URI Template

URI Template Google

joe@bitworking.org http://bitworking.org/

Adobe Systems Incorporated

fielding@gbiv.com http://roy.gbiv.com/

Oracle

Marc.Hadley@oracle.com http://oracle.com/

mnot@pobox.com http://mnot.net/

http://www.pacificspirit.com/

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. To provide feedback on this Internet-Draft, join the W3C URI mailing list (http://lists.w3.org/Archives/Public/uri/).

A Uniform Resource Identifier (URI) is often used to identify a specific resource within a common space of similar resources. For example, personal web spaces are often delegated using a common pattern, such as

http://example.com/~fred/ http://example.com/~mark/ or a set of dictionary entries might be grouped in a hierarchy by the first letter of the term, as in

http://example.com/dictionary/c/cat http://example.com/dictionary/d/dog or a service interface might be invoked with various user input in a common pattern, as in

http://example.com/search?q=cat&lang=en http://example.com/search?q=chien&lang=fr URI Templates provide a mechanism for abstracting a space of resource identifiers such that the variable parts can be easily identified and described. URI templates can have many uses, including discovery of available services, configuring resource mappings, defining computed links, specifying interfaces, and other forms of programmatic interaction with resources. For example, the above resources could be described by the following URI templates:

http://example.com/~{username}/ http://example.com/dictionary/{term:1}/{term} http://example.com/search{?q,lang} We define the following terms: expression - The text between '{' and '}', including the enclosing braces, as defined in . 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 . template processor - A program or library that, given a URI Template and a set of variables with values, transforms the template string into a URI-reference by parsing the template for expressions and substituting each one with its corresponding expansion. A URI Template provides both a structural description of a URI space and, when variable values are provided, 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 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 . 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 .

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 plus ("+") operator for expansion of values that are allowed to include reserved characters.

.-----------------------------------------------------------------. | Level 2 examples, with variables having values of | | | | var := "value" | | hello := "Hello World!" | | empty := "" | | undef := null | | path := "/foo/bar" | | | |-----------------------------------------------------------------| | Op Expression Expansion | |-----------------------------------------------------------------| | + | Reserved string expansion (Sec 3.2.3) | | | | | | {+var} value | | | {+hello} Hello%20World! | | | O{+empty}X OX | | | O{+undef}X OX | | | {+path}/here /foo/bar/here | | | here?ref={+path} here?ref=/foo/bar | | | up{+path}{var}/here up/foo/barvalue/here | | | | `-----------------------------------------------------------------' 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" | | | |-----------------------------------------------------------------| | 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 | | | | |-----+-----------------------------------------------------------| | + | 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 | | | {/var,x}/here /value/1024/here | | | | |-----+-----------------------------------------------------------| | ; | 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 | | | | `-----------------------------------------------------------------' 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","."),("comma",",")] | | empty_keys := [] | | | | Op Expression Expansion | |-----------------------------------------------------------------| | | String expansion with value modifiers (Sec 3.2.2) | | | | | | {var:3} val | | | {var:30} value | | | {list} red,green,blue | | | {list*} red,green,blue | | | {keys} semi,%3B,dot,.,comma,%2C | | | {keys*} semi=%3B,dot=.,comma=%2C | | | | |-----+-----------------------------------------------------------| | + | Reserved expansion with value modifiers (Sec 3.2.3) | | | | | | {+path:6}/here /foo/b/here | | | {+list} red,green,blue | | | {+list*} red,green,blue | | | {+keys} semi,;,dot,.,comma,, | | | {+keys*} semi=;,dot=.,comma=, | | | | |-----+-----------------------------------------------------------| | . | 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 | | | X{.empty_keys} X | | | X{.empty_keys*} X | | | | |-----+-----------------------------------------------------------| | / | Path segments, slash-prefixed (Sec 3.2.5) | | | | | | {/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} ;list=red,green,blue | | | {;list*} ;red;green;blue | | | {;keys} ;keys=semi,%3B,dot,.,comma,%2C | | | {;keys*} ;semi=%3B;dot=.;comma=%2C | | | | |-----+-----------------------------------------------------------| | ? | Form-style query, ampersand-separated (Sec 3.2.7) | | | | | | {?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 | | | | `-----------------------------------------------------------------'

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. 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.

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. 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.

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 . This specification uses the Augmented Backus-Naur Form (ABNF) notation of . The following ABNF rules are imported from the normative references , , and .

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

This specification uses the terms "character" and "coded character set" in accordance with the definitions provided in , and "character encoding" in place of what 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 . This specification defines terminal values as codepoints within the Unicode coded character set . In spite of the syntax and template expansion process being defined in terms of Unicode codepoints, it should be understood that templates occur in practice as a sequence of characters in whatever form or encoding is suitable for the context in which they occur, whether that be octets embedded in a network protocol element or paint applied to the side of a bus. This specification does not mandate any particular character encoding for mapping between URI Template characters and the octets used to store or transmit those characters. When a URI Template appears in a protocol element, the character encoding is defined by that protocol; without such a definition, a URI Template is assumed to be in the same character encoding as the surrounding text. It is only during the process of template expansion that a string of characters in a URI Template is REQUIRED to be processed as a sequence of Unicode codepoints. The Unicode Standard defines various equivalences between sequences of characters for various purposes. Unicode Standard Annex #15 defines various Normalization Forms for these equivalences. The normalization form determines how to consistently encode equivalent strings. In theory, all URI processing implementations, including template processors, should use the same normalization form for generating a URI reference. In practice, they do not. If a value has been provided by the same server as the resource, then it can be assumed that the string is already in the form expected by that server. If a value is provided by a user, such as via a data-entry dialog, then the string SHOULD be normalized as Normalization Form C (NFC: Canonical Decomposition, followed by Canonical Composition) prior to being used in expansions by a template processor. Likewise, when non-ASCII data that represents readable strings is pct-encoded for use in a URI reference, a template processor MUST first encode the string as UTF-8 and then pct-encode any octets that are not allowed in a URI reference.

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.

The characters outside of expressions in a URI Template string are intended to be copied literally to the URI-reference if the character is allowed in a URI (reserved / unreserved / pct-encoded) or, if not allowed, copied to the URI-reference in its UTF-8 pct-encoded form.

literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B / %x5D-5F / %x61-7A / %x7E / ucschar / iprivate / pct-encoded ; any Unicode character except: CTL, SP, ; DQUOTE, "'", "%" (aside from pct-encoded), ; "<", ">", "\", "^", "`", "{", "|", "}"

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: "$" / "(" / ")" The operator characters have been chosen to reflect each of their roles as reserved characters in the URI generic syntax. The operators defined by this specification include: plus ("+") for substituting values that may contain reserved characters; dot (".") for substituting values as a sequence of name labels prefixed by "."; slash ("/") for substituting values as a sequence of path segments prefixed by "/"; semicolon (";") for substituting key=value pairs as path parameters prefixed by ";"; and, question-mark ("?") for substituting a query component beginning with "?" and consisting of key=value pairs separated by "&". These operators will be described in detail in . The operator characters equals ("="), comma (","), exclamation ("!"), at-sign ("@"), and pipe ("|") are reserved for future extensions. The expression syntax specifically excludes use of the dollar ("$") and parentheses ["(" and ")"] characters so that they remain available for local language extensions outside the scope of this specification.

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 ] varname = varchar *( varchar / "." ) varchar = ALPHA / DIGIT / "_" / ucschar / iprivate / pct-encoded A varname MAY contain one or more pct-encoded triplets. If so, these triplets MUST be decoded to octets prior to mapping the varname to a variable name. If the expression type calls for the variable name to be included in the expansion result, then the template processor MAY use either the original pct-encoded varname or the string corresponding to a minimally encoded version of the mapped variable name. I.e., a varname containing unnecessarily pct-encoded characters might be expanded to use those characters directly instead. 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).

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

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 = ":" max-length max-length = %x31-39 *DIGIT ; positive integer The max-length is a positive integer that refers to a maximum number of characters from the beginning of the variable's value as a Unicode string. Note that this numbering is in characters, not octets, in order to avoid splitting between the octets of a multi-octet UTF-8 encoded character or within a pct-encoded triplet. If the max-length is greater than the length of the variable's value, then the entire value string is used. 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

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 , is dependent on both the operator being used and the type or schema of the value being substituted.

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's value MUST be formed prior to template expansion. If a template processor encounters an error outside of an expression, such as a character sequence that does not match the <URI-Template> grammar, then processing of the template SHOULD cease, the URI-reference result SHOULD be undefined, and the location and type of error SHOULD be indicated to the invoking application. If an error is encountered inside an expression, such as an operator or value modifier that it does not recognize or cannot support, then the expression SHOULD be copied to the result unexpanded, processing of the remainder of the template SHOULD continue, and the location and type of error SHOULD be indicated to the invoking application. In this latter case, the result returned will not be a valid URI reference; it will be an incompletely expanded template string that is only intended for diagnostic use.

If the literal character is allowed anywhere in the URI syntax (unreserved / reserved / pct-encoded ), then it is copied directly to the result string. Otherwise, the pct-encoded equivalent of the literal character is copied to the result string by encoding the character in UTF-8 (a sequence of octets) and then encoding each octet as a pct-encoded triplet.

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.

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 all of the variables in an expression are undefined, then the expression's expansion is the empty string.

Simple string expansion is the default expression type when no operator is given. 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 (",").

For example, foo := "fred" "{foo}" -> "fred" "{foo,foo}" -> "fred,fred" "{bar,foo}" -> "fred" "{bar|wilma}" -> "wilma" Level 1 and 2 templates are limited to single variable expressions without modifiers or value structures. 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.

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).

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 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.

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 that has a defined value: 1) modify its value as indicated by any optional modifiers; 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. 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.

The slash ("/") operator indicates that the expression type is hierarchical path segment expansion, which can be useful for describing URI path hierarchies.

For each variable in the variable-list with a defined value: 1) modify its value as indicated by any optional modifiers; 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. 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 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.

TBD.

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 .

No IANA actions are required by this document.

The following people made significant contributions to this specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton, John Cowan, James H. Manger, Marc Portier, and James Snell.

&ASCII; &UNIV4; &UTR15; &rfc2119; &rfc2978; &rfc3986; &rfc3987; &rfc3629; &rfc5234;

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. TBD.

06 - Removed template defaults for undefined variables. Added paragraph on limitations of variable matching templates. Replaced "offset" with "max-length" in definition of prefix. Noted that IRI literals may need to be pct-encoded for the URI. Removed NFKC requirement -- state that NFC should be applied only for user-supplied data-entry values. Removed pct-decode normalization process for entire template, since that is a lot more than necessary -- just normalize the variable names before mapping. Fixed a lot of last-minute paste-o's in the examples. Fixed Level 4 examples for unexploded keys to include the keys= in the expansion. 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.