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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;
}
h1 { color: #900; background-color: transparent; text-align: right; }
h3 { color: #333; background-color: transparent; }
td.RFCbug {
font-size: x-small; text-decoration: none;
width: 30px; height: 30px; padding-top: 2px;
text-align: justify; vertical-align: middle;
background-color: #000;
}
td.RFCbug span.RFC {
font-family: monaco, charcoal, geneva, "MS Sans Serif", helvetica, verdana, sans-serif;
font-weight: bold; color: #666;
}
td.RFCbug span.hotText {
font-family: charcoal, monaco, geneva, "MS Sans Serif", helvetica, verdana, sans-serif;
font-weight: normal; text-align: center; color: #FFF;
}
table.TOCbug { width: 30px; height: 15px; }
td.TOCbug {
text-align: center; width: 30px; height: 15px;
color: #FFF; background-color: #900;
}
td.TOCbug a {
font-family: monaco, charcoal, geneva, "MS Sans Serif", helvetica, sans-serif;
font-weight: bold; font-size: x-small; text-decoration: none;
color: #FFF; background-color: transparent;
}
td.header {
font-family: arial, helvetica, sans-serif; font-size: x-small;
vertical-align: top; width: 33%;
color: #FFF; background-color: #666;
}
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;
text-decoration: none;
}
a.info:hover {
z-index: 25;
color: #FFF; background-color: #900;
}
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. */
table.all, table.full, table.headers, table.none {
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vertical-align: top; border-collapse: collapse;
}
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table.all td {
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}
table.full td, table.headers td, table.none td { border-style: none; }
hr { height: 1px; }
hr.insert {
width: 80%; border-style: none; border-width: 0;
color: #CCC; background-color: #CCC;
}
--></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, Ed.</td></tr>
<tr><td class="header">Expires: January 12, 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">Oracle</td></tr>
<tr><td class="header"> </td><td class="header">M. Nottingham</td></tr>
<tr><td class="header"> </td><td class="header">D. Orchard</td></tr>
<tr><td class="header"> </td><td class="header">Jul 11, 2011</td></tr>
</table></td></tr></table>
<h1><br />URI Template<br />draft-gregorio-uritemplate-05</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 January 12, 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>
<p>
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.</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="#component-values">2.4.2.</a>
Component Values<br />
<a href="#defaults">2.5.</a>
Value Defaults<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="#undef-expansion">3.2.1.</a>
Undefined 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-dot">3.2.4.</a>
Label expansion with dot-prefix: {.var}<br />
<a href="#op-slash">3.2.5.</a>
Path segment expansion: {/var}<br />
<a href="#op-semicolon">3.2.6.</a>
Path-style parameter expansion: {;var}<br />
<a href="#op-question">3.2.7.</a>
Form-style query expansion: {?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="#parser">Appendix A.</a>
Example URI Template Parser<br />
<a href="#revisions">Appendix B.</a>
Revision History (to be removed by RFC Editor)<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=dog&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, a simple instruction
on how to construct a URI corresponding to those values.
A URI Template is transformed into a URI-reference by replacing each
delimited expression with its value as defined by the
expression type and the values of variables named within the
expression. The expression types range from simple string
expansion to multiple key=value lists. The expansions
are based on the URI generic syntax, allowing an implementation
to process any URI Template without knowing the scheme-specific
requirements of every possible resulting URI.
</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>
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.
</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 rendered as an IRI by transforming
each of the pct-encoded sequences to their corresponding Unicode
character if that character is not in the reserved set.
</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!" |
| empty := "" |
| undef := null |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | Simple string expansion (Sec 3.2.2) |
| | |
| | {var} value |
| | {hello} Hello%20World%21 |
| | O{empty}X OX |
| | O{undef}X OX |
`-----------------------------------------------------------------'
</pre></div>
<p>
Level 2 templates add the ability to define a default string, which is
substituted if the variable is undefined, and the plus ("+") operator
for expansion values that are allowed to include reserved characters.
</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!" |
| empty := "" |
| undef := null |
| path := "/foo/bar" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with defaults (Sec 3.2.2) |
| | |
| | {var|default} value |
| | O{empty|default}X OX |
| | O{undef|default}X OdefaultX |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with defaults (Sec 3.2.3) |
| | |
| | {+var} value |
| | {+hello} Hello%20World! |
| | {+path}/here /foo/bar/here |
| | here?ref={+path} here?ref=/foo/bar |
| | up{+path}{x}/here up/foo/bar1024/here |
| | up{+empty|/1}/here up/here |
| | up{+undef|/1}/here up/1/here |
| | |
`-----------------------------------------------------------------'
</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 key=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 := "" |
| undef := null |
| path := "/foo/bar" |
| x := "1024" |
| y := "768" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with multiple variables (Sec 3.2.2) |
| | |
| | {x,y} 1024,768 |
| | {x,hello,y} 1024,Hello%20World%21,768 |
| | ?{x,empty} ?1024, |
| | ?{x,undef} ?1024 |
| | ?{undef,y} ?768 |
| | ?{x,undef|0} ?1024,0 |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with multiple variables (Sec 3.2.3) |
| | |
| | {+x,hello,y} 1024,Hello%20World!,768 |
| | {+path,x}/here /foo/bar/1024/here |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var} X.value |
| | X{.empty} X. |
| | X{.undef} X |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
| | |
| | {/var} /value |
| | {/var,empty} /value/ |
| | {/var,undef} /value |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.6) |
| | |
| | {;x,y} ;x=1024;y=768 |
| | {;x,y,empty} ;x=1024;y=768;empty |
| | {;x,y,undef} ;x=1024;y=768 |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?x,y} ?x=1024&y=768 |
| | {?x,y,empty} ?x=1024&y=768&empty= |
| | {?x,y,undef} ?x=1024&y=768 |
| | |
`-----------------------------------------------------------------'
</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 tells
the expansion process to treat the value as a multivalued structure ---
a list of values or key=value pairs -- rather than as a single string.
</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", ".")] |
| |
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with value modifiers (Sec 3.2.2) |
| | |
| | {var:3} val |
| | {var:30} value |
| | {list} red,green,blue |
| | {list*} red,green,blue |
| | {keys} semi,%3B,dot,.,comma,%2C |
| | {keys*} semi=%3B,dot=.,comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with value modifiers (Sec 3.2.3) |
| | |
| | {+path:6}/here /foo/b/here |
| | {+list} red,green,blue |
| | {+list*} red,green,blue |
| | {+keys} semi,;,dot,.,comma,, |
| | {+keys*} semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var:3} X.val |
| | X{.list} X.red,green,blue |
| | X{.list*} X.red.green.blue |
| | X{.keys} X.semi,%3B,dot,.,comma,%2C |
| | X{.keys*} X.semi=%3B.dot=..comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
| | |
| | {/var:1,var} /v/value |
| | {/list} /red,green,blue |
| | {/list*} /red/green/blue |
| | {/list*,path:4} /red/green/blue/%2Ffoo |
| | {/keys} /semi,%3B,dot,.,comma,%2C |
| | {/keys*} /semi=%3B/dot=./comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.6) |
| | |
| | {;hello:5} ;hello=Hello |
| | {;list} ;red,green,blue |
| | {;list*} ;red;green;blue |
| | {;keys} ;semi,%3B,dot,.,comma,%2C |
| | {;keys*} ;semi=%3B;dot=.;comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?var:3} ?var=val |
| | {?list} ?list=red,green,blue |
| | {?list*} ?list=red&list=green&list=blue |
| | {?keys} ?keys=semi,%3B,dot,.,comma,%2C |
| | {?keys*} ?semi=%3B&dot=.&comma=%2C |
| | |
`-----------------------------------------------------------------'
</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. 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.
</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 and will be able to cope with any errors that might
occur when the resulting URI is used for name resolution or access.
</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>
<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 MUST be
transformed into its corresponding sequence of normalized
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, in particular
Normalization Form KC (NFKC: Compatibility Decomposition followed
by Canonical Composition). The normalization form determines how
to consistently encode the equivalent strings.
</p>
<p>
In theory, all URI processing implementations, including template
processors, should use the same normalization form for generating
or handling a URI reference. In practice, they do not. However,
template processors transform the template literals and variable
values from characters to codepoints, expand the expressions, and
then transform the resulting codepoints back to characters again.
In order to promote consistent generation of URI references from
templates, template processors MUST ensure that the template and
each variable value is in NFKC when they are transformed to
a sequence of Unicode codepoints. The final step of template
processing will encode the expanded sequence of Unicode
codepoints, which should still be in NFKC, as UTF-8
<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 further pct-encode any octets
within the UTF-8 string that are not allowed in a URI.
</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>
<p>
Each character in a URI Template MUST be transcoded to an NFKC
Unicode codepoint prior to evaluation. If a character sequence
in the template consists of pct-encoded triplets corresponding
to a valid sequence of octets in UTF-8 that represent a character
not in the reserved set, then the triplets MUST be replaced with
their corresponding Unicode codepoint. In other words, pct-encoded
UTF-8 found within the template is decoded prior to further processing
unless the corresponding character is in the reserved set.
</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 translated literally to the URI-reference.
</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>
<p>
A sequence of pct-encoded literals that corresponds to a UTF-8
encoded character that is not within the reserved set will be
transcoded during template processing, as describe above. If
such a character is not in the unreserved set, it will be
re-pct-encoded during the final step of template processing.
Unreserved characters will therefore be normalized to their
unencoded form as a side-effect of template processing.
</p>
<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>" / <cite class='id'>op-reserve</cite>
<dfn>op-reserve</dfn> = "<span class='str'>,</span>" / "<span class='str'>!</span>" / "<span class='str'>@</span>"
<em>; reserved for local use: "$" / "(" / ")"</em>
</pre></div>
<p>
</p>
<p>
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 <a class='info' href='#expansion'>Section 3<span> (</span><span class='info'>Expansion</span><span>)</span></a>.
</p>
<p>
The operator characters comma (","), exclamation ("!"), and at-sign ("@")
are reserved for future extensions. A processor that unexpectedly
encounters such an extension operator SHOULD pass the expression
through unexpanded and MAY also indicate a warning to the invoking
application.
</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 string to use
for each key on key=value expansions.
</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> ] [ "<span class='str'>|</span>" <cite class='id'>default</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'>ucschar</cite> / <cite class='id'>iprivate</cite>
/ <cite class='id'>pct-encoded</cite>
</pre></div>
<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 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