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.
Gregorio, et al. Standards Track [Page 10]RFC 6570 URI Template March 2012
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
Gregorio, et al. Standards Track [Page 11]RFC 6570 URI Template March 2012
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.
Gregorio, et al. Standards Track [Page 12]
RFC 6570 URI Template March 2012
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 = "=" / "," / "!" / "@" / "|"
Gregorio, et al. Standards Track [Page 13]RFC 6570 URI Template March 2012
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
Gregorio, et al. Standards Track [Page 14]RFC 6570 URI Template March 2012
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.
Gregorio, et al. Standards Track [Page 15]RFC 6570 URI Template March 2012
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.
Gregorio, et al. Standards Track [Page 16]RFC 6570 URI Template March 2012
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
Gregorio, et al. Standards Track [Page 17]RFC 6570 URI Template March 2012
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
Each expression is indicated by an opening brace ("{") character and
continues until the next closing brace ("}"). Expressions cannot be
nested.
Gregorio, et al. Standards Track [Page 18]RFC 6570 URI Template March 2012
An expression is expanded by determining its 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.
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:
count := ("one", "two", "three")
dom := ("example", "com")
dub := "me/too"
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 (Section 2.3) has no value and is
ignored by the expansion process. 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. As described in Section 1.6,
the expansion process is defined in terms of Unicode code points in
order to ensure that non-ASCII characters are consistently pct-
encoded in the resulting URI reference. One way for a template
Gregorio, et al. Standards Track [Page 19]RFC 6570 URI Template March 2012
processor to obtain a consistent expansion is to transcode the value
string to UTF-8 (if it is not already in UTF-8) and then transform
each octet that is not in the allowed set into the corresponding pct-
encoded triplet. Another is to map directly from the value's native
character encoding to the set of allowed URI characters, with any
remaining disallowed characters mapping to the sequence of pct-
encoded triplets that correspond to the octet(s) of that character
when encoded as UTF-8 [RFC3629].
The allowed set for a given expansion depends on the expression type:
reserved ("+") and fragment ("#") expansions allow the set of
characters in the union of ( 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 character 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. An explode
modifier has no effect. A prefix modifier limits the expansion to
the first max-length characters of the decoded value. If the value
contains multi-octet or pct-encoded characters, care must be taken to
avoid splitting the value in mid-character: count each Unicode code
point as one character.
For a variable that is an associative array, expansion depends on
both the expression type and the presence of an explode modifier. If
there is no explode modifier, expansion consists of appending a
comma-separated concatenation of each (name, value) pair that has a
defined value. If there is an explode modifier, expansion consists
of appending each pair that has a defined value as either
"name=value" or, if the value is the empty string and the expression
type does not indicate form-style parameters (i.e., not a "?" or "&"
type), simply "name". Both name and value strings are encoded in the
same way as simple string values. A separator string is appended
between defined pairs according to the expression type, as defined by
the following table:
Gregorio, et al. Standards Track [Page 20]RFC 6570 URI Template March 2012
Type Separator
"," (default)
+ ","
# ","
. "."
/ "/"
; ";"
? "&"
& "&"
For a variable that is a list of values, expansion depends on both
the expression type and the presence of an explode modifier. If
there is no explode modifier, the expansion consists of a comma-
separated concatenation of the defined member string values. If
there is an explode modifier and the expression type expands named
parameters (";", "?", or "&"), then the list is expanded as if it
were an associative array in which each member value is paired with
the list's varname. Otherwise, the value will be expanded as if it
were a list of separate variable values, each value separated by the
expression type's associated separator as defined by the table above.
Example Template Expansion
{count} one,two,three
{count*} one,two,three
{/count} /one,two,three
{/count*} /one/two/three
{;count} ;count=one,two,three
{;count*} ;count=one;count=two;count=three
{?count} ?count=one,two,three
{?count*} ?count=one&count=two&count=three
{&count*} &count=one&count=two&count=three
3.2.2. Simple String Expansion: {var}
Simple string expansion is the default expression type when no
operator is given.
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.
Gregorio, et al. Standards Track [Page 21]RFC 6570 URI Template March 2012
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.
Gregorio, et al. Standards Track [Page 22]RFC 6570 URI Template March 2012
Example Template Expansion
{+var} value
{+hello} Hello%20World!
{+half} 50%25
{base}index http%3A%2F%2Fexample.com%2Fhome%2Findex
{+base}index http://example.com/home/index
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
{#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=,
Gregorio, et al. Standards Track [Page 23]RFC 6570 URI Template March 2012
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.
Gregorio, et al. Standards Track [Page 24]RFC 6570 URI Template March 2012
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 "path;property" or "path;name=value".
For each defined variable in the variable-list:
o append ";" to the result string;
o if the variable has a simple string value or no explode modifier
is given, then:
* append the variable name (encoded as if it were a literal
string) to the result string;
* if the variable's value is not empty, append "=" to the result
string;
o perform variable expansion, as defined in Section 3.2.1, with the
allowed characters being those in the unreserved set.
Gregorio, et al. Standards Track [Page 25]RFC 6570 URI Template March 2012
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*} ;list=red;list=green;list=blue
{;keys} ;keys=semi,%3B,dot,.,comma,%2C
{;keys*} ;semi=%3B;dot=.;comma=%2C
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 the variable has a simple string value or no explode modifier
is given, 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
Gregorio, et al. Standards Track [Page 26]RFC 6570 URI Template March 2012
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 the variable has a simple string value or no explode modifier
is given, 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
?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.
However, it might be possible to craft unanticipated URIs if an
attacker is given control over the template or over the variable
values within an expression that allows reserved characters in the
expansion. In either case, the security considerations are largely
determined by who provides the template, who provides the values to
use for variables within the template, in what execution context the
expansion occurs (client or server), and where the resulting URIs are
used.
Gregorio, et al. Standards Track [Page 27]RFC 6570 URI Template March 2012
This specification does not limit where URI Templates might be used.
Current implementations exist within server-side development
frameworks and within client-side javascript for computed links or
forms.
Within frameworks, templates usually act as guides for where data
might occur within later (request-time) URIs in client requests.
Hence, the security concerns are not in the templates themselves, but
rather in how the server extracts and processes the user-provided
data within a normal Web request.
Within client-side implementations, a URI Template has many of the
same properties as HTML forms, except limited to URI characters and
possibly included in HTTP header field values instead of just message
body content. Care ought to be taken to ensure that potentially
dangerous URI reference strings, such as those beginning with
"javascript:", do not appear in the expansion unless both the
template and the values are provided by a trusted source.
Other security considerations are the same as those for URIs, as
described in Section 7 of [RFC3986].
5. Acknowledgments
The following people made contributions to this specification: Mike
Burrows, Michaeljohn Clement, DeWitt Clinton, John Cowan, Stephen
Farrell, Robbie Gates, Vijay K. Gurbani, Peter Johanson, Murray S.
Kucherawy, James H. Manger, Tom Petch, Marc Portier, Pete Resnick,
James Snell, and Jiankang Yao.
6. References
6.1. 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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
Gregorio, et al. Standards Track [Page 28]RFC 6570 URI Template March 2012
[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.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[UNIV6] The Unicode Consortium, "The Unicode Standard, Version
6.0.0", (Mountain View, CA: The Unicode Consortium,
2011. ISBN 978-1-936213-01-6),
<http://www.unicode.org/versions/Unicode6.0.0/>.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex # 15, April 2003,
<http://www.unicode.org/unicode/reports/tr15/
tr15-23.html>.
6.2. Informative References
[OpenSearch] Clinton, D., "OpenSearch 1.1", Draft 5, December 2011,
<http://www.opensearch.org/Specifications/OpenSearch>.
[UPU-S42] Universal Postal Union, "International Postal Address
Components and Templates", UPU S42-1, November 2002,
<http://www.upu.int/en/activities/addressing/
standards.html>.
[WADL] Hadley, M., "Web Application Description Language",
World Wide Web Consortium Member Submission
SUBM-wadl-20090831, August 2009,
<http://www.w3.org/Submission/2009/
SUBM-wadl-20090831/>.
[WSDL] Weerawarana, S., Moreau, J., Ryman, A., and R.
Chinnici, "Web Services Description Language (WSDL)
Version 2.0 Part 1: Core Language", World Wide Web
Consortium Recommendation REC-wsdl20-20070626,
June 2007, <http://www.w3.org/TR/2007/
REC-wsdl20-20070626>.
Gregorio, et al. Standards Track [Page 29]RFC 6570 URI Template March 2012
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 non-normative 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 "{", an error is
indicated by the presence of a non-literals character other than "{",
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
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 or key name when no explode modifier is given.
The entry for "ifemp" is a string to append to the name if its
corresponding value is empty. The entry for "allow" indicates what
Gregorio, et al. Standards Track [Page 30]RFC 6570 URI Template March 2012
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 union of (unreserved / reserved / pct-
encoding) will be encoded; and, for both cases, each disallowed
character is first encoded as its sequence of octets in UTF-8 and
then each such 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 a variable name and optional modifier from
the expression by scanning the variable-list until a character not in
the varname set is found or the end of the expression is reached.
o If it is the end of the expression and the varname is empty, go
back to scan the remainder of the template.
o If it is not the end of the expression and the last character
found indicates a modifier ("*" or ":"), remember that modifier.
If it is an explode ("*"), scan the next character. If it is a
prefix (":"), continue scanning the next one to four characters
for the max-length represented as a decimal integer and then, if
it is still not the end of the expression, scan the next
character.
o If it is not the end of the expression and the last character
found is not a comma (","), append "{", the stored operator (if
any), the scanned varname and modifier, the remaining 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.
Lookup the value for the scanned variable name, and then
o If the varname is unknown or corresponds to a variable with an
undefined value (Section 2.3), then skip to the next varspec.
Gregorio, et al. Standards Track [Page 31]RFC 6570 URI Template March 2012
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 this variable's value is a string, 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 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 pct-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 code point;
* otherwise, append the value to the result string after pct-
encoding any characters that are not in the allow set.
o else if no explode modifier is given, 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; and
* if this variable's value is a list, append each defined list
member to the result string, after pct-encoding any characters
that are not in the allow set, with a comma (",") appended to
the result between each defined list member;
* if this variable's value is an associative array or any other
form of paired (name, value) structure, append each pair with a
defined value to the result string as "name,value", after pct-
encoding any characters that are not in the allow set, with a
comma (",") appended to the result between each defined pair.
Gregorio, et al. Standards Track [Page 32]RFC 6570 URI Template March 2012
o else if an explode modifier is given, then
* if named is true, then for each defined list member or array
(name, value) pair with a defined value, do:
+ if this is not the first defined member/value, append the
sep string to the result string;
+ if this is a list, append the varname to the result string
using the same encoding process as for literals;
+ if this is a pair, append the name to the result string
using the same encoding process as for literals;
+ if the member/value is empty, append the ifemp string to the
result string; otherwise, append "=" and the member/value to
the result string, after pct-encoding any member/value
characters that are not in the allow set.
* else if named is false, then
+ if this is a list, append each defined list member to the
result string, after pct-encoding any characters that are
not in the allow set, with the sep string appended to the
result between each defined list member.
+ if this is an array of (name, value) pairs, append each pair
with a defined value to the result string as "name=value",
after pct-encoding any characters that are not in the allow
set, with the sep string appended to the result between each
defined pair.
When the variable-list for this expression is exhausted, go back to
scan the remainder of the template.
Gregorio, et al. Standards Track [Page 33]RFC 6570 URI Template March 2012
Authors' Addresses
Joe Gregorio
Google
EMail: joe@bitworking.org
URI: http://bitworking.org/
Roy T. Fielding
Adobe Systems Incorporated
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Marc Hadley
The MITRE Corporation
EMail: mhadley@mitre.org
URI: http://mitre.org/
Mark Nottingham
Rackspace
EMail: mnot@mnot.net
URI: http://www.mnot.net/
David Orchard
Salesforce.com
EMail: orchard@pacificspirit.com
URI: http://www.pacificspirit.com/
Gregorio, et al. Standards Track [Page 34]
