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rfc6570.txt
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Internet Engineering Task Force (IETF) J. Gregorio
Request for Comments: 6570 Google
Category: Standards Track R. Fielding
ISSN: 2070-1721 Adobe
M. Hadley
MITRE
M. Nottingham
Rackspace
D. Orchard
Salesforce.com
March 2012
URI Template
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6570.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Gregorio, et al. Standards Track [Page 1]
RFC 6570 URI Template March 2012
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.
Table of Contents
1. Introduction ....................................................3
1.1. Overview ...................................................3
1.2. Levels and Expression Types ................................5
1.3. Design Considerations ......................................9
1.4. Limitations ...............................................10
1.5. Notational Conventions ....................................11
1.6. Character Encoding and Unicode Normalization ..............12
2. Syntax .........................................................13
2.1. Literals ..................................................13
2.2. Expressions ...............................................13
2.3. Variables .................................................14
2.4. Value Modifiers ...........................................15
2.4.1. Prefix Values ......................................15
2.4.2. Composite Values ...................................16
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} .....................21
3.2.3. Reserved Expansion: {+var} .........................22
3.2.4. Fragment Expansion: {#var} .........................23
3.2.5. Label Expansion with Dot-Prefix: {.var} ............24
3.2.6. Path Segment Expansion: {/var} .....................24
3.2.7. Path-Style Parameter Expansion: {;var} .............25
3.2.8. Form-Style Query Expansion: {?var} .................26
3.2.9. Form-Style Query Continuation: {&var} ..............27
4. Security Considerations ........................................27
5. Acknowledgments ................................................28
6. References .....................................................28
6.1. Normative References ......................................28
6.2. Informative References ....................................29
Appendix A. Implementation Hints ..................................30
Gregorio, et al. Standards Track [Page 2]
RFC 6570 URI Template March 2012
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 (informally, a "URI space"). 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
A URI Template is a compact sequence of characters for describing a
range of Uniform Resource Identifiers through variable expansion.
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 the 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:
expression: The text between '{' and '}', including the enclosing
braces, as defined in Section 2.
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.
Gregorio, et al. Standards Track [Page 3]
RFC 6570 URI Template March 2012
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.
A URI Template provides both a structural description of a URI space
and, when variable values are provided, machine-readable instructions
on how to construct a URI corresponding to those values. A URI
Template is transformed into a URI reference by replacing each
delimited expression with its value as defined by the expression type
and the values of variables named within the expression. The
expression types range from simple string expansion to multiple
name=value lists. The expansions are based on the URI generic
syntax, allowing an implementation to process any URI Template
without knowing the scheme-specific requirements of every possible
resulting URI.
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
Gregorio, et al. Standards Track [Page 4]
RFC 6570 URI Template March 2012
or if both variables are undefined, then it would be
http://www.example.com/foo
A URI Template may be provided in absolute form, as in the examples
above, or in relative form. A template is expanded before the
resulting reference is 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 Internationalized Resource Identifier (IRI) references [RFC3987].
Therefore, a URI Template is 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 pct-encoding any characters not in the set of unreserved URI
characters (Section 1.5).
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!" |
| |
|-----------------------------------------------------------------|
| 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 URI characters
(Section 1.5), and the crosshatch ("#") operator for expansion of
fragment identifiers.
Gregorio, et al. Standards Track [Page 5]
RFC 6570 URI Template March 2012
.-----------------------------------------------------------------.
| 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 allow multiple variables per expression, each
separated by a comma, and add more complex operators for dot-prefixed
labels, slash-prefixed path segments, semicolon-prefixed path
parameters, and the form-style construction of a query syntax
consisting of name=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 |
| | |
Gregorio, et al. Standards Track [Page 6]
RFC 6570 URI Template March 2012
|-----+-----------------------------------------------------------|
| + | 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 value modifiers as an optional suffix
to each variable name. A prefix modifier (":") indicates that only a
limited number of characters from the beginning of the value are used
by the expansion (Section 2.4.1). An explode ("*") modifier
Gregorio, et al. Standards Track [Page 7]
RFC 6570 URI Template March 2012
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 (Section 2.4.2).
.-----------------------------------------------------------------.
| Level 4 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| list := ("red", "green", "blue") |
| keys := [("semi",";"),("dot","."),("comma",",")] |
| |
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with value modifiers (Sec 3.2.2) |
| | |
| | {var:3} val |
| | {var:30} value |
| | {list} red,green,blue |
| | {list*} red,green,blue |
| | {keys} semi,%3B,dot,.,comma,%2C |
| | {keys*} semi=%3B,dot=.,comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with value modifiers (Sec 3.2.3) |
| | |
| | {+path:6}/here /foo/b/here |
| | {+list} red,green,blue |
| | {+list*} red,green,blue |
| | {+keys} semi,;,dot,.,comma,, |
| | {+keys*} semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| # | Fragment expansion with value modifiers (Sec 3.2.4) |
| | |
| | {#path:6}/here #/foo/b/here |
| | {#list} #red,green,blue |
| | {#list*} #red,green,blue |
| | {#keys} #semi,;,dot,.,comma,, |
| | {#keys*} #semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.5) |
| | |
| | X{.var:3} X.val |
| | X{.list} X.red,green,blue |
Gregorio, et al. Standards Track [Page 8]
RFC 6570 URI Template March 2012
| | 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*} ;list=red;list=green;list=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 [WSDL], WADL [WADL], and OpenSearch
[OpenSearch]. This specification extends and formally defines the
Gregorio, et al. Standards Track [Page 9]
RFC 6570 URI Template March 2012
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.
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. Therefore, the template syntax 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.
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URI Templates are not URIs: they do not identify an abstract or
physical resource, they are not parsed as URIs, and they 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"
; case-insensitive
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", "character encoding
scheme", "code point", "coded character set", "glyph", "non-ASCII",
"normalization", "protocol element", and "regular expression" as they
are defined in [RFC6365].
The ABNF notation defines its terminal values to be non-negative
integers (code points) that are a superset of the US-ASCII coded
character set [ASCII]. This specification defines terminal values as
code points within the Unicode coded character set [UNIV6].
In spite of the syntax and template expansion process being defined
in terms of Unicode code points, 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
glyphs painted on the side of a bus. This specification does not
mandate any particular character encoding scheme 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 scheme is defined by that protocol; without
such a definition, a URI Template is assumed to be in the same
character encoding scheme 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
code points.
The Unicode Standard [UNIV6] 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 as the sequence of pct-encoded
triplets corresponding to that character's encoding in UTF-8
[RFC3629].
literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
/ %x5D / %x5F / %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-level2 / op-level3 / op-reserve
op-level2 = "+" / "#"
op-level3 = "." / "/" / ";" / "?" / "&"
op-reserve = "=" / "," / "!" / "@" / "|"
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The operator characters have been chosen to reflect each of their
roles as reserved characters in the URI generic syntax. The
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 name or name=value pairs prefixed by ";";
? Query component beginning with "?" and consisting of
name=value pairs separated by "&"; and,
& Continuation of query-style &name=value pairs within
a literal query component.
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 use outside the scope of this specification. For
example, a macro language might use these characters to apply macro
substitution to a string prior to that string being processed as a
URI Template.
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 names are case-sensitive because the name might be expanded
within a case-sensitive URI component.
variable-list = varspec *( "," varspec )
varspec = varname [ modifier-level4 ]
varname = varchar *( ["."] varchar )
varchar = ALPHA / DIGIT / "_" / pct-encoded
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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.
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 (Section 3.2.1).
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
In Level 4 templates, 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 they do have an impact on the expansion process
(Section 3.2.1).
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 are
associated with undefined values.
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-level4 = 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.
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prefix = ":" max-length
max-length = %x31-39 0*3DIGIT ; positive integer < 10000
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-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.
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 is to be
treated as a composite value consisting of either a list of values or
an associative array of (name, value) pairs. Hence, the expansion
process is applied to each member of the composite as if it were
listed as a separate variable. This kind of variable specification
is significantly less self-documenting than non-exploded variables,
since there is less correspondence between the variable name and how
the URI reference appears after expansion.
explode = "*"
Since URI Templates do not contain an indication of type or schema,
the type for an exploded variable is assumed to be determined by
context. For example, the processor might be supplied values in a
form that differentiates values as strings, lists, or associative
arrays. Likewise, the context in which the template is used (script,
mark-up language, Interface Definition Language, etc.) might define
rules for associating variable names with types, structures, or
schema.
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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]). 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
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.
If a variable has a composite structure and only some of the fields
in that structure have defined values, then only the defined pairs
are present in the expansion. This can be useful for templates that
consist of a large number of potential query terms.
An explode modifier applied to a list variable causes the expansion
to iterate over the list's member values. For path and query
parameter expansions, each member value is paired with the variable's
name as a (varname, value) pair. This allows path and query
parameters to be repeated for multiple values, as in
Given the variable assignments
year := ("1965", "2000", "2012")
dom := ("example", "com")
Example Template Expansion
find{?year*} find?year=1965&year=2000&year=2012
www{.dom*} www.example.com
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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.
The requirements on expansion for each aspect of the URI Template
grammar are defined in this section. A non-normative algorithm for
the expansion process as a whole is provided in Appendix A.
If a template processor encounters a character sequence outside an
expression that does not match the <URI-Template> grammar, then
processing of the template SHOULD cease, the URI reference result
SHOULD contain the expanded part of the template followed by the
remainder unexpanded, and the location and type of error SHOULD be
indicated to the invoking application.
If an error is encountered in an expression, such as an operator or
value modifier that the template processor does not recognize or does
not yet support, or a character is found that is not allowed by the
<expression> grammar, then the unprocessed parts of 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.
If an error occurs, the result returned might 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 first encoding
the character as its sequence of octets in UTF-8 and then encoding
each such octet as a pct-encoded triplet.
3.2. Expression Expansion