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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 v1.34 (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; }
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;
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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;
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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;
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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; }
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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, Ed.</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: September 9, 2010</td><td class="header">Day Software</td></tr>
<tr><td class="header"> </td><td class="header">M. Hadley, Ed.</td></tr>
<tr><td class="header"> </td><td class="header">Oracle</td></tr>
<tr><td class="header"> </td><td class="header">M. Nottingham, Ed.</td></tr>
<tr><td class="header"> </td><td class="header">D. Orchard</td></tr>
<tr><td class="header"> </td><td class="header">Mar 08, 2010</td></tr>
</table></td></tr></table>
<h1><br />URI Template<br />draft-gregorio-uritemplate-04</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, 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 to IETF 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), its areas, and its working groups.
Note that other groups may also distribute working documents as
Internet-Drafts.</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>
The list of current Internet-Drafts can be accessed at
<a href='http://www.ietf.org/ietf/1id-abstracts.txt'>http://www.ietf.org/ietf/1id-abstracts.txt</a>.</p>
<p>
The list of Internet-Draft Shadow Directories can be accessed at
<a href='http://www.ietf.org/shadow.html'>http://www.ietf.org/shadow.html</a>.</p>
<p>
This Internet-Draft will expire on September 9, 2010.</p>
<h3>Copyright Notice</h3>
<p>
Copyright (c) 2010 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your
rights and restrictions with respect to this document.</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>
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="#syntax">2.</a>
URI Template 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="#component-values">2.4.1.</a>
Component Values<br />
<a href="#partial-values">2.4.2.</a>
Prefix and Suffix Values<br />
<a href="#defaults">2.5.</a>
Value Defaults<br />
<a href="#expansion">3.</a>
URI Template Expansion<br />
<a href="#unicode-normal">3.1.</a>
Unicode normalization<br />
<a href="#literal-expansion">3.2.</a>
Literal expansion<br />
<a href="#expression-expansion">3.3.</a>
Expression expansion<br />
<a href="#variable-expansion">3.4.</a>
Variable and modifier expansion<br />
<a href="#op-default">3.5.</a>
Simple expansion: {var}<br />
<a href="#op-plus">3.6.</a>
Reserved expansion: {+var}<br />
<a href="#op-semicolon">3.7.</a>
Path-style parameter expansion: {;var}<br />
<a href="#op-question">3.8.</a>
Form-style parameter expansion: {?var}<br />
<a href="#op-slash">3.9.</a>
Hierarchical path expansion: {/var}<br />
<a href="#op-dot">3.10.</a>
Label expansion with dot-prefix: {.var}<br />
<a href="#examples">4.</a>
Examples<br />
<a href="#anchor1">5.</a>
Security Considerations<br />
<a href="#iana">6.</a>
IANA Considerations<br />
<a href="#ack">7.</a>
Acknowledgments<br />
<a href="#rfc.references1">8.</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'>URI Template 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'>URI Template 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 value
expansion to multiple key=value lists. The expansions
are based on the URI generic syntax, allowing an implementation
to process any URI Template without knowing the scheme-specific
requirements of every possible resulting URI.
</p>
<p>
A URI Template may be provided in absolute form, as in the examples
above, or in relative form if a suitable base URI is defined.
</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
the pct-encoded sequences to their corresponding Unicode
character if the 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.
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. 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>
The following table summarizes each type of template
expression by its associated operator and cross-references
the section of this document that defines the operator and
its specific expansion process. The example expansions are
based on the following variables and values:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
var := "value";
hello := "Hello World!";
undef := null;
empty := "";
list := [ "val1", "val2", "val3" ];
keys := [("key1", "val1"), ("key2", "val2")];
path := "/foo/bar"
x := "1024";
y := "768";
.-----------------------------------------------------------------.
| Sec | Op | Description |
| | | Expression | Expansion |
|-----+-----+-----------------------------------------------------|
| 3.3 | | Simple expansion with comma-separated values |
| | | |
| | | {var} value |
| | | {hello} Hello%20World%21 |
| | | {path}/here %2Ffoo%2Fbar/here |
| | | {x,y} 1024,768 |
| | | {var=default} value |
| | | {undef=default} default |
| | | {list} val1,val2,val3 |
| | | {list*} val1,val2,val3 |
| | | {list+} list.val1,list.val2,list.val3 |
| | | {keys} key1,val1,key2,val2 |
| | | {keys*} key1,val1,key2,val2 |
| | | {keys+} keys.key1,val1,keys.key2,val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.4 | + | Reserved expansion with comma-separated values |
| | | |
| | | {+var} value |
| | | {+hello} Hello%20World! |
| | | {+path}/here /foo/bar/here |
| | | {+path,x}/here /foo/bar,1024/here |
| | | {+path}{x}/here /foo/bar1024/here |
| | | {+empty}/here /here |
| | | {+undef}/here /here |
| | | {+list} val1,val2,val3 |
| | | {+list*} val1,val2,val3 |
| | | {+list+} list.val1,list.val2,list.val3 |
| | | {+keys} key1,val1,key2,val2 |
| | | {+keys*} key1,val1,key2,val2 |
| | | {+keys+} keys.key1,val1,keys.key2,val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.5 | ; | Path-style parameters, semicolon-prefixed |
| | | |
| | | {;x,y} ;x=1024;y=768 |
| | | {;x,y,empty} ;x=1024;y=768;empty |
| | | {;x,y,undef} ;x=1024;y=768 |
| | | {;list} ;val1,val2,val3 |
| | | {;list*} ;val1;val2;val3 |
| | | {;list+} ;list=val1;list=val2;list=val3 |
| | | {;keys} ;key1,val1,key2,val2 |
| | | {;keys*} ;key1=val1;key2=val2 |
| | | {;keys+} ;keys.key1=val1;keys.key2=val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.6 | ? | Form-style parameters, ampersand-separated |
| | | |
| | | {?x,y} ?x=1024&y=768 |
| | | {?x,y,empty} ?x=1024&y=768&empty= |
| | | {?x,y,undef} ?x=1024&y=768 |
| | | {?list} ?list=val1,val2,val3 |
| | | {?list*} ?val1&val2&val3 |
| | | {?list+} ?list=val1&list=val2&list=val3 |
| | | {?keys} ?keys=key1,val1,key2,val2 |
| | | {?keys*} ?key1=val1&key2=val2 |
| | | {?keys+} ?keys.key1=val1&keys.key2=val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.7 | / | Hierarchical path segments, slash-separated |
| | | |
| | | {/var} /value |
| | | {/var,empty} /value/ |
| | | {/var,undef} /value |
| | | {/list} /val1,val2,val3 |
| | | {/list*} /val1/val2/val3 |
| | | {/list*,x} /val1/val2/val3/1024 |
| | | {/list+} /list.val1/list.val2/list.val3 |
| | | {/keys} /key1,val1,key2,val2 |
| | | {/keys*} /key1/val1/key2/val2 |
| | | {/keys+} /keys.key1/val1/keys.key2/val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.8 | . | Label expansion, dot-prefixed |
| | | |
| | | X{.var} X.value |
| | | X{.empty} X. |
| | | X{.undef} X |
| | | X{.list} X.val1,val2,val3 |
| | | X{.list*} X.val1.val2.val3 |
| | | X{.list*,x} X.val1.val2.val3.1024 |
| | | X{.list+} X.list.val1.list.val2.list.val3 |
| | | X{.keys} X.key1,val1,key2,val2 |
| | | X{.keys*} X.key1.val1.key2.val2 |
| | | X{.keys+} X.keys.key1.val1.keys.key2.val2 |
| | | |
`-----+-----+-----------------------------------------------------'
</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>
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
simple URI Template expressions. If we were only concerned with
URI generation, then the template syntax could be limited to
just simple variable expansion, since more complex forms could
be generated by changing the variable values. However, URI
Templates have the additional goal of describing the layout of
identifiers in terms of preexisting data values. The template
syntax therefore includes operators that reflect how
resource identifiers are commonly allocated. Likewise, since
prefix and suffix substrings are often used to partition large
spaces of resources, modifiers on variable values provide a way
to specify those substrings.
</p>
<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.
</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 Template expressions are not URIs: they do not identify
an abstract or physical resource, they are not parsed as URIs,
and should not be used in places where a URI would be expected
unless the template expressions will be expanded by a template
processor prior to use. Distinct field, element, or attribute
names should be used to differentiate protocol elements that
carry a URI Template from those that expect a URI-reference.
</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>
<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>
A URI Template is defined as a sequence of characters and therefore
has the same issues as URIs with regard to codepoints and character sets.
That is, URI Template characters are frequently encoded as octets
for transport or presentation. This specification does not mandate
any particular character encoding for mapping between URI Template
characters and the octets used to store or transmit those characters.
When a URI Template appears in a protocol element, the character
encoding is defined by that protocol; without such a definition,
a URI Template is assumed to be in the same character encoding as
the surrounding text.
</p>
<p>
A URI Template and its associated variable values are converted
to a normal form of 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> prior to
template processing, as defined in <a class='info' href='#unicode-normal'>Section 3.1<span> (</span><span class='info'>Unicode normalization</span><span>)</span></a>.
</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>. Thus, a string
of characters in a URI Template is assumed to be transformed into
its corresponding sequence of Unicode codepoints prior to testing
for a match with the URI Template grammar.
</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.
URI Template 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>
<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>
<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'>URI Template Expansion</span><span>)</span></a>.
</p>
<p>
The operator characters pipe ("|"), 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 URI 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
syntax, but do have an impact on the expansion process. A
composite or structured value with zero member values is
considered undefined.
</p>
<p>
If a variable appears more than once in an expression or
within multiple expressions of a URI Template, the value of
that variable MUST remain static throughout the expansion
process (i.e., the variable must have the same value for the
purpose of calculating each expansion).
</p>
<a name="modifiers"></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.2.4"></a><h3>2.4.
Value Modifiers</h3>
<p>
Any of the variables can have a modifier indicating that its
value is exploded into components or is limited to a prefix,
suffix, or the remainder of a prefix or suffix of the variable value.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>modifier</dfn> = <cite class='id'>explode</cite> / <cite class='id'>partial</cite>
</pre></div>
<a name="component-values"></a><br /><hr />
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<a name="rfc.section.2.4.1"></a><h3>2.4.1.
Component Values</h3>
<p>
The explode modifiers ("*" and "+") indicate that the
variable represents a composite value that may be substituted
in full or partial forms, depending on the variable's type or
schema. Since URI Templates do not contain an indication of
type or schema, this is assumed to be determined by context.
An example context is a mark-up element or header field that
contains one attribute that is a template and one or more other
attributes that define the schema applicable to variables found
in the template. Likewise, a typed programming language might
differentiate variables as strings, lists, associative arrays,
or structures.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>explode</dfn> = ( "<span class='str'>*</span>" / "<span class='str'>+</span>" )
</pre></div>
<p>
The primary difference between the two explode modifiers is
that an asterisk ("*") indicates that just the component names
and values are included in the expansion, whereas the plus
("+") indicates that each component name is prefixed with the
given variable name and a period ("."), thereby enabling multiple
variables with the same component names to be disambiguated.
</p>
<p>
Component modifiers improve brevity in the URI Template syntax.
For example, a resource that provides a geographic map for a
given street address might accept a hundred permutations on
fields for address input, including partial addresses (e.g.,
just the city or postal code). Such a resource could be
described as a template with each and every address component
listed in order, or with a far more simple template that makes
use of an explode modifier, as in
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
/mapper{?address*}
</pre></div>
<p>
or
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
/directions{?from+,to+}
</pre></div>
<p>
along with some context that defines each variable (address, from,
and to) as adhering to a given addressing standard (e.g., UPU S42
or AS/NZS 4819:2003). A recipient aware of the schema can then
provide appropriate expansions, such as:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
/mapper?city=Newport%20Beach&state=CA
/directions?from.zipcode=92660&to.zipcode=90210
</pre></div>
<p>
The expansion process for variables, as defined in
<a class='info' href='#expansion'>Section 3<span> (</span><span class='info'>URI Template Expansion</span><span>)</span></a>, is dependent on both the
operator being used and, if one of the explode modifiers is
used, the type and schema of the variable being substituted.
</p>
<a name="partial-values"></a><br /><hr />
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<a name="rfc.section.2.4.2"></a><h3>2.4.2.
Prefix and Suffix Values</h3>
<p>
Prefix and suffix modifiers are often used to partition an identifier
space hierarchically, as is common in reference indices and hash-based
storage, or to limit the substituted value to a maximum number of
characters.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>partial</dfn> = ( <cite class='id'>substring</cite> / <cite class='id'>remainder</cite> ) <cite class='id'>offset</cite>
<dfn>substring</dfn> = "<span class='str'>:</span>"
<dfn>remainder</dfn> = "<span class='str'>^</span>"
<dfn>offset</dfn> = [ <cite class='id'>from-end</cite> ] <span class='rep'>1*</span><cite class='key'>DIGIT</cite>
<dfn>from-end</dfn> = "<span class='str'>-</span>"
</pre></div>
<p>
The offset refers to a maximum number of characters from either
the beginning (prefix) or end (suffix) of the variable's value as
a Unicode string. Note that this numbering is in characters, not
octets, in order to avoid substituting improperly encoded values
due to splitting a multi-octet UTF-8 encoded character or a
pct-encoded triplet.
</p>
<p>
A substring modifier requires that only the indicated prefix or
suffix be used in the expansion. A remainder modifier requires
that only the remainder of the value, excluding the indicated prefix
or suffix, be used in the expansion. If the offset is greater than
the length of the variable's value, then the entire string is used
for a substring and the empty string is used for a remainder.
</p>
<p>
The following examples illustrate how modifiers work with the
different variable types. More complex examples are provided
in <a class='info' href='#examples'>Section 4<span> (</span><span class='info'>Examples</span><span>)</span></a>.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
Given the variable assignments:
var := "value";
name := [ "Fred", "Wilma", "Pebbles" ];
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{var^3} ue
{var:-3} lue
{var^-3} va
{?name} ?name=Fred,Wilma,Pebbles
{?name:1} ?name=F
</pre></div>
<a name="defaults"></a><br /><hr />
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<a name="rfc.section.2.5"></a><h3>2.5.
Value Defaults</h3>
<p>
Any of the variables may also be supplied with a default value
to be used when a template processor determines the variable to be
undefined. The default value is limited to the unreserved and
pct-encoded characters of a URI-reference, since our intention
is for the default to be presented in the exact form that it would
appear in the resulting URI. The default is not affected by the
variable modifiers; it is assumed that the default string
provided in the expression already reflects any necessary
substring or remainder processing.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
<dfn>default</dfn> = <span class='rep'>*</span>( <cite class='id'>unreserved</cite> / <cite class='id'>pct-encoded</cite> )
</pre></div>
<p>
The following examples illustrate how default values work with
different variable types. More complex examples are provided
in <a class='info' href='#examples'>Section 4<span> (</span><span class='info'>Examples</span><span>)</span></a>.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>