### ----------------------------------------------------
### -- URI::Template
### -- Licenses: Artistic-2.0
### -- Authors: Jonathan Stowe <jns+git@gellyfish.co.uk>
### -- File: doc/rfc6570.txt
### ----------------------------------------------------
### -- Chunk 1 of 3






Internet Engineering Task Force (IETF)                       J. Gregorio
Request for Comments: 6570                                        Google
Category: Standards Track                                    R. Fielding
ISSN: 2070-1721                                                    Adobe
                                                               M. Hadley
                                                                   MITRE
                                                           M. Nottingham
                                                               Rackspace
                                                              D. Orchard
                                                          Salesforce.com
                                                              March 2012


                              URI Template

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6570.

Copyright Notice

   Copyright (c) 2012 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



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   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 ......................................................18
      3.1. Literal Expansion .........................................18
      3.2. Expression Expansion ......................................18
           3.2.1. Variable Expansion .................................19
           3.2.2. Simple String Expansion: {var} .....................21
           3.2.3. Reserved Expansion: {+var} .........................22
           3.2.4. Fragment Expansion: {#var} .........................23
           3.2.5. Label Expansion with Dot-Prefix: {.var} ............24
           3.2.6. Path Segment Expansion: {/var} .....................24
           3.2.7. Path-Style Parameter Expansion: {;var} .............25
           3.2.8. Form-Style Query Expansion: {?var} .................26
           3.2.9. Form-Style Query Continuation: {&var} ..............27
   4. Security Considerations ........................................27
   5. Acknowledgments ................................................28
   6. References .....................................................28
      6.1. Normative References ......................................28
      6.2. Informative References ....................................29
   Appendix A. Implementation Hints ..................................30













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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 (informally, a "URI space").  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

   A URI Template is a compact sequence of characters for describing a
   range of Uniform Resource Identifiers through variable expansion.

   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 the 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:

   expression:  The text between '{' and '}', including the enclosing
      braces, as defined in Section 2.

   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.




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   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.

   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



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   or if both variables are undefined, then it would be

     http://www.example.com/foo

   A URI Template may be provided in absolute form, as in the examples
   above, or in relative form.  A template is expanded before the
   resulting reference is 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 Internationalized Resource Identifier (IRI) references [RFC3987].
   Therefore, a URI Template is 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 pct-encoding any characters not in the set of unreserved URI
   characters (Section 1.5).

   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 URI characters
   (Section 1.5), 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 allow multiple variables per expression, each
   separated by a comma, and add more complex operators for dot-prefixed
   labels, slash-prefixed path segments, semicolon-prefixed path
   parameters, and the form-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 value modifiers as an optional suffix
   to each variable name.  A prefix modifier (":") indicates that only a
   limited number of characters from the beginning of the value are used
   by the expansion (Section 2.4.1).  An explode ("*") modifier



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   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, that is expanded as if each member were a
   separate variable (Section 2.4.2).

   .-----------------------------------------------------------------.
   | 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                 |



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   |     |    X{.list*}             X.red.green.blue                 |
   |     |    X{.keys}              X.semi,%3B,dot,.,comma,%2C       |
   |     |    X{.keys*}             X.semi=%3B.dot=..comma=%2C       |
   |     |                                                           |
   |-----+-----------------------------------------------------------|
   |  /  | Path segments, slash-prefixed                 (Sec 3.2.6) |
   |     |                                                           |
   |     |    {/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*}              ;list=red;list=green;list=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*}              ?list=red&list=green&list=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*}              &list=red&list=green&list=blue   |
   |     |    {&keys}               &keys=semi,%3B,dot,.,comma,%2C   |
   |     |    {&keys*}              &semi=%3B&dot=.&comma=%2C        |
   |     |                                                           |
   `-----------------------------------------------------------------'

1.3.  Design Considerations

   Mechanisms similar to URI Templates have been defined within several
   specifications, including WSDL [WSDL], WADL [WADL], and OpenSearch
   [OpenSearch].  This specification extends and formally defines the



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   syntax so that URI Templates can be used consistently across multiple
   Internet applications and within Internet message fields, while at
   the same time retaining compatibility with those earlier definitions.

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

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





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

   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"
                       ; case-insensitive

     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", "character encoding
   scheme", "code point", "coded character set", "glyph", "non-ASCII",
   "normalization", "protocol element", and "regular expression" as they
   are defined in [RFC6365].

   The ABNF notation defines its terminal values to be non-negative
   integers (code points) that are a superset of the US-ASCII coded
   character set [ASCII].  This specification defines terminal values as
   code points within the Unicode coded character set [UNIV6].

   In spite of the syntax and template expansion process being defined
   in terms of Unicode code points, 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
   glyphs painted on the side of a bus.  This specification does not
   mandate any particular character encoding scheme 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 scheme is defined by that protocol; without
   such a definition, a URI Template is assumed to be in the same
   character encoding scheme 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
   code points.

   The Unicode Standard [UNIV6] 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 as the sequence of pct-encoded
   triplets corresponding to that character's encoding in UTF-8
   [RFC3629].

     literals      =  %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
                   /  %x5D / %x5F / %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-level2 / op-level3 / op-reserve
     op-level2     =  "+" / "#"
     op-level3     =  "." / "/" / ";" / "?" / "&"
     op-reserve    =  "=" / "," / "!" / "@" / "|"




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RFC 6570                      URI Template                    March 2012


   The operator characters have been chosen to reflect each of their
   roles as reserved characters in the URI generic syntax.  The
   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 use outside the scope of this specification.  For
   example, a macro language might use these characters to apply macro
   substitution to a string prior to that string being processed as a
   URI Template.

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
