One document matched: draft-gregorio-uritemplate-08.xml
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<rfc category="std" ipr="trust200902" docName="draft-gregorio-uritemplate-08">
<front>
<title>URI Template</title>
<author initials='J.C.' surname="Gregorio" fullname='Joe Gregorio'>
<organization>Google</organization>
<address>
<email>joe@bitworking.org</email>
<uri>http://bitworking.org/</uri>
</address>
</author>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address>
<email>fielding@gbiv.com</email>
<uri>http://roy.gbiv.com/</uri>
</address>
</author>
<author initials='M.H.' surname="Hadley" fullname='Marc Hadley'>
<organization abbrev="MITRE">The MITRE Corporation</organization>
<address>
<email>mhadley@mitre.org</email>
<uri>http://mitre.org/</uri>
</address>
</author>
<author initials='M.N.' surname="Nottingham" fullname='Mark Nottingham'>
<organization>Rackspace</organization>
<address>
<email>mnot@mnot.net</email>
<uri>http://www.mnot.net/</uri>
</address>
</author>
<author initials='D.O.' surname="Orchard" fullname='David Orchard'>
<organization>Salesforce.com</organization>
<address>
<email>orchard@pacificspirit.com</email>
<uri>http://www.pacificspirit.com/</uri>
</address>
</author>
<date day="26" month="Jan" year="2012"/>
<abstract>
<t>
A URI Template is a compact sequence of characters
for describing a range of Uniform Resource Identifiers
through variable expansion.
This specification defines the URI Template
syntax and the process for expanding a URI Template into a
URI reference, along with guidelines for the
use of URI Templates on the Internet.
</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="intro">
<section title="Overview" anchor="overview">
<t>
A Uniform Resource Identifier (URI) <xref target="RFC3986"/> is often
used to identify a specific resource within a common space of similar
resources (informally, a "URI space"). For example, personal web spaces
are often delegated using a common pattern, such as
</t>
<figure><artwork>
http://example.com/~fred/
http://example.com/~mark/
</artwork></figure>
<t>
or a set of dictionary entries might be grouped in a hierarchy
by the first letter of the term, as in
</t>
<figure><artwork>
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
</artwork></figure>
<t>
or a service interface might be invoked with various user input
in a common pattern, as in
</t>
<figure><artwork>
http://example.com/search?q=cat&lang=en
http://example.com/search?q=chien&lang=fr
</artwork></figure>
<t>
A URI Template is a compact sequence of characters for describing a
range of Uniform Resource Identifiers through variable expansion.
</t>
<t>
URI Templates provide a mechanism for abstracting a space of
resource identifiers such that the variable parts can be easily
identified and described. URI templates can have many uses,
including discovery of available services, configuring resource
mappings, defining computed links, specifying interfaces, and
other forms of programmatic interaction with resources.
For example, the above resources could be described by the
following URI templates:
</t>
<figure><artwork>
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
</artwork></figure>
<t>
We define the following terms:
<list style="symbols">
<t>expression -
The text between '{' and '}', including the enclosing braces,
as defined in <xref target="syntax"/>.</t>
<t>expansion -
The string result obtained from a template expression after
processing it according to its expression type, list of variable
names, and value modifiers, as defined in <xref target="expansion"/>.</t>
<t>template processor -
A program or library that, given a URI Template and a set of
variables with values, transforms the template string into a
URI-reference by parsing the template for expressions and
substituting each one with its corresponding expansion.</t>
</list>
</t>
<t>
A URI Template provides both a structural description of a URI
space and, when variable values are provided, machine-readable
instructions on how to construct a URI corresponding to those values.
A URI Template is transformed into a URI-reference by replacing each
delimited expression with its value as defined by the
expression type and the values of variables named within the
expression. The expression types range from simple string
expansion to multiple 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.
</t>
<t>
For example, the following URI Template includes a
form-style parameter expression, as indicated by the "?" operator
appearing before the variable names.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
</artwork></figure>
<t>
The expansion process for expressions beginning with the
question-mark ("?") operator follows the same pattern
as form-style interfaces on the World Wide Web:
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
\_____________/
|
|
For each defined variable in [ 'query', 'number' ],
substitute "?" if it is the first substitution or "&"
thereafter, followed by the variable name, '=', and the
variable's value.
</artwork></figure>
<t>
If the variables have the values
</t>
<figure><artwork>
query := "mycelium"
number := 100
</artwork></figure>
<t>
then the expansion of the above URI Template is
</t>
<figure><artwork>
http://www.example.com/foo?query=mycelium&number=100
</artwork></figure>
<t>
Alternatively, if 'query' is undefined, then the expansion
would be
</t>
<figure><artwork>
http://www.example.com/foo?number=100
</artwork></figure>
<t>
or if both variables are undefined, then it would be
</t>
<figure><artwork>
http://www.example.com/foo
</artwork></figure>
<t>
A URI Template may be provided in absolute form, as in the examples
above, or in relative form. A template is expanded before the
resulting reference is resolved from relative to absolute form.
</t>
<t>
Although the URI syntax is used for the result, the template
string is allowed to contain the broader set of characters
that can be found in IRI references <xref target="RFC3987"/>.
A URI Template is therefore also an IRI template, and the result
of template processing can be transformed to an IRI by following
the process defined in Section 3.2 of <xref target="RFC3987"/>.
</t>
</section>
<section title="Levels and Expression Types" anchor="types">
<t>
URI Templates are similar to a macro language with a fixed set of
macro definitions: the expression type determines the expansion
process. The default expression type is simple string expansion, wherein
a single named variable is replaced by its value as a string after
pct-encoding any characters
not in the set of unreserved URI characters (<xref target="notation"/>).
</t>
<t>
Since most template processors implemented prior to this specification
have only implemented the default expression type, we refer to these as
Level 1 templates.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 1 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | Simple string expansion (Sec 3.2.2) |
| | |
| | {var} value |
| | {hello} Hello%20World%21 |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Level 2 templates add the plus ("+") operator, for expansion of
values that are allowed to include reserved URI characters
(<xref target="notation"/>), and the crosshatch ("#") operator
for expansion of fragment identifiers.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 2 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| + | Reserved string expansion (Sec 3.2.3) |
| | |
| | {+var} value |
| | {+hello} Hello%20World! |
| | {+path}/here /foo/bar/here |
| | here?ref={+path} here?ref=/foo/bar |
|-----+-----------------------------------------------------------|
| # | Fragment expansion, crosshatch-prefixed (Sec 3.2.4) |
| | |
| | X{#var} X#value |
| | X{#hello} X#Hello%20World! |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Level 3 templates 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 forms-style construction of a query syntax consisting of
name=value pairs that are separated by an ampersand character.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 3 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| path := "/foo/bar" |
| x := "1024" |
| y := "768" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with multiple variables (Sec 3.2.2) |
| | |
| | map?{x,y} map?1024,768 |
| | {x,hello,y} 1024,Hello%20World%21,768 |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with multiple variables (Sec 3.2.3) |
| | |
| | {+x,hello,y} 1024,Hello%20World!,768 |
| | {+path,x}/here /foo/bar,1024/here |
| | |
|-----+-----------------------------------------------------------|
| # | Fragment expansion with multiple variables (Sec 3.2.4) |
| | |
| | {#x,hello,y} #1024,Hello%20World!,768 |
| | {#path,x}/here #/foo/bar,1024/here |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.5) |
| | |
| | X{.var} X.value |
| | X{.x,y} X.1024.768 |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.6) |
| | |
| | {/var} /value |
| | {/var,x}/here /value/1024/here |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.7) |
| | |
| | {;x,y} ;x=1024;y=768 |
| | {;x,y,empty} ;x=1024;y=768;empty |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.8) |
| | |
| | {?x,y} ?x=1024&y=768 |
| | {?x,y,empty} ?x=1024&y=768&empty= |
| | |
|-----+-----------------------------------------------------------|
| & | Form-style query continuation (Sec 3.2.9) |
| | |
| | ?fixed=yes{&x} ?fixed=yes&x=1024 |
| | {&x,y,empty} &x=1024&y=768&empty= |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Finally, Level 4 templates add 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 (<xref target="prefix-values"/>).
An explode ("*") modifier indicates that the variable is to be treated
as a composite value, consisting of either a list of names or an
associative array of (name, value) pairs, that is expanded as if each
member were a separate variable (<xref target="composite-values"/>).
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 4 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| list := ("red", "green", "blue") |
| keys := [("semi",";"),("dot","."),("comma",",")] |
| |
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with value modifiers (Sec 3.2.2) |
| | |
| | {var:3} val |
| | {var:30} value |
| | {list} red,green,blue |
| | {list*} red,green,blue |
| | {keys} semi,%3B,dot,.,comma,%2C |
| | {keys*} semi=%3B,dot=.,comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with value modifiers (Sec 3.2.3) |
| | |
| | {+path:6}/here /foo/b/here |
| | {+list} red,green,blue |
| | {+list*} red,green,blue |
| | {+keys} semi,;,dot,.,comma,, |
| | {+keys*} semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| # | Fragment expansion with value modifiers (Sec 3.2.4) |
| | |
| | {#path:6}/here #/foo/b/here |
| | {#list} #red,green,blue |
| | {#list*} #red,green,blue |
| | {#keys} #semi,;,dot,.,comma,, |
| | {#keys*} #semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.5) |
| | |
| | X{.var:3} X.val |
| | X{.list} X.red,green,blue |
| | X{.list*} X.red.green.blue |
| | X{.keys} X.semi,%3B,dot,.,comma,%2C |
| | X{.keys*} X.semi=%3B.dot=..comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.6) |
| | |
| | {/var:1,var} /v/value |
| | {/list} /red,green,blue |
| | {/list*} /red/green/blue |
| | {/list*,path:4} /red/green/blue/%2Ffoo |
| | {/keys} /semi,%3B,dot,.,comma,%2C |
| | {/keys*} /semi=%3B/dot=./comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.7) |
| | |
| | {;hello:5} ;hello=Hello |
| | {;list} ;list=red,green,blue |
| | {;list*} ;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 |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
</section>
<section title="Design Considerations" anchor="design">
<t>
Mechanisms similar to URI Templates have been defined within several
specifications, including WSDL <xref target="WSDL"/>,
WADL <xref target="WADL"/> and OpenSearch <xref target="OpenSearch"/>.
This specification extends and formally defines the syntax so that URI
Templates can be used consistently across multiple Internet applications
and within Internet message fields, while at the same time retaining
compatibility with those earlier definitions.
</t>
<t>
The URI Template syntax has been designed to carefully balance
the need for a powerful expansion mechanism with the need for
ease of implementation. The syntax is designed to be trivial
to parse while at the same time providing enough flexibility to
express many common template scenarios. Implementations are able
to parse the template and perform the expansions in a single pass.
</t>
<t>
Templates are simple and readable when used with common
examples because the single-character operators match the URI
generic syntax delimiters. The operator's associated delimiter
(".", ";", "/", "?", "&", and "#") is omitted when none of the
listed variables are defined. Likewise, the expansion process for ";"
(path-style parameters) will omit the "=" when the variable value
is empty, whereas the process for "?" (form-style parameters)
will not omit the "=" when the value is empty. Multiple variables
and list values have their values joined with "," if there is no
predefined joining mechanism for the operator. The "+" and "#"
operators will substitute unencoded reserved characters found
inside the variable values; the other operators will pct-encode
reserved characters found in the variable values prior to expansion.
</t>
<t>
The most common cases for URI spaces can be described with
Level 1 template expressions. If we were only concerned with
URI generation, then the template syntax could be limited to
just simple variable expansion, since more complex forms could
be generated by changing the variable values. However, URI
Templates have the additional goal of describing the layout of
identifiers in terms of preexisting data values. The template
syntax therefore includes operators that reflect how
resource identifiers are commonly allocated. Likewise, since
prefix substrings are often used to partition large spaces of
resources, modifiers on variable values provide a way
to specify both the substring and the full value string
with a single variable name.
</t>
</section>
<section title="Limitations" anchor="limitations">
<t>
Since a URI Template describes a superset of the identifiers,
there is no implication that every possible expansion for
each delimited variable expression corresponds to a URI of an
existing resource. Our expectation is that an application
constructing URIs according to the template will be provided
with an appropriate set of values for the variables being
substituted, or at least a means of validating user data-entry
for those values.
</t>
<t>
URI Templates are not URIs: they do not identify
an abstract or physical resource, they are not parsed as URIs,
and should not be used in places where a URI would be expected
unless the template expressions will be expanded by a template
processor prior to use. Distinct field, element, or attribute
names should be used to differentiate protocol elements that
carry a URI Template from those that expect a URI reference.
</t>
<t>
Some URI Templates can be used in reverse for the purpose of
variable matching: comparing the template to a fully formed
URI in order to extract the variable parts from that URI and
assign them to the named variables. Variable matching only works
well if the template expressions are delimited by the beginning or
end of the URI or by characters that cannot be part of the
expansion, such as reserved characters surrounding a simple
string expression. In general, regular expression languages
are better suited for variable matching.
</t>
</section>
<section title="Notational Conventions" anchor="notation">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <xref target="RFC5234"/>. The following ABNF rules
are imported from the normative references <xref target="RFC5234"/>,
<xref target="RFC3986"/>, and <xref target="RFC3987"/>.
</t>
<figure><artwork type="abnf">
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
; 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
</artwork></figure>
</section>
<section title='Character Encoding and Unicode Normalization'
anchor="character-encoding">
<t>
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 <xref target="RFC6365"/>.
</t>
<t>
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 <xref target="ASCII"/>. This
specification defines terminal values as code points within the
Unicode coded character set <xref target="UNIV6"/>.
</t>
<t>
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.
</t>
<t>
The Unicode Standard <xref target="UNIV6"/> defines various
equivalences between sequences of characters for various purposes.
Unicode Standard Annex #15 <xref target="UTR15"/> defines various
Normalization Forms for these equivalences. The normalization form
determines how to consistently encode equivalent strings.
In theory, all URI processing implementations, including template
processors, should use the same normalization form for generating
a URI reference. In practice, they do not. If a value has been
provided by the same server as the resource, then it can be assumed
that the string is already in the form expected by that server.
If a value is provided by a user, such as via a data-entry dialog,
then the string SHOULD be normalized as Normalization Form C
(NFC: Canonical Decomposition, followed by Canonical Composition)
prior to being used in expansions by a template processor.
</t>
<t>
Likewise, when non-ASCII data that represents readable strings is
pct-encoded for use in a URI reference, a template processor MUST
first encode the string as UTF-8 <xref target="RFC3629"/> and then
pct-encode any octets that are not allowed in a URI reference.
</t>
</section>
</section>
<section title="Syntax" anchor="syntax">
<t>
A URI Template is a string of printable Unicode characters
that contains zero or more embedded variable expressions, each
expression being delimited by a matching pair of braces ('{', '}').
</t>
<figure><artwork type="abnf">
URI-Template = *( literals / expression )
</artwork></figure>
<t>
Although templates (and template processor implementations) are
described above in terms of four gradual levels, we define the
URI-Template syntax in terms of the ABNF for Level 4. A template
processor limited to lower level templates MAY exclude the ABNF
rules applicable only to higher levels. However, it is RECOMMENDED
that all parsers implement the full syntax such that unsupported
levels can be properly identified as such to the end user.
</t>
<section title="Literals" anchor="literals">
<t>
The characters outside of expressions in a URI Template string
are intended to be copied literally to the URI-reference if the
character is allowed in a URI (reserved / unreserved / pct-encoded)
or, if not allowed, copied to the URI-reference as the sequence of
pct-encoded triplets corresponding to that character's encoding
in UTF-8 <xref target="RFC3629"/>.
</t>
<figure><artwork type="abnf">
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),
; "<", ">", "\", "^", "`", "{", "|", "}"
</artwork></figure>
</section>
<section title="Expressions" anchor="expressions">
<t>
Template expressions are the parameterized parts of a URI Template.
Each expression contains an optional operator, which defines the
expression type and its corresponding expansion process, followed by a
comma-separated list of variable specifiers (variable names and
optional value modifiers). If no operator is provided, the expression
defaults to simple variable expansion of unreserved values.
</t>
<figure><artwork type="abnf">
expression = "{" [ operator ] variable-list "}"
operator = op-level2 / op-level3 / op-reserve
op-level2 = "+" / "#"
op-level3 = "." / "/" / ";" / "?" / "&"
op-reserve = "=" / "," / "!" / "@" / "|"
</artwork></figure>
<t>
The operator characters have been chosen to reflect each of their roles
as reserved characters in the URI generic syntax. The operators defined
in <xref target="expansion"/> of this specification include:
</t>
<figure><artwork>
+ Reserved character strings;
# Fragment identifiers prefixed by "#";
. Name labels or extensions prefixed by ".";
/ Path segments prefixed by "/";
; Path parameter 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.
</artwork></figure>
<t>
The operator characters equals ("="), comma (","), exclamation ("!"),
at-sign ("@"), and pipe ("|") are reserved for future extensions.
</t>
<t>
The expression syntax specifically excludes use of the dollar ("$")
and parentheses ["(" and ")"] characters so that they remain
available for 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.
</t>
</section>
<section title="Variables" anchor="variables">
<t>
After the operator (if any), each expression contains a list of
one or more comma-separated variable specifiers (varspec).
The variable names serve multiple purposes: documentation for
what kinds of values are expected, identifiers for associating
values within a template processor, and the literal string to use
for the name in name=value expansions (aside from when exploding
an associative array). Variable names are case-sensitive because
the name might be expanded within a case-sensitive URI component.
</t>
<figure><artwork type="abnf">
variable-list = varspec *( "," varspec )
varspec = varname [ modifier-level4 ]
varname = varchar *( ["."] varchar )
varchar = ALPHA / DIGIT / "_" / pct-encoded
</artwork></figure>
<t>
A varname MAY contain one or more pct-encoded triplets.
These triplets are considered an essential part of the variable name
and are not decoded during processing.
A varname containing pct-encoded characters is not the same variable
as a varname with those same characters decoded. Applications that
provide URI Templates are expected to be consistent in their use of
pct-encoding within variable names.
</t>
<t>
An expression MAY reference variables that are unknown to the template
processor or whose value is set to a special "undefined" value, such as
undef or null. Such undefined variables are given special treatment by
the expansion process (<xref target="variable-expansion"/>).
</t>
<t>
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
</t>
<t>
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
do have an impact on the expansion process
(<xref target="variable-expansion"/>).
</t>
<t>
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.
</t>
</section>
<section title="Value Modifiers" anchor="modifiers">
<t>
Each of the variables in a Level 4 template expression can have
a modifier indicating either that its expansion is limited to a
prefix of the variable's value string or that its expansion is
exploded as a composite value in the form of a value list or
an associative array of (name, value) pairs.
</t>
<figure><artwork type="abnf">
modifier-level4 = prefix / explode
</artwork></figure>
<section title="Prefix Values" anchor="prefix-values">
<t>
A prefix modifier indicates that the variable expansion is limited
to a prefix of the variable's value string. Prefix modifiers are
often used to partition an identifier space hierarchically, as is
common in reference indices and hash-based storage. It also serves
to limit the expanded value to a maximum number of characters.
Prefix modifiers are not applicable to variables that have composite
values.
</t>
<figure><artwork type="abnf">
prefix = ":" max-length
max-length = %x31-39 0*3DIGIT ; positive integer < 10000
</artwork></figure>
<t>
The max-length is a positive integer that refers to a maximum number
of characters from the beginning of the variable's value as a
Unicode string.
Note that this numbering is in characters, not octets, in order to
avoid splitting between the octets of a multi-octet encoded
character or within a pct-encoded triplet.
If the max-length is greater than the length of the variable's
value, then the entire value string is used.
</t>
<figure><preamble>For example,</preamble><artwork>
Given the variable assignments
var := "value"
semi := ";"
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{semi} %3B
{semi:2} %3B
</artwork></figure>
</section>
<section title="Composite Values" anchor="composite-values">
<t>
An explode ("*") modifier indicates that the variable 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.
</t>
<figure><artwork type="abnf">
explode = "*"
</artwork></figure>
<t>
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, IDL, etc.) might define rules for
associating variable names with types, structures, or schema.
</t>
<t>
Explode modifiers improve brevity in the URI Template syntax.
For example, a resource that provides a geographic map for a
given street address might accept a hundred permutations on
fields for address input, including partial addresses (e.g.,
just the city or postal code). Such a resource could be
described as a template with each and every address component
listed in order, or with a far more simple template that makes
use of an explode modifier, as in
</t>
<figure><artwork>
/mapper{?address*}
</artwork></figure>
<t>
along with some context that defines what the variable named
"address" can include, such as by reference to some other
standard for addressing (e.g., <xref target="UPU-S42"/>).
A recipient aware of the schema can then provide appropriate
expansions, such as:
</t>
<figure><artwork>
/mapper?city=Newport%20Beach&state=CA
</artwork></figure>
<t>
The expansion process for exploded variables is dependent on
both the operator being used and whether the composite value is to
be treated as a list of values or as an associative array of
(name, value) pairs. Structures are processed as if they are an
associative array with names corresponding to the fields in the
structure definition and "." separators used to indicate name
hierarchy in substructures.
</t>
<t>
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.
</t>
<t>
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
</t>
<figure><artwork>
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
</artwork></figure>
</section>
</section>
</section>
<section title="Expansion" anchor="expansion">
<t>
The process of URI Template expansion is to scan the template
string from beginning to end, copying literal characters
and replacing each expression with the result of applying the
expression's operator to the value of each variable named in
the expression. Each variable's value MUST be formed prior to
template expansion.
</t>
<t>
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
<xref target="implementation"/>.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<section title='Literal Expansion' anchor="literal-expansion">
<t>
If the literal character is allowed anywhere in the URI
syntax (unreserved / reserved / pct-encoded ), then it is copied
directly to the result string. Otherwise, the pct-encoded equivalent
of the literal character is copied to the result string by first
encoding the character as its sequence of octets in UTF-8 and
then encoding each such octet as a pct-encoded triplet.
</t>
</section>
<section title='Expression Expansion' anchor="expression-expansion">
<t>
Each expression is indicated by an opening brace ("{") character
and continues until the next closing brace ("}"). Expressions cannot
be nested.
</t>
<t>
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.
</t>
<t>
The expression type is determined by looking at the first
character after the opening brace. If the character is an
operator, then remember the expression type associated with
that operator for later expansion decisions and skip to the
next character for the variable-list. If the first character
is not an operator, then the expression type is simple string
expansion and the first character is the beginning of the
variable-list.
</t>
<figure><preamble>The examples in the subsections below use
the following definitions for variable values:</preamble><artwork>
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
</artwork></figure>
<section title='Variable Expansion' anchor="variable-expansion">
<t>
A variable that is undefined (<xref target="variables"/>)
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.
</t>
<t>
Variable expansion of a defined, non-empty value results in a
substring of allowed URI characters. As described in
<xref target="character-encoding"/>, 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 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 <xref target="RFC3629"/>.
</t>
<t>
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.
</t>
<t>
If a variable appears more than once in an expression or
within multiple expressions of a URI Template, the value of
that variable MUST remain static throughout the expansion
process (i.e., the variable must have the same value for the
purpose of calculating each expansion). However, if reserved
characters or pct-encoded triplets occur in the value, they will be
pct-encoded by some expression types and not by others.
</t>
<t>
For a variable that is a simple string value, expansion consists
of appending the encoded value to the result string. 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.
</t>
<t>
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:
</t>
<figure><artwork>
Type Separator
"," (default)
+ ","
# ","
. "."
/ "/"
; ";"
? "&"
& "&"
</artwork></figure>
<t>
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.
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Simple String Expansion: {var}' anchor="op-simple">
<t>
Simple string expansion is the default expression type when no
operator is given.
</t>
<t>
For each defined variable in the variable-list,
perform variable expansion, as defined in
<xref target="variable-expansion"/>, 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.
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Reserved expansion: {+var}' anchor="op-plus">
<t>
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.
</t>
<t>
For each defined variable in the variable-list, perform variable
expansion, as defined in <xref target="variable-expansion"/>, 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.
</t>
<figure><artwork>
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=,
</artwork></figure>
</section>
<section title='Fragment expansion: {#var}' anchor="op-crosshatch">
<t>
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.
</t>
<figure><artwork>
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=,
</artwork></figure>
</section>
<section title="Label expansion with dot-prefix: {.var}" anchor="op-dot">
<t>
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).
</t>
<t>
For each defined variable in the variable-list, append "." to the
result string and then perform variable expansion, as defined in
<xref target="variable-expansion"/>, with the allowed characters
being those in the unreserved set.
</t>
<t>
Since "." is in the unreserved set, a value that contains
a "." has the effect of adding multiple labels.
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Path segment expansion: {/var}' anchor="op-slash">
<t>
Path segment expansion, as indicated by the slash ("/") operator
in Level 3 and above templates, is useful for describing URI
path hierarchies.
</t>
<t>
For each defined variable in the variable-list, append "/" to the
result string and then perform variable expansion, as defined in
<xref target="variable-expansion"/>, with the allowed characters
being those in the unreserved set.
</t>
<t>
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.
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Path-style parameter expansion: {;var}' anchor="op-semicolon">
<t>
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".
</t>
<t>
For each defined variable in the variable-list:
<list style="symbols">
<t>append ";" to the result string;</t>
<t>if the variable has a simple string value or
no explode modifier is given, then:
<list style="symbols">
<t>append the variable name (encoded as if it were a literal
string) to the result string;</t>
<t>if the variable's value is not empty, append "=" to the
result string;</t>
</list>
</t>
<t>perform variable expansion, as defined in
<xref target="variable-expansion"/>, with the allowed characters
being those in the unreserved set.</t>
</list>
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Form-style query expansion: {?var}' anchor="op-question">
<t>
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.
</t>
<t>
For each defined variable in the variable-list:
<list style="symbols">
<t>append "?" to the result string if this is the first defined value
or append "&" thereafter;</t>
<t>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,</t>
<t>perform variable expansion, as defined in
<xref target="variable-expansion"/>, with the allowed characters
being those in the unreserved set.</t>
</list>
</t>
<figure><artwork>
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
</artwork></figure>
</section>
<section title='Form-style query continuation: {&var}' anchor="op-ampersand">
<t>
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.
</t>
<t>
For each defined variable in the variable-list:
<list style="symbols">
<t>append "&" to the result string;</t>
<t>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,</t>
<t>perform variable expansion, as defined in
<xref target="variable-expansion"/>, with the allowed characters
being those in the unreserved set.</t>
</list>
</t>
<figure><artwork>
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
</artwork></figure>
</section>
</section>
</section>
<section title="Security Considerations">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Other security considerations are the same as those for URIs, as described
in section 7 of <xref target="RFC3986"/>.
</t>
</section>
<section title="IANA Considerations" anchor="iana">
<t>
No IANA actions are required by this document.
</t>
</section>
<section title="Acknowledgments" anchor="ack">
<t>
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.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
<reference anchor="ASCII">
<front>
<title>Coded Character Set - 7-bit American Standard Code for Information Interchange</title>
<author>
<organization>American National Standards Institute</organization>
</author>
<date month="" year="1986" />
</front>
<seriesInfo name="ANSI" value="X3.4" />
</reference>
<reference anchor='RFC2119'>
<front>
<title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='Scott Bradner'>
<organization /></author>
<date year='1997' month='March' />
<area>General</area>
<keyword>keyword</keyword>
</front>
<seriesInfo name='BCP' value='14' />
<seriesInfo name='RFC' value='2119' />
</reference>
<reference anchor='RFC3629'>
<front>
<title>UTF-8, a transformation format of ISO 10646</title>
<author initials='F.' surname='Yergeau' fullname='F. Yergeau'>
<organization /></author>
<date year='2003' month='November' />
</front>
<seriesInfo name='STD' value='63' />
<seriesInfo name='RFC' value='3629' />
</reference>
<reference anchor='RFC3986'>
<front>
<title>Uniform Resource Identifier (URI): Generic Syntax</title>
<author initials='T.' surname='Berners-Lee' fullname='T. Berners-Lee'>
<organization /></author>
<author initials='R.' surname='Fielding' fullname='R. Fielding'>
<organization /></author>
<author initials='L.' surname='Masinter' fullname='L. Masinter'>
<organization /></author>
<date year='2005' month='January' />
</front>
<seriesInfo name='STD' value='66' />
<seriesInfo name='RFC' value='3986' />
</reference>
<reference anchor='RFC3987'>
<front>
<title>Internationalized Resource Identifiers (IRIs)</title>
<author initials='M.' surname='Duerst' fullname='M. Duerst'>
<organization /></author>
<author initials='M.' surname='Suignard' fullname='M. Suignard'>
<organization /></author>
<date year='2005' month='January' />
</front>
<seriesInfo name='RFC' value='3987' />
</reference>
<reference anchor='RFC5234'>
<front>
<title>Augmented BNF for Syntax Specifications: ABNF</title>
<author initials='D.' surname='Crocker' fullname='D. Crocker'>
<organization /></author>
<author initials='P.' surname='Overell' fullname='P. Overell'>
<organization /></author>
<date year='2008' month='January' />
</front>
<seriesInfo name='STD' value='68' />
<seriesInfo name='RFC' value='5234' />
</reference>
<reference anchor='RFC6365'>
<front>
<title>Terminology Used in Internationalization in the IETF</title>
<author initials='P.' surname='Hoffman' fullname='P. Hoffman'>
<organization /></author>
<author initials='J.' surname='Klensin' fullname='J. Klensin'>
<organization /></author>
<date year='2011' month='September' />
</front>
<seriesInfo name='BCP' value='166' />
<seriesInfo name='RFC' value='6365' />
</reference>
<reference anchor="UNIV6"
target="http://www.unicode.org/versions/Unicode6.0.0/">
<front>
<title>The Unicode Standard, Version 6.0.0</title>
<author>
<organization>The Unicode Consortium</organization>
<address><postal>
<street>The Unicode Consortium</street>
<city>Mountain View</city><region>CA</region></postal>
</address>
</author>
<date month="October" year="2011" />
</front>
<seriesInfo name="ISBN" value="978-1-936213-01-6" />
</reference>
<reference anchor="UTR15"
target='http://www.unicode.org/unicode/reports/tr15/tr15-23.html'>
<front>
<title>Unicode Normalization Forms</title>
<author initials='M.' surname='Davis' fullname='M. Davis'>
<organization /></author>
<author initials='M.' surname='Duerst' fullname='M. Duerst'>
<organization /></author>
<date month="April" year="2003" />
</front>
<seriesInfo name='Unicode Standard Annex #' value='15' />
</reference>
</references>
<references title="Informative References">
<reference anchor='OpenSearch' target='http://www.opensearch.org/Specifications/OpenSearch'>
<front>
<title>OpenSearch 1.1</title>
<author initials='D.' surname='Clinton' fullname='DeWitt Clinton'>
<organization /></author>
<date month='December' day='13' year='2011' />
</front>
<seriesInfo name='Draft' value='5' />
</reference>
<reference anchor='UPU-S42' target='http://www.upu.int/en/activities/addressing/standards.html'>
<front>
<title>International Postal Address Components and Templates</title>
<author>
<organization>Universal Postal Union</organization></author>
<date month='November' day='4' year='2002' />
</front>
<seriesInfo name='UPU' value='S42-1' />
</reference>
<reference anchor='WSDL'
target='http://www.w3.org/TR/2007/REC-wsdl20-20070626'>
<front>
<title>Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language</title>
<author initials='S.' surname='Weerawarana' fullname='Sanjiva Weerawarana'>
<organization />
</author>
<author initials='J.' surname='Moreau' fullname='Jean-Jacques Moreau'>
<organization />
</author>
<author initials='A.' surname='Ryman' fullname='Arthur Ryman'>
<organization />
</author>
<author initials='R.' surname='Chinnici' fullname='Roberto Chinnici'>
<organization />
</author>
<date month='June' day='26' year='2007' />
</front>
<seriesInfo name='World Wide Web Consortium Recommendation' value='REC-wsdl20-20070626' />
</reference>
<reference anchor='WADL'
target='http://www.w3.org/Submission/2009/SUBM-wadl-20090831/'>
<front>
<title>Web Application Description Language</title>
<author initials='M.' surname='Hadley' fullname='Marc Hadley'>
<organization>Sun Microsystems</organization>
</author>
<date month='August' day='31' year='2009' />
</front>
<seriesInfo name='World Wide Web Consortium Member Submission' value='SUBM-wadl-20090831' />
</reference>
</references>
<section title="Implementation Hints" anchor="implementation">
<t>
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.
</t>
<t>
Initialize an empty result string and its non-error state.
</t>
<t>
Scan the template and copy literals to the result string (as in
<xref target="literal-expansion"/>) 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.
<list style="symbols">
<t>If an expression is found, scan the template to the next "}"
and extract the characters in between the braces.</t>
<t>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.</t>
</list>
</t>
<t>
Examine the first character of the extracted expression for an operator.
<list style="symbols">
<t>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 (<xref target="variables"/>), 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.</t>
<t>If a known and implemented operator is found, store the operator
and skip to the next character to begin the varspec-list.</t>
<t>Otherwise, store the operator as NUL (simple string expansion).</t>
</list>
</t>
<t>
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 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.
</t>
<figure><artwork>
.------------------------------------------------------------------.
| NUL + . / ; ? & # |
|------------------------------------------------------------------|
| first | "" "" "." "/" ";" "?" "&" "#" |
| sep | "," "," "." "/" ";" "&" "&" "," |
| named | false false false false true true true false |
| ifemp | "" "" "" "" "" "=" "=" "" |
| allow | U U+R U U U U U U+R |
`------------------------------------------------------------------'
</artwork></figure>
<t>
With the above table in mind, process the variable-list as follows:
</t>
<t>
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.
<list style="symbols">
<t>If it is the end of the expression and the varname is empty,
go back to scan the remainder of the template.</t>
<t>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.</t>
<t>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.</t>
</list>
Lookup the value for the scanned variable name, and then
<list style="symbols">
<t>If the varname is unknown or corresponds to a variable with an
undefined value (<xref target="variables"/>), then skip to
the next varspec.</t>
<t>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.</t>
<t>If this variable's value is a string, then
<list style="symbols">
<t>if named is true, append the varname to the result string using
the same encoding process as for literals, and
<list style="symbols">
<t>if the value is empty, append the ifemp string to the
result string and skip to the next varspec;</t>
<t>otherwise, append "=" to the result string.</t>
</list>
</t>
<t>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;</t>
<t>otherwise, append the value to the result string after
pct-encoding any characters that are not in the allow set.</t>
</list>
</t>
<t>else if no explode modifier is given, then
<list style="symbols">
<t>if named is true, append the varname to the result string
using the same encoding process as for literals, and
<list style="symbols">
<t>if the value is empty, append the ifemp string to the
result string and skip to the next varspec;</t>
<t>otherwise, append "=" to the result string; and</t>
</list>
</t>
<t>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;</t>
<t>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.</t>
</list>
</t>
<t>else if an explode modifier is given, then
<list style="symbols">
<t>if named is true, then for each defined list member or array
(name, value) pair with a defined value, do:
<list style="symbols">
<t>if this is not the first defined member/value, append the
sep string to the result string;</t>
<t>if this is a list, append the varname to the result string
using the same encoding process as for literals;</t>
<t>if this is a pair, append the name to the result string
using the same encoding process as for literals;</t>
<t>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.</t>
</list></t>
<t>else if named is false, then
<list style="symbols">
<t>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.</t>
<t>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.</t>
</list>
</t>
</list>
</t>
</list>
When the variable-list for this expression is exhausted,
go back to scan the remainder of the template.
</t>
</section>
</back>
</rfc>
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