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

Network Working Group                                   J. Gregorio, Ed.
Internet-Draft                                                    Google
Intended status: Standards Track                        R. Fielding, Ed.
Expires: September 9, 2010                                  Day Software
                                                          M. Hadley, Ed.
                                                                  Oracle
                                                      M. Nottingham, Ed.
                                                              D. Orchard
                                                            Mar 08, 2010


                              URI Template
                     draft-gregorio-uritemplate-04

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, 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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   http://www.ietf.org/shadow.html.




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   This Internet-Draft will expire on September 9, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
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   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.  Expression Types . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Design Considerations  . . . . . . . . . . . . . . . . . .  8
     1.4.  Limitations  . . . . . . . . . . . . . . . . . . . . . . .  9
     1.5.  Notational Conventions . . . . . . . . . . . . . . . . . .  9
   2.  URI Template Syntax  . . . . . . . . . . . . . . . . . . . . . 11
     2.1.  Literals . . . . . . . . . . . . . . . . . . . . . . . . . 11
     2.2.  Expressions  . . . . . . . . . . . . . . . . . . . . . . . 11
     2.3.  Variables  . . . . . . . . . . . . . . . . . . . . . . . . 12
     2.4.  Value Modifiers  . . . . . . . . . . . . . . . . . . . . . 13
       2.4.1.  Component Values . . . . . . . . . . . . . . . . . . . 13
       2.4.2.  Prefix and Suffix Values . . . . . . . . . . . . . . . 14
     2.5.  Value Defaults . . . . . . . . . . . . . . . . . . . . . . 15
   3.  URI Template Expansion . . . . . . . . . . . . . . . . . . . . 18
     3.1.  Unicode normalization  . . . . . . . . . . . . . . . . . . 18
     3.2.  Literal expansion  . . . . . . . . . . . . . . . . . . . . 18
     3.3.  Expression expansion . . . . . . . . . . . . . . . . . . . 19
     3.4.  Variable and modifier expansion  . . . . . . . . . . . . . 19
     3.5.  Simple expansion: {var}  . . . . . . . . . . . . . . . . . 20
     3.6.  Reserved expansion: {+var} . . . . . . . . . . . . . . . . 21
     3.7.  Path-style parameter expansion: {;var} . . . . . . . . . . 21
     3.8.  Form-style parameter expansion: {?var} . . . . . . . . . . 21
     3.9.  Hierarchical path expansion: {/var}  . . . . . . . . . . . 21
     3.10. Label expansion with dot-prefix: {.var}  . . . . . . . . . 22
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Example URI Template Parser . . . . . . . . . . . . . 23
   Appendix B.  Revision History (to be removed by RFC Editor)  . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
















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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 value 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.

   A URI Template may be provided in absolute form, as in the examples
   above, or in relative form if a suitable base URI is defined.

   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 the pct-encoded sequences to their corresponding
   Unicode character if the character is not in the reserved set.

1.2.  Expression Types

   URI Templates are similar to a macro language with a fixed set of
   macro definitions: the expression type determines the expansion
   process.  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"



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     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

   The following table summarizes each type of template expression by
   its associated operator and cross-references the section of this
   document that defines the operator and its specific expansion
   process.  The example expansions are based on the following variables
   and values:

      var   := "value";
      hello := "Hello World!";
      undef := null;
      empty := "";
      list  := [ "val1", "val2", "val3" ];
      keys  := [("key1", "val1"), ("key2", "val2")];
      path  := "/foo/bar"
      x     := "1024";
      y     := "768";

   .-----------------------------------------------------------------.
   | Sec | Op  | Description                                         |
   |     |     |    Expression      | Expansion                      |
   |-----+-----+-----------------------------------------------------|
   | 3.3 |     | Simple expansion with comma-separated values        |
   |     |     |                                                     |
   |     |     |    {var}             value                          |
   |     |     |    {hello}           Hello%20World%21               |
   |     |     |    {path}/here       %2Ffoo%2Fbar/here              |
   |     |     |    {x,y}             1024,768                       |
   |     |     |    {var=default}     value                          |
   |     |     |    {undef=default}   default                        |
   |     |     |    {list}            val1,val2,val3                 |
   |     |     |    {list*}           val1,val2,val3                 |
   |     |     |    {list+}           list.val1,list.val2,list.val3  |
   |     |     |    {keys}            key1,val1,key2,val2            |
   |     |     |    {keys*}           key1,val1,key2,val2            |
   |     |     |    {keys+}           keys.key1,val1,keys.key2,val2  |



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   |     |     |                                                     |
   |-----+-----+-----------------------------------------------------|
   | 3.4 |  +  | Reserved expansion with comma-separated values      |
   |     |     |                                                     |
   |     |     |    {+var}            value                          |
   |     |     |    {+hello}          Hello%20World!                 |
   |     |     |    {+path}/here      /foo/bar/here                  |
   |     |     |    {+path,x}/here    /foo/bar,1024/here             |
   |     |     |    {+path}{x}/here   /foo/bar1024/here              |
   |     |     |    {+empty}/here     /here                          |
   |     |     |    {+undef}/here     /here                          |
   |     |     |    {+list}           val1,val2,val3                 |
   |     |     |    {+list*}          val1,val2,val3                 |
   |     |     |    {+list+}          list.val1,list.val2,list.val3  |
   |     |     |    {+keys}           key1,val1,key2,val2            |
   |     |     |    {+keys*}          key1,val1,key2,val2            |
   |     |     |    {+keys+}          keys.key1,val1,keys.key2,val2  |
   |     |     |                                                     |
   |-----+-----+-----------------------------------------------------|
   | 3.5 |  ;  | Path-style parameters, semicolon-prefixed           |
   |     |     |                                                     |
   |     |     |    {;x,y}            ;x=1024;y=768                  |
   |     |     |    {;x,y,empty}      ;x=1024;y=768;empty            |
   |     |     |    {;x,y,undef}      ;x=1024;y=768                  |
   |     |     |    {;list}           ;val1,val2,val3                |
   |     |     |    {;list*}          ;val1;val2;val3                |
   |     |     |    {;list+}          ;list=val1;list=val2;list=val3 |
   |     |     |    {;keys}           ;key1,val1,key2,val2           |
   |     |     |    {;keys*}          ;key1=val1;key2=val2           |
   |     |     |    {;keys+}          ;keys.key1=val1;keys.key2=val2 |
   |     |     |                                                     |
   |-----+-----+-----------------------------------------------------|
   | 3.6 |  ?  | Form-style parameters, ampersand-separated          |
   |     |     |                                                     |
   |     |     |    {?x,y}            ?x=1024&y=768                  |
   |     |     |    {?x,y,empty}      ?x=1024&y=768&empty=           |
   |     |     |    {?x,y,undef}      ?x=1024&y=768                  |
   |     |     |    {?list}           ?list=val1,val2,val3           |
   |     |     |    {?list*}          ?val1&val2&val3                |
   |     |     |    {?list+}          ?list=val1&list=val2&list=val3 |
   |     |     |    {?keys}           ?keys=key1,val1,key2,val2      |
   |     |     |    {?keys*}          ?key1=val1&key2=val2           |
   |     |     |    {?keys+}          ?keys.key1=val1&keys.key2=val2 |
   |     |     |                                                     |
   |-----+-----+-----------------------------------------------------|
   | 3.7 |  /  | Hierarchical path segments, slash-separated         |
   |     |     |                                                     |
   |     |     |    {/var}            /value                         |



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   |     |     |    {/var,empty}      /value/                        |
   |     |     |    {/var,undef}      /value                         |
   |     |     |    {/list}           /val1,val2,val3                |
   |     |     |    {/list*}          /val1/val2/val3                |
   |     |     |    {/list*,x}        /val1/val2/val3/1024           |
   |     |     |    {/list+}          /list.val1/list.val2/list.val3 |
   |     |     |    {/keys}           /key1,val1,key2,val2           |
   |     |     |    {/keys*}          /key1/val1/key2/val2           |
   |     |     |    {/keys+}          /keys.key1/val1/keys.key2/val2 |
   |     |     |                                                     |
   |-----+-----+-----------------------------------------------------|
   | 3.8 |  .  | Label expansion, dot-prefixed                       |
   |     |     |                                                     |
   |     |     |    X{.var}          X.value                         |
   |     |     |    X{.empty}        X.                              |
   |     |     |    X{.undef}        X                               |
   |     |     |    X{.list}         X.val1,val2,val3                |
   |     |     |    X{.list*}        X.val1.val2.val3                |
   |     |     |    X{.list*,x}      X.val1.val2.val3.1024           |
   |     |     |    X{.list+}        X.list.val1.list.val2.list.val3 |
   |     |     |    X{.keys}         X.key1,val1,key2,val2           |
   |     |     |    X{.keys*}        X.key1.val1.key2.val2           |
   |     |     |    X{.keys+}        X.keys.key1.val1.keys.key2.val2 |
   |     |     |                                                     |
   `-----+-----+-----------------------------------------------------'

1.3.  Design Considerations

   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.  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.



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   The most common cases for URI spaces can be described with simple URI
   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 and suffix substrings are often used to partition large
   spaces of resources, modifiers on variable values provide a way to
   specify those substrings.

   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.

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





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     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

   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".

   A URI Template is defined as a sequence of characters and therefore
   has the same issues as URIs with regard to codepoints and character
   sets.  That is, URI Template characters are frequently encoded as
   octets for transport or presentation.  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.

   A URI Template and its associated variable values are converted to a
   normal form of UTF-8 [RFC3629] prior to template processing, as
   defined in Section 3.1.

   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].  Thus, a
   string of characters in a URI Template is assumed to be transformed
   into its corresponding sequence of Unicode codepoints prior to
   testing for a match with the URI Template grammar.




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2.  URI Template 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 )

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

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
   in detail in Section 3.

   The operator characters pipe ("|"), exclamation ("!"), and at-sign



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   ("@") 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 URI
   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

   Any of the variables can have a modifier indicating that its value is
   exploded into components or is limited to a prefix, suffix, or the
   remainder of a prefix or suffix of the variable value.

     modifier      =  explode / partial

2.4.1.  Component Values

   The explode modifiers ("*" and "+") indicate 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       =  ( "*" / "+" )

   The primary difference between the two explode modifiers is that an
   asterisk ("*") indicates that just the component names and values are
   included in the expansion, whereas the plus ("+") indicates that each
   component name is prefixed with the given variable name and a period
   ("."), thereby enabling multiple variables with the same component
   names to be disambiguated.

   Component 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*}

   or

      /directions{?from+,to+}

   along with some context that defines each variable (address, from,
   and to) as adhering to a given addressing standard (e.g., UPU S42 or
   AS/NZS 4819:2003).  A recipient aware of the schema can then provide
   appropriate expansions, such as:



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      /mapper?city=Newport%20Beach&state=CA
      /directions?from.zipcode=92660&to.zipcode=90210

   The expansion process for variables, as defined in Section 3, is
   dependent on both the operator being used and, if one of the explode
   modifiers is used, the type and schema of the variable being
   substituted.

2.4.2.  Prefix and Suffix Values

   Prefix and suffix modifiers are often used to partition an identifier
   space hierarchically, as is common in reference indices and hash-
   based storage, or to limit the substituted value to a maximum number
   of characters.

     partial       =  ( substring / remainder ) offset
     substring     =  ":"
     remainder     =  "^"
     offset        =  [ from-end ] 1*DIGIT
     from-end      =  "-"

   The offset refers to a maximum number of characters from either the
   beginning (prefix) or end (suffix) of the variable's value as a
   Unicode string.  Note that this numbering is in characters, not
   octets, in order to avoid substituting improperly encoded values due
   to splitting a multi-octet UTF-8 encoded character or a pct-encoded
   triplet.

   A substring modifier requires that only the indicated prefix or
   suffix be used in the expansion.  A remainder modifier requires that
   only the remainder of the value, excluding the indicated prefix or
   suffix, be used in the expansion.  If the offset is greater than the
   length of the variable's value, then the entire string is used for a
   substring and the empty string is used for a remainder.

   The following examples illustrate how modifiers work with the
   different variable types.  More complex examples are provided in
   Section 4.













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     Given the variable assignments:
       var   := "value";
       name  := [ "Fred", "Wilma", "Pebbles" ];

     Example Template     Expansion

       {var}              value
       {var:20}           value
       {var:3}            val
       {var^3}            ue
       {var:-3}           lue
       {var^-3}           va

       {?name}            ?name=Fred,Wilma,Pebbles
       {?name:1}          ?name=F

2.5.  Value Defaults

   Any of the variables may also be supplied with a default value to be
   used when a template processor determines the variable to be
   undefined.  The default value is limited to the unreserved and pct-
   encoded characters of a URI-reference, since our intention is for the
   default to be presented in the exact form that it would appear in the
   resulting URI.  The default is not affected by the variable
   modifiers; it is assumed that the default string provided in the
   expression already reflects any necessary substring or remainder
   processing.

     default       =  *( unreserved / pct-encoded )

   The following examples illustrate how default values work with
   different variable types.  More complex examples are provided in
   Section 4.

     Given the variable assignments:
       var   := "value";
       empty := "";
       undef := null;
       name  := [ "Fred", "Wilma", "Pebbles" ];
       favs  := [("color","red"), ("volume","high")];
       empty_list := [];
       empty_keys := [];

     Example Template        Expansion

       {var=default}         value
       {undef=default}       default




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

       x{empty_list}y        xy
       x{empty_list=_}y      xy
       x{empty_list*}y       xy
       x{empty_list*=_}y     x_y
       x{empty_list+}y       xy
       x{empty_list+=_}y     xempty_list._y

       x{empty_keys}y        xy
       x{empty_keys=_}y      xy
       x{empty_keys*}y       xy
       x{empty_keys*=_}y     x_y
       x{empty_keys+}y       xy
       x{empty_keys+=_}y     xempty_keys._y

       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{?favs+=none}        x?favs.color=red&favs.volume=high

       x{?undef}             x
       x{?undef=none}        x?undef=none
       x{?empty}             x?empty=
       x{?empty=none}        x?empty=

       x{?empty_list}        x
       x{?empty_list=none}   x?empty_list=none
       x{?empty_list*}       x
       x{?empty_list*=none}  x?none
       x{?empty_list+}       x
       x{?empty_list+=none}  x?empty_list.none

       x{?empty_keys}        x
       x{?empty_keys=none}   x?empty_keys=none
       x{?empty_keys*}       x
       x{?empty_keys*=none}  x?none
       x{?empty_keys+}       x
       x{?empty_keys+=none}  x?empty_keys.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{;favs+=none}        x;favs.color=red;favs.volume=high




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       x{;undef}             x
       x{;undef=none}        x;undef=none
       x{;empty}             x;empty
       x{;empty=none}        x;empty

       x{;empty_list}        x
       x{;empty_list=none}   x;empty_list=none
       x{;empty_list*}       x
       x{;empty_list*=none}  x;none
       x{;empty_list+}       x
       x{;empty_list+=none}  x;empty_list.none

       x{;empty_keys}        x
       x{;empty_keys=none}   x;empty_keys=none
       x{;empty_keys*}       x
       x{;empty_keys*=none}  x;none
       x{;empty_keys+}       x
       x{;empty_keys+=none}  x;empty_keys.none

       x{/name=none}         x/Fred,Wilma,Pebbles
       x{/name*=none}        x/Fred/Wilma/Pebbles
       x{/name+=none}        x/name.Fred/name.Wilma/name.Pebbles
       x{/favs=none}         x/color,red,volume,high
       x{/favs*=none}        x/color/red/volume/high
       x{/favs+=none}        x/favs.color/red/favs.volume/high

       x{/undef}             x
       x{/undef=none}        x/none
       x{/empty}             x/
       x{/empty=none}        x/

       x{/empty_list}        x
       x{/empty_list=none}   x/none
       x{/empty_list*}       x
       x{/empty_list*=none}  x/none
       x{/empty_list+}       x
       x{/empty_list+=none}  x/empty_list.none

       x{/empty_keys}        x
       x{/empty_keys=none}   x/none
       x{/empty_keys*}       x
       x{/empty_keys*=none}  x/none
       x{/empty_keys+}       x
       x{/empty_keys+=none}  x/empty_keys.none







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3.  URI Template 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.

   If a template processor encounters an error, such as an operator that
   it does not understand or 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 a template processor encounters a warning, such as the use of an
   operator character reserved for future extension, then the processing
   of the template SHOULD NOT cease, and the location and type of
   warning SHOULD be indicated to the invoking application.

3.1.  Unicode normalization

   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).
   Since different Normalized Form unicode strings will have different
   UTF-8 representations, the only way to guarantee that template
   processors will produce the same URI is to require a common
   Normalized Form.

   The string values for the URI Template and template variables MUST be
   in NFKC and encoded as UTF-8 [RFC3629] prior to use in the template
   expansion process (US-ASCII is a proper subset of NFKC UTF-8).  The
   remaining sections defining the expansion process assume strings are
   in NFKC UTF-8.

3.2.  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-
   encoded triplet.







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3.3.  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.

   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.

   If the expression does not contain any varspec, as in "{}" or "{,}",
   then a template processor SHOULD copy that invalid expression to the
   result string, continue processing the remainder of the template, and
   indicate that an error occurred to the caller.

   If the template contains an opening brace without a corresponding
   closing brace (the template ends in mid-expression), then a processor
   SHOULD attempt to process the template as if it ended in a closing
   brace and indicate that an error occurred to the caller.

3.4.  Variable and modifier expansion

   A variable that is undefined has no value and thus is excluded from
   the expansion.  A variable defined as composite or component values
   is undefined if it contains zero members or all of its components are
   undefined.  If all of the variables are undefined, then the
   expression's expansion is the empty string.

   A variable defined as a single value is expanded by converting its
   value to a NFKC UTF-8 string, replacing any character within the
   string that is not in the unreserved set with its corresponding
   sequence of pct-encoded octets, applying any prefix or suffix
   modifier (Section 2.4.2), and then appending the result to the URI-
   reference.

   A variable defined as a list of values is substituted as a string of
   comma-separated single values when no explode modifier is given.  If
   the "*" modifier is used, then each value is separated by the default
   delimiter for the expression type.  If the "+" operator is used, then
   the variable name is prepended to the expansion list as if it were
   the initial value in the list.  If a partial modifier is indicated,
   the modifier is applied to the combined string of values.  The list



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   expansion is then appended to the result string.

   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.  If no explode modifier is used,
   then the list is substituted as comma-separated single values.  If
   the "*" modifier is given, then the list is delimited as key=value
   pairs according to the default delimiters defined by the expression
   type.  If the "+" modifier is used, the values are substituted as in
   the "*" case, except that each key name is prefixed by the variable
   name and a ".", as in "name.key=value".

   When a variable containing component values is given without an
   explode modifier, the value of each defined component is substituted,
   separated by a comma (",") character, in the order indicated by the
   variable's schema or, if the schema is unknown, in the order provided
   by the variable's value.  A structure of component values is expanded
   as a list of the component values in the order implied by a preorder
   (depth-first) traversal of that structure, excluding any components
   that are undefined.

   When an explode modifier is used with an operator that substitutes
   variables as key=value pairs, the key is determined as follows.  If
   the modifier is an asterisk ("*"), then each "key" is the name of the
   component.  If the modifier is a plus ("+"), then each key is the
   variable name followed by a period (".") and the component name.  In
   both cases, if the component names have a hierarchical structure,
   then the component subnames are also appended to the key, each
   separated by a period.

   When an explode modifier is used with the hierarchical ("/")
   operator, the slash delimiter is substituted before each defined
   component's value if the modifier is "*", or before each conjunction
   of component name and value (e.g., "name.value") if the modifier is
   "+".

3.5.  Simple expansion: {var}

   The default expression type when no operator is given is simple
   expansion: the value of each defined variable is substituted in the
   order given, modified as indicated by the optional modifiers, with
   each value separated by a comma character (",").  A variable that is
   undefined has no value and thus is excluded from the expansion.  If
   all of the variables are undefined, then the expansion is the empty
   string.






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   For example,

     foo := "fred"

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

3.6.  Reserved expansion: {+var}

   Reserved expansion is identical to simple expansion except that the
   substituted values may contain characters in the reserved set.

   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 3.5 except that,
   instead of replacing any character within each value string that is
   not in the unreserved set with its corresponding sequence of pct-
   encoded octets, replace any character within each value string that
   is not in the set of unreserved or reserved characters with its
   corresponding sequence of pct-encoded octets.

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

   TBD.

3.8.  Form-style parameter expansion: {?var}

   TBD.

3.9.  Hierarchical path expansion: {/var}

   TBD.







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

   TBD.


4.  Examples

   TBD.


5.  Security Considerations

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


6.  IANA Considerations

   No IANA actions are required by this document.


7.  Acknowledgments

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


8.  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
              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.




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   [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.

   TBD.


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

   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



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   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.


Authors' Addresses

   Joe Gregorio (editor)
   Google

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


   Roy T. Fielding (editor)
   Day Software

   Email: fielding@day.com
   URI:   http://www.day.com/


   Marc Hadley (editor)
   Oracle

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


   Mark Nottingham (editor)

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






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   David Orchard

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
















































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