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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en"><head><title>URI Template</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="description" content="URI Template">
<meta name="generator" content="xml2rfc v1.36 (http://xml.resource.org/)">
<style type='text/css'><!--
body {
font-family: verdana, charcoal, helvetica, arial, sans-serif;
font-size: small; color: #000; background-color: #FFF;
margin: 2em;
}
h1, h2, h3, h4, h5, h6 {
font-family: helvetica, monaco, "MS Sans Serif", arial, sans-serif;
font-weight: bold; font-style: normal;
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h1 { color: #900; background-color: transparent; text-align: right; }
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td.RFCbug {
font-size: x-small; text-decoration: none;
width: 30px; height: 30px; padding-top: 2px;
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td.RFCbug span.RFC {
font-family: monaco, charcoal, geneva, "MS Sans Serif", helvetica, verdana, sans-serif;
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font-family: charcoal, monaco, geneva, "MS Sans Serif", helvetica, verdana, sans-serif;
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table.TOCbug { width: 30px; height: 15px; }
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font-weight: bold; font-size: x-small; text-decoration: none;
color: #FFF; background-color: transparent;
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td.header {
font-family: arial, helvetica, sans-serif; font-size: x-small;
vertical-align: top; width: 33%;
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td.author { font-weight: bold; font-size: x-small; margin-left: 4em; }
td.author-text { font-size: x-small; }
/* info code from SantaKlauss at http://www.madaboutstyle.com/tooltip2.html */
a.info {
/* This is the key. */
position: relative;
z-index: 24;
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}
a.info:hover {
z-index: 25;
color: #FFF; background-color: #900;
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a.info span { display: none; }
a.info:hover span.info {
/* The span will display just on :hover state. */
display: block;
position: absolute;
font-size: smaller;
top: 2em; left: -5em; width: 15em;
padding: 2px; border: 1px solid #333;
color: #900; background-color: #EEE;
text-align: left;
}
a { font-weight: bold; }
a:link { color: #900; background-color: transparent; }
a:visited { color: #633; background-color: transparent; }
a:active { color: #633; background-color: transparent; }
p { margin-left: 2em; margin-right: 2em; }
p.copyright { font-size: x-small; }
p.toc { font-size: small; font-weight: bold; margin-left: 3em; }
table.toc { margin: 0 0 0 3em; padding: 0; border: 0; vertical-align: text-top; }
td.toc { font-size: small; font-weight: bold; vertical-align: text-top; }
ol.text { margin-left: 2em; margin-right: 2em; }
ul.text { margin-left: 2em; margin-right: 2em; }
li { margin-left: 3em; }
/* RFC-2629 <spanx>s and <artwork>s. */
em { font-style: italic; }
strong { font-weight: bold; }
dfn { font-weight: bold; font-style: normal; }
cite { font-weight: normal; font-style: normal; }
tt { color: #036; }
tt, pre, pre dfn, pre em, pre cite, pre span {
font-family: "Courier New", Courier, monospace; font-size: small;
}
pre {
text-align: left; padding: 4px;
color: #000; background-color: #CCC;
}
pre dfn { color: #900; }
pre em { color: #66F; background-color: #FFC; font-weight: normal; }
pre .key { color: #33C; font-weight: bold; }
pre .id { color: #900; }
pre .str { color: #000; background-color: #CFF; }
pre .val { color: #066; }
pre .rep { color: #909; }
pre .oth { color: #000; background-color: #FCF; }
pre .err { background-color: #FCC; }
/* RFC-2629 <texttable>s. */
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hr { height: 1px; }
hr.insert {
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--></style>
</head>
<body>
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<table summary="layout" width="66%" border="0" cellpadding="0" cellspacing="0"><tr><td><table summary="layout" width="100%" border="0" cellpadding="2" cellspacing="1">
<tr><td class="header">Network Working Group</td><td class="header">J. Gregorio</td></tr>
<tr><td class="header">Internet-Draft</td><td class="header">Google</td></tr>
<tr><td class="header">Intended status: Standards Track</td><td class="header">R. Fielding</td></tr>
<tr><td class="header">Expires: March 29, 2012</td><td class="header">Adobe</td></tr>
<tr><td class="header"> </td><td class="header">M. Hadley</td></tr>
<tr><td class="header"> </td><td class="header">MITRE</td></tr>
<tr><td class="header"> </td><td class="header">M. Nottingham</td></tr>
<tr><td class="header"> </td><td class="header">Rackspace</td></tr>
<tr><td class="header"> </td><td class="header">D. Orchard</td></tr>
<tr><td class="header"> </td><td class="header">Salesforce.com</td></tr>
<tr><td class="header"> </td><td class="header">Sep 26, 2011</td></tr>
</table></td></tr></table>
<h1><br />URI Template<br />draft-gregorio-uritemplate-07</h1>
<h3>Abstract</h3>
<p>
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.
</p>
<h3>Editorial Note (to be removed by RFC Editor)</h3>
<p>
To provide feedback on this Internet-Draft, join the
<a href='http://lists.w3.org/Archives/Public/uri/'>W3C URI
mailing list (http://lists.w3.org/Archives/Public/uri/)</a>.
</p>
<h3>Status of this Memo</h3>
<p>
This Internet-Draft is submitted in full
conformance with the provisions of BCP 78 and BCP 79.</p>
<p>
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current
Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.</p>
<p>
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any time.
It is inappropriate to use Internet-Drafts as reference material or to cite
them other than as “work in progress.”</p>
<p>
This Internet-Draft will expire on March 29, 2012.</p>
<h3>Copyright Notice</h3>
<p>
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.</p>
<p>
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.</p>
<a name="toc"></a><br /><hr />
<h3>Table of Contents</h3>
<p class="toc">
<a href="#intro">1.</a>
Introduction<br />
<a href="#overview">1.1.</a>
Overview<br />
<a href="#types">1.2.</a>
Levels and Expression Types<br />
<a href="#design">1.3.</a>
Design Considerations<br />
<a href="#limitations">1.4.</a>
Limitations<br />
<a href="#notation">1.5.</a>
Notational Conventions<br />
<a href="#unicode-normal">1.6.</a>
Character Encoding and Unicode Normalization<br />
<a href="#syntax">2.</a>
Syntax<br />
<a href="#literals">2.1.</a>
Literals<br />
<a href="#expressions">2.2.</a>
Expressions<br />
<a href="#variables">2.3.</a>
Variables<br />
<a href="#modifiers">2.4.</a>
Value Modifiers<br />
<a href="#prefix-values">2.4.1.</a>
Prefix Values<br />
<a href="#composite-values">2.4.2.</a>
Composite Values<br />
<a href="#expansion">3.</a>
Expansion<br />
<a href="#literal-expansion">3.1.</a>
Literal Expansion<br />
<a href="#expression-expansion">3.2.</a>
Expression Expansion<br />
<a href="#variable-expansion">3.2.1.</a>
Variable Expansion<br />
<a href="#op-simple">3.2.2.</a>
Simple String Expansion: {var}<br />
<a href="#op-plus">3.2.3.</a>
Reserved expansion: {+var}<br />
<a href="#op-crosshatch">3.2.4.</a>
Fragment expansion: {#var}<br />
<a href="#op-dot">3.2.5.</a>
Label expansion with dot-prefix: {.var}<br />
<a href="#op-slash">3.2.6.</a>
Path segment expansion: {/var}<br />
<a href="#op-semicolon">3.2.7.</a>
Path-style parameter expansion: {;var}<br />
<a href="#op-question">3.2.8.</a>
Form-style query expansion: {?var}<br />
<a href="#op-ampersand">3.2.9.</a>
Form-style query continuation: {&var}<br />
<a href="#anchor1">4.</a>
Security Considerations<br />
<a href="#iana">5.</a>
IANA Considerations<br />
<a href="#ack">6.</a>
Acknowledgments<br />
<a href="#rfc.references1">7.</a>
Normative References<br />
<a href="#implementation">Appendix A.</a>
Implementation Hints<br />
<a href="#rfc.authors">§</a>
Authors' Addresses<br />
</p>
<br clear="all" />
<a name="intro"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1"></a><h3>1.
Introduction</h3>
<a name="overview"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.1"></a><h3>1.1.
Overview</h3>
<p>
A Uniform Resource Identifier (URI) <a class='info' href='#RFC3986'>[RFC3986]<span> (</span><span class='info'>Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.</span><span>)</span></a>
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
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://example.com/~fred/
http://example.com/~mark/
</pre></div>
<p>
or a set of dictionary entries might be grouped in a hierarchy
by the first letter of the term, as in
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
</pre></div>
<p>
or a service interface might be invoked with various user input
in a common pattern, as in
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://example.com/search?q=cat&lang=en
http://example.com/search?q=chien&lang=fr
</pre></div>
<p>
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:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
</pre></div>
<p>
We define the following terms:
</p>
<ul class="text">
<li>expression -
The text between '{' and '}', including the enclosing braces,
as defined in <a class='info' href='#syntax'>Section 2<span> (</span><span class='info'>Syntax</span><span>)</span></a>.
</li>
<li>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 <a class='info' href='#expansion'>Section 3<span> (</span><span class='info'>Expansion</span><span>)</span></a>.
</li>
<li>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.
</li>
</ul><p>
</p>
<p>
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.
</p>
<p>
For example, the following URI Template includes a
form-style parameter expression, as indicated by the "?" operator
appearing before the variable names.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://www.example.com/foo{?query,number}
</pre></div>
<p>
The expansion process for expressions beginning with the
question-mark ("?") operator follows the same pattern
as form-style interfaces on the World Wide Web:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
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.
</pre></div>
<p>
If the variables have the values
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
query := "mycelium"
number := 100
</pre></div>
<p>
then the expansion of the above URI Template is
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://www.example.com/foo?query=mycelium&number=100
</pre></div>
<p>
Alternatively, if 'query' is undefined, then the expansion
would be
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://www.example.com/foo?number=100
</pre></div>
<p>
or if both variables are undefined, then it would be
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
http://www.example.com/foo
</pre></div>
<p>
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.
</p>
<p>
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 <a class='info' href='#RFC3987'>[RFC3987]<span> (</span><span class='info'>Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.</span><span>)</span></a>.
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 <a class='info' href='#RFC3987'>[RFC3987]<span> (</span><span class='info'>Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.</span><span>)</span></a>.
</p>
<a name="types"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.2"></a><h3>1.2.
Levels and Expression Types</h3>
<p>
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.
</p>
<p>
Since most template processors implemented prior to this specification
have only implemented the default expression type, we refer to these as
Level 1 templates.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
.-----------------------------------------------------------------.
| 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 |
`-----------------------------------------------------------------'
</pre></div>
<p>
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.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
.-----------------------------------------------------------------.
| 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! |
`-----------------------------------------------------------------'
</pre></div>
<p>
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.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
.-----------------------------------------------------------------.
| 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 |
| | |
|-----+-----------------------------------------------------------|
| + | 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= |
| | |
`-----------------------------------------------------------------'
</pre></div>
<p>
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, 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.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
.-----------------------------------------------------------------.
| 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 |
| | 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 |
| | |
`-----------------------------------------------------------------'
</pre></div>
<a name="design"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.3"></a><h3>1.3.
Design Considerations</h3>
<p>
Mechanisms similar to URI Templates have been defined within
several specifications, including WSDL, WADL and OpenSearch.
This specification extends and formally defines the syntax so
that URI Templates can be used consistently across multiple
Internet applications and within Internet message fields,
while at the same time retaining compatibility with those
earlier definitions.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<a name="limitations"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.4"></a><h3>1.4.
Limitations</h3>
<p>
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.
</p>
<p>
URI Templates are not URIs: they do not identify
an abstract or physical resource, they are not parsed as URIs,
and should not be used in places where a URI would be expected
unless the template expressions will be expanded by a template
processor prior to use. Distinct field, element, or attribute
names should be used to differentiate protocol elements that
carry a URI Template from those that expect a URI reference.
</p>
<p>
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.
</p>
<a name="notation"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.5"></a><h3>1.5.
Notational Conventions</h3>
<p>
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 <a class='info' href='#RFC2119'>[RFC2119]<span> (</span><span class='info'>Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.</span><span>)</span></a>.
</p>
<p>
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <a class='info' href='#RFC5234'>[RFC5234]<span> (</span><span class='info'>Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.</span><span>)</span></a>. The following ABNF rules
are imported from the normative references <a class='info' href='#RFC5234'>[RFC5234]<span> (</span><span class='info'>Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.</span><span>)</span></a>,
<a class='info' href='#RFC3986'>[RFC3986]<span> (</span><span class='info'>Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.</span><span>)</span></a>, and <a class='info' href='#RFC3987'>[RFC3987]<span> (</span><span class='info'>Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.</span><span>)</span></a>.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>ALPHA</dfn> = <span class='val'>%x41-5A</span> / <span class='val'>%x61-7A</span> <em>; A-Z / a-z</em>
<dfn>DIGIT</dfn> = <span class='val'>%x30-39</span> <em>; 0-9</em>
<dfn>HEXDIG</dfn> = <cite class='key'>DIGIT</cite> / "<span class='str'>A</span>" / "<span class='str'>B</span>" / "<span class='str'>C</span>" / "<span class='str'>D</span>" / "<span class='str'>E</span>" / "<span class='str'>F</span>"
<dfn>pct-encoded</dfn> = "<span class='str'>%</span>" <cite class='key'>HEXDIG</cite> <cite class='key'>HEXDIG</cite>
<dfn>unreserved</dfn> = <cite class='key'>ALPHA</cite> / <cite class='key'>DIGIT</cite> / "<span class='str'>-</span>" / "<span class='str'>.</span>" / "<span class='str'>_</span>" / "<span class='str'>~</span>"
<dfn>reserved</dfn> = <cite class='id'>gen-delims</cite> / <cite class='id'>sub-delims</cite>
<dfn>gen-delims</dfn> = "<span class='str'>:</span>" / "<span class='str'>/</span>" / "<span class='str'>?</span>" / "<span class='str'>#</span>" / "<span class='str'>[</span>" / "<span class='str'>]</span>" / "<span class='str'>@</span>"
<dfn>sub-delims</dfn> = "<span class='str'>!</span>" / "<span class='str'>$</span>" / "<span class='str'>&</span>" / "<span class='str'>'</span>" / "<span class='str'>(</span>" / "<span class='str'>)</span>"
/ "<span class='str'>*</span>" / "<span class='str'>+</span>" / "<span class='str'>,</span>" / "<span class='str'>;</span>" / "<span class='str'>=</span>"
<dfn>ucschar</dfn> = <span class='val'>%xA0-D7FF</span> / <span class='val'>%xF900-FDCF</span> / <span class='val'>%xFDF0-FFEF</span>
/ <span class='val'>%x10000-1FFFD</span> / <span class='val'>%x20000-2FFFD</span> / <span class='val'>%x30000-3FFFD</span>
/ <span class='val'>%x40000-4FFFD</span> / <span class='val'>%x50000-5FFFD</span> / <span class='val'>%x60000-6FFFD</span>
/ <span class='val'>%x70000-7FFFD</span> / <span class='val'>%x80000-8FFFD</span> / <span class='val'>%x90000-9FFFD</span>
/ <span class='val'>%xA0000-AFFFD</span> / <span class='val'>%xB0000-BFFFD</span> / <span class='val'>%xC0000-CFFFD</span>
/ <span class='val'>%xD0000-DFFFD</span> / <span class='val'>%xE1000-EFFFD</span>
<dfn>iprivate</dfn> = <span class='val'>%xE000-F8FF</span> / <span class='val'>%xF0000-FFFFD</span> / <span class='val'>%x100000-10FFFD</span>
</pre></div>
<a name="unicode-normal"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1.6"></a><h3>1.6.
Character Encoding and Unicode Normalization</h3>
<p>
This specification uses the terms "character" and "coded
character set" in accordance with the definitions provided
in <a class='info' href='#RFC2978'>[RFC2978]<span> (</span><span class='info'>Freed, N. and J. Postel, “IANA Charset Registration Procedures,” October 2000.</span><span>)</span></a>, and "character encoding" in
place of what <a class='info' href='#RFC2978'>[RFC2978]<span> (</span><span class='info'>Freed, N. and J. Postel, “IANA Charset Registration Procedures,” October 2000.</span><span>)</span></a> refers to as a "charset".
</p>
<p>
The ABNF notation defines its terminal values to be
non-negative integers (codepoints) that are a superset of the
US-ASCII coded character set <a class='info' href='#ASCII'>[ASCII]<span> (</span><span class='info'>American National Standards Institute, “Coded Character Set - 7-bit American Standard Code for Information Interchange,” 1986.</span><span>)</span></a>. This
specification defines terminal values as codepoints within the
Unicode coded character set <a class='info' href='#UNIV4'>[UNIV4]<span> (</span><span class='info'>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.</span><span>)</span></a>.
</p>
<p>
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.
</p>
<p>
The Unicode Standard <a class='info' href='#UNIV4'>[UNIV4]<span> (</span><span class='info'>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.</span><span>)</span></a> defines various
equivalences between sequences of characters for various purposes.
Unicode Standard Annex #15 <a class='info' href='#UTR15'>[UTR15]<span> (</span><span class='info'>Davis, M. and M. Duerst, “Unicode Normalization Forms,” April 2003.</span><span>)</span></a> 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.
</p>
<p>
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 <a class='info' href='#RFC3629'>[RFC3629]<span> (</span><span class='info'>Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.</span><span>)</span></a> and then
pct-encode any octets that are not allowed in a URI reference.
</p>
<a name="syntax"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2"></a><h3>2.
Syntax</h3>
<p>
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 ('{', '}').
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>URI-Template</dfn> = <span class='rep'>*</span>( <cite class='id'>literals</cite> / <cite class='id'>expression</cite> )
</pre></div>
<p>
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.
</p>
<a name="literals"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2.1"></a><h3>2.1.
Literals</h3>
<p>
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.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>literals</dfn> = <span class='val'>%x21</span> / <span class='val'>%x23-24</span> / <span class='val'>%x26</span> / <span class='val'>%x28-3B</span> / <span class='val'>%x3D</span> / <span class='val'>%x3F-5B</span>
/ <span class='val'>%x5D-5F</span> / <span class='val'>%x61-7A</span> / <span class='val'>%x7E</span> / <cite class='id'>ucschar</cite> / <cite class='id'>iprivate</cite>
/ <cite class='id'>pct-encoded</cite>
<em>; any Unicode character except: CTL, SP,</em>
<em>; DQUOTE, "'", "%" (aside from pct-encoded),</em>
<em>; "<", ">", "\", "^", "`", "{", "|", "}"</em>
</pre></div>
<a name="expressions"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2.2"></a><h3>2.2.
Expressions</h3>
<p>
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.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>expression</dfn> = "<span class='str'>{</span>" [ <cite class='id'>operator</cite> ] <cite class='id'>variable-list</cite> "<span class='str'>}</span>"
<dfn>operator</dfn> = "<span class='str'>+</span>" / "<span class='str'>#</span>" / "<span class='str'>.</span>" / "<span class='str'>/</span>" / "<span class='str'>;</span>" / "<span class='str'>?</span>" / "<span class='str'>&</span>"
/ <cite class='id'>op-reserve</cite>
<dfn>op-reserve</dfn> = "<span class='str'>=</span>" / "<span class='str'>,</span>" / "<span class='str'>!</span>" / "<span class='str'>@</span>" / "<span class='str'>|</span>"
<em>; reserved for local use: "$" / "(" / ")"</em>
</pre></div>
<p>
The operator characters have been chosen to reflect each of their roles
as reserved characters in the URI generic syntax. The operators defined
in <a class='info' href='#expansion'>Section 3<span> (</span><span class='info'>Expansion</span><span>)</span></a> of this specification include:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
+ 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.
</pre></div>
<p>
The operator characters equals ("="), comma (","), exclamation ("!"),
at-sign ("@"), and pipe ("|") are reserved for future extensions.
</p>
<p>
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.
</p>
<a name="variables"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2.3"></a><h3>2.3.
Variables</h3>
<p>
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).
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>variable-list</dfn> = <cite class='id'>varspec</cite> <span class='rep'>*</span>( "<span class='str'>,</span>" <cite class='id'>varspec</cite> )
<dfn>varspec</dfn> = <cite class='id'>varname</cite> [ <cite class='id'>modifier</cite> ]
<dfn>varname</dfn> = <cite class='id'>varchar</cite> <span class='rep'>*</span>( <cite class='id'>varchar</cite> / "<span class='str'>.</span>" )
<dfn>varchar</dfn> = <cite class='key'>ALPHA</cite> / <cite class='key'>DIGIT</cite> / "<span class='str'>_</span>" / <cite class='id'>pct-encoded</cite>
</pre></div>
<p>
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.
</p>
<p>
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.
</p>
<p>
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
</p>
<p>
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.