JSONPath: Query expressions for JSON
draft-ietf-jsonpath-base-03
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Stefan Gössner , Glyn Normington , Carsten Bormann | ||
| Last updated | 2022-01-16 | ||
| Replaces | draft-normington-jsonpath | ||
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draft-ietf-jsonpath-base-03
JSONPath WG S. Gössner, Ed.
Internet-Draft Fachhochschule Dortmund
Intended status: Standards Track G. Normington, Ed.
Expires: 20 July 2022
C. Bormann, Ed.
Universität Bremen TZI
16 January 2022
JSONPath: Query expressions for JSON
draft-ietf-jsonpath-base-03
Abstract
JSONPath defines a string syntax for selecting and extracting values
within a JavaScript Object Notation (JSON, RFC 8259) value.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-jsonpath-base/.
Discussion of this document takes place on the JSON Path Working
Group mailing list (mailto:jsonpath@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/jsonpath/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath-base.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 20 July 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. History . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Overview of JSONPath Expressions . . . . . . . . . . . . 5
2. JSONPath Examples . . . . . . . . . . . . . . . . . . . . . . 6
3. JSONPath Syntax and Semantics . . . . . . . . . . . . . . . . 8
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Processing Model . . . . . . . . . . . . . . . . . . . . 9
3.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5.1. Root Selector . . . . . . . . . . . . . . . . . . . . 12
3.5.2. Dot Selector . . . . . . . . . . . . . . . . . . . . 12
3.5.3. Dot Wild Card Selector . . . . . . . . . . . . . . . 13
3.5.4. Index Selector . . . . . . . . . . . . . . . . . . . 13
3.5.5. Index Wild Card Selector . . . . . . . . . . . . . . 16
3.5.6. Array Slice Selector . . . . . . . . . . . . . . . . 16
3.5.7. Descendant Selector . . . . . . . . . . . . . . . . . 20
3.5.8. Union Selector . . . . . . . . . . . . . . . . . . . 20
3.5.9. Filter Selector . . . . . . . . . . . . . . . . . . . 21
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
5. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1. Normative References . . . . . . . . . . . . . . . . . . 24
6.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Inspired by XPath . . . . . . . . . . . . . . . . . 26
A.1. JSONPath and XPath . . . . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 30
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
JavaScript Object Notation (JSON, [RFC8259]) is a popular
representation format for structured data values. JSONPath defines a
string syntax for identifying values within a JSON value.
JSONPath is not intended as a replacement, but as a more powerful
companion, to JSON Pointer [RFC6901].
// Insert reference to section where the relationship is detailed.
// The purposes of the two syntaxes are different. Pointer is for
// isolating a single location within a JSON document. Path is a
// query syntax that can also be used to pull multiple locations.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The grammatical rules in this document are to be interpreted as ABNF,
as described in [RFC5234]. ABNF terminal values in this document
define Unicode code points rather than their UTF-8 encoding. For
example, the Unicode PLACE OF INTEREST SIGN (U+2318) would be defined
in ABNF as %x2318.
The terminology of [RFC8259] applies except where clarified below.
The terms "Primitive" and "Structured" are used to group the types as
in Section 1 of [RFC8259]. Definitions for "Object", "Array",
"Number", and "String" remain unchanged. Importantly "object" and
"array" in particular do not take on a generic meaning, such as they
would in a general programming context.
Additional terms used in this specification are defined below.
Value: As per [RFC8259], a structure complying to the generic data
model of JSON, i.e., composed of components such as structured
values, namely JSON objects and arrays, and primitive data, namely
numbers and text strings as well as the special values null, true,
and false.
Member: A name/value pair in an object. (Not itself a value.)
Name: The name in a name/value pair constituting a member. (Also
known as "key", "tag", or "label".) This is also used in
[RFC8259], but that specification does not formally define it. It
is included here for completeness.
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Element: A value in an array. (Not to be confused with XML
element.)
Index: A non-negative integer that identifies a specific element in
an array. Note that the term _indexing_ is also used for
accessing elements using negative integers (Section "Semantics"),
and for accessing member values in an object using their member
name.
Query: Short name for JSONPath expression.
Argument: Short name for the value a JSONPath expression is applied
to.
Node: The pair of a value along with its location within the
argument.
Root Node: The unique node whose value is the entire argument.
Children (of a node): If the node is an array, each of its elements,
or if the node is an object, each its member values (but not its
member names).
Descendants (of a node): The node itself, plus the descendants of
each of its children.
// Note that this is often more selectively called descendant-or-
// self. Should we define descendants non-inclusive of the node
// itself? We do have the language to say "node + descendants" in
// several places.
Nodelist: A list of nodes. The output of applying a query to an
argument is manifested as a list of nodes. While this list can be
represented in JSON, e.g. as an array, the nodelist is an abstract
concept unrelated to JSON values.
Normalized Path: A simple form of JSONPath expression that
identifies a node by providing a query that results in exactly
that node. Similar to, but syntactically different from, a JSON
Pointer [RFC6901].
For the purposes of this specification, a value as defined by
[RFC8259] is also viewed as a tree of nodes. Each node, in turn,
holds a value. Further nodes within each value are the elements of
arrays and the member values of objects and are themselves values.
(The type of the value held by a node may also be referred to as the
type of the node.)
A query is applied to an argument, and the output is a nodelist.
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1.2. History
This document picks up Stefan Goessner's popular JSONPath proposal
dated 2007-02-21 [JSONPath-orig] and provides a normative definition
for it.
Appendix A describes how JSONPath was inspired by XML's XPath
[XPath].
JSONPath was intended as a light-weight companion to JSON
implementations on platforms such as PHP and JavaScript, so instead
of defining its own expression language like XPath did, JSONPath
delegated this to the expression language of the platform. While the
languages in which JSONPath is used do have significant
commonalities, over time this caused non-portability of JSONPath
expressions between the ensuing platform-specific dialects.
The present specification intends to remove platform dependencies and
serve as a common JSONPath specification that can be used across
platforms. Obviously, this means that backwards compatibility could
not always be achieved; a design principle of this specification is
to go with a "consensus" between implementations even if it is rough,
as long as that does not jeopardize the objective of obtaining a
usable, stable JSON query language.
1.3. Overview of JSONPath Expressions
JSONPath expressions are applied to a JSON value, the _argument_.
Within the JSONPath expression, the abstract name $ is used to refer
to the _root node_ of the argument, i.e., to the argument as a whole.
JSONPath expressions can use the _dot notation_
$.store.book[0].title
or the _bracket notation_
$['store']['book'][0]['title']
to build paths that are input to a JSONPath processor. Bracket
notation is more general than dot notation and can serve as a
canonical form (for instance, when a JSONPath processor uses JSONPath
expressions as output paths).
JSONPath allows the wildcard symbol * to select any member of an
object or any element of an array (Section 3.5.3). The descendant
operator .. selects the node and all its descendants (Section 3.5.7).
The array slice syntax [start:end:step] allows selecting a regular
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selection of an element from an array, giving a start position, an
end position, and possibly a step value that moves the position from
the start to the end (Section 3.5.6).
Filter expressions are supported via the syntax ?(<boolean expr>) as
in
$.store.book[?(@.price < 10)].title
Table 1 provides a quick overview of the JSONPath syntax elements.
+==================+==============================================+
| JSONPath | Description |
+==================+==============================================+
| $ | the root node |
+------------------+----------------------------------------------+
| @ | the current node |
+------------------+----------------------------------------------+
| . or [] | child operator |
+------------------+----------------------------------------------+
| n/a | parent operator |
+------------------+----------------------------------------------+
| .. | nested descendants |
+------------------+----------------------------------------------+
| * | wildcard: all member values/array elements |
| | regardless of their names/indices |
+------------------+----------------------------------------------+
| [] | subscript operator: index current node as an |
| | array (from 0) |
+------------------+----------------------------------------------+
| [,] | Union operator JSONPath allows alternate(??) |
| | names or array indices as a set |
+------------------+----------------------------------------------+
| [start:end:step] | array slice operator |
+------------------+----------------------------------------------+
| ?() | applies a filter expression |
+------------------+----------------------------------------------+
| () | expression, e.g., for indexing |
+------------------+----------------------------------------------+
Table 1: Overview over JSONPath
2. JSONPath Examples
This section provides some more examples for JSONPath expressions.
The examples are based on the simple JSON value shown in Figure 1,
which was patterned after a typical XML example representing a
bookstore (that also has bicycles).
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{ "store": {
"book": [
{ "category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
},
{ "category": "fiction",
"author": "Evelyn Waugh",
"title": "Sword of Honour",
"price": 12.99
},
{ "category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
},
{ "category": "fiction",
"author": "J. R. R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
}
],
"bicycle": {
"color": "red",
"price": 19.95
}
}
}
Figure 1: Example JSON value
The examples in Table 2 use the expression mechanism to obtain the
number of elements in an array, to test for the presence of a member
in a object, and to perform numeric comparisons of member values with
a constant.
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+========================+===================================+
| JSONPath | Result |
+========================+===================================+
| $.store.book[*].author | the authors of all books in the |
| | store |
+------------------------+-----------------------------------+
| $..author | all authors |
+------------------------+-----------------------------------+
| $.store.* | all things in store, which are |
| | some books and a red bicycle |
+------------------------+-----------------------------------+
| $.store..price | the prices of everything in the |
| | store |
+------------------------+-----------------------------------+
| $..book[2] | the third book |
+------------------------+-----------------------------------+
| $..book[(@.length-1)] | the last book in order |
| $..book[-1] | |
+------------------------+-----------------------------------+
| $..book[0,1] | the first two books |
| $..book[:2] | |
+------------------------+-----------------------------------+
| $..book[?(@.isbn)] | filter all books with isbn number |
+------------------------+-----------------------------------+
| $..book[?(@.price<10)] | filter all books cheaper than 10 |
+------------------------+-----------------------------------+
| $..* | all elements in XML document; all |
| | member values and array elements |
| | contained in input value |
+------------------------+-----------------------------------+
Table 2: Example JSONPath expressions applied to the
example JSON value
3. JSONPath Syntax and Semantics
3.1. Overview
A JSONPath query is a string which selects zero or more nodes of a
piece of JSON.
A query MUST be encoded using UTF-8. The grammar for queries given
in this document assumes that its UTF-8 form is first decoded into
Unicode code points as described in [RFC3629]; implementation
approaches that lead to an equivalent result are possible.
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A string to be used as a JSONPath query needs to be _well-formed_ and
_valid_. A string is a well-formed JSONPath query if it conforms to
the ABNF syntax in this document. A well-formed JSONPath query is
valid if it also fulfills all semantic requirements posed by this
document.
To be valid, integer numbers in the JSONPath query that are relevant
to the JSONPath processing (e.g., index values and steps) MUST be
within the range of exact values defined in I-JSON [RFC7493], namely
within the interval [-(2^53)+1, (2^53)-1]).
The well-formedness and the validity of JSONPath queries are
independent of the JSON value the query is applied to; no further
errors can be raised during application of the query to a value.
Obviously, an implementation can still fail when executing a JSONPath
query, e.g., because of resource depletion, but this is not modeled
in the present specification. However, the implementation MUST NOT
silently malfunction. Specifically, if a valid JSONPath query is
evaluated against a structured value whose size doesn't fit in the
range of exact values, interfering with the correct interpretation of
the query, the implementation MUST provide an indication of overflow.
(Readers familiar with the HTTP error model may be reminded of 400
type errors when pondering well-formedness and validity, while
resource depletion and related errors are comparable to 500 type
errors.)
3.2. Processing Model
In this specification, the semantics of a JSONPath query are defined
in terms of a _processing model_. That model is not prescriptive of
the internal workings of an implementation: Implementations may wish
(or need) to design a different process that yields results that are
consistent with this model.
In the processing model, a valid query is executed against a value,
the _argument_, and produces a list of zero or more nodes of the
value.
The query is a sequence of zero or more _selectors_, each of which is
applied to the result of the previous selector and provides input to
the next selector. These results and inputs take the form of a
_nodelist_, i.e., a sequence of zero or more nodes.
The nodelist going into the first selector contains a single node,
the argument. The nodelist resulting from the last selector is
presented as the result of the query; depending on the specific API,
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it might be presented as an array of the JSON values at the nodes, an
array of Output Paths referencing the nodes, or both -- or some other
representation as desired by the implementation. Note that the API
must be capable of presenting an empty nodelist as the result of the
query.
A selector performs its function on each of the nodes in its input
nodelist, during such a function execution, such a node is referred
to as the "current node". Each of these function executions produces
a nodelist, which are then concatenated into the result of the
selector.
The processing within a selector may execute nested queries, which
are in turn handled with the processing model defined here.
Typically, the argument to that query will be the current node of the
selector or a set of nodes subordinate to that current node.
3.3. Syntax
Syntactically, a JSONPath query consists of a root selector ($),
which stands for a nodelist that contains the root node of the
argument, followed by a possibly empty sequence of _selectors_.
json-path = root-selector *(S (dot-selector /
dot-wild-selector /
index-selector /
index-wild-selector /
union-selector /
slice-selector /
descendant-selector /
filter-selector))
The syntax and semantics of each selector is defined below.
3.4. Semantics
The root selector $ not only selects the root node of the argument,
but it also produces as output a list consisting of one node: the
argument itself.
A selector may select zero or more nodes for further processing. A
syntactically valid selector MUST NOT produce errors. This means
that some operations which might be considered erroneous, such as
indexing beyond the end of an array, simply result in fewer nodes
being selected.
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But a selector doesn't just act on a single node: a selector acts on
each of the nodes in its input nodelist and concatenates the
resultant nodelists to form the result nodelist of the selector.
For each node in the list, the selector selects zero or more nodes,
each of which is a descendant of the node or the node itself.
For instance, with the argument {"a":[{"b":0},{"b":1},{"c":2}]}, the
query $.a[*].b selects the following list of nodes: 0, 1 (denoted
here by their value). Let's walk through this in detail.
The query consists of $ followed by three selectors: .a, [*], and .b.
Firstly, $ selects the root node which is the argument. So the
result is a list consisting of just the root node.
Next, .a selects from any input node of type object and selects the
node of any member value of the input node corresponding to the
member name "a". The result is again a list of one node:
[{"b":0},{"b":1},{"c":2}].
Next, [*] selects from an input node of type array all its elements
(if the input note were of type object, it would select all its
member values, but not the member names). The result is a list of
three nodes: {"b":0}, {"b":1}, and {"c":2}.
Finally, .b selects from any input node of type object with a member
name b and selects the node of the member value of the input node
corresponding to that name. The result is a list containing 0, 1.
This is the concatenation of three lists, two of length one
containing 0, 1, respectively, and one of length zero.
As a consequence of this approach, if any of the selectors selects no
nodes, then the whole query selects no nodes.
In what follows, the semantics of each selector are defined for each
type of node.
3.5. Selectors
A JSONPath query consists of a sequence of selectors. Valid
selectors are
* Root selector $ (used at the start of a query and in expressions)
* Dot selector .<name>, used with object member names exclusively.
* Dot wild card selector .*.
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* Index selector [<index>], where <index> is either a (possibly
negative, see Section "Semantics") array index or an object member
name.
* Index wild card selector [*].
* Array slice selector [<start>:<end>:<step>], where the optional
values <start>, <end>, and <step> are integer literals.
* Nested descendants selector ...
* Union selector [<sel1>,<sel2>,...,<selN>], holding a comma
delimited list of index, index wild card, array slice, and filter
selectors.
* Filter selector [?(<expr>)]
* Current item selector @ (used in expressions)
3.5.1. Root Selector
Syntax
Every valid JSONPath query MUST begin with the root selector $.
root-selector = "$"
Semantics
The Argument -- the root JSON value -- becomes the root node, which
is addressed by the root selector $.
3.5.2. Dot Selector
Syntax
A dot selector starts with a dot . followed by an object's member
name.
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dot-selector = "." dot-member-name
dot-member-name = name-first *name-char
name-first =
ALPHA /
"_" / ; _
%x80-10FFFF ; any non-ASCII Unicode character
name-char = DIGIT / name-first
DIGIT = %x30-39 ; 0-9
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
Member names containing characters other than allowed by dot-selector
-- such as space ` , minus-, or dot.characters -- MUST NOT be used
with thedot-selector. (Such member names can be addressed by
theindex-selector` instead.)
Semantics
The dot-selector selects the node of the member value corresponding
to the member name from any JSON object in its input nodelist. It
selects no nodes from any other JSON value.
3.5.3. Dot Wild Card Selector
Syntax
The dot wild card selector has the form .* as defined in the
following syntax:
dot-wild-selector = "." "*" ; dot followed by asterisk
Semantics
A dot-wild-selector acts as a wild card by selecting the nodes of all
member values of an object in its input nodelist as well as all
element nodes of an array in its input nodelist. Applying the dot-
wild-selector to a primitive JSON value (number, string, or
true/false/null) selects no node.
3.5.4. Index Selector
Syntax
An index selector [<index>] addresses at most one object member value
or at most one array element value.
index-selector = "[" S (quoted-member-name / element-index) S "]"
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Applying the index-selector to an object value in its input nodelist,
a quoted-member-name string is required to select the corresponding
member value. In contrast to JSON, the JSONPath syntax allows
strings to be enclosed in _single_ or _double_ quotes.
quoted-member-name = string-literal
string-literal = %x22 *double-quoted %x22 / ; "string"
%x27 *single-quoted %x27 ; 'string'
double-quoted = unescaped /
%x27 / ; '
ESC %x22 / ; \"
ESC escapable
single-quoted = unescaped /
%x22 / ; "
ESC %x27 / ; \'
ESC escapable
ESC = %x5C ; \ backslash
unescaped = %x20-21 / ; s. RFC 8259
%x23-26 / ; omit "
%x28-5B / ; omit '
%x5D-10FFFF ; omit \
escapable = ( %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
; b / ; BS backspace U+0008
; t / ; HT horizontal tab U+0009
; n / ; LF line feed U+000A
; f / ; FF form feed U+000C
; r / ; CR carriage return U+000D
"/" / ; / slash (solidus)
"\" / ; \ backslash (reverse solidus)
(%x75 hexchar) ; uXXXX U+XXXX
)
hexchar = non-surrogate / (high-surrogate "\" %x75 low-surrogate)
non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) /
("D" %x30-37 2HEXDIG )
high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG
low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
; Task from 2021-06-15 interim: update ABNF later
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Applying the index-selector to an array, a numerical element-index is
required to select the corresponding element. JSONPath allows it to
be negative (see Section "Semantics").
element-index = int ; decimal integer
int = ["-"] ( "0" / (DIGIT1 *DIGIT) ) ; - optional
DIGIT1 = %x31-39 ; 1-9 non-zero digit
Notes: 1. double-quoted strings follow the JSON string syntax
(Section 7 of [RFC8259]); single-quoted strings follow an analogous
pattern (Section "Syntax"). 2. An element-index is an integer (in
base 10, as in JSON numbers). 3. As in JSON numbers, the syntax does
not allow octal-like integers with leading zeros such as 01 or -01.
Semantics
A quoted-member-name string MUST be converted to a member name by
removing the surrounding quotes and replacing each escape sequence
with its equivalent Unicode character, as in the table below:
+=================+===================+=============================+
| Escape Sequence | Unicode Character | Description |
+=================+===================+=============================+
| \b | U+0008 | BS backspace |
+-----------------+-------------------+-----------------------------+
| \t | U+0009 | HT horizontal tab |
+-----------------+-------------------+-----------------------------+
| \n | U+000A | LF line feed |
+-----------------+-------------------+-----------------------------+
| \f | U+000C | FF form feed |
+-----------------+-------------------+-----------------------------+
| \r | U+000D | CR carriage return |
+-----------------+-------------------+-----------------------------+
| \" | U+0022 | quotation mark |
+-----------------+-------------------+-----------------------------+
| \' | U+0027 | apostrophe |
+-----------------+-------------------+-----------------------------+
| \/ | U+002F | slash (solidus) |
+-----------------+-------------------+-----------------------------+
| \\ | U+005C | backslash (reverse |
| | | solidus) |
+-----------------+-------------------+-----------------------------+
| \uXXXX | U+XXXX | unicode character |
+-----------------+-------------------+-----------------------------+
Table 3: Escape Sequence Replacements
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The index-selector applied with a quoted-member-name to an object
selects the node of the corresponding member value from it, if and
only if that object has a member with that name. Nothing is selected
from a value which is not a object.
Array indexing via element-index is a way of selecting a particular
array element using a zero-based index. For example, selector [0]
selects the first and selector [4] the fifth element of a
sufficiently long array.
A negative element-index counts from the array end. For example,
selector [-1] selects the last and selector [-2] selects the
penultimate element of an array with at least two elements. As with
non-negative indexes, it is not an error if such an element does not
exist; this simply means that no element is selected.
3.5.5. Index Wild Card Selector
Syntax
The index wild card selector has the form [*].
index-wild-selector = "[" "*" "]" ; asterisk enclosed by brackets
Semantics
An index-wild-selector selects the nodes of all member values of an
object as well as of all elements of an array. Applying the index-
wild-selector to a primitive JSON value (such as a number, string, or
true/false/null) selects no node.
The index-wild-selector behaves identically to the dot-wild-selector.
3.5.6. Array Slice Selector
Syntax
The array slice selector has the form [<start>:<end>:<step>]. It
selects elements starting at index <start>, ending at -- but not
including -- <end>, while incrementing by step.
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slice-selector = "[" S [start S] ":" S [end S] [":" S [step S]] "]"
start = int ; included in selection
end = int ; not included in selection
step = int ; default: 1
B = %x20 / ; Space
%x09 / ; Horizontal tab
%x0A / ; Line feed or New line
%x0D ; Carriage return
S = *B ; optional blank space
RS = 1*B ; required blank space
The slice-selector consists of three optional decimal integers
separated by colons.
Semantics
The slice-selector was inspired by the slice operator of ECMAScript 4
(ES4), which was deprecated in 2014, and that of Python.
Informal Introduction
This section is non-normative.
Array indexing is a way of selecting a particular element of an array
using a 0-based index. For example, the expression [0] selects the
first element of a non-empty array.
Negative indices index from the end of an array. For example, the
expression [-2] selects the last but one element of an array with at
least two elements.
Array slicing is inspired by the behaviour of the
Array.prototype.slice method of the JavaScript language as defined by
the ECMA-262 standard [ECMA-262], with the addition of the step
parameter, which is inspired by the Python slice expression.
The array slice expression [start:end:step] selects elements at
indices starting at start, incrementing by step, and ending with end
(which is itself excluded). So, for example, the expression [1:3]
(where step defaults to 1) selects elements with indices 1 and 2 (in
that order) whereas [1:5:2] selects elements with indices 1 and 3.
When step is negative, elements are selected in reverse order. Thus,
for example, [5:1:-2] selects elements with indices 5 and 3, in that
order and [::-1] selects all the elements of an array in reverse
order.
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When step is 0, no elements are selected. (This is the one case
which differs from the behaviour of Python, which raises an error in
this case.)
The following section specifies the behaviour fully, without
depending on JavaScript or Python behaviour.
Detailed Semantics
An array selector is either an array slice or an array index, which
is defined in terms of an array slice.
A slice expression selects a subset of the elements of the input
array, in the same order as the array or the reverse order, depending
on the sign of the step parameter. It selects no nodes from a node
which is not an array.
A slice is defined by the two slice parameters, start and end, and an
iteration delta, step. Each of these parameters is optional. len is
the length of the input array.
The default value for step is 1. The default values for start and
end depend on the sign of step, as follows:
+===========+=========+==========+
| Condition | start | end |
+===========+=========+==========+
| step >= 0 | 0 | len |
+-----------+---------+----------+
| step < 0 | len - 1 | -len - 1 |
+-----------+---------+----------+
Table 4: Default array slice
start and end values
Slice expression parameters start and end are not directly usable as
slice bounds and must first be normalized. Normalization for this
purpose is defined as:
FUNCTION Normalize(i, len):
IF i >= 0 THEN
RETURN i
ELSE
RETURN len + i
END IF
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The result of the array indexing expression [i] applied to an array
of length len is defined to be the result of the array slicing
expression [i:Normalize(i, len)+1:1].
Slice expression parameters start and end are used to derive slice
bounds lower and upper. The direction of the iteration, defined by
the sign of step, determines which of the parameters is the lower
bound and which is the upper bound:
FUNCTION Bounds(start, end, step, len):
n_start = Normalize(start, len)
n_end = Normalize(end, len)
IF step >= 0 THEN
lower = MIN(MAX(n_start, 0), len)
upper = MIN(MAX(n_end, 0), len)
ELSE
upper = MIN(MAX(n_start, -1), len-1)
lower = MIN(MAX(n_end, -1), len-1)
END IF
RETURN (lower, upper)
The slice expression selects elements with indices between the lower
and upper bounds. In the following pseudocode, the a(i) construct
expresses the 0-based indexing operation on the underlying array.
IF step > 0 THEN
i = lower
WHILE i < upper:
SELECT a(i)
i = i + step
END WHILE
ELSE if step < 0 THEN
i = upper
WHILE lower < i:
SELECT a(i)
i = i + step
END WHILE
END IF
When step = 0, no elements are selected and the result array is
empty.
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To be valid, the slice expression parameters MUST be in the I-JSON
range of exact values, see Section 3.1.
3.5.7. Descendant Selector
Syntax
The descendant selector starts with a double dot .. and can be
followed by an object member name (similar to the dot-selector), by
an index-selector acting on objects or arrays, or by a wild card.
descendant-selector = ".." ( dot-member-name / ; ..<name>
index-selector / ; ..[<index>]
index-wild-selector / ; ..[*]
"*" ; ..*
)
Semantics
The descendant-selector selects the node and all its descendants.
In the resultant nodelist: * nodes occur before their children, and *
nodes of an array occur in array order.
Children of an object may occur in any order, since JSON objects are
unordered.
3.5.8. Union Selector
Syntax
The union selector is syntactically related to the index-selector.
It contains two or more entries, separated by commas.
union-selector = "[" S union-entry 1*(S "," S union-entry) S "]"
union-entry = ( quoted-member-name /
element-index /
slice-index
)
Semantics
A union selects any node which is selected by at least one of the
union selectors and selects the concatenation of the lists (in the
order of the selectors) of nodes selected by the union elements.
Note that any node selected in more than one of the union selectors
is kept as many times in the node list.
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To be valid, integer values in the element-index and slice-index
components MUST be in the I-JSON range of exact values, see
Section 3.1.
3.5.9. Filter Selector
Syntax
The filter selector has the form [?<expr>]. It works via iterating
over structured values, i.e. arrays and objects.
filter-selector = "[" S "?" S boolean-expr S "]"
During iteration process each array element or object member is
visited and its value -- accessible via symbol @ -- or one of its
descendants -- uniquely defined by a relative path -- is tested
against a boolean expression boolean-expr.
The current item is selected if and only if the result is true.
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boolean-expr = logical-or-expr
logical-or-expr = logical-and-expr *(S "||" S logical-and-expr)
; disjunction
; binds less tightly than conjunction
logical-and-expr = basic-expr *(S "&&" S basic-expr) ; conjunction
; binds more tightly than disjunction
basic-expr = exist-expr /
paren-expr /
relation-expr
exist-expr = [neg-op S] path ; path existence or non-existence
path = rel-path / json-path
rel-path = "@" *(S (dot-selector / index-selector))
paren-expr = [neg-op S] "(" S boolean-expr S ")" ; parenthesized expression
neg-op = "!" ; not operator
relation-expr = comp-expr / ; comparison test
regex-expr ; regular expression test
comp-expr = comparable S comp-op S comparable
comparable = number / string-literal / ; primitive ...
true / false / null / ; values only
path ; path value
comp-op = "==" / "!=" / ; comparison ...
"<" / ">" / ; operators
"<=" / ">="
number = int [ frac ] [ exp ] ; decimal number
frac = "." 1*DIGIT ; decimal fraction
exp = "e" [ "-" / "+" ] 1*DIGIT ; decimal exponent
true = %x74.72.75.65 ; true
false = %x66.61.6c.73.65 ; false
null = %x6e.75.6c.6c ; null
regex-expr = (path / string-literal) S regex-op S regex
regex-op = "=~" ; regular expression match
regex = <TO BE DEFINED>
Notes:
* Parentheses can be used with boolean-expr for grouping. So filter
selection syntax in the original proposal [?(<expr>)] is naturally
contained in the current lean syntax [?<expr>] as a special case.
* Comparisons are restricted to primitive values (such as number,
string, true, false, null). Comparisons with complex values will
fail, i.e. no selection occurs.
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* Types are not implicitly converted in comparisons. So "13 ==
'13'" selects no node.
* A member or element value by itself in a Boolean context is
interpreted as false only if it does not exist. Otherwise it is
interpreted as true. To be more specific about the actual value,
explicit comparisons are necessary. This existence test -- as an
exception to the general rule -- also works with structured
values.
* Regular expression tests can be applied to string values only.
* Alphabetic characters in ABNF are case-insensitive, so "e" can be
either "e" or "E".
* false, null, true are lower-case only (case-sensitive).
The following table lists filter expression operators in order of
precedence from highest (binds most tightly) to lowest (binds least
tightly).
+============+===========+===========+
| Precedence | Operator | Syntax |
| | type | |
+============+===========+===========+
| 5 | Grouping | (...) |
+------------+-----------+-----------+
| 4 | Logical | ! |
| | NOT | |
+------------+-----------+-----------+
| 3 | Relations | == != |
| | | < <= > >= |
| | | =~ |
| | | in |
+------------+-----------+-----------+
| 2 | Logical | && |
| | AND | |
+------------+-----------+-----------+
| 1 | Logical | || |
| | OR | |
+------------+-----------+-----------+
Table 5: Filter expression
operator precedence
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Semantics
The filter-selector works with arrays and objects exclusively. Its
result might be a list of _zero_, _one_, _multiple_ or _all_ of their
element or member values then. Applied to other value types, it will
select nothing.
Some examples:
+===================+=====================+===============+=========+
|JSON | Query | Result |Comment |
+===================+=====================+===============+=========+
|{"a":1,"b":2} | $[?@] | [1,2] |Same as |
|[2,3,4] | | [2,3,4] |$.* or |
| | | |$[*] |
+-------------------+---------------------+---------------+---------+
|./. | $[?@==2] | [2] |Select by|
| | | [2] |value. |
+-------------------+---------------------+---------------+---------+
|{"a":{"b":{"c":{}}}| $[?@.b] |[{"b":{"c":{}}]|Existence|
| | $[?@.b.c] | | |
+-------------------+---------------------+---------------+---------+
|{"key":false} | $[?index(@)=='key'] | [false] |Select |
| | $[?index(@)==0] | [] |object |
| | | |member |
+-------------------+---------------------+---------------+---------+
|[3,4,5] | $[?index(@)==2] | [5] |Select |
| | $[?index(@)==17] | [] |array |
| | | |element |
+-------------------+---------------------+---------------+---------+
|{"a":{"b":{5},c:0}}| $[?@.b==5 && !@.c] |[{"b":{5},c:0}]|Existence|
+-------------------+---------------------+---------------+---------+
Table 6
4. IANA Considerations
TBD: Define a media type for JSONPath expressions.
5. Security Considerations
This section gives security considerations, as required by [RFC3552].
6. References
6.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
6.2. Informative References
[E4X] ISO, "Information technology — ECMAScript for XML (E4X)
specification", ISO/IEC 22537:2006 , 2006.
[ECMA-262] Ecma International, "ECMAScript Language Specification,
Standard ECMA-262, Third Edition", December 1999,
<http://www.ecma-international.org/publications/files/
ECMA-ST-ARCH/ECMA-
262,%203rd%20edition,%20December%201999.pdf>.
[JSONPath-orig]
Gössner, S., "JSONPath — XPath for JSON", 21 February
2007, <https://goessner.net/articles/JsonPath/>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
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[RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
"JavaScript Object Notation (JSON) Pointer", RFC 6901,
DOI 10.17487/RFC6901, April 2013,
<https://www.rfc-editor.org/info/rfc6901>.
[SLICE] "Slice notation", n.d.,
<https://github.com/tc39/proposal-slice-notation>.
[XPath] Berglund, A., Boag, S., Chamberlin, D., Fernandez, M.,
Kay, M., Robie, J., and J. Simeon, "XML Path Language
(XPath) 2.0 (Second Edition)", World Wide Web Consortium
Recommendation REC-xpath20-20101214, 14 December 2010,
<https://www.w3.org/TR/2010/REC-xpath20-20101214>.
Appendix A. Inspired by XPath
This appendix is informative.
At the time JSONPath was invented, XML was noted for the availability
of powerful tools to analyse, transform and selectively extract data
from XML documents. [XPath] is one of these tools.
In 2007, the need for something solving the same class of problems
for the emerging JSON community became apparent, specifically for:
* Finding data interactively and extracting them out of [RFC8259]
JSON values without special scripting.
* Specifying the relevant parts of the JSON data in a request by a
client, so the server can reduce the amount of data in its
response, minimizing bandwidth usage.
(Note that XPath has evolved since 2007, and recent versions even
nominally support operating inside JSON values. This appendix only
discusses the more widely used version of XPath that was available in
2007.)
JSONPath picks up the overall feeling of XPath, but maps the concepts
to syntax (and partially semantics) that would be familiar to someone
using JSON in a dynamic language.
E.g., in popular dynamic programming languages such as JavaScript,
Python and PHP, the semantics of the XPath expression
/store/book[1]/title
can be realized in the expression
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x.store.book[0].title
or, in bracket notation,
x['store']['book'][0]['title']
with the variable x holding the argument.
The JSONPath language was designed to:
* be naturally based on those language characteristics;
* cover only the most essential parts of XPath 1.0;
* be lightweight in code size and memory consumption;
* be runtime efficient.
A.1. JSONPath and XPath
JSONPath expressions apply to JSON values in the same way as XPath
expressions are used in combination with an XML document. JSONPath
uses $ to refer to the root node of the argument, similar to XPath's
/ at the front.
JSONPath expressions move further down the hierarchy using _dot
notation_ ($.store.book[0].title) or the _bracket notation_
($['store']['book'][0]['title']), a lightweight/limited, and a more
heavyweight syntax replacing XPath's / within query expressions.
Both JSONPath and XPath use * for a wildcard. The descendant
operator .., borrowed from [E4X], is similar to XPath's //. The array
slicing construct [start:end:step] is unique to JSONPath, inspired by
[SLICE] from ECMASCRIPT 4.
Filter expressions are supported via the syntax ?(<boolean expr>) as
in
$.store.book[?(@.price < 10)].title
Table 7 extends Table 1 by providing a comparison with similar XPath
concepts.
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+==========+==================+=================================+
| XPath | JSONPath | Description |
+==========+==================+=================================+
| / | $ | the root XML element |
+----------+------------------+---------------------------------+
| . | @ | the current XML element |
+----------+------------------+---------------------------------+
| / | . or [] | child operator |
+----------+------------------+---------------------------------+
| .. | n/a | parent operator |
+----------+------------------+---------------------------------+
| // | .. | nested descendants (JSONPath |
| | | borrows this syntax from E4X) |
+----------+------------------+---------------------------------+
| * | * | wildcard: All XML elements |
| | | regardless of their names |
+----------+------------------+---------------------------------+
| @ | n/a | attribute access: JSON values |
| | | do not have attributes |
+----------+------------------+---------------------------------+
| [] | [] | subscript operator used to |
| | | iterate over XML element |
| | | collections and for predicates |
+----------+------------------+---------------------------------+
| | | [,] | Union operator (results in a |
| | | combination of node sets); |
| | | JSONPath allows alternate names |
| | | or array indices as a set |
+----------+------------------+---------------------------------+
| n/a | [start:end:step] | array slice operator borrowed |
| | | from ES4 |
+----------+------------------+---------------------------------+
| [] | ?() | applies a filter (script) |
| | | expression |
+----------+------------------+---------------------------------+
| seamless | n/a | expression engine |
+----------+------------------+---------------------------------+
| () | n/a | grouping |
+----------+------------------+---------------------------------+
Table 7: XPath syntax compared to JSONPath
For further illustration, Table 8 shows some XPath expressions and
their JSONPath equivalents.
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+======================+========================+===================+
| XPath | JSONPath | Result |
+======================+========================+===================+
| /store/book/author | $.store.book[*].author | the authors of |
| | | all books in |
| | | the store |
+----------------------+------------------------+-------------------+
| //author | $..author | all authors |
+----------------------+------------------------+-------------------+
| /store/* | $.store.* | all things in |
| | | store, which |
| | | are some books |
| | | and a red |
| | | bicycle |
+----------------------+------------------------+-------------------+
| /store//price | $.store..price | the prices of |
| | | everything in |
| | | the store |
+----------------------+------------------------+-------------------+
| //book[3] | $..book[2] | the third book |
+----------------------+------------------------+-------------------+
| //book[last()] | $..book[(@.length-1)] | the last book |
| | $..book[-1] | in order |
+----------------------+------------------------+-------------------+
| //book[position()<3] | $..book[0,1] | the first two |
| | $..book[:2] | books |
+----------------------+------------------------+-------------------+
| //book[isbn] | $..book[?(@.isbn)] | filter all |
| | | books with isbn |
| | | number |
+----------------------+------------------------+-------------------+
| //book[price<10] | $..book[?(@.price<10)] | filter all |
| | | books cheaper |
| | | than 10 |
+----------------------+------------------------+-------------------+
| //* | $..* | all elements in |
| | | XML document; |
| | | all member |
| | | values and |
| | | array elements |
| | | contained in |
| | | input value |
+----------------------+------------------------+-------------------+
Table 8: Example XPath expressions and their JSONPath equivalents
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XPath has a lot more functionality (location paths in unabbreviated
syntax, operators and functions) than listed in this comparison.
Moreover, there are significant differences in how the subscript
operator works in XPath and JSONPath:
* Square brackets in XPath expressions always operate on the _node
set_ resulting from the previous path fragment. Indices always
start at 1.
* With JSONPath, square brackets operate on the _object_ or _array_
addressed by the previous path fragment. Array indices always
start at 0.
Acknowledgements
This specification is based on Stefan Gössner's original online
article defining JSONPath [JSONPath-orig].
The books example was taken from http://coli.lili.uni-
bielefeld.de/~andreas/Seminare/sommer02/books.xml -- a dead link now.
Contributors
Marko Mikulicic
InfluxData, Inc.
Pisa
Italy
Email: mmikulicic@gmail.com
Edward Surov
TheSoul Publishing Ltd.
Limassol
Cyprus
Email: esurov.tsp@gmail.com
Authors' Addresses
Stefan Gössner (editor)
Fachhochschule Dortmund
Sonnenstraße 96
D-44139 Dortmund
Germany
Email: stefan.goessner@fh-dortmund.de
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Glyn Normington (editor)
Winchester
United Kingdom
Email: glyn.normington@gmail.com
Carsten Bormann (editor)
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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