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draft-gregorio-uritemplate-06.xml
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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY rfc2119 SYSTEM 'bibxml/reference.RFC.2119.xml'>
<!ENTITY rfc3986 SYSTEM 'bibxml/reference.RFC.3986.xml'>
<!ENTITY rfc3987 SYSTEM 'bibxml/reference.RFC.3987.xml'>
<!ENTITY rfc5234 SYSTEM 'bibxml/reference.RFC.5234.xml'>
<!ENTITY rfc2978 SYSTEM 'bibxml/reference.RFC.2978.xml'>
<!ENTITY rfc3629 SYSTEM 'bibxml/reference.RFC.3629.xml'>
<!ENTITY ASCII SYSTEM 'bibxml/_reference.ANSI.X3-4.1986.xml'>
<!ENTITY UNIV4 SYSTEM 'bibxml/univ4.xml'>
<!ENTITY UTR15 SYSTEM 'bibxml/utr15.xml'>
]>
<!--<?xml-stylesheet type='text/xsl' href='rfc2629.xslt'/ ?>-->
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes"?>
<?rfc iprnotified="yes" ?>
<?rfc strict="yes" ?>
<?rfc compact="yes" ?>
<?rfc comments="yes" ?>
<?rfc inline="yes" ?>
<?rfc linkmailto="no" ?>
<?rfc tocdepth="3" ?>
<!--
1. Update the docName
2. Update the date
3. Update the Revision History.
-->
<rfc category="std" ipr="pre5378Trust200902" docName="draft-gregorio-uritemplate-06">
<front>
<title>URI Template</title>
<author initials='J.C.' surname="Gregorio" fullname='Joe Gregorio'>
<organization>Google</organization>
<address>
<email>[email protected]</email>
<uri>http://bitworking.org/</uri>
</address>
</author>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address>
<email>[email protected]</email>
<uri>http://roy.gbiv.com/</uri>
</address>
</author>
<author initials='M.H.' surname="Hadley" fullname='Marc Hadley'>
<organization>Oracle</organization>
<address>
<email>[email protected]</email>
<uri>http://oracle.com/</uri>
</address>
</author>
<author initials='M.N.' surname="Nottingham" fullname='Mark Nottingham'>
<organization></organization>
<address>
<email>[email protected]</email>
<uri>http://mnot.net/</uri>
</address>
</author>
<author initials='D.O.' surname="Orchard" fullname='David Orchard'>
<organization></organization>
<address>
<uri>http://www.pacificspirit.com/</uri>
</address>
</author>
<date day="20" month="Aug" year="2011"/>
<abstract>
<t>
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.
</t>
</abstract>
<note title="Editorial Note (to be removed by RFC Editor)">
<t>
To provide feedback on this Internet-Draft, join the
<eref target="http://lists.w3.org/Archives/Public/uri/">W3C URI
mailing list (http://lists.w3.org/Archives/Public/uri/)</eref>.
</t>
</note>
</front>
<middle>
<section title="Introduction" anchor="intro">
<section title="Overview" anchor="overview">
<t>
A Uniform Resource Identifier (URI) <xref target="RFC3986"/>
is often used to identify a specific resource within a common
space of similar resources. For example, personal web spaces
are often delegated using a common pattern, such as
</t>
<figure><artwork>
http://example.com/~fred/
http://example.com/~mark/
</artwork></figure>
<t>
or a set of dictionary entries might be grouped in a hierarchy
by the first letter of the term, as in
</t>
<figure><artwork>
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
</artwork></figure>
<t>
or a service interface might be invoked with various user input
in a common pattern, as in
</t>
<figure><artwork>
http://example.com/search?q=cat&lang=en
http://example.com/search?q=chien&lang=fr
</artwork></figure>
<t>
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:
</t>
<figure><artwork>
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
</artwork></figure>
<t>
We define the following terms:
<list style="symbols">
<t>expression -
The text between '{' and '}', including the enclosing braces,
as defined in <xref target="syntax"/>.</t>
<t>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 <xref target="expansion"/>.</t>
<t>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.</t>
</list>
</t>
<t>
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 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.
</t>
<t>
For example, the following URI Template includes a
form-style parameter expression, as indicated by the "?" operator
appearing before the variable names.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
</artwork></figure>
<t>
The expansion process for expressions beginning with the
question-mark ("?") operator follows the same pattern
as form-style interfaces on the World Wide Web:
</t>
<figure><artwork>
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.
</artwork></figure>
<t>
If the variables have the values
</t>
<figure><artwork>
query := "mycelium"
number := 100
</artwork></figure>
<t>
then the expansion of the above URI Template is
</t>
<figure><artwork>
http://www.example.com/foo?query=mycelium&number=100
</artwork></figure>
<t>
Alternatively, if 'query' is undefined, then the expansion
would be
</t>
<figure><artwork>
http://www.example.com/foo?number=100
</artwork></figure>
<t>
or if both variables are undefined, then it would be
</t>
<figure><artwork>
http://www.example.com/foo
</artwork></figure>
<t>
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.
</t>
<t>
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 <xref target="RFC3987"/>.
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 <xref target="RFC3987"/>.
</t>
</section>
<section title="Levels and Expression Types" anchor="types">
<t>
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.
</t>
<t>
Since most template processors implemented prior to this specification
have only implemented the default expression type, we refer to these as
Level 1 templates.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| 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 |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
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.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| 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! |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
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.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| 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= |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
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.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 4 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| list := [ "red", "green", "blue" ] |
| keys := [("semi",";"),("dot","."),("comma",",")] |
| empty_keys := [] |
| |
| 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*} ?list=red&list=green&list=blue |
| | {?keys} ?keys=semi,%3B,dot,.,comma,%2C |
| | {?keys*} ?semi=%3B&dot=.&comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| & | Form-style query continuation (Sec 3.2.9) |
| | |
| | {&var:3} &var=val |
| | {&list} &list=red,green,blue |
| | {&list*} &list=red&list=green&list=blue |
| | {&keys} &keys=semi,%3B,dot,.,comma,%2C |
| | {&keys*} &semi=%3B&dot=.&comma=%2C |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
</section>
<section title="Design Considerations" anchor="design">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Limitations" anchor="limitations">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Notational Conventions" anchor="notation">
<t>
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 <xref target="RFC2119"/>.
</t>
<t>
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <xref target="RFC5234"/>. The following ABNF rules
are imported from the normative references <xref target="RFC5234"/>,
<xref target="RFC3986"/>, and <xref target="RFC3987"/>.
</t>
<figure><artwork type="abnf">
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
</artwork></figure>
</section>
<section title='Character Encoding and Unicode Normalization'
anchor="unicode-normal">
<t>
This specification uses the terms "character" and "coded
character set" in accordance with the definitions provided
in <xref target="RFC2978"/>, and "character encoding" in
place of what <xref target="RFC2978"/> refers to as a "charset".
</t>
<t>
The ABNF notation defines its terminal values to be
non-negative integers (codepoints) that are a superset of the
US-ASCII coded character set <xref target="ASCII"/>. This
specification defines terminal values as codepoints within the
Unicode coded character set <xref target="UNIV4"/>.
</t>
<t>
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.
</t>
<t>
The Unicode Standard <xref target="UNIV4"/> defines various
equivalences between sequences of characters for various purposes.
Unicode Standard Annex #15 <xref target="UTR15"/> defines various
Normalization Forms for these equivalences. The normalization form
determines how to consistently encode equivalent strings.
In theory, all URI processing implementations, including template
processors, should use the same normalization form for generating
a URI reference. In practice, they do not. If a value has been
provided by the same server as the resource, then it can be assumed
that the string is already in the form expected by that server.
If a value is provided by a user, such as via a data-entry dialog,
then the string SHOULD be normalized as Normalization Form C
(NFC: Canonical Decomposition, followed by Canonical Composition)
prior to being used in expansions by a template processor.
</t>
<t>
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 <xref target="RFC3629"/> and then
pct-encode any octets that are not allowed in a URI reference.
</t>
</section>
</section>
<section title="Syntax" anchor="syntax">
<t>
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 ('{', '}').
</t>
<figure><artwork type="abnf">
URI-Template = *( literals / expression )
</artwork></figure>
<t>
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.
</t>
<section title="Literals" anchor="literals">
<t>
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.
</t>
<figure><artwork type="abnf">
literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
/ %x5D-5F / %x61-7A / %x7E / ucschar / iprivate
/ pct-encoded
; any Unicode character except: CTL, SP,
; DQUOTE, "'", "%" (aside from pct-encoded),
; "<", ">", "\", "^", "`", "{", "|", "}"
</artwork></figure>
</section>
<section title="Expressions" anchor="expressions">
<t>
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.
</t>
<figure><artwork type="abnf">
expression = "{" [ operator ] variable-list "}"
operator = "+" / "#" / "." / "/" / ";" / "?" / "&"
/ op-reserve
op-reserve = "=" / "," / "!" / "@" / "|"
; reserved for local use: "$" / "(" / ")"
</artwork></figure>
<t>
The operator characters have been chosen to reflect each of their roles
as reserved characters in the URI generic syntax. The operators defined
in <xref target="expansion"/> of this specification include:
</t>
<figure><artwork>
+ Reserved character strings;
# Fragment identifiers prefixed by "#";
. Name labels or extensions prefixed by ".";
/ Path segments prefixed by "/";
; Path parameter key or key=value pairs prefixed by ";";
? Query component beginning with "?" and consisting of
key=value pairs separated by "&"; and,
& Continuation of query-style &key=value pairs within
a literal query component.
</artwork></figure>
<t>
The operator characters equals ("="), comma (","), exclamation ("!"),
at-sign ("@"), and pipe ("|") are reserved for future extensions.
</t>
<t>
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.
</t>
</section>
<section title="Variables" anchor="variables">
<t>
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).
</t>
<figure><artwork type="abnf">
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / pct-encoded
</artwork></figure>
<t>
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.
</t>
<t>
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.
</t>
<t>
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
</t>
<t>
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.
</t>
</section>
<section title="Value Modifiers" anchor="modifiers">
<t>
Each of the variables in a Level 4 template expression can have
a modifier indicating either that its expansion is limited to a
prefix of the variable's value string or that its expansion is
exploded as a composite value in the form of a value list or
an associative array of (name, value) pairs.
</t>
<figure><artwork type="abnf">
modifier = prefix / explode
</artwork></figure>
<section title="Prefix Values" anchor="prefix-values">
<t>
A prefix modifier indicates that the variable expansion is limited
to a prefix of the variable's value string. Prefix modifiers are
often used to partition an identifier space hierarchically, as is
common in reference indices and hash-based storage. It also serves
to limit the expanded value to a maximum number of characters.
Prefix modifiers are not applicable to variables that have composite
values.
</t>
<figure><artwork type="abnf">
prefix = ":" max-length
max-length = %x31-39 *DIGIT ; positive integer
</artwork></figure>
<t>
The max-length is a positive integer that refers to a maximum number
of characters from the beginning of the variable's value as a
Unicode string.
Note that this numbering is in characters, not octets, in order to
avoid splitting between the octets of a multi-octet UTF-8 encoded
character or within a pct-encoded triplet.
If the max-length is greater than the length of the variable's
value, then the entire value string is used.
</t>
<figure><preamble>For example,</preamble><artwork>
Given the variable assignments
var := "value"
semi := ";"
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{semi} %3B
{semi:2} %3B
</artwork></figure>
</section>
<section title="Composite Values" anchor="composite-values">
<t>
An explode modifier ("*") indicates that the
variable represents a composite value that may be substituted
in full or partial forms, depending on the variable's type and
value set. 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.
</t>
<figure><artwork type="abnf">
explode = "*"
</artwork></figure>
<t>
Explode 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
</t>
<figure><artwork>
/mapper{?address*}
</artwork></figure>
<t>
along with some context that defines what the variable named
"address" can include, such as by reference to some other
standard for addressing (e.g., UPU S42 or AS/NZS 4819:2003).
A recipient aware of the schema can then provide appropriate
expansions, such as:
</t>
<figure><artwork>
/mapper?city=Newport%20Beach&state=CA
</artwork></figure>
<t>
The expansion process for exploded variables is dependent on
both the operator being used and whether the composite value is to
be treated as a list of values or as an associative array of
(name, value) pairs. Structures are processed as if they are an
associative array with names corresponding to the fields in the
structure definition and "." separators used to indicate name
hierarchy in substructures.
</t>
</section>
</section>
</section>
<section title="Expansion" anchor="expansion">
<t>
The process of URI Template expansion is to scan the template
string from beginning to end, copying literal characters
and replacing each expression with the result of applying the
expression's operator to the value of each variable named in
the expression. Each variable's value MUST be formed prior to
template expansion.
</t>
<t>
If a template processor encounters an error outside of an expression,
such as a character sequence that does not match the <URI-Template>
grammar, then processing of the template SHOULD cease, the URI-reference
result SHOULD be undefined, and the location and type of error SHOULD be
indicated to the invoking application. If an error is encountered inside
an expression, such as an operator or value modifier that it does not
recognize or cannot support, then the expression SHOULD be copied to
the result unexpanded, processing of the remainder of the template
SHOULD continue, and the location and type of error SHOULD be indicated
to the invoking application. In this latter case, the result returned
will not be a valid URI reference; it will be an incompletely expanded
template string that is only intended for diagnostic use.
</t>
<section title='Literal Expansion' anchor="literal-expansion">
<t>
If the literal character is allowed anywhere in the URI
syntax (unreserved / reserved / pct-encoded ), then it is copied
directly to the result string. Otherwise, the pct-encoded equivalent
of the literal character is copied to the result string by
encoding the character in UTF-8 (a sequence of octets) and
then encoding each octet as a pct-encoded triplet.
</t>
</section>
<section title='Expression Expansion' anchor="expression-expansion">
<t>
Each expression is indicated by an opening brace ("{") character
and continues until the next closing brace ("}"). The expression
is expanded by determining the expression type and then following
that type's expansion process for each comma-separated varspec
in the expression. Level 1 templates are limited to the default
operator (simple string value expansion) and a single variable
per expression. Level 2 templates are limited to a single
varspec per expression.
</t>
<t>
The expression type is determined by looking at the first
character after the opening brace. If the character is an
operator, then remember the expression type associated with
that operator for later expansion decisions and skip to the
next character for the variable-list. If the first character
is not an operator, then the expression type is simple string
expansion and the first character is the beginning of the
variable-list.
</t>
<figure><preamble>The examples in the subsections below use
the following definitions for variable values:</preamble><artwork>
dom := "example.com"
dub := "me/too"
foo := "That's right!"
hello := "Hello World!"
half := "50%"
var := "value"
who := "fred"
base := "http://example.com/home/"
path := "/foo/bar"
list := [ "red", "green", "blue" ]
keys := [("semi",";"),("dot","."),("comma",",")]
v := "6"
x := "1024"
y := "768"
empty := ""
empty_keys := []
undef := null
</artwork></figure>
<section title='Variable Expansion' anchor="variable-expansion">
<t>
A variable that is undefined has no value and is ignored by the
expansion process. A variable defined as a list value is considered
undefined if the list contains zero members. A variable defined as
an associative array of (name, value) pairs is considered undefined
if the array contains zero members or if all member names in the array
have undefined values.
If all of the variables in an expression are undefined, then the
expression's expansion is the empty string.
</t>
<t>
Variable expansion of a defined, non-empty value results in a
substring of allowed URI characters. A template processor MUST
encode the value string as UTF-8 and transform each octet that
is not in the allowed set into the corresponding pct-encoded triplet.
The allowed set depends on the expression type: reserved ("+")
and fragment ("#") expansions allow the set of characters in
( unreserved / reserved / pct-encoded ) to be passed through without
pct-encoding, whereas all other expression types allow only unreserved
characters to be passed through without pct-encoding. Note that
the percent character ("%") is only allowed as part of a pct-encoded
triplet and only for reserved/fragment expansion: in all other cases,
a value of "%" MUST be pct-encoded as "%25" by variable expansion.
</t>
<t>
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). However, if reserved
characters or pct-encoded triplets occur in the value, they will be
pct-encoded by some expression types and not by others.
</t>
<t>
For a variable that is a simple string value, expansion consists
of appending the encoded value to the result string. The explode
modifier has no effect. The prefix modifier limits the expansion to
the first max-length characters of the decoded value. If the value
contains multibyte UTF-8, care must be taken to avoid splitting the
value in mid-character: count each Unicode codepoint as one character.
</t>
<t>
For a variable that is a list of values, expansion consists of