Network Working Group J. Gregorio
Internet-Draft Google
Intended status: Standards Track R. Fielding, Ed.
Expires: February 21, 2012 Adobe
M. Hadley
Oracle
M. Nottingham
D. Orchard
Aug 20, 2011
URI Template
draft-gregorio-uritemplate-06
Abstract
A URI Template is a compact sequence of characters for describing a
range of Uniform Resource Identifiers through variable expansion.
This specification defines the URI Template syntax and the process
for expanding a URI Template into a URI reference, along with
guidelines for the use of URI Templates on the Internet.
Editorial Note (to be removed by RFC Editor)
To provide feedback on this Internet-Draft, join the W3C URI mailing
list (http://lists.w3.org/Archives/Public/uri/) [1].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on February 21, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Levels and Expression Types . . . . . . . . . . . . . . . 6
1.3. Design Considerations . . . . . . . . . . . . . . . . . . 10
1.4. Limitations . . . . . . . . . . . . . . . . . . . . . . . 11
1.5. Notational Conventions . . . . . . . . . . . . . . . . . . 12
1.6. Character Encoding and Unicode Normalization . . . . . . . 13
2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1. Literals . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2. Expressions . . . . . . . . . . . . . . . . . . . . . . . 14
2.3. Variables . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4. Value Modifiers . . . . . . . . . . . . . . . . . . . . . 16
2.4.1. Prefix Values . . . . . . . . . . . . . . . . . . . . 16
2.4.2. Composite Values . . . . . . . . . . . . . . . . . . . 17
3. Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1. Literal Expansion . . . . . . . . . . . . . . . . . . . . 18
3.2. Expression Expansion . . . . . . . . . . . . . . . . . . . 18
3.2.1. Variable Expansion . . . . . . . . . . . . . . . . . . 19
3.2.2. Simple String Expansion: {var} . . . . . . . . . . . . 20
3.2.3. Reserved expansion: {+var} . . . . . . . . . . . . . . 21
3.2.4. Fragment expansion: {#var} . . . . . . . . . . . . . . 22
3.2.5. Label expansion with dot-prefix: {.var} . . . . . . . 23
3.2.6. Path segment expansion: {/var} . . . . . . . . . . . . 23
3.2.7. Path-style parameter expansion: {;var} . . . . . . . . 24
3.2.8. Form-style query expansion: {?var} . . . . . . . . . . 25
3.2.9. Form-style query continuation: {&var} . . . . . . . . 25
4. Security Considerations . . . . . . . . . . . . . . . . . . . 26
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
7. Normative References . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Implementation Hints . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
1.1. Overview
A Uniform Resource Identifier (URI) [RFC3986] is often used to
identify a specific resource within a common space of similar
resources. For example, personal web spaces are often delegated
using a common pattern, such as
http://example.com/~fred/
http://example.com/~mark/
or a set of dictionary entries might be grouped in a hierarchy by the
first letter of the term, as in
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
or a service interface might be invoked with various user input in a
common pattern, as in
http://example.com/search?q=cat&lang=en
http://example.com/search?q=chien&lang=fr
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:
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
We define the following terms:
o expression - The text between '{' and '}', including the enclosing
braces, as defined in Section 2.
o expansion - The string result obtained from a template expression
after processing it according to its expression type, list of
variable names, and value modifiers, as defined in Section 3.
o 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.
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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.
For example, the following URI Template includes a form-style
parameter expression, as indicated by the "?" operator appearing
before the variable names.
http://www.example.com/foo{?query,number}
The expansion process for expressions beginning with the question-
mark ("?") operator follows the same pattern as form-style interfaces
on the World Wide Web:
http://www.example.com/foo{?query,number}
\_____________/
|
|
For each defined variable in [ 'query', 'number' ],
substitute "?" if it is the first substitution or "&"
thereafter, followed by the variable name, '=', and the
variable's value.
If the variables have the values
query := "mycelium"
number := 100
then the expansion of the above URI Template is
http://www.example.com/foo?query=mycelium&number=100
Alternatively, if 'query' is undefined, then the expansion would be
http://www.example.com/foo?number=100
or if both variables are undefined, then it would be
http://www.example.com/foo
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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.
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 [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 [RFC3987].
1.2. Levels and Expression Types
URI Templates are similar to a macro language with a fixed set of
macro definitions: the expression type determines the expansion
process. The default expression type is simple string expansion,
wherein a single named variable is replaced by its value as a string
after UTF-8 encoding the characters and then pct-encoding any octets
that are not in the unreserved set.
Since most template processors implemented prior to this
specification have only implemented the default expression type, we
refer to these as Level 1 templates.
.-----------------------------------------------------------------.
| Level 1 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | Simple string expansion (Sec 3.2.2) |
| | |
| | {var} value |
| | {hello} Hello%20World%21 |
`-----------------------------------------------------------------'
Level 2 templates add the plus ("+") operator, for expansion of
values that are allowed to include reserved characters, and the
crosshatch ("#") operator for expansion of fragment identifiers.
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.-----------------------------------------------------------------.
| Level 2 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| + | Reserved string expansion (Sec 3.2.3) |
| | |
| | {+var} value |
| | {+hello} Hello%20World! |
| | {+path}/here /foo/bar/here |
| | here?ref={+path} here?ref=/foo/bar |
|-----+-----------------------------------------------------------|
| # | Fragment expansion, crosshatch-prefixed (Sec 3.2.4) |
| | |
| | X{#var} X#value |
| | X{#hello} X#Hello%20World! |
`-----------------------------------------------------------------'
Level 3 templates 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.
.-----------------------------------------------------------------.
| Level 3 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| path := "/foo/bar" |
| x := "1024" |
| y := "768" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with multiple variables (Sec 3.2.2) |
| | |
| | map?{x,y} map?1024,768 |
| | {x,hello,y} 1024,Hello%20World%21,768 |
| | |
|-----+-----------------------------------------------------------|
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| + | Reserved expansion with multiple variables (Sec 3.2.3) |
| | |
| | {+x,hello,y} 1024,Hello%20World!,768 |
| | {+path,x}/here /foo/bar,1024/here |
| | |
|-----+-----------------------------------------------------------|
| # | Fragment expansion with multiple variables (Sec 3.2.4) |
| | |
| | {#x,hello,y} #1024,Hello%20World!,768 |
| | {#path,x}/here #/foo/bar,1024/here |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.5) |
| | |
| | X{.var} X.value |
| | X{.x,y} X.1024.768 |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.6) |
| | |
| | {/var} /value |
| | {/var,x}/here /value/1024/here |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.7) |
| | |
| | {;x,y} ;x=1024;y=768 |
| | {;x,y,empty} ;x=1024;y=768;empty |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.8) |
| | |
| | {?x,y} ?x=1024&y=768 |
| | {?x,y,empty} ?x=1024&y=768&empty= |
| | |
|-----+-----------------------------------------------------------|
| & | Form-style query continuation (Sec 3.2.9) |
| | |
| | ?fixed=yes{&x} ?fixed=yes&x=1024 |
| | {&x,y,empty} &x=1024&y=768&empty= |
| | |
`-----------------------------------------------------------------'
Finally, Level 4 templates add 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,
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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.
.-----------------------------------------------------------------.
| 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 |
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| | X{.list*} X.red.green.blue |
| | X{.keys} X.semi,%3B,dot,.,comma,%2C |
| | X{.keys*} X.semi=%3B.dot=..comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.6) |
| | |
| | {/var:1,var} /v/value |
| | {/list} /red,green,blue |
| | {/list*} /red/green/blue |
| | {/list*,path:4} /red/green/blue/%2Ffoo |
| | {/keys} /semi,%3B,dot,.,comma,%2C |
| | {/keys*} /semi=%3B/dot=./comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.7) |
| | |
| | {;hello:5} ;hello=Hello |
| | {;list} ;list=red,green,blue |
| | {;list*} ;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 |
| | |
`-----------------------------------------------------------------'
1.3. Design Considerations
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
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Templates can be used consistently across multiple Internet
applications and within Internet message fields, while at the same
time retaining compatibility with those earlier definitions.
The URI Template syntax has been designed to carefully balance the
need for a powerful expansion mechanism with the need for ease of
implementation. The syntax is designed to be trivial to parse while
at the same time providing enough flexibility to express many common
template scenarios. Implementations are able to parse the template
and perform the expansions in a single pass.
Templates are simple and readable when used with common examples
because the single-character operators match the URI generic syntax
delimiters. The operator's associated delimiter (".", ";", "/", "?",
"&", and "#") is omitted when none of the listed variables are
defined. Likewise, the expansion process for ";" (path-style
parameters) will omit the "=" when the variable value is empty,
whereas the process for "?" (form-style parameters) will not omit the
"=" when the value is empty. Multiple variables and list values have
their values joined with "," if there is no predefined joining
mechanism for the operator. The "+" and "#" operators will
substitute unencoded reserved characters found inside the variable
values; the other operators will pct-encode reserved characters found
in the variable values prior to expansion.
The most common cases for URI spaces can be described with Level 1
template expressions. If we were only concerned with URI generation,
then the template syntax could be limited to just simple variable
expansion, since more complex forms could be generated by changing
the variable values. However, URI Templates have the additional goal
of describing the layout of identifiers in terms of preexisting data
values. 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.
1.4. Limitations
Since a URI Template describes a superset of the identifiers, there
is no implication that every possible expansion for each delimited
variable expression corresponds to a URI of an existing resource.
Our expectation is that an application constructing URIs according to
the template will be provided with an appropriate set of values for
the variables being substituted, or at least a means of validating
user data-entry for those values.
URI Templates are not URIs: they do not identify an abstract or
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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.
Some URI Templates can be used in reverse for the purpose of variable
matching: comparing the template to a fully formed URI in order to
extract the variable parts from that URI and assign them to the named
variables. Variable matching only works well if the template
expressions are delimited by the beginning or end of the URI or by
characters that cannot be part of the expansion, such as reserved
characters surrounding a simple string expression. In general,
regular expression languages are better suited for variable matching.
1.5. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234]. The following ABNF rules are imported from
the normative references [RFC5234], [RFC3986], and [RFC3987].
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
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1.6. Character Encoding and Unicode Normalization
This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"charset".
The ABNF notation defines its terminal values to be non-negative
integers (codepoints) that are a superset of the US-ASCII coded
character set [ASCII]. This specification defines terminal values as
codepoints within the Unicode coded character set [UNIV4].
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.
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences. The normalization form determines how to consistently
encode equivalent strings. In theory, all URI processing
implementations, including template processors, should use the same
normalization form for generating a URI reference. In practice, they
do not. If a value has been provided by the same server as the
resource, then it can be assumed that the string is already in the
form expected by that server. If a value is provided by a user, such
as via a data-entry dialog, then the string SHOULD be normalized as
Normalization Form C (NFC: Canonical Decomposition, followed by
Canonical Composition) prior to being used in expansions by a
template processor.
Likewise, when non-ASCII data that represents readable strings is
pct-encoded for use in a URI reference, a template processor MUST
first encode the string as UTF-8 [RFC3629] and then pct-encode any
octets that are not allowed in a URI reference.
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2. Syntax
A URI Template is a string of printable Unicode characters that
contains zero or more embedded variable expressions, each expression
being delimited by a matching pair of braces ('{', '}').
URI-Template = *( literals / expression )
Although templates (and template processor implementations) are
described above in terms of four gradual levels, we define the URI-
Template syntax in terms of the ABNF for Level 4. A template
processor limited to lower level templates MAY exclude the ABNF rules
applicable only to higher levels. However, it is RECOMMENDED that
all parsers implement the full syntax such that unsupported levels
can be properly identified as such to the end user.
2.1. Literals
The characters outside of expressions in a URI Template string are
intended to be copied literally to the URI-reference if the character
is allowed in a URI (reserved / unreserved / pct-encoded) or, if not
allowed, copied to the URI-reference in its UTF-8 pct-encoded form.
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),
; "<", ">", "\", "^", "`", "{", "|", "}"
2.2. Expressions
Template expressions are the parameterized parts of a URI Template.
Each expression contains an optional operator, which defines the
expression type and its corresponding expansion process, followed by
a comma-separated list of variable specifiers (variable names and
optional value modifiers). If no operator is provided, the
expression defaults to simple variable expansion of unreserved
values.
expression = "{" [ operator ] variable-list "}"
operator = "+" / "#" / "." / "/" / ";" / "?" / "&"
/ op-reserve
op-reserve = "=" / "," / "!" / "@" / "|"
; reserved for local use: "$" / "(" / ")"
The operator characters have been chosen to reflect each of their
roles as reserved characters in the URI generic syntax. The
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operators defined in Section 3 of this specification include:
+ Reserved character strings;
# Fragment identifiers prefixed by "#";
. Name labels or extensions prefixed by ".";
/ Path segments prefixed by "/";
; Path parameter 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.
The operator characters equals ("="), comma (","), exclamation ("!"),
at-sign ("@"), and pipe ("|") are reserved for future extensions.
The expression syntax specifically excludes use of the dollar ("$")
and parentheses ["(" and ")"] characters so that they remain
available for local language extensions outside the scope of this
specification.
2.3. Variables
After the operator (if any), each expression contains a list of one
or more comma-separated variable specifiers (varspec). The variable
names serve multiple purposes: documentation for what kinds of values
are expected, identifiers for associating values within a template
processor, and the literal string to use for the name in name=value
expansions (aside from when exploding an associative array).
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / pct-encoded
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.
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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.
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
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.
2.4. Value Modifiers
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.
modifier = prefix / explode
2.4.1. Prefix Values
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.
prefix = ":" max-length
max-length = %x31-39 *DIGIT ; positive integer
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.
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For example,
Given the variable assignments
var := "value"
semi := ";"
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{semi} %3B
{semi:2} %3B
2.4.2. Composite Values
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.
explode = "*"
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
/mapper{?address*}
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:
/mapper?city=Newport%20Beach&state=CA
The expansion process for exploded variables is dependent on both the
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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.
3. Expansion
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.
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.
3.1. Literal Expansion
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.
3.2. Expression Expansion
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.
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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.
The examples in the subsections below use the following definitions
for variable values:
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
3.2.1. Variable Expansion
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.
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,
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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.
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.
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.
For a variable that is a list of values, expansion consists of
concatenating the defined member string values, encoded as above,
with a separator string inserted between those values. A prefix
modifier has no effect. If no explode modifier is given, the
separator string is a comma (","). If an explode modifier is given,
the separator string is defined per operator by the following table,
where NUL is the default expression type:
.-----------------------------------------------------------------.
| operator: NUL + . / ; ? & # |
| separator: "," "," "." "/" ";" "&" "&" "," |
`-----------------------------------------------------------------'
For a variable that is an associative array, expansion consists of a
list of either "name,value" (without explode modifier) or
"name=value" (with explode modifier) pairs, excluding any pairs for
which the corresponding value is undefined, with a separator string
inserted between defined pairs. The separator string is defined in
the same way as for list variables above. Both the name and value
strings are encoded in the same way as simple string values.
3.2.2. Simple String Expansion: {var}
Simple string expansion is the default expression type when no
operator is given.
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For each defined variable in the variable-list, perform variable
expansion, as defined in Section 3.2.1, with the allowed characters
being those in the unreserved set. If more than one variable has a
defined value, append a comma (",") to the result string as a
separator between variable expansions.
Example Template Expansion
{var} value
{hello} Hello%20World%21
{half} 50%25
O{empty}X OX
O{undef}X OX
{x,y} 1024,768
{x,hello,y} 1024,Hello%20World%21,768
?{x,empty} ?1024,
?{x,undef} ?1024
?{undef,y} ?768
{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
3.2.3. Reserved expansion: {+var}
Reserved expansion, as indicated by the plus ("+") operator for Level
2 and above templates, is identical to simple string expansion except
that the substituted values may also contain pct-encoded triplets and
characters in the reserved set.
For each defined variable in the variable-list, perform variable
expansion, as defined in Section 3.2.1, with the allowed characters
being those in the set (unreserved / reserved / pct-encoded). If
more than one variable has a defined value, append a comma (",") to
the result string as a separator between variable expansions.
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Example Template Expansion
{foo} That%27s%20right%21
{+foo} That%27s%20right!
{+half} 50%25
{base}index http%3A%2F%2Fexample.com%2Fhome%2Findex
{+base}index http://example.com/home/index
{+var} value
{+hello} Hello%20World!
O{+empty}X OX
O{+undef}X OX
{+path}/here /foo/bar/here
here?ref={+path} here?ref=/foo/bar
up{+path}{var}/here up/foo/barvalue/here
{+x,hello,y} 1024,Hello%20World!,768
{+path,x}/here /foo/bar,1024/here
{+path:6}/here /foo/b/here
{+list} red,green,blue
{+list*} red,green,blue
{+keys} semi,;,dot,.,comma,,
{+keys*} semi=;,dot=.,comma=,
3.2.4. Fragment expansion: {#var}
Fragment expansion, as indicated by the crosshatch ("#") operator for
Level 2 and above templates, is identical to reserved expansion
except that a crosshatch character (fragment delimiter) is appended
first to the result string if any of the variables are defined.
Example Template Expansion
{#foo} #That%27s%20right!
{#var} #value
{#hello} #Hello%20World!
{#half} #50%25
foo{#empty} foo#
foo{#undef} foo
{#x,hello,y} #1024,Hello%20World!,768
{#path,x}/here #/foo/bar,1024/here
{#path:6}/here #/foo/b/here
{#list} #red,green,blue
{#list*} #red,green,blue
{#keys} #semi,;,dot,.,comma,,
{#keys*} #semi=;,dot=.,comma=,
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3.2.5. Label expansion with dot-prefix: {.var}
Label expansion, as indicated by the dot (".") operator for Level 3
and above templates, is useful for describing URI spaces with varying
domain names or path selectors (e.g., filename extensions).
For each defined variable in the variable-list, append "." to the
result string and then perform variable expansion, as defined in
Section 3.2.1, with the allowed characters being those in the
unreserved set.
Since "." is in the unreserved set, a value that contains a "." has
the effect of adding multiple labels.
Example Template Expansion
{.who} .fred
{.who,who} .fred.fred
{.half,who} .50%25.fred
www{.dom} www.example.com
X{.var} X.value
X{.empty} X.
X{.undef} X
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
X{.empty_keys} X
X{.empty_keys*} X
3.2.6. Path segment expansion: {/var}
Path segment expansion, as indicated by the slash ("/") operator in
Level 3 and above templates, is useful for describing URI path
hierarchies.
For each defined variable in the variable-list, append "/" to the
result string and then perform variable expansion, as defined in
Section 3.2.1, with the allowed characters being those in the
unreserved set.
Note that the expansion process for path segment expansion is
identical to that of label expansion aside from the substitution of
"/" instead of ".". However, unlike ".", a "/" is a reserved
character and will be pct-encoded if found in a value.
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Example Template Expansion
{/who} /fred
{/who,who} /fred/fred
{/half,who} /50%25/fred
{/who,dub} /fred/me%2Ftoo
{/var} /value
{/var,empty} /value/
{/var,undef} /value
{/var,x}/here /value/1024/here
{/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
3.2.7. Path-style parameter expansion: {;var}
Path-style parameter expansion, as indicated by the semicolon (";")
operator in Level 3 and above templates, is useful for describing URI
path parameters, such as "pathname;property" or "pathname;key=value".
For each defined variable in the variable-list:
o append ";" to the result string;
o if no explode modifier is present or the variable does not have a
composite value, append the variable name (encoded as if it were a
literal string) to the result string and, if the variable's value
is not empty, append "=" to the result string; and,
o perform variable expansion, as defined in Section 3.2.1, with the
allowed characters being those in the unreserved set.
Example Template Expansion
{;who} ;who=fred
{;half} ;half=50%25
{;empty} ;empty
{;v,empty,who} ;v=6;empty;who=fred
{;v,bar,who} ;v=6;who=fred
{;x,y} ;x=1024;y=768
{;x,y,empty} ;x=1024;y=768;empty
{;x,y,undef} ;x=1024;y=768
{;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
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3.2.8. Form-style query expansion: {?var}
Form-style query expansion, as indicated by the question-mark ("?")
operator in Level 3 and above templates, is useful for describing an
entire optional query component.
For each defined variable in the variable-list:
o append "?" to the result string if this is the first defined value
or append "&" thereafter;
o if no explode modifier is present or the variable does not have a
composite value, append the variable name (encoded as if it were a
literal string) and an equals character ("=") to the result
string; and,
o perform variable expansion, as defined in Section 3.2.1, with the
allowed characters being those in the unreserved set.
Example Template Expansion
{?who} ?who=fred
{?half} ?half=50%25
{?x,y} ?x=1024&y=768
{?x,y,empty} ?x=1024&y=768&empty=
{?x,y,undef} ?x=1024&y=768
{?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
3.2.9. Form-style query continuation: {&var}
Form-style query continuation, as indicated by the ampersand ("&")
operator in Level 3 and above templates, is useful for describing
optional &name=value pairs in a template that already contains a
literal query component with fixed parameters.
For each defined variable in the variable-list:
o append "&" to the result string;
o if no explode modifier is present or the variable does not have a
composite value, append the variable name (encoded as if it were a
literal string) and an equals character ("=") to the result
string; and,
o perform variable expansion, as defined in Section 3.2.1, with the
allowed characters being those in the unreserved set.
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Example Template Expansion
{&who} &who=fred
{&half} &half=50%25
?fixed=yes{&x} ?fixed=yes&x=1024
{&x,y,empty} &x=1024&y=768&empty=
{&x,y,undef} &x=1024&y=768
{&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
4. Security Considerations
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, as described
in section 7 of [RFC3986].
5. IANA Considerations
No IANA actions are required by this document.
6. Acknowledgments
The following people made significant contributions to this
specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton,
John Cowan, James H. Manger, Marc Portier, and James Snell.
7. Normative References
[ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[UNIV4] 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.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex # 15, April 2003.
[1] <http://lists.w3.org/Archives/Public/uri/>
Appendix A. Implementation Hints
The normative sections on expansion describe each operator with a
separate expansion process for the sake of descriptive clarity. In
actual implementations, we expect the expressions to be processed
left-to-right using a common algorithm that has only minor variations
in process per operator. This appendix describes one such algorithm.
Initialize an empty result string and its non-error state.
Scan the template and copy literals to the result string (as in
Section 3.1) until an expression is indicated by a "{" or the
template ends. When it ends, return the result string and its
current error or non-error state.
o If an expression is found, scan the template to the next "}" and
extract the characters in between the braces.
o If the template ends before a "}", then append the "{" and
extracted characters to the result string and return with an error
status indicating the expression is malformed.
Examine the first character of the extracted expression for an
operator.
o If the expression ended (i.e., is "{}"), an operator is found that
is unknown or unimplemented, or the character is not in the
varchar set (Section 2.3), then append "{", the extracted
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expression, and "}" to the result string, remember that the result
is in an error state, and then go back to scan the remainder of
the template.
o If a known and implemented operator is found, store the operator
and skip to the next character to begin the varspec-list.
o Otherwise, store the operator as NUL (simple string expansion).
Use the following value table to determine the processing behavior by
expression type operator. The entry for "first" is the string to
append to the result first if any of the expression's variables are
defined. The entry for "sep" is the separator to append to the
result before any second (or subsequent) defined variable expansion.
The entry for "named" is a boolean for whether or not the expansion
includes the variable name when no explode modifier is given. The
entry for "ifemp" is a string to append to the name when the
expansion includes the variable (or key) name and that variable (or
key) has an empty value. The entry for "allow" indicates what
characters to allow unencoded within the value expansion: (U) means
any character not in the unreserved set will be encoded; (U+R) means
any character not in the (unreserved / reserved / pct-encoding) set
will be encoded; and, for both cases, disallowed characters are
encoded as UTF-8 (a sequence of octets) and then each octet is
encoded as a pct-encoded triplet.
.------------------------------------------------------------------.
| NUL + . / ; ? & # |
|------------------------------------------------------------------|
| first | "" "" "." "/" ";" "?" "&" "#" |
| sep | "," "," "." "/" ";" "&" "&" "," |
| named | false false false false true true true false |
| ifemp | "" "" "" "" "" "=" "=" "" |
| allow | U U+R U U U U U U+R |
`------------------------------------------------------------------'
With the above table in mind, process the variable-list as follows:
For each varspec, extract the varname and optional modifier, lookup
the value for that variable, and then:
o If the varname is unknown or corresponds to a variable with an
undefined value (Section 3.2.1), then skip to the next varspec.
o If this is the first defined variable for this expression, append
the first string for this expression type to the result string and
remember that it has been done. Otherwise, append the sep string
to the result string.
o If an explode modifier is present, then
* If the variable is a list, then append each defined list member
to the result string, after encoding any characters that are
not in the allow set, with the sep string appended to the
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result between each defined list member.
* If the variable is an associative array of (name, value) pairs,
then append each pair with a defined value to the result string
as "name=value", after encoding any characters that are not in
the allow set, with the sep string appended to the result
between each defined pair.
* Otherwise (the variable is a string):
+ If named is true, append the varname to the result string
using the same encoding process as for literals, and
- if the value is empty, append the ifemp string to the
result string and skip to the next varspec;
- otherwise, append "=" to the result string.
+ Append the value to the result string after encoding any
characters that are not in the allow set.
o If an explode modifier is not present, then
* If named is true, append the varname to the result string using
the same encoding process as for literals, and
+ if the value is empty, append the ifemp string to the result
string and skip to the next varspec;
+ otherwise, append "=" to the result string.
* If the variable is a list, append each defined list member to
the result string, after encoding any characters that are not
in the allow set, with a comma (",") appended to the result
between each defined list member.
* If the variable is an associative array of (name, value) pairs,
append each pair with a defined value to the result string as
"name,value", after encoding any characters that are not in the
allow set, with a comma (",") appended to the result between
each defined pair.
* Otherwise (the variable is a string), then
+ if a prefix modifier is present and the prefix length is
less than the value string length in number of Unicode
characters, append that number of characters from the
beginning of the value string to the result string, after
encoding any characters that are not in the allow set, while
taking care not to split multi-octet or pct-encoded triplet
characters that represent a single Unicode codepoint;
+ otherwise, append the value to the result string after
encoding any characters that are not in the allow set.
When the variable-list for this expression is exhausted, go back to
scan the remainder of the template.
Gregorio, et al. Expires February 21, 2012 [Page 29]
Internet-Draft URI Template Aug 2011
Authors' Addresses
Joe Gregorio
Google
Email: joe@bitworking.org
URI: http://bitworking.org/
Roy T. Fielding (editor)
Adobe Systems Incorporated
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/
Marc Hadley
Oracle
Email: Marc.Hadley@oracle.com
URI: http://oracle.com/
Mark Nottingham
Email: mnot@pobox.com
URI: http://mnot.net/
David Orchard
URI: http://www.pacificspirit.com/
Gregorio, et al. Expires February 21, 2012 [Page 30]
