Network Working Group J. Gregorio
Internet-Draft Google
Intended status: Standards Track R. Fielding, Ed.
Expires: January 12, 2012 Adobe
M. Hadley
Oracle
M. Nottingham
D. Orchard
Jul 11, 2011
URI Template
draft-gregorio-uritemplate-05
Abstract
A URI Template is a compact sequence of characters for describing a
range of Uniform Resource Identifiers through variable expansion.
This specification defines the URI Template syntax and the process
for expanding a URI Template into a URI reference, along with
guidelines for the use of URI Templates on the Internet.
Editorial Note (to be removed by RFC Editor)
To provide feedback on this Internet-Draft, join the W3C URI mailing
list (http://lists.w3.org/Archives/Public/uri/) [1].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
Gregorio, et al. Expires January 12, 2012 [Page 1]
Internet-Draft URI Template Jul 2011
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
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Gregorio, et al. Expires January 12, 2012 [Page 2]
Internet-Draft URI Template Jul 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Levels and Expression Types . . . . . . . . . . . . . . . 6
1.3. Design Considerations . . . . . . . . . . . . . . . . . . 10
1.4. Limitations . . . . . . . . . . . . . . . . . . . . . . . 11
1.5. Notational Conventions . . . . . . . . . . . . . . . . . . 11
1.6. Character Encoding and Unicode Normalization . . . . . . . 12
2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1. Literals . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2. Expressions . . . . . . . . . . . . . . . . . . . . . . . 14
2.3. Variables . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4. Value Modifiers . . . . . . . . . . . . . . . . . . . . . 16
2.4.1. Prefix Values . . . . . . . . . . . . . . . . . . . . 16
2.4.2. Component Values . . . . . . . . . . . . . . . . . . . 17
2.5. Value Defaults . . . . . . . . . . . . . . . . . . . . . . 17
3. Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1. Literal Expansion . . . . . . . . . . . . . . . . . . . . 19
3.2. Expression Expansion . . . . . . . . . . . . . . . . . . . 20
3.2.1. Undefined Variable Expansion . . . . . . . . . . . . . 20
3.2.2. Simple String Expansion: {var} . . . . . . . . . . . . 20
3.2.3. Reserved expansion: {+var} . . . . . . . . . . . . . . 21
3.2.4. Label expansion with dot-prefix: {.var} . . . . . . . 22
3.2.5. Path segment expansion: {/var} . . . . . . . . . . . . 23
3.2.6. Path-style parameter expansion: {;var} . . . . . . . . 25
3.2.7. Form-style query expansion: {?var} . . . . . . . . . . 25
4. Security Considerations . . . . . . . . . . . . . . . . . . . 25
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
7. Normative References . . . . . . . . . . . . . . . . . . . . . 25
Appendix A. Example URI Template Parser . . . . . . . . . . . . . 26
Appendix B. Revision History (to be removed by RFC Editor) . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Gregorio, et al. Expires January 12, 2012 [Page 3]
Internet-Draft URI Template Jul 2011
1. Introduction
1.1. Overview
A Uniform Resource Identifier (URI) [RFC3986] is often used to
identify a specific resource within a common space of similar
resources. For example, personal web spaces are often delegated
using a common pattern, such as
http://example.com/~fred/
http://example.com/~mark/
or a set of dictionary entries might be grouped in a hierarchy by the
first letter of the term, as in
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
or a service interface might be invoked with various user input in a
common pattern, as in
http://example.com/search?q=cat&lang=en
http://example.com/search?q=dog&lang=fr
URI Templates provide a mechanism for abstracting a space of resource
identifiers such that the variable parts can be easily identified and
described. URI templates can have many uses, including discovery of
available services, configuring resource mappings, defining computed
links, specifying interfaces, and other forms of programmatic
interaction with resources. For example, the above resources could
be described by the following URI templates:
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
We define the following terms:
o expression - The text between '{' and '}', including the enclosing
braces, as defined in Section 2.
o expansion - The string result obtained from a template expression
after processing it according to its expression type, list of
variable names, and value modifiers, as defined in Section 3.
o template processor - A program or library that, given a URI
Template and a set of variables with values, transforms the
template string into a URI-reference by parsing the template for
expressions and substituting each one with its corresponding
expansion.
Gregorio, et al. Expires January 12, 2012 [Page 4]
Internet-Draft URI Template Jul 2011
A URI Template provides both a structural description of a URI space
and, when variable values are provided, a simple instruction on how
to construct a URI corresponding to those values. A URI Template is
transformed into a URI-reference by replacing each delimited
expression with its value as defined by the expression type and the
values of variables named within the expression. The expression
types range from simple string expansion to multiple key=value lists.
The expansions are based on the URI generic syntax, allowing an
implementation to process any URI Template without knowing the
scheme-specific requirements of every possible resulting URI.
For example, the following URI Template includes a form-style
parameter expression, as indicated by the "?" operator appearing
before the variable names.
http://www.example.com/foo{?query,number}
Each template expression describes, in a machine-readable manner, how
a URI is to be constructed. In this example, the expansion process
for templates beginning with the question-mark ("?") operator follows
the same pattern as form-style interfaces on the World Wide Web.
http://www.example.com/foo{?query,number}
\_____________/
|
|
For each defined variable in [ 'query', 'number' ],
substitute "?" if it is the first substitution or "&"
thereafter, followed by the variable name, '=', and the
variable's value.
If the variables have the values
query := "mycelium"
number := 100
then the expansion of the above URI Template is
http://www.example.com/foo?query=mycelium&number=100
Alternatively, if 'query' is undefined, then the expansion would be
http://www.example.com/foo?number=100
or if both variables are undefined, then it would be
http://www.example.com/foo
Gregorio, et al. Expires January 12, 2012 [Page 5]
Internet-Draft URI Template Jul 2011
A URI Template may be provided in absolute form, as in the examples
above, or in relative form. A template MUST be expanded before the
resulting reference can be resolved from relative to absolute form.
Although the URI syntax is used for the result, the template string
is allowed to contain the broader set of characters that can be found
in IRI references [RFC3987]. A URI Template is therefore also an IRI
template, and the result of template processing can be rendered as an
IRI by transforming each of the pct-encoded sequences to their
corresponding Unicode character if that character is not in the
reserved set.
1.2. Levels and Expression Types
URI Templates are similar to a macro language with a fixed set of
macro definitions: the expression type determines the expansion
process. The default expression type is simple string expansion,
wherein a single named variable is replaced by its value as a string
after UTF-8 encoding the characters and then pct-encoding any octets
that are not in the unreserved set.
Since most template processors implemented prior to this
specification have only implemented the default expression type, we
refer to these as Level 1 templates.
.-----------------------------------------------------------------.
| Level 1 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | Simple string expansion (Sec 3.2.2) |
| | |
| | {var} value |
| | {hello} Hello%20World%21 |
| | O{empty}X OX |
| | O{undef}X OX |
`-----------------------------------------------------------------'
Level 2 templates add the ability to define a default string, which
is substituted if the variable is undefined, and the plus ("+")
operator for expansion values that are allowed to include reserved
characters.
Gregorio, et al. Expires January 12, 2012 [Page 6]
Internet-Draft URI Template Jul 2011
.-----------------------------------------------------------------.
| Level 2 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| path := "/foo/bar" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with defaults (Sec 3.2.2) |
| | |
| | {var|default} value |
| | O{empty|default}X OX |
| | O{undef|default}X OdefaultX |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with defaults (Sec 3.2.3) |
| | |
| | {+var} value |
| | {+hello} Hello%20World! |
| | {+path}/here /foo/bar/here |
| | here?ref={+path} here?ref=/foo/bar |
| | up{+path}{x}/here up/foo/bar1024/here |
| | up{+empty|/1}/here up/here |
| | up{+undef|/1}/here up/1/here |
| | |
`-----------------------------------------------------------------'
Level 3 templates add more complex operators for lists of comma-
separated values, dot-prefixed labels, slash-prefixed path segments,
semicolon-prefixed path parameters, and the forms-style construction
of a query syntax consisting of key=value pairs that are separated by
an ampersand character.
.-----------------------------------------------------------------.
| Level 3 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| path := "/foo/bar" |
| x := "1024" |
| y := "768" |
| |
Gregorio, et al. Expires January 12, 2012 [Page 7]
Internet-Draft URI Template Jul 2011
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with multiple variables (Sec 3.2.2) |
| | |
| | {x,y} 1024,768 |
| | {x,hello,y} 1024,Hello%20World%21,768 |
| | ?{x,empty} ?1024, |
| | ?{x,undef} ?1024 |
| | ?{undef,y} ?768 |
| | ?{x,undef|0} ?1024,0 |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with multiple variables (Sec 3.2.3) |
| | |
| | {+x,hello,y} 1024,Hello%20World!,768 |
| | {+path,x}/here /foo/bar/1024/here |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var} X.value |
| | X{.empty} X. |
| | X{.undef} X |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
| | |
| | {/var} /value |
| | {/var,empty} /value/ |
| | {/var,undef} /value |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.6) |
| | |
| | {;x,y} ;x=1024;y=768 |
| | {;x,y,empty} ;x=1024;y=768;empty |
| | {;x,y,undef} ;x=1024;y=768 |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?x,y} ?x=1024&y=768 |
| | {?x,y,empty} ?x=1024&y=768&empty= |
| | {?x,y,undef} ?x=1024&y=768 |
| | |
`-----------------------------------------------------------------'
Gregorio, et al. Expires January 12, 2012 [Page 8]
Internet-Draft URI Template Jul 2011
Finally, Level 4 templates add the ability to specify value modifiers
as a suffix to the variable name. The prefix modifier (":")
indicates that only a limited number of characters from the beginning
of the value are used by the expansion. The explode ("*") modifier
tells the expansion process to treat the value as a multivalued
structure --- a list of values or key=value pairs -- rather than as a
single string.
.-----------------------------------------------------------------.
| Level 4 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| list := [ "red", "green", "blue" ] |
| keys := [("semi", ";"), ("dot", ".")] |
| |
| 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=, |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var:3} X.val |
| | X{.list} X.red,green,blue |
| | X{.list*} X.red.green.blue |
| | X{.keys} X.semi,%3B,dot,.,comma,%2C |
| | X{.keys*} X.semi=%3B.dot=..comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
Gregorio, et al. Expires January 12, 2012 [Page 9]
Internet-Draft URI Template Jul 2011
| | |
| | {/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 3.2.6) |
| | |
| | {;hello:5} ;hello=Hello |
| | {;list} ;red,green,blue |
| | {;list*} ;red;green;blue |
| | {;keys} ;semi,%3B,dot,.,comma,%2C |
| | {;keys*} ;semi=%3B;dot=.;comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?var:3} ?var=val |
| | {?list} ?list=red,green,blue |
| | {?list*} ?list=red&list=green&list=blue |
| | {?keys} ?keys=semi,%3B,dot,.,comma,%2C |
| | {?keys*} ?semi=%3B&dot=.&comma=%2C |
| | |
`-----------------------------------------------------------------'
1.3. Design Considerations
Mechanisms similar to URI Templates have been defined within several
specifications, including WSDL, WADL and OpenSearch. This
specification extends and formally defines the syntax so that URI
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.
Gregorio, et al. Expires January 12, 2012 [Page 10]
Internet-Draft URI Template Jul 2011
Likewise, the expansion process for ";" (path-style parameters) will
omit the "=" when the variable value is empty, whereas the process
for "?" (form-style parameters) will not omit the "=" when the value
is empty. Multiple variables and list values have their values
joined with "," if there is no predefined joining mechanism for the
operator. Only one operator, plus ("+"), will substitute unencoded
reserved characters found inside the variable values; the other
operators will pct-encode reserved characters found in the variable
values prior to expansion.
The most common cases for URI spaces can be described with Level 1
template expressions. If we were only concerned with URI generation,
then the template syntax could be limited to just simple variable
expansion, since more complex forms could be generated by changing
the variable values. However, URI Templates have the additional goal
of describing the layout of identifiers in terms of preexisting data
values. The template syntax therefore includes operators that
reflect how resource identifiers are commonly allocated. Likewise,
since prefix substrings are often used to partition large spaces of
resources, modifiers on variable values provide a way to specify both
the substring and the full value string with a single variable name.
1.4. Limitations
Since a URI Template describes a superset of the identifiers, there
is no implication that every possible expansion for each delimited
variable expression corresponds to a URI of an existing resource.
Our expectation is that an application constructing URIs according to
the template will be provided with an appropriate set of values for
the variables being substituted and will be able to cope with any
errors that might occur when the resulting URI is used for name
resolution or access.
URI Templates are not URIs: they do not identify an abstract or
physical resource, they are not parsed as URIs, and should not be
used in places where a URI would be expected unless the template
expressions will be expanded by a template processor prior to use.
Distinct field, element, or attribute names should be used to
differentiate protocol elements that carry a URI Template from those
that expect a URI reference.
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)
Gregorio, et al. Expires January 12, 2012 [Page 11]
Internet-Draft URI Template Jul 2011
notation of [RFC5234]. The following ABNF rules are imported from
the normative references [RFC5234], [RFC3986], and [RFC3987].
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
1.6. Character Encoding and Unicode Normalization
This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"charset".
The ABNF notation defines its terminal values to be non-negative
integers (codepoints) that are a superset of the US-ASCII coded
character set [ASCII]. This specification defines terminal values as
codepoints within the Unicode coded character set [UNIV4].
In spite of the syntax and template expansion process being defined
in terms of Unicode codepoints, it should be understood that
templates occur in practice as a sequence of characters in whatever
form or encoding is suitable for the context in which they occur,
whether that be octets embedded in a network protocol element or
paint applied to the side of a bus. This specification does not
mandate any particular character encoding for mapping between URI
Template characters and the octets used to store or transmit those
characters. When a URI Template appears in a protocol element, the
character encoding is defined by that protocol; without such a
definition, a URI Template is assumed to be in the same character
encoding as the surrounding text. It is only during the process of
template expansion that a string of characters in a URI Template MUST
Gregorio, et al. Expires January 12, 2012 [Page 12]
Internet-Draft URI Template Jul 2011
be transformed into its corresponding sequence of normalized Unicode
codepoints.
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form KC (NFKC:
Compatibility Decomposition followed by Canonical Composition). The
normalization form determines how to consistently encode the
equivalent strings.
In theory, all URI processing implementations, including template
processors, should use the same normalization form for generating or
handling a URI reference. In practice, they do not. However,
template processors transform the template literals and variable
values from characters to codepoints, expand the expressions, and
then transform the resulting codepoints back to characters again. In
order to promote consistent generation of URI references from
templates, template processors MUST ensure that the template and each
variable value is in NFKC when they are transformed to a sequence of
Unicode codepoints. The final step of template processing will
encode the expanded sequence of Unicode codepoints, which should
still be in NFKC, as UTF-8 [RFC3629] and then further pct-encode any
octets within the UTF-8 string that are not allowed in a URI.
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.
Each character in a URI Template MUST be transcoded to an NFKC
Unicode codepoint prior to evaluation. If a character sequence in
the template consists of pct-encoded triplets corresponding to a
valid sequence of octets in UTF-8 that represent a character not in
the reserved set, then the triplets MUST be replaced with their
corresponding Unicode codepoint. In other words, pct-encoded UTF-8
Gregorio, et al. Expires January 12, 2012 [Page 13]
Internet-Draft URI Template Jul 2011
found within the template is decoded prior to further processing
unless the corresponding character is in the reserved set.
2.1. Literals
The characters outside of expressions in a URI Template string are
intended to be translated literally to the URI-reference.
literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
/ %x5D-5F / %x61-7A / %x7E / ucschar / iprivate
/ pct-encoded
; any Unicode character except: CTL, SP,
; DQUOTE, "'", "%" (aside from pct-encoded),
; "<", ">", "\", "^", "`", "{", "|", "}"
A sequence of pct-encoded literals that corresponds to a UTF-8
encoded character that is not within the reserved set will be
transcoded during template processing, as describe above. If such a
character is not in the unreserved set, it will be re-pct-encoded
during the final step of template processing. Unreserved characters
will therefore be normalized to their unencoded form as a side-effect
of template processing.
2.2. Expressions
Template expressions are the parameterized parts of a URI Template.
Each expression contains an optional operator, which defines the
expression type and its corresponding expansion process, followed by
a comma-separated list of variable specifiers (variable names and
optional value modifiers). If no operator is provided, the
expression defaults to simple variable expansion of unreserved
values.
expression = "{" [ operator ] variable-list "}"
operator = "+" / "." / "/" / ";" / "?" / op-reserve
op-reserve = "," / "!" / "@"
; reserved for local use: "$" / "(" / ")"
The operator characters have been chosen to reflect each of their
roles as reserved characters in the URI generic syntax. The
operators defined by this specification include: plus ("+") for
substituting values that may contain reserved characters; dot (".")
for substituting values as a sequence of name labels prefixed by ".";
slash ("/") for substituting values as a sequence of path segments
separated by "/"; semicolon (";") for substituting key=value pairs as
path parameters prefixed by ";"; and, question-mark ("?") for
substituting a query component beginning with "?" and consisting of
key=value pairs separated by "&". These operators will be described
Gregorio, et al. Expires January 12, 2012 [Page 14]
Internet-Draft URI Template Jul 2011
in detail in Section 3.
The operator characters comma (","), exclamation ("!"), and at-sign
("@") are reserved for future extensions. A processor that
unexpectedly encounters such an extension operator SHOULD pass the
expression through unexpanded and MAY also indicate a warning to the
invoking application.
The expression syntax specifically excludes use of the dollar ("$")
and parentheses ["(" and ")"] characters so that they remain
available for local language extensions outside the scope of this
specification.
2.3. Variables
After the operator (if any), each expression contains a list of one
or more comma-separated variable specifiers (varspec). The variable
names serve multiple purposes: documentation for what kinds of values
are expected, identifiers for associating values within a template
processor, and the string to use for each key on key=value
expansions.
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ] [ "|" default ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / ucschar / iprivate
/ pct-encoded
An expression MAY reference variables that are unknown to the
template processor or whose value is set to a special "undefined"
value, such as undef or null. Such undefined variables are given
special treatment by the expansion process.
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
A variable may have a composite or structured value, such as a list
of values, an associative array of (key, value) pairs, or a structure
of components defined by some separate schema. Such value types are
not directly indicated by the template syntax, but do have an impact
on the expansion process. A composite or structured value with zero
member values is considered undefined.
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).
Gregorio, et al. Expires January 12, 2012 [Page 15]
Internet-Draft URI Template Jul 2011
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 into
components based on an external type or schema associated with that
variable.
modifier = prefix / explode
2.4.1. Prefix Values
A prefix modifier indicates that the variable expansion is limited to
a prefix of the variable's value string. Prefix modifiers are often
used to partition an identifier space hierarchically, as is common in
reference indices and hash-based storage. It also serves to limit
the expanded value to a maximum number of characters.
prefix = ":" offset
max-length = %x31-39 *DIGIT ; positive integer
The max-length is a positive integer that refers to a maximum number
of characters from the beginning of the variable's value as a Unicode
string. Note that this numbering is in characters, not octets, in
order to avoid splitting between the octets of a multi-octet UTF-8
encoded character or within a pct-encoded triplet. If the max-length
is greater than the length of the variable's value, then the entire
value string is used.
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
Gregorio, et al. Expires January 12, 2012 [Page 16]
Internet-Draft URI Template Jul 2011
2.4.2. Component Values
An explode modifier ("*") indicates that the variable represents a
composite value that may be substituted in full or partial forms,
depending on the variable's type or schema. Since URI Templates do
not contain an indication of type or schema, this is assumed to be
determined by context. An example context is a mark-up element or
header field that contains one attribute that is a template and one
or more other attributes that define the schema applicable to
variables found in the template. Likewise, a typed programming
language might differentiate variables as strings, lists, associative
arrays, or structures.
explode = "*"
Explode modifiers improve brevity in the URI Template syntax. For
example, a resource that provides a geographic map for a given street
address might accept a hundred permutations on fields for address
input, including partial addresses (e.g., just the city or postal
code). Such a resource could be described as a template with each
and every address component listed in order, or with a far more
simple template that makes use of an explode modifier, as in
/mapper{?address*}
along with some context that defines what the variable named
"address" can include, such as by reference to some other standard
for addressing (e.g., UPU S42 or AS/NZS 4819:2003). A recipient
aware of the schema can then provide appropriate expansions, such as:
/mapper?city=Newport%20Beach&state=CA
If an explode modifier is present, the expansion process for that
variable, as defined in Section 3, is dependent on both the operator
being used and the type or schema of the value being substituted.
2.5. Value Defaults
Each of the variables in Level 2 (and above) templates may also be
supplied with a default value to be used when a template processor
determines that variable to be undefined. The default value is
limited to unreserved characters, the equal sign ("="), and pct-
encoded triplets, since the default is provided in the exact form
that it would appear in the expanded URI reference. The default is
not affected by any variable modifiers, since we assume that the
default string provided in the expression already reflects the
desired value.
Gregorio, et al. Expires January 12, 2012 [Page 17]
Internet-Draft URI Template Jul 2011
default = *( unreserved / "=" / pct-encoded )
The following examples illustrate how default values work with
different variable types.
Given the variable assignments:
var := "value"
name := [ "Fred", "Wilma", "Pebbles" ]
favs := [("color","red"), ("volume","high")]
empty_keys := []
empty := ""
undef := null
Example Template Expansion
{var|default} value
{undef|default} default
{undef:3|default} default
x{empty}y xy
x{empty|_}y xy
x{undef}y xy
x{undef|_}y x_y
x{.name|none} x.Fred,Wilma,Pebbles
x{.name*|none} x.Fred.Wilma.Pebbles
x{.empty} x.
x{.empty|none} x.
x{.undef} x
x{.undef|none} x.none
x{/name|none} x/Fred,Wilma,Pebbles
x{/name*|none} x/Fred/Wilma/Pebbles
x{/undef} x
x{/undef|none} x/none
x{/empty} x/
x{/empty|none} x/
x{/empty_keys} x
x{/empty_keys|none} x/none
x{/empty_keys*} x
x{/empty_keys*|none} x/none
x{;name|none} x;name=Fred,Wilma,Pebbles
x{;favs|none} x;favs=color,red,volume,high
x{;favs*|none} x;color=red;volume=high
x{;empty} x;empty
x{;empty|none} x;empty
Gregorio, et al. Expires January 12, 2012 [Page 18]
Internet-Draft URI Template Jul 2011
x{;undef} x
x{;undef|none} x;none
x{;undef|foo=y} x;foo=y
x{?var|none} x?var=value
x{?favs|none} x?favs=color,red,volume,high
x{?favs*|none} x?color=red&volume=high
x{?empty} x?empty=
x{?empty|foo=none} x?empty=
x{?undef} x
x{?undef|foo=none} x?foo=none
x{?empty_keys} x
x{?empty_keys|none} x?none
x{?empty_keys|y=z} x?y=z
x{?empty_keys*|y=z} x?y=z
3. Expansion
The process of URI Template expansion is to scan the template string
from beginning to end, copying literal characters as-is 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 value MUST be formed as a sequence of NFKC Unicode
codepoints prior to template expansion. A value is assumed to be raw
data that might need to be encoded by the template processor during
expansion, depending on the expression type.
If a template processor encounters an error outside of an expression,
such as a character sequence that does not match the <URI-Template>
grammar, then processing of the template SHOULD cease, the URI-
reference result SHOULD be undefined, and the location and type of
error SHOULD be indicated to the invoking application. If an error
is encountered inside an expression, such as an operator or value
modifier that it does not recognize or cannot support, then the
expression SHOULD be copied to the result unexpanded, processing of
the remainder of the template SHOULD continue, and the location and
type of error SHOULD be indicated to the invoking application.
3.1. Literal Expansion
If the literal character is allowed anywhere in the URI syntax
(unreserved / reserved), then it is copied directly to the result
string. Otherwise, the pct-encoded equivalent of the literal
character is copied to the result string by encoding the character in
UTF-8 (a sequence of octets) and then encoding each octet as a pct-
Gregorio, et al. Expires January 12, 2012 [Page 19]
Internet-Draft URI Template Jul 2011
encoded triplet.
3.2. Expression Expansion
Each expression is indicated by an opening brace ("{") character and
continues until the next closing brace ("}"). The expression is
expanded by determining the expression type and then following that
type's expansion process for each comma-separated varspec in the
expression. Level 1 templates are limited to the default operator
(simple string value expansion) and a single variable per expression.
Level 2 templates are limited to a single varspec per expression.
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 varspec
list. If the first character is not an operator, then the expression
type is simple expansion and the first character is the beginning of
the varspec list.
3.2.1. Undefined Variable Expansion
Regardless of the expression type, a variable that is undefined has
no value. A variable defined as a list or structure of component
values is considered undefined if the list contains zero members or
all of the structure's components are undefined.
If a default value is provided for an undefined variable, as
described for Level 2 (and above) templates in Section 2.5, then the
default string is substituted in its place as the entire expansion
for that varspec. If all of the variables in an expression are
undefined and no defaults are given, then the expression's expansion
is the empty string.
3.2.2. Simple String Expansion: {var}
The default expression type when no operator is given is simple
string expansion. For each defined variable in the variable-list,
modify its value as indicated by the optional modifiers (if any),
encode the value as UTF-8, pct-encode any octets that are not in the
unreserved set, and then append the encoded value to the result
string. If more than one value is appended, separate each value with
a comma (",").
Gregorio, et al. Expires January 12, 2012 [Page 20]
Internet-Draft URI Template Jul 2011
For example,
foo := "fred"
"{foo}" -> "fred"
"{foo,foo}" -> "fred,fred"
"{bar,foo}" -> "fred"
"{bar|wilma}" -> "wilma"
Level 1 templates are limited to single variable expressions with no
modifiers, defaults, or value structures. Simple string expansion in
Level 2 templates adds variable defaulting, as defined in
Section 2.5. Level 3 templates allow a list of variables. Level 4
templates add compound variable types and value modifiers, as
follows:
For a variable defined as a single value string, the explode modifier
has no effect. The prefix modifier limits the expansion to the first
max-length characters of that single value. If the value contains
pct-encoded triplets, multibyte UTF-8, or both, care must be taken to
avoid splitting the value in mid-character: count each Unicode
codepoint as one character.
For a variable defined as a list of values, when no value modifier is
present or the explode modifier is given, the variable's string
expansion consists of a concatenation of the individual values with
each value separated by a comma (","). A prefix modifier has no
effect.
A variable defined as an associative array is expanded as a list of
alternating key and value pairs, excluding any keys for which the
corresponding value is undefined, when no value modifier is present.
If the explode modifier is given, then the keys with defined values
are expanded as "key=value" pairs instead of "key,value". A prefix
modifier has no effect.
3.2.3. Reserved expansion: {+var}
Reserved expansion is identical to simple expansion except that the
substituted values may contain characters in the reserved set. The
reserved expansion operator "+" is defined for Level 2 templates (and
above).
Gregorio, et al. Expires January 12, 2012 [Page 21]
Internet-Draft URI Template Jul 2011
For example,
foo := "That's right!"
"{foo}" -> "That%27s%20right%21"
"{+foo}" -> "That%27s%20right!"
base := "http://example.com/home/"
"{base}index" -> "http%3A%2F%2Fexample.com%2Fhome%2Findex"
"{+base}index" -> "http://example.com/home/index"
The same expansion process is followed as in Section 3.2.2 except
that, instead of "pct-encode any octets that are not in the
unreserved set", we pct-encode any octets that are not in either the
reserved or unreserved sets.
3.2.4. Label expansion with dot-prefix: {.var}
The dot (".") operator indicates that the expression type is label
expansion, which can be useful for describing URI spaces with varying
domain names or path selectors (e.g., filename extensions).
For each variable in the variable-list, do one of
a) if the variable has a defined value
1) modify its value as indicated by the optional
modifiers (if any);
2) encode the value as UTF-8;
3) pct-encode any octets that are not in the
unreserved set;
4) append "." to the result string; and,
5) append the encoded value to the result string.
b) if the variable is undefined and a default is given,
1) encode the default string as UTF-8;
2) append "." to the result string; and,
3) append the default to the result string.
c) the variable is undefined and has no default,
Gregorio, et al. Expires January 12, 2012 [Page 22]
Internet-Draft URI Template Jul 2011
so ignore this variable.
Since "." is not in the reserved set, a value that contains a "." has
the effect of adding multiple labels.
For example,
foo := "fred"
"{foo}" -> ".fred"
"{foo,foo}" -> ".fred.fred"
"{bar,foo}" -> ".fred"
"{bar|wilma}" -> ".wilma"
Label expansion only applies to Level 3 and Level 4 templates. Level
4 templates add compound variable types and value modifiers, as
follows:
For a variable defined as a single value string, the explode modifier
has no effect. The prefix modifier limits the expansion to the first
max-length characters of that single value. If the value contains
pct-encoded triplets, multibyte UTF-8, or both, care must be taken to
avoid splitting the value in mid-character: count each Unicode
codepoint as one character.
For a variable defined as a list of values, when no value modifier is
present or the explode modifier is given, the variable's string
expansion consists of a concatenation of the individual defined
values with each value prepended by a dot ("."). A prefix modifier
has no effect.
When no value modifier is present, a variable defined as an
associative array is expanded by appending the (key, value) pairs as
alternating labels (i.e., ".key.value"), but excluding any keys for
which the corresponding value is undefined. If the explode modifier
is given, then the keys with defined values are expanded as
".key=value" instead of ".key.value". A prefix modifier has no
effect.
3.2.5. Path segment expansion: {/var}
The slash ("/") operator indicates that the expression type is
hierarchical path segment expansion, which can be useful for
describing URI path hierarchies.
Gregorio, et al. Expires January 12, 2012 [Page 23]
Internet-Draft URI Template Jul 2011
For each variable in the variable-list, do one of
a) if the variable has a defined value
1) modify its value as indicated by the optional
modifiers (if any);
2) encode the value as UTF-8;
3) pct-encode any octets that are not in the
unreserved set;
4) append "/" to the result string; and,
5) append the encoded value to the result string.
b) if the variable is undefined and a default is given,
1) encode the default string as UTF-8;
2) append "/" to the result string; and,
3) append the default to the result string.
c) the variable is undefined and has no default,
so ignore this variable.
Note that the expansion process for path segment expansion is
identical to that of label expansion aside from the substitution of
"/" instead of ".".
For example,
foo := "fred"
"{foo}" -> "/fred"
"{foo,foo}" -> "/fred/fred"
"{bar,foo}" -> "/fred"
"{bar|wilma}" -> "/wilma"
Label expansion only applies to Level 3 and Level 4 templates. Level
4 templates add compound variable types and value modifiers, as
follows:
For a variable defined as a single value string, the explode modifier
has no effect. The prefix modifier limits the expansion to the first
max-length characters of that single value. If the value contains
pct-encoded triplets, multibyte UTF-8, or both, care must be taken to
Gregorio, et al. Expires January 12, 2012 [Page 24]
Internet-Draft URI Template Jul 2011
avoid splitting the value in mid-character: count each Unicode
codepoint as one character.
For a variable defined as a list of values, when no value modifier is
present or the explode modifier is given, the variable's string
expansion consists of a concatenation of the individual defined
values with each value prepended by a slash ("/"). A prefix modifier
has no effect.
When no value modifier is present, a variable defined as an
associative array is expanded by appending the (key, value) pairs as
alternating segments (i.e., "/key/value"), but excluding any keys for
which the corresponding value is undefined. If the explode modifier
is given, then the keys with defined values are expanded as
"/key=value" instead of "/key/value". A prefix modifier has no
effect.
3.2.6. Path-style parameter expansion: {;var}
TBD.
3.2.7. Form-style query expansion: {?var}
TBD.
4. Security Considerations
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, as described
in section 7 of [RFC3986].
5. IANA Considerations
No IANA actions are required by this document.
6. Acknowledgments
The following people made significant contributions to this
specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton,
John Cowan, James H. Manger, and James Snell.
7. Normative References
[ASCII] American National Standards Institute, "Coded Character
Gregorio, et al. Expires January 12, 2012 [Page 25]
Internet-Draft URI Template Jul 2011
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
4.0.1, defined by: The Unicode Standard, Version 4.0
(Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
as amended by Unicode 4.0.1
(http://www.unicode.org/versions/Unicode4.0.1/)",
March 2004.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex # 15, April 2003.
[1] <http://lists.w3.org/Archives/Public/uri/>
Appendix A. Example URI Template Parser
Parsing a valid URI Template expression does not require building a
parser from the given ABNF. Instead, the set of allowed characters
in each part of URI Template expression has been chosen to avoid
complex parsing, and breaking an expression into its component parts
can be achieved by a series of splits of the character string.
Here is example Python code that parses a URI Template expression and
returns the operator, argument, and variables as a tuple. The
variables are returned as a dictionary of variable names mapped to
their default values. If no default is given then the name maps to
None.
Gregorio, et al. Expires January 12, 2012 [Page 26]
Internet-Draft URI Template Jul 2011
TBD.
Appendix B. Revision History (to be removed by RFC Editor)
05 - Introduced levels to differentiate between legacy, partial, and
full implementations of URI Templates. Changed the default indicator
to pipe ("|") to allow the defaults to contain the equals character
and thus remove the need for complex defaulting for the different
variable types. Removed suffix, remainder, and labelled value
expansion because there didn't seem much interest in them. Clarified
that templates and values are processed as sequences of Unicode
codepoints rather than prematurely encoded as UTF-8, since that is
easier to explain and more consistent with common language routines
for processing Unicode strings.
04 - Changed the operator syntax to a single character that is
analogous to its reserved role within the URI generic syntax,
resulting in templates that are far more readable for the common
cases. Added value modifiers for prefix and suffix expansion. Added
explode modifiers to allow expansion of complex variables and lists
according to (external) variable types or schema. Replaced use of
"expansion" with "expression", since expansion is traditionally used
to refer to the result after expanding a macro (not the macro
itself). Made applicable to any hypertext reference string, such
that the process for template expansion also includes transforming
the surrounding string into a proper URI-reference rather than
assuming it is already in absolute URI form. Rewrote the text
accordingly.
03 - Added more examples. Introduced error conditions and defined
their handling. Changed listjoin to list. Changed -append to
-suffix, and allowed -prefix and -suffix to accept list variables.
Clarified the handling of unicode.
02 - Added operators and came up with coherent percent-encoding and
reserved character story. Added large examples section which is
extracted and tested against the implementation.
01
00 - Initial Revision.
Gregorio, et al. Expires January 12, 2012 [Page 27]
Internet-Draft URI Template Jul 2011
Authors' Addresses
Joe Gregorio
Google
Email: joe@bitworking.org
URI: http://bitworking.org/
Roy T. Fielding (editor)
Adobe Systems Incorporated
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/
Marc Hadley
Oracle
Email: Marc.Hadley@oracle.com
URI: http://oracle.com/
Mark Nottingham
Email: mnot@pobox.com
URI: http://mnot.net/
David Orchard
URI: http://www.pacificspirit.com/
Gregorio, et al. Expires January 12, 2012 [Page 28]
