form for generating or
   handling a URI reference.  In practice, they do not.  However,
   template processors transform the template literals and variable
   values from characters to codepoints, expand the expressions, and
   then transform the resulting codepoints back to characters again.  In
   order to promote consistent generation of URI references from
   templates, template processors MUST ensure that the template and each
   variable value is in NFKC when they are transformed to a sequence of
   Unicode codepoints.  The final step of template processing will
   encode the expanded sequence of Unicode codepoints, which should
   still be in NFKC, as UTF-8 [RFC3629] and then further pct-encode any
   octets within the UTF-8 string that are not allowed in a URI.


2.  Syntax

   A URI Template is a string of printable Unicode characters that
   contains zero or more embedded variable expressions, each expression
   being delimited by a matching pair of braces ('{', '}').

     URI-Template  = *( literals / expression )

   Although templates (and template processor implementations) are
   described above in terms of four gradual levels, we define the URI-
   Template syntax in terms of the ABNF for Level 4.  A template
   processor limited to lower level templates MAY exclude the ABNF rules
   applicable only to higher levels.  However, it is RECOMMENDED that
   all parsers implement the full syntax such that unsupported levels
   can be properly identified as such to the end user.

   Each character in a URI Template MUST be transcoded to an NFKC
   Unicode codepoint prior to evaluation.  If a character sequence in
   the template consists of pct-encoded triplets corresponding to a
   valid sequence of octets in UTF-8 that represent a character not in
   the reserved set, then the triplets MUST be replaced with their
   corresponding Unicode codepoint.  In other words, pct-encoded UTF-8



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   found within the template is decoded prior to further processing
   unless the corresponding character is in the reserved set.

2.1.  Literals

   The characters outside of expressions in a URI Template string are
   intended to be translated literally to the URI-reference.

     literals      =  %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
                   /  %x5D-5F / %x61-7A / %x7E / ucschar / iprivate
                   /  pct-encoded
                        ; any Unicode character except: CTL, SP,
                        ;  DQUOTE, "'", "%" (aside from pct-encoded),
                        ;  "<", ">", "\", "^", "`", "{", "|", "}"

   A sequence of pct-encoded literals that corresponds to a UTF-8
   encoded character that is not within the reserved set will be
   transcoded during template processing, as describe above.  If such a
   character is not in the unreserved set, it will be re-pct-encoded
   during the final step of template processing.  Unreserved characters
   will therefore be normalized to their unencoded form as a side-effect
   of template processing.

2.2.  Expressions

   Template expressions are the parameterized parts of a URI Template.
   Each expression contains an optional operator, which defines the
   expression type and its corresponding expansion process, followed by
   a comma-separated list of variable specifiers (variable names and
   optional value modifiers).  If no operator is provided, the
   expression defaults to simple variable expansion of unreserved
   values.

     expression    =  "{" [ operator ] variable-list "}"
     operator      =  "+" / "." / "/" / ";" / "?" / op-reserve
     op-reserve    =  "," / "!" / "@"
                          ; reserved for local use: "$" / "(" / ")"

   The operator characters have been chosen to reflect each of their
   roles as reserved characters in the URI generic syntax.  The
   operators defined by this specification include: plus ("+") for
   substituting values that may contain reserved characters; dot (".")
   for substituting values as a sequence of name labels prefixed by ".";
   slash ("/") for substituting values as a sequence of path segments
   separated by "/"; semicolon (";") for substituting key=value pairs as
   path parameters prefixed by ";"; and, question-mark ("?") for
   substituting a query component beginning with "?" and consisting of
   key=value pairs separated by "&".  These operators will be described



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   in detail in Section 3.

   The operator characters comma (","), exclamation ("!"), and at-sign
   ("@") are reserved for future extensions.  A processor that
   unexpectedly encounters such an extension operator SHOULD pass the
   expression through unexpanded and MAY also indicate a warning to the
   invoking application.

   The expression syntax specifically excludes use of the dollar ("$")
   and parentheses ["(" and ")"] characters so that they remain
   available for local language extensions outside the scope of this
   specification.

2.3.  Variables

   After the operator (if any), each expression contains a list of one
   or more comma-separated variable specifiers (varspec).  The variable
   names serve multiple purposes: documentation for what kinds of values
   are expected, identifiers for associating values within a template
   processor, and the string to use for each key on key=value
   expansions.

     variable-list =  varspec *( "," varspec )
     varspec       =  varname [ modifier ] [ "|" default ]
     varname       =  varchar *( varchar / "." )
     varchar       =  ALPHA / DIGIT / "_" / ucschar / iprivate
                   /  pct-encoded

   An expression MAY reference variables that are unknown to the
   template processor or whose value is set to a special "undefined"
   value, such as undef or null.  Such undefined variables are given
   special treatment by the expansion process.

   A variable value that is a string of length zero is not considered
   undefined; it has the defined value of an empty string.

   A variable may have a composite or structured value, such as a list
   of values, an associative array of (key, value) pairs, or a structure
   of components defined by some separate schema.  Such value types are
   not directly indicated by the template syntax, but do have an impact
   on the expansion process.  A composite or structured value with zero
   member values is considered undefined.

   If a variable appears more than once in an expression or within
   multiple expressions of a URI Template, the value of that variable
   MUST remain static throughout the expansion process (i.e., the
   variable must have the same value for the purpose of calculating each
   expansion).



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2.4.  Value Modifiers

   Each of the variables in a Level 4 template expression can have a
   modifier indicating either that its expansion is limited to a prefix
   of the variable's value string or that its expansion is exploded into
   components based on an external type or schema associated with that
   variable.

     modifier      =  prefix / explode

2.4.1.  Prefix Values

   A prefix modifier indicates that the variable expansion is limited to
   a prefix of the variable's value string.  Prefix modifiers are often
   used to partition an identifier space hierarchically, as is common in
   reference indices and hash-based storage.  It also serves to limit
   the expanded value to a maximum number of characters.

     prefix        =  ":" offset
     max-length    =  %x31-39 *DIGIT   ; positive integer

   The max-length is a positive integer that refers to a maximum number
   of characters from the beginning of the variable's value as a Unicode
   string.  Note that this numbering is in characters, not octets, in
   order to avoid splitting between the octets of a multi-octet UTF-8
   encoded character or within a pct-encoded triplet.  If the max-length
   is greater than the length of the variable's value, then the entire
   value string is used.

   For example,

     Given the variable assignments

       var   := "value"
       semi  := ";"

     Example Template     Expansion

       {var}              value
       {var:20}           value
       {var:3}            val
       {semi}             %3B
       {semi:2}           %3B








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2.4.2.  Component Values

   An explode modifier ("*") indicates that the variable represents a
   composite value that may be substituted in full or partial forms,
   depending on the variable's type or schema.  Since URI Templates do
   not contain an indication of type or schema, this is assumed to be
   determined by context.  An example context is a mark-up element or
   header field that contains one attribute that is a template and one
   or more other attributes that define the schema applicable to
   variables found in the template.  Likewise, a typed programming
   language might differentiate variables as strings, lists, associative
   arrays, or structures.

     explode       =  "*"

   Explode modifiers improve brevity in the URI Template syntax.  For
   example, a resource that provides a geographic map for a given street
   address might accept a hundred permutations on fields for address
   input, including partial addresses (e.g., just the city or postal
   code).  Such a resource could be described as a template with each
   and every address component listed in order, or with a far more
   simple template that makes use of an explode modifier, as in

      /mapper{?address*}

   along with some context that defines what the variable named
   "address" can include, such as by reference to some other standard
   for addressing (e.g., UPU S42 or AS/NZS 4819:2003).  A recipient
   aware of the schema can then provide appropriate expansions, such as:

      /mapper?city=Newport%20Beach&state=CA

   If an explode modifier is present, the expansion process for that
   variable, as defined in Section 3, is dependent on both the operator
   being used and the type or schema of the value being substituted.

2.5.  Value Defaults

   Each of the variables in Level 2 (and above) templates may also be
   supplied with a default value to be used when a template processor
   determines that variable to be undefined.  The default value is
   limited to unreserved characters, the equal sign ("="), and pct-
   encoded triplets, since the default is provided in the exact form
   that it would appear in the expanded URI reference.  The default is
   not affected by any variable modifiers, since we assume that the
   default string provided in the expression already reflects the
   desired value.




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     default       =  *( unreserved / "=" / pct-encoded )

   The following examples illustrate how default values work with
   different variable types.

     Given the variable assignments:

       var   := "value"
       name  := [ "Fred", "Wilma", "Pebbles" ]
       favs  := [("color","red"), ("volume","high")]
       empty_keys := []
       empty := ""
       undef := null

     Example Template        Expansion

       {var|default}         value
       {undef|default}       default
       {undef:3|default}     default

       x{empty}y             xy
       x{empty|_}y           xy
       x{undef}y             xy
       x{undef|_}y           x_y

       x{.name|none}         x.Fred,Wilma,Pebbles
       x{.name*|none}        x.Fred.Wilma.Pebbles
       x{.empty}             x.
       x{.empty|none}        x.
       x{.undef}             x
       x{.undef|none}        x.none

       x{/name|none}         x/Fred,Wilma,Pebbles
       x{/name*|none}        x/Fred/Wilma/Pebbles
       x{/undef}             x
       x{/undef|none}        x/none
       x{/empty}             x/
       x{/empty|none}        x/
       x{/empty_keys}        x
       x{/empty_keys|none}   x/none
       x{/empty_keys*}       x
       x{/empty_keys*|none}  x/none

       x{;name|none}         x;name=Fred,Wilma,Pebbles
       x{;favs|none}         x;favs=color,red,volume,high
       x{;favs*|none}        x;color=red;volume=high
       x{;empty}             x;empty
       x{;empty|none}        x;empty



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       x{;undef}             x
       x{;undef|none}        x;none
       x{;undef|foo=y}       x;foo=y

       x{?var|none}          x?var=value
       x{?favs|none}         x?favs=color,red,volume,high
       x{?favs*|none}        x?color=red&volume=high
       x{?empty}             x?empty=
       x{?empty|foo=none}    x?empty=
       x{?undef}             x
       x{?undef|foo=none}    x?foo=none
       x{?empty_keys}        x
       x{?empty_keys|none}   x?none
       x{?empty_keys|y=z}    x?y=z
       x{?empty_keys*|y=z}   x?y=z



3.  Expansion

   The process of URI Template expansion is to scan the template string
   from beginning to end, copying literal characters as-is and replacing
   each expression with the result of applying the expression's operator
   to the value of each variable named in the expression.

   Each variable value MUST be formed as a sequence of NFKC Unicode
   codepoints prior to template expansion.  A value is assumed to be raw
   data that might need to be encoded by the template processor during
   expansion, depending on the expression type.

   If a template processor encounters an error outside of an expression,
   such as a character sequence that does not match the <URI-Template>
   grammar, then processing of the template SHOULD cease, the URI-
   reference result SHOULD be undefined, and the location and type of
   error SHOULD be indicated to the invoking application.  If an error
   is encountered inside an expression, such as an operator or value
   modifier that it does not recognize or cannot support, then the
   expression SHOULD be copied to the result unexpanded, processing of
   the remainder of the template SHOULD continue, and the location and
   type of error SHOULD be indicated to the invoking application.

3.1.  Literal Expansion

   If the literal character is allowed anywhere in the URI syntax
   (unreserved / reserved), then it is copied directly to the result
   string.  Otherwise, the pct-encoded equivalent of the literal
   character is copied to the result string by encoding the character in
   UTF-8 (a sequence of octets) and then encoding each octet as a pct-



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   encoded triplet.

3.2.  Expression Expansion

   Each expression is indicated by an opening brace ("{") character and
   continues until the next closing brace ("}").  The expression is
   expanded by determining the expression type and then following that
   type's expansion process for each comma-separated varspec in the
   expression.  Level 1 templates are limited to the default operator
   (simple string value expansion) and a single variable per expression.
   Level 2 templates are limited to a single varspec per expression.

   The expression type is determined by looking at the first character
   after the opening brace.  If the character is an operator, then
   remember the expression type associated with that operator for later
   expansion decisions and skip to the next character for the varspec
   list.  If the first character is not an operator, then the expression
   type is simple expansion and the first character is the beginning of
   the varspec list.

3.2.1.  Undefined Variable Expansion

   Regardless of the expression type, a variable that is undefined has
   no value.  A variable defined as a list or structure of component
   values is considered undefined if the list contains zero members or
   all of the structure's components are undefined.

   If a default value is provided for an undefined variable, as
   described for Level 2 (and above) templates in Section 2.5, then the
   default string is substituted in its place as the entire expansion
   for that varspec.  If all of the variables in an expression are
   undefined and no defaults are given, then the expression's expansion
   is the empty string.

3.2.2.  Simple String Expansion: {var}

   The default expression type when no operator is given is simple
   string expansion.  For each defined variable in the variable-list,
   modify its value as indicated by the optional modifiers (if any),
   encode the value as UTF-8, pct-encode any octets that are not in the
   unreserved set, and then append the encoded value to the result
   string.  If more than one value is appended, separate each value with
   a comma (",").








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   For example,

     foo := "fred"

     "{foo}"        -> "fred"
     "{foo,foo}"    -> "fred,fred"
     "{bar,foo}"    -> "fred"
     "{bar|wilma}"  -> "wilma"

   Level 1 templates are limited to single variable expressions with no
   modifiers, defaults, or value structures.  Simple string expansion in
   Level 2 templates adds variable defaulting, as defined in
   Section 2.5.  Level 3 templates allow a list of variables.  Level 4
   templates add compound variable types and value modifiers, as
   follows:

   For a variable defined as a single value string, the explode modifier
   has no effect.  The prefix modifier limits the expansion to the first
   max-length characters of that single value.  If the value contains
   pct-encoded triplets, multibyte UTF-8, or both, care must be taken to
   avoid splitting the value in mid-character: count each Unicode
   codepoint as one character.

   For a variable defined as a list of values, when no value modifier is
   present or the explode modifier is given, the variable's string
   expansion consists of a concatenation of the individual values with
   each value separated by a comma (",").  A prefix modifier has no
   effect.

   A variable defined as an associative array is expanded as a list of
   alternating key and value pairs, excluding any keys for which the
   corresponding value is undefined, when no value modifier is present.
   If the explode modifier is given, then the keys with defined values
   are expanded as "key=value" pairs instead of "key,value".  A prefix
   modifier has no effect.

3.2.3.  Reserved expansion: {+var}

   Reserved expansion is identical to simple expansion except that the
   substituted values may contain characters in the reserved set.  The
   reserved expansion operator "+" is defined for Level 2 templates (and
   above).









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   For example,

     foo  := "That's right!"

     "{foo}"        -> "That%27s%20right%21"
     "{+foo}"       -> "That%27s%20right!"

     base := "http://example.com/home/"

     "{base}index"  -> "http%3A%2F%2Fexample.com%2Fhome%2Findex"
     "{+base}index" -> "http://example.com/home/index"

   The same expansion process is followed as in Section 3.2.2 except
   that, instead of "pct-encode any octets that are not in the
   unreserved set", we pct-encode any octets that are not in either the
   reserved or unreserved sets.

3.2.4.  Label expansion with dot-prefix: {.var}

   The dot (".") operator indicates that the expression type is label
   expansion, which can be useful for describing URI spaces with varying
   domain names or path selectors (e.g., filename extensions).

     For each variable in the variable-list, do one of

       a) if the variable has a defined value

           1) modify its value as indicated by the optional
              modifiers (if any);

           2) encode the value as UTF-8;

           3) pct-encode any octets that are not in the
              unreserved set;

           4) append "." to the result string; and,

           5) append the encoded value to the result string.

       b) if the variable is undefined and a default is given,

           1) encode the default string as UTF-8;

           2) append "." to the result string; and,

           3) append the default to the result string.

       c) the variable is undefined and has no default,



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          so ignore this variable.

   Since "." is not in the reserved set, a value that contains a "." has
   the effect of adding multiple labels.

   For example,

     foo := "fred"

     "{foo}"        -> ".fred"
     "{foo,foo}"    -> ".fred.fred"
     "{bar,foo}"    -> ".fred"
     "{bar|wilma}"  -> ".wilma"

   Label expansion only applies to Level 3 and Level 4 templates.  Level
   4 templates add compound variable types and value modifiers, as
   follows:

   For a variable defined as a single value string, the explode modifier
   has no effect.  The prefix modifier limits the expansion to the first
   max-length characters of that single value.  If the value contains
   pct-encoded triplets, multibyte UTF-8, or both, care must be taken to
   avoid splitting the value in mid-character: count each Unicode
   codepoint as one character.

   For a variable defined as a list of values, when no value modifier is
   present or the explode modifier is given, the variable's string
   expansion consists of a concatenation of the individual defined
   values with each value prepended by a dot (".").  A prefix modifier
   has no effect.

   When no value modifier is present, a variable defined as an
   associative array is expanded by appending the (key, value) pairs as
   alternating labels (i.e., ".key.value"), but excluding any keys for
   which the corresponding value is undefined.  If the explode modifier
   is given, then the keys with defined values are expanded as
   ".key=value" instead of ".key.value".  A prefix modifier has no
   effect.

3.2.5.  Path segment expansion: {/var}

   The slash ("/") operator indicates that the expression type is
   hierarchical path segment expansion, which can be useful for
   describing URI path hierarchies.







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     For each variable in the variable-list, do one of

       a) if the variable has a defined value

           1) modify its value as indicated by the optional
              modifiers (if any);

           2) encode the value as UTF-8;

           3) pct-encode any octets that are not in the
              unreserved set;

           4) append "/" to the result string; and,

           5) append the encoded value to the result string.

       b) if the variable is undefined and a default is given,

           1) encode the default string as UTF-8;

           2) append "/" to the result string; and,

           3) append the default to the result string.

       c) the variable is undefined and has no default,
          so ignore this variable.

   Note that the expansion process for path segment expansion is
   identical to that of label expansion aside from the substitution of
   "/" instead of ".".

   For example,

     foo := "fred"

     "{foo}"        -> "/fred"
     "{foo,foo}"    -> "/fred/fred"
     "{bar,foo}"    -> "/fred"
     "{bar|wilma}"  -> "/wilma"

   Label expansion only applies to Level 3 and Level 4 templates.  Level
   4 templates add compound variable types and value modifiers, as
   follows:

   For a variable defined as a single value string, the explode modifier
   has no effect.  The prefix modifier limits the expansion to the first
   max-length characters of that single value.  If the value contains
   pct-encoded triplets, multibyte UTF-8, or both, care must be taken to



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   avoid splitting the value in mid-character: count each Unicode
   codepoint as one character.

   For a variable defined as a list of values, when no value modifier is
   present or the explode modifier is given, the variable's string
   expansion consists of a concatenation of the individual defined
   values with each value prepended by a slash ("/").  A prefix modifier
   has no effect.

   When no value modifier is present, a variable defined as an
   associative array is expanded by appending the (key, value) pairs as
   alternating segments (i.e., "/key/value"), but excluding any keys for
   which the corresponding value is undefined.  If the explode modifier
   is given, then the keys with defined values are expanded as
   "/key=value" instead of "/key/value".  A prefix modifier has no
   effect.

3.2.6.  Path-style parameter expansion: {;var}

   TBD.

3.2.7.  Form-style query expansion: {?var}

   TBD.


4.  Security Considerations

   A URI Template does not contain active or executable content.  Other
   security considerations are the same as those for URIs, as described
   in section 7 of [RFC3986].


5.  IANA Considerations

   No IANA actions are required by this document.


6.  Acknowledgments

   The following people made significant contributions to this
   specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton,
   John Cowan, James H. Manger, and James Snell.


7.  Normative References

   [ASCII]    American National Standards Institute, "Coded Character



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              Set - 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

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

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [UNIV4]    The Unicode Consortium, "The Unicode Standard, Version
              4.0.1, defined by: The Unicode Standard, Version 4.0
              (Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
              as amended by Unicode 4.0.1
              (http://www.unicode.org/versions/Unicode4.0.1/)",
              March 2004.

   [UTR15]    Davis, M. and M. Duerst, "Unicode Normalization Forms",
              Unicode Standard Annex # 15, April 2003.

   [1]  <http://lists.w3.org/Archives/Public/uri/>


Appendix A.  Example URI Template Parser

   Parsing a valid URI Template expression does not require building a
   parser from the given ABNF.  Instead, the set of allowed characters
   in each part of URI Template expression has been chosen to avoid
   complex parsing, and breaking an expression into its component parts
   can be achieved by a series of splits of the character string.

   Here is example Python code that parses a URI Template expression and
   returns the operator, argument, and variables as a tuple.  The
   variables are returned as a dictionary of variable names mapped to
   their default values.  If no default is given then the name maps to
   None.



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   TBD.


Appendix B.  Revision History (to be removed by RFC Editor)

   05 - Introduced levels to differentiate between legacy, partial, and
   full implementations of URI Templates.  Changed the default indicator
   to pipe ("|") to allow the defaults to contain the equals character
   and thus remove the need for complex defaulting for the different
   variable types.  Removed suffix, remainder, and labelled value
   expansion because there didn't seem much interest in them.  Clarified
   that templates and values are processed as sequences of Unicode
   codepoints rather than prematurely encoded as UTF-8, since that is
   easier to explain and more consistent with common language routines
   for processing Unicode strings.

   04 - Changed the operator syntax to a single character that is
   analogous to its reserved role within the URI generic syntax,
   resulting in templates that are far more readable for the common
   cases.  Added value modifiers for prefix and suffix expansion.  Added
   explode modifiers to allow expansion of complex variables and lists
   according to (external) variable types or schema.  Replaced use of
   "expansion" with "expression", since expansion is traditionally used
   to refer to the result after expanding a macro (not the macro
   itself).  Made applicable to any hypertext reference string, such
   that the process for template expansion also includes transforming
   the surrounding string into a proper URI-reference rather than
   assuming it is already in absolute URI form.  Rewrote the text
   accordingly.

   03 - Added more examples.  Introduced error conditions and defined
   their handling.  Changed listjoin to list.  Changed -append to
   -suffix, and allowed -prefix and -suffix to accept list variables.
   Clarified the handling of unicode.

   02 - Added operators and came up with coherent percent-encoding and
   reserved character story.  Added large examples section which is
   extracted and tested against the implementation.

   01

   00 - Initial Revision.









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Internet-Draft                URI Template                      Jul 2011


Authors' Addresses

   Joe Gregorio
   Google

   Email: joe@bitworking.org
   URI:   http://bitworking.org/


   Roy T. Fielding (editor)
   Adobe Systems Incorporated

   Email: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Marc Hadley
   Oracle

   Email: Marc.Hadley@oracle.com
   URI:   http://oracle.com/


   Mark Nottingham

   Email: mnot@pobox.com
   URI:   http://mnot.net/


   David Orchard

   URI:   http://www.pacificspirit.com/



















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