draft-gregorio-uritemplate-07.txt

Network Working Group                                        J. Gregorio
Internet-Draft                                                    Google
Intended status: Standards Track                             R. Fielding
Expires: March 29, 2012                                            Adobe
                                                               M. Hadley
                                                                   MITRE
                                                           M. Nottingham
                                                               Rackspace
                                                              D. Orchard
                                                          Salesforce.com
                                                            Sep 26, 2011


                              URI Template
                     draft-gregorio-uritemplate-07

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
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   This Internet-Draft will expire on March 29, 2012.

Copyright Notice




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

   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Levels and Expression Types  . . . . . . . . . . . . . . .  5
     1.3.  Design Considerations  . . . . . . . . . . . . . . . . . .  9
     1.4.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . 10
     1.5.  Notational Conventions . . . . . . . . . . . . . . . . . . 11
     1.6.  Character Encoding and Unicode Normalization . . . . . . . 12
   2.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.1.  Literals . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.2.  Expressions  . . . . . . . . . . . . . . . . . . . . . . . 13
     2.3.  Variables  . . . . . . . . . . . . . . . . . . . . . . . . 14
     2.4.  Value Modifiers  . . . . . . . . . . . . . . . . . . . . . 15
       2.4.1.  Prefix Values  . . . . . . . . . . . . . . . . . . . . 15
       2.4.2.  Composite Values . . . . . . . . . . . . . . . . . . . 16
   3.  Expansion  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.1.  Literal Expansion  . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Expression Expansion . . . . . . . . . . . . . . . . . . . 18
       3.2.1.  Variable Expansion . . . . . . . . . . . . . . . . . . 18
       3.2.2.  Simple String Expansion: {var} . . . . . . . . . . . . 20
       3.2.3.  Reserved expansion: {+var} . . . . . . . . . . . . . . 20
       3.2.4.  Fragment expansion: {#var} . . . . . . . . . . . . . . 21
       3.2.5.  Label expansion with dot-prefix: {.var}  . . . . . . . 22
       3.2.6.  Path segment expansion: {/var} . . . . . . . . . . . . 22
       3.2.7.  Path-style parameter expansion: {;var} . . . . . . . . 23
       3.2.8.  Form-style query expansion: {?var} . . . . . . . . . . 24
       3.2.9.  Form-style query continuation: {&var}  . . . . . . . . 24
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Implementation Hints  . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29



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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=chien&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, machine-readable instructions
   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
   name=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}

   The expansion process for expressions 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 transformed to
   an IRI by following the process defined in Section 3.2 of [RFC3987].

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!"                             |
   |                                                                 |
   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | Simple string expansion                       (Sec 3.2.2) |
   |     |                                                           |
   |     |    {var}                 value                            |
   |     |    {hello}               Hello%20World%21                 |
   `-----------------------------------------------------------------'

   Level 2 templates add the plus ("+") operator, for expansion of
   values that are allowed to include reserved characters, and the
   crosshatch ("#") operator for expansion of fragment identifiers.










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   .-----------------------------------------------------------------.
   | Level 2 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             path  := "/foo/bar"                                 |
   |                                                                 |
   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |  +  | Reserved string expansion                     (Sec 3.2.3) |
   |     |                                                           |
   |     |    {+var}                value                            |
   |     |    {+hello}              Hello%20World!                   |
   |     |    {+path}/here          /foo/bar/here                    |
   |     |    here?ref={+path}      here?ref=/foo/bar                |
   |-----+-----------------------------------------------------------|
   |  #  | Fragment expansion, crosshatch-prefixed       (Sec 3.2.4) |
   |     |                                                           |
   |     |    X{#var}               X#value                          |
   |     |    X{#hello}             X#Hello%20World!                 |
   `-----------------------------------------------------------------'

   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 name=value pairs that are separated
   by an ampersand character.

   .-----------------------------------------------------------------.
   | Level 3 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             empty := ""                                         |
   |             path  := "/foo/bar"                                 |
   |             x     := "1024"                                     |
   |             y     := "768"                                      |
   |                                                                 |
   |-----------------------------------------------------------------|
   | Op       Expression            Expansion                        |
   |-----------------------------------------------------------------|
   |     | String expansion with multiple variables      (Sec 3.2.2) |
   |     |                                                           |
   |     |    map?{x,y}             map?1024,768                     |
   |     |    {x,hello,y}           1024,Hello%20World%21,768        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|



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   |  +  | Reserved expansion with multiple variables    (Sec 3.2.3) |
   |     |                                                           |
   |     |    {+x,hello,y}          1024,Hello%20World!,768          |
   |     |    {+path,x}/here        /foo/bar,1024/here               |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  #  | Fragment expansion with multiple variables    (Sec 3.2.4) |
   |     |                                                           |
   |     |    {#x,hello,y}          #1024,Hello%20World!,768         |
   |     |    {#path,x}/here        #/foo/bar,1024/here              |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  .  | Label expansion, dot-prefixed                 (Sec 3.2.5) |
   |     |                                                           |
   |     |    X{.var}               X.value                          |
   |     |    X{.x,y}               X.1024.768                       |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  /  | Path segments, slash-prefixed                 (Sec 3.2.6) |
   |     |                                                           |
   |     |    {/var}                /value                           |
   |     |    {/var,x}/here         /value/1024/here                 |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ;  | Path-style parameters, semicolon-prefixed     (Sec 3.2.7) |
   |     |                                                           |
   |     |    {;x,y}                ;x=1024;y=768                    |
   |     |    {;x,y,empty}          ;x=1024;y=768;empty              |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ?  | Form-style query, ampersand-separated         (Sec 3.2.8) |
   |     |                                                           |
   |     |    {?x,y}                ?x=1024&y=768                    |
   |     |    {?x,y,empty}          ?x=1024&y=768&empty=             |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  &  | Form-style query continuation                 (Sec 3.2.9) |
   |     |                                                           |
   |     |    ?fixed=yes{&x}        ?fixed=yes&x=1024                |
   |     |    {&x,y,empty}          &x=1024&y=768&empty=             |
   |     |                                                           |
   `-----------------------------------------------------------------'

   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
   indicates that the variable is to be treated as a composite value,



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   consisting of either a list of names or an associative array of
   (name, value) pairs, that is expanded as if each member were a
   separate variable.

   .-----------------------------------------------------------------.
   | Level 4 examples, with variables having values of               |
   |                                                                 |
   |             var   := "value"                                    |
   |             hello := "Hello World!"                             |
   |             path  := "/foo/bar"                                 |
   |             list  := [ "red", "green", "blue" ]                 |
   |             keys  := [("semi",";"),("dot","."),("comma",",")]   |
   |                                                                 |
   | 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=,             |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  #  | Fragment expansion with value modifiers       (Sec 3.2.4) |
   |     |                                                           |
   |     |    {#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 3.2.5) |
   |     |                                                           |
   |     |    X{.var:3}             X.val                            |
   |     |    X{.list}              X.red,green,blue                 |
   |     |    X{.list*}             X.red.green.blue                 |



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   |     |    X{.keys}              X.semi,%3B,dot,.,comma,%2C       |
   |     |    X{.keys*}             X.semi=%3B.dot=..comma=%2C       |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  /  | Path segments, slash-prefixed                 (Sec 3.2.6) |
   |     |                                                           |
   |     |    {/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 3.2.7) |
   |     |                                                           |
   |     |    {;hello:5}            ;hello=Hello                     |
   |     |    {;list}               ;list=red,green,blue             |
   |     |    {;list*}              ;red;green;blue                  |
   |     |    {;keys}               ;keys=semi,%3B,dot,.,comma,%2C   |
   |     |    {;keys*}              ;semi=%3B;dot=.;comma=%2C        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  ?  | Form-style query, ampersand-separated         (Sec 3.2.8) |
   |     |                                                           |
   |     |    {?var:3}              ?var=val                         |
   |     |    {?list}               ?list=red,green,blue             |
   |     |    {?list*}              ?red&green&blue                  |
   |     |    {?keys}               ?keys=semi,%3B,dot,.,comma,%2C   |
   |     |    {?keys*}              ?semi=%3B&dot=.&comma=%2C        |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  &  | Form-style query continuation                 (Sec 3.2.9) |
   |     |                                                           |
   |     |    {&var:3}              &var=val                         |
   |     |    {&list}               &list=red,green,blue             |
   |     |    {&list*}              &red&green&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



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   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.  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.  The "+" and "#" operators 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, or at least a means of validating
   user data-entry for those values.

   URI Templates are not URIs: they do not identify an abstract or
   physical resource, they are not parsed as URIs, and should not be



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   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.

   Some URI Templates can be used in reverse for the purpose of variable
   matching: comparing the template to a fully formed URI in order to
   extract the variable parts from that URI and assign them to the named
   variables.  Variable matching only works well if the template
   expressions are delimited by the beginning or end of the URI or by
   characters that cannot be part of the expansion, such as reserved
   characters surrounding a simple string expression.  In general,
   regular expression languages are better suited for variable matching.

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)
   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







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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 is
   REQUIRED to be processed as a sequence of 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.  The normalization form determines how to consistently
   encode equivalent strings.  In theory, all URI processing
   implementations, including template processors, should use the same
   normalization form for generating a URI reference.  In practice, they
   do not.  If a value has been provided by the same server as the
   resource, then it can be assumed that the string is already in the
   form expected by that server.  If a value is provided by a user, such
   as via a data-entry dialog, then the string SHOULD be normalized as
   Normalization Form C (NFC: Canonical Decomposition, followed by
   Canonical Composition) prior to being used in expansions by a
   template processor.

   Likewise, when non-ASCII data that represents readable strings is
   pct-encoded for use in a URI reference, a template processor MUST
   first encode the string as UTF-8 [RFC3629] and then pct-encode any
   octets that are not allowed in a URI reference.





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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.

2.1.  Literals

   The characters outside of expressions in a URI Template string are
   intended to be copied literally to the URI-reference if the character
   is allowed in a URI (reserved / unreserved / pct-encoded) or, if not
   allowed, copied to the URI-reference in its UTF-8 pct-encoded form.

     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),
                        ;  "<", ">", "\", "^", "`", "{", "|", "}"

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



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   operators defined in Section 3 of this specification include:

      +   Reserved character strings;

      #   Fragment identifiers prefixed by "#";

      .   Name labels or extensions prefixed by ".";

      /   Path segments prefixed by "/";

      ;   Path parameter name or name=value pairs prefixed by ";";

      ?   Query component beginning with "?" and consisting of
          name=value pairs separated by "&"; and,

      &   Continuation of query-style &name=value pairs within
          a literal query component.

   The operator characters equals ("="), comma (","), exclamation ("!"),
   at-sign ("@"), and pipe ("|") are reserved for future extensions.

   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 literal string to use for the name in name=value
   expansions (aside from when exploding an associative array).

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

   A varname MAY contain one or more pct-encoded triplets.  These
   triplets are considered an essential part of the variable name and
   are not decoded during processing.  A varname containing pct-encoded
   characters is not the same variable as a varname with those same
   characters decoded.  Applications that provide URI Templates are
   expected to be consistent in their use of pct-encoding within
   variable names.




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   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 value in the form of a list of values
   or an associative array of (name, value) pairs.  Such value types are
   not directly indicated by the template syntax, but do have an impact
   on the expansion process.  A composite value with zero member values
   is considered undefined.

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 as a
   composite value in the form of a value list or an associative array
   of (name, value) pairs.

     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
   modifiers are not applicable to variables that have composite values.

     prefix        =  ":" max-length
     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.








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   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

2.4.2.  Composite Values

   An explode ("*") modifier indicates that the variable is to be
   treated as a composite value consisting of either a list of names or
   an associative array of (name, value) pairs.  Hence, the expansion
   process is applied to each member of the composite as if it were
   listed as a separate variable.  This kind of variable specification
   is significantly less self-documenting than non-exploded variables,
   since there is less correspondence between the variable name and how
   the URI reference appears after expansion.

     explode       =  "*"

   Since URI Templates do not contain an indication of type or schema,
   the type for an exploded variable is assumed to be determined by
   context.  For example, the processor might be supplied values in a
   form that differentiates values as strings, lists, or associative
   arrays.  Likewise, the context in which the template is used (script,
   mark-up language, IDL, etc.) might define rules for associating
   variable names with types, structures, or schema.

   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



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   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

   The expansion process for exploded variables is dependent on both the
   operator being used and whether the composite value is to be treated
   as a list of values or as an associative array of (name, value)
   pairs.  Structures are processed as if they are an associative array
   with names corresponding to the fields in the structure definition
   and "." separators used to indicate name hierarchy in substructures.


3.  Expansion

   The process of URI Template expansion is to scan the template string
   from beginning to end, copying literal characters 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's
   value MUST be formed prior to template expansion.

   The requirements on expansion for each aspect of the URI Template
   grammar are defined in this section.  A non-normative algorithm for
   the expansion process as a whole is provided in Appendix A.

   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 contain the expanded part of the template
   followed by the remainder unexpanded, 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.  If an
   error occurs, the result returned might not be a valid URI reference;
   it will be an incompletely expanded template string that is only
   intended for diagnostic use.

3.1.  Literal Expansion

   If the literal character is allowed anywhere in the URI syntax
   (unreserved / reserved / pct-encoded ), 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-encoded triplet.



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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 variable-
   list.  If the first character is not an operator, then the expression
   type is simple string expansion and the first character is the
   beginning of the variable-list.

   The examples in the subsections below use the following definitions
   for variable values:


         dom   := "example.com"
         dub   := "me/too"
         hello := "Hello World!"
         half  := "50%"
         var   := "value"
         who   := "fred"
         base  := "http://example.com/home/"
         path  := "/foo/bar"
         list  := [ "red", "green", "blue" ]
         keys  := [("semi",";"),("dot","."),("comma",",")]
         v     := "6"
         x     := "1024"
         y     := "768"
         empty := ""
         empty_keys  := []
         undef := null

3.2.1.  Variable Expansion

   A variable that is undefined has no value and is ignored by the
   expansion process.  A variable defined as a list value is considered
   undefined if the list contains zero members.  A variable defined as
   an associative array of (name, value) pairs is considered undefined
   if the array contains zero members or if all member names in the
   array have undefined values.  If all of the variables in an
   expression are undefined, then the expression's expansion is the



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   empty string.

   Variable expansion of a defined, non-empty value results in a
   substring of allowed URI characters.  A template processor MUST
   encode the value string as UTF-8 and transform each octet that is not
   in the allowed set into the corresponding pct-encoded triplet.  The
   allowed set depends on the expression type: reserved ("+") and
   fragment ("#") expansions allow the set of characters in ( unreserved
   / reserved / pct-encoded ) to be passed through without pct-encoding,
   whereas all other expression types allow only unreserved characters
   to be passed through without pct-encoding.  Note that the percent
   character ("%") is only allowed as part of a pct-encoded triplet and
   only for reserved/fragment expansion: in all other cases, a value of
   "%" MUST be pct-encoded as "%25" by variable expansion.

   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).  However, if reserved characters or pct-encoded triplets
   occur in the value, they will be pct-encoded by some expression types
   and not by others.

   For a variable that is a simple string value, expansion consists of
   appending the encoded value to the result string.  The explode
   modifier has no effect.  The prefix modifier limits the expansion to
   the first max-length characters of the decoded value.  If the value
   contains multibyte UTF-8, care must be taken to avoid splitting the
   value in mid-character: count each Unicode codepoint as one
   character.

   For a variable that is a list of values, expansion consists of
   concatenating the defined member string values, encoded as above,
   with a separator string inserted between those values.  If no explode
   modifier is given, the separator string is a comma (",").  If an
   explode modifier is given, the separator string is defined per
   operator by the following table, where NUL is the default expression
   type:

   .-----------------------------------------------------------------.
   |  operator:  NUL     +      .      /      ;      ?      &     #  |
   | separator:  ","    ","    "."    "/"    ";"    "&"    "&"   "," |
   `-----------------------------------------------------------------'

   For a variable that is an associative array, expansion consists of a
   list of either "name,value" (without explode modifier) or
   "name=value" (with explode modifier) pairs, excluding any pairs for
   which the corresponding value is undefined, with a separator string



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   inserted between defined pairs.  The separator string is defined in
   the same way as for list variables above.  Both the name and value
   strings are encoded in the same way as simple string values.

3.2.2.  Simple String Expansion: {var}

   Simple string expansion is the default expression type when no
   operator is given.

   For each defined variable in the variable-list, perform variable
   expansion, as defined in Section 3.2.1, with the allowed characters
   being those in the unreserved set.  If more than one variable has a
   defined value, append a comma (",") to the result string as a
   separator between variable expansions.

     Example Template     Expansion

       {var}              value
       {hello}            Hello%20World%21
       {half}             50%25
       O{empty}X          OX
       O{undef}X          OX
       {x,y}              1024,768
       {x,hello,y}        1024,Hello%20World%21,768
       ?{x,empty}         ?1024,
       ?{x,undef}         ?1024
       ?{undef,y}         ?768
       {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

3.2.3.  Reserved expansion: {+var}

   Reserved expansion, as indicated by the plus ("+") operator for Level
   2 and above templates, is identical to simple string expansion except
   that the substituted values may also contain pct-encoded triplets and
   characters in the reserved set.

   For each defined variable in the variable-list, perform variable
   expansion, as defined in Section 3.2.1, with the allowed characters
   being those in the set (unreserved / reserved / pct-encoded).  If
   more than one variable has a defined value, append a comma (",") to
   the result string as a separator between variable expansions.





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     Example Template        Expansion

       {+var}                value
       {+hello}              Hello%20World!
       {+half}               50%25

       {base}index           http%3A%2F%2Fexample.com%2Fhome%2Findex
       {+base}index          http://example.com/home/index
       O{+empty}X            OX
       O{+undef}X            OX

       {+path}/here          /foo/bar/here
       here?ref={+path}      here?ref=/foo/bar
       up{+path}{var}/here   up/foo/barvalue/here
       {+x,hello,y}          1024,Hello%20World!,768
       {+path,x}/here        /foo/bar,1024/here

       {+path:6}/here        /foo/b/here
       {+list}               red,green,blue
       {+list*}              red,green,blue
       {+keys}               semi,;,dot,.,comma,,
       {+keys*}              semi=;,dot=.,comma=,

3.2.4.  Fragment expansion: {#var}

   Fragment expansion, as indicated by the crosshatch ("#") operator for
   Level 2 and above templates, is identical to reserved expansion
   except that a crosshatch character (fragment delimiter) is appended
   first to the result string if any of the variables are defined.

     Example Template     Expansion

       {#var}             #value
       {#hello}           #Hello%20World!
       {#half}            #50%25
       foo{#empty}        foo#
       foo{#undef}        foo
       {#x,hello,y}       #1024,Hello%20World!,768
       {#path,x}/here     #/foo/bar,1024/here
       {#path:6}/here     #/foo/b/here
       {#list}            #red,green,blue
       {#list*}           #red,green,blue
       {#keys}            #semi,;,dot,.,comma,,
       {#keys*}           #semi=;,dot=.,comma=,







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

   Label expansion, as indicated by the dot (".") operator for Level 3
   and above templates, is useful for describing URI spaces with varying
   domain names or path selectors (e.g., filename extensions).

   For each defined variable in the variable-list, append "." to the
   result string and then perform variable expansion, as defined in
   Section 3.2.1, with the allowed characters being those in the
   unreserved set.

   Since "." is in the unreserved set, a value that contains a "." has
   the effect of adding multiple labels.

     Example Template     Expansion

       {.who}             .fred
       {.who,who}         .fred.fred
       {.half,who}        .50%25.fred
       www{.dom}          www.example.com
       X{.var}            X.value
       X{.empty}          X.
       X{.undef}          X
       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
       X{.empty_keys}     X
       X{.empty_keys*}    X

3.2.6.  Path segment expansion: {/var}

   Path segment expansion, as indicated by the slash ("/") operator in
   Level 3 and above templates, is useful for describing URI path
   hierarchies.

   For each defined variable in the variable-list, append "/" to the
   result string and then perform variable expansion, as defined in
   Section 3.2.1, with the allowed characters being those in the
   unreserved set.

   Note that the expansion process for path segment expansion is
   identical to that of label expansion aside from the substitution of
   "/" instead of ".".  However, unlike ".", a "/" is a reserved
   character and will be pct-encoded if found in a value.





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     Example Template     Expansion

       {/who}             /fred
       {/who,who}         /fred/fred
       {/half,who}        /50%25/fred
       {/who,dub}         /fred/me%2Ftoo
       {/var}             /value
       {/var,empty}       /value/
       {/var,undef}       /value
       {/var,x}/here      /value/1024/here
       {/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

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

   Path-style parameter expansion, as indicated by the semicolon (";")
   operator in Level 3 and above templates, is useful for describing URI
   path parameters, such as "path;property" or "path;name=value".

   For each defined variable in the variable-list:
   o  append ";" to the result string;
   o  if the variable has a simple string value or no explode modifier
      is given, then:
      *  append the variable name (encoded as if it were a literal
         string) to the result string;
      *  if the variable's value is not empty, append "=" to the result
         string;
   o  perform variable expansion, as defined in Section 3.2.1, with the
      allowed characters being those in the unreserved set.

     Example Template     Expansion

       {;who}             ;who=fred
       {;half}            ;half=50%25
       {;empty}           ;empty
       {;v,empty,who}     ;v=6;empty;who=fred
       {;v,bar,who}       ;v=6;who=fred
       {;x,y}             ;x=1024;y=768
       {;x,y,empty}       ;x=1024;y=768;empty
       {;x,y,undef}       ;x=1024;y=768
       {;hello:5}         ;hello=Hello
       {;list}            ;list=red,green,blue
       {;list*}           ;red;green;blue
       {;keys}            ;keys=semi,%3B,dot,.,comma,%2C



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       {;keys*}           ;semi=%3B;dot=.;comma=%2C

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

   Form-style query expansion, as indicated by the question-mark ("?")
   operator in Level 3 and above templates, is useful for describing an
   entire optional query component.

   For each defined variable in the variable-list:
   o  append "?" to the result string if this is the first defined value
      or append "&" thereafter;
   o  if the variable has a simple string value or no explode modifier
      is given, append the variable name (encoded as if it were a
      literal string) and an equals character ("=") to the result
      string; and,
   o  perform variable expansion, as defined in Section 3.2.1, with the
      allowed characters being those in the unreserved set.

     Example Template     Expansion

       {?who}             ?who=fred
       {?half}            ?half=50%25
       {?x,y}             ?x=1024&y=768
       {?x,y,empty}       ?x=1024&y=768&empty=
       {?x,y,undef}       ?x=1024&y=768
       {?var:3}           ?var=val
       {?list}            ?list=red,green,blue
       {?list*}           ?red&green&blue
       {?keys}            ?keys=semi,%3B,dot,.,comma,%2C
       {?keys*}           ?semi=%3B&dot=.&comma=%2C

3.2.9.  Form-style query continuation: {&var}

   Form-style query continuation, as indicated by the ampersand ("&")
   operator in Level 3 and above templates, is useful for describing
   optional &name=value pairs in a template that already contains a
   literal query component with fixed parameters.

   For each defined variable in the variable-list:
   o  append "&" to the result string;
   o  if the variable has a simple string value or no explode modifier
      is given, append the variable name (encoded as if it were a
      literal string) and an equals character ("=") to the result
      string; and,
   o  perform variable expansion, as defined in Section 3.2.1, with the
      allowed characters being those in the unreserved set.





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     Example Template     Expansion

       {&who}             &who=fred
       {&half}            &half=50%25
       ?fixed=yes{&x}     ?fixed=yes&x=1024
       {&x,y,empty}       &x=1024&y=768&empty=
       {&x,y,undef}       &x=1024&y=768

       {&var:3}           &var=val
       {&list}            &list=red,green,blue
       {&list*}           &red&green&blue
       {&keys}            &keys=semi,%3B,dot,.,comma,%2C
       {&keys*}           &semi=%3B&dot=.&comma=%2C


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, Robbie Gates, James H. Manger, Marc Portier, and James
   Snell.


7.  Normative References

   [ASCII]    American National Standards Institute, "Coded Character
              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



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              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.  Implementation Hints

   The normative sections on expansion describe each operator with a
   separate expansion process for the sake of descriptive clarity.  In
   actual implementations, we expect the expressions to be processed
   left-to-right using a common algorithm that has only minor variations
   in process per operator.  This appendix describes one such algorithm.

   Initialize an empty result string and its non-error state.

   Scan the template and copy literals to the result string (as in
   Section 3.1) until an expression is indicated by a "{" or the
   template ends.  When it ends, return the result string and its
   current error or non-error state.
   o  If an expression is found, scan the template to the next "}" and
      extract the characters in between the braces.
   o  If the template ends before a "}", then append the "{" and
      extracted characters to the result string and return with an error
      status indicating the expression is malformed.

   Examine the first character of the extracted expression for an
   operator.




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   o  If the expression ended (i.e., is "{}"), an operator is found that
      is unknown or unimplemented, or the character is not in the
      varchar set (Section 2.3), then append "{", the extracted
      expression, and "}" to the result string, remember that the result
      is in an error state, and then go back to scan the remainder of
      the template.
   o  If a known and implemented operator is found, store the operator
      and skip to the next character to begin the varspec-list.
   o  Otherwise, store the operator as NUL (simple string expansion).

   Use the following value table to determine the processing behavior by
   expression type operator.  The entry for "first" is the string to
   append to the result first if any of the expression's variables are
   defined.  The entry for "sep" is the separator to append to the
   result before any second (or subsequent) defined variable expansion.
   The entry for "named" is a boolean for whether or not the expansion
   includes the variable or key name when no explode modifier is given.
   The entry for "ifemp" is a string to append to the name if its
   corresponding value is empty.  The entry for "allow" indicates what
   characters to allow unencoded within the value expansion: (U) means
   any character not in the unreserved set will be encoded; (U+R) means
   any character not in the (unreserved / reserved / pct-encoding) set
   will be encoded; and, for both cases, disallowed characters are
   encoded as UTF-8 (a sequence of octets) and then each octet is
   encoded as a pct-encoded triplet.

   .------------------------------------------------------------------.
   |          NUL     +      .       /       ;      ?      &      #   |
   |------------------------------------------------------------------|
   | first |  ""     ""     "."     "/"     ";"    "?"    "&"    "#"  |
   | sep   |  ","    ","    "."     "/"     ";"    "&"    "&"    ","  |
   | named | false  false  false   false   true   true   true   false |
   | ifemp |  ""     ""     ""      ""      ""     "="    "="    ""   |
   | allow |   U     U+R     U       U       U      U      U     U+R  |
   `------------------------------------------------------------------'

   With the above table in mind, process the variable-list as follows:

   For each varspec, extract the varname and optional modifier, lookup
   the value for that variable, and then:
   o  If the varname is unknown or corresponds to a variable with an
      undefined value (Section 3.2.1), then skip to the next varspec.
   o  If this is the first defined variable for this expression, append
      the first string for this expression type to the result string and
      remember that it has been done.  Otherwise, append the sep string
      to the result string.





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   o  If this variable's value is a string, then:
      *  if named is true, append the varname to the result string using
         the same encoding process as for literals, and
         +  if the value is empty, append the ifemp string to the result
            string and skip to the next varspec;
         +  otherwise, append "=" to the result string.
      *  if a prefix modifier is present and the prefix length is less
         than the value string length in number of Unicode characters,
         append that number of characters from the beginning of the
         value string to the result string, after encoding any
         characters that are not in the allow set, while taking care not
         to split multi-octet or pct-encoded triplet characters that
         represent a single Unicode codepoint;
      *  otherwise, append the value to the result string after encoding
         any characters that are not in the allow set.
   o  If this variable's value is a list, then:
      *  if no explode modifier is given, then:
         +  if named is true, append the varname to the result string
            using the same encoding process as for literals, and
            -  if the value is empty, append the ifemp string to the
               result string and skip to the next varspec;
            -  otherwise, append "=" to the result string; and
         +  append each defined list member to the result string, after
            encoding any characters that are not in the allow set, with
            a comma (",") appended to the result between each defined
            list member.
      *  else if an explode modifier is given, then:
         +  append each defined list member to the result string, after
            encoding any characters that are not in the allow set, with
            the sep string appended to the result between each defined
            list member.
   o  If this variable's value is an associative array or any other form
      of paired (name, value) structure, then:
      *  if no explode modifier is given, then:
         +  if named is true, append the varname to the result string
            using the same encoding process as for literals, and
            -  if the value is empty, append the ifemp string to the
               result string and skip to the next varspec;
            -  otherwise, append "=" to the result string; and
         +  append each pair with a defined value to the result string
            as "name,value", after encoding any characters that are not
            in the allow set, with a comma (",") appended to the result
            between each defined pair.
      *  else if an explode modifier is given, then:
         +  append each pair with a defined value to the result string
            as "name=value", after encoding any characters that are not
            in the allow set, with the sep string appended to the result
            between each defined pair.



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   When the variable-list for this expression is exhausted, go back to
   scan the remainder of the template.


Authors' Addresses

   Joe Gregorio
   Google

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


   Roy T. Fielding
   Adobe Systems Incorporated

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


   Marc Hadley
   The MITRE Corporation

   Email: mhadley@mitre.org
   URI:   http://mitre.org/


   Mark Nottingham
   Rackspace

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/


   David Orchard
   Salesforce.com

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













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