Internet Engineering Task Force (IETF) S. Gössner, Ed.
Request for Comments: 9535 Fachhochschule Dortmund
Category: Standards Track G. Normington, Ed.
ISSN: 2070-1721
C. Bormann, Ed.
Universität Bremen TZI
February 2024
JSONPath: Query Expressions for JSON
Abstract
JSONPath defines a string syntax for selecting and extracting JSON
(RFC 8259) values from within a given JSON value.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9535.
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Table of Contents
1. Introduction
1.1. Terminology
1.1.1. JSON Values as Trees of Nodes
1.2. History
1.3. JSON Values
1.4. Overview of JSONPath Expressions
1.4.1. Identifiers
1.4.2. Segments
1.4.3. Selectors
1.4.4. Summary
1.5. JSONPath Examples
2. JSONPath Syntax and Semantics
2.1. Overview
2.1.1. Syntax
2.1.2. Semantics
2.1.3. Example
2.2. Root Identifier
2.2.1. Syntax
2.2.2. Semantics
2.2.3. Examples
2.3. Selectors
2.3.1. Name Selector
2.3.1.1. Syntax
2.3.1.2. Semantics
2.3.1.3. Examples
2.3.2. Wildcard Selector
2.3.2.1. Syntax
2.3.2.2. Semantics
2.3.2.3. Examples
2.3.3. Index Selector
2.3.3.1. Syntax
2.3.3.2. Semantics
2.3.3.3. Examples
2.3.4. Array Slice Selector
2.3.4.1. Syntax
2.3.4.2. Semantics
2.3.4.3. Examples
2.3.5. Filter Selector
2.3.5.1. Syntax
2.3.5.2. Semantics
2.3.5.3. Examples
2.4. Function Extensions
2.4.1. Type System for Function Expressions
2.4.2. Type Conversion
2.4.3. Well-Typedness of Function Expressions
2.4.4. length() Function Extension
2.4.5. count() Function Extension
2.4.6. match() Function Extension
2.4.7. search() Function Extension
2.4.8. value() Function Extension
2.4.9. Examples
2.5. Segments
2.5.1. Child Segment
2.5.1.1. Syntax
2.5.1.2. Semantics
2.5.1.3. Examples
2.5.2. Descendant Segment
2.5.2.1. Syntax
2.5.2.2. Semantics
2.5.2.3. Examples
2.6. Semantics of null
2.6.1. Examples
2.7. Normalized Paths
2.7.1. Examples
3. IANA Considerations
3.1. Registration of Media Type application/jsonpath
3.2. Function Extensions Subregistry
4. Security Considerations
4.1. Attack Vectors on JSONPath Implementations
4.2. Attack Vectors on How JSONPath Queries Are Formed
4.3. Attacks on Security Mechanisms That Employ JSONPath
5. References
5.1. Normative References
5.2. Informative References
Appendix A. Collected ABNF Grammars
Appendix B. Inspired by XPath
B.1. JSONPath and XPath
Appendix C. JSON Pointer
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
JSON [RFC8259] is a popular representation format for structured data
values. JSONPath defines a string syntax for selecting and
extracting JSON values from within a given JSON value.
In relation to JSON Pointer [RFC6901], JSONPath is not intended as a
replacement but as a more powerful companion. See Appendix C.
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 scalar values rather than their UTF-8 encoding. For
example, the Unicode PLACE OF INTEREST SIGN (U+2318) would be defined
in ABNF as %x2318.
Functions are referred to using the function name followed by a pair
of parentheses, as in fname().
The terminology of [RFC8259] applies except where clarified below.
The terms "primitive" and "structured" are used to group different
kinds of values as in Section 1 of [RFC8259]. JSON objects and
arrays are structured; all other values are primitive. 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.
The terminology of [RFC9485] applies.
Additional terms used in this document are defined below.
Value: As per [RFC8259], a data item conforming to the generic data
model of JSON, i.e., primitive data (numbers, text strings, and
the special values null, true, and false), or structured data
(JSON objects and arrays). [RFC8259] focuses on the textual
representation of JSON values and does not fully define the value
abstraction assumed here.
Member: A name/value pair in an object. (A member is not itself a
value.)
Name: The name (a string) in a name/value pair constituting a
member. This is also used in [RFC8259], but that specification
does not formally define it. It is included here for
completeness.
Element: A value in a JSON array.
Index: An integer that identifies a specific element in an array.
Query: Short name for a JSONPath expression.
Query Argument: Short name for the value a JSONPath expression is
applied to.
Location: The position of a value within the query argument. This
can be thought of as a sequence of names and indexes navigating to
the value through the objects and arrays in the query argument,
with the empty sequence indicating the query argument itself. A
location can be represented as a Normalized Path (defined below).
Node: The pair of a value along with its location within the query
argument.
Root Node: The unique node whose value is the entire query argument.
Root Node Identifier: The expression $, which refers to the root
node of the query argument.
Current Node Identifier: The expression @, which refers to the
current node in the context of the evaluation of a filter
expression (described later).
Children (of a node): If the node is an array, the nodes of its
elements; if the node is an object, the nodes of its member
values. If the node is neither an array nor an object, it has no
children.
Descendants (of a node): The children of the node, together with the
children of its children, and so forth recursively. More
formally, the "descendants" relation between nodes is the
transitive closure of the "children" relation.
Depth (of a descendant node within a value): The number of ancestors
of the node within the value. The root node of the value has
depth zero, the children of the root node have depth one, their
children have depth two, and so forth.
Nodelist: A list of nodes. While a nodelist can be represented in
JSON, e.g., as an array, this document does not require or assume
any particular representation.
Parameter: Formal parameter (of a function) that can take a function
argument (an actual parameter) in a function expression.
Normalized Path: A form of JSONPath expression that identifies a
node in a value by providing a query that results in exactly that
node. Each node in a query argument is identified by exactly one
Normalized Path (we say that the Normalized Path is "unique" for
that node), and to be a Normalized Path for a specific query
argument, the Normalized Path needs to identify exactly one node.
This is similar to, but syntactically different from, a JSON
Pointer [RFC6901]. Note: This definition is based on the
syntactical definition in Section 2.7; JSONPath expressions that
identify a node in a value but do not conform to that syntax are
not Normalized Paths.
Unicode Scalar Value: Any Unicode [UNICODE] code point except high-
surrogate and low-surrogate code points (in other words, integers
in the inclusive base 16 ranges, either 0 to D7FF or E000 to
10FFFF). JSONPath queries are sequences of Unicode scalar values.
Segment: One of the constructs that selects children ([<selectors>])
or descendants (..[<selectors>]) of an input value.
Selector: A single item within a segment that takes the input value
and produces a nodelist consisting of child nodes of the input
value.
Singular Query: A JSONPath expression built from segments that have
been syntactically restricted in a certain way (Section 2.3.5.1)
so that, regardless of the input value, the expression produces a
nodelist containing at most one node. Note: JSONPath expressions
that always produce a singular nodelist but do not conform to the
syntax in Section 2.3.5.1 are not singular queries.
1.1.1. JSON Values as Trees of Nodes
This document models the query argument as a tree of JSON values,
each with its own node. A node is either the root node or one of its
descendants.
This document models the result of applying a query to the query
argument as a nodelist (a list of nodes).
Nodes are the selectable parts of the query argument. The only parts
of an object that can be selected by a query are the member values.
Member names and members (name/value pairs) cannot be selected.
Thus, member values have nodes, but members and member names do not.
Similarly, member values are children of an object, but members and
member names are not.
1.2. History
This document is based on Stefan Gössner's popular JSONPath proposal
(dated 2007-02-21) [JSONPath-orig], builds on the experience from the
widespread deployment of its implementations, and provides a
normative specification for it.
Appendix B describes how JSONPath was inspired by XML's XPath
[XPath].
JSONPath was intended as a lightweight companion to JSON
implementations in programming languages such as PHP and JavaScript,
so instead of defining its own expression language, like XPath did,
JSONPath delegated parts of a query to the underlying runtime, e.g.,
JavaScript's eval() function. As JSONPath was implemented in more
environments, JSONPath expressions became decreasingly portable. For
example, regular expression processing was often delegated to a
convenient regular expression engine.
This document aims to remove such implementation-specific
dependencies and serve as a common JSONPath specification that can be
used across programming languages and environments. This means that
backwards compatibility is not always achieved; a design principle of
this document 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.
The term _JSONPath_ was chosen because of the XPath inspiration and
also because the outcome of a query consists of _paths_ identifying
nodes in the JSON query argument.
1.3. JSON Values
The JSON value a JSONPath query is applied to is, by definition, a
valid JSON value. A JSON value is often constructed by parsing a
JSON text.
The parsing of a JSON text into a JSON value and what happens if a
JSON text does not represent valid JSON are not defined by this
document. Sections 4 and 8 of [RFC8259] identify specific situations
that may conform to the grammar for JSON texts but are not
interoperable uses of JSON, as they may cause unpredictable behavior.
This document does not attempt to define predictable behavior for
JSONPath queries in these situations.
Specifically, the "Semantics" subsections of Sections 2.3.1, 2.3.2,
2.3.5, and 2.5.2 describe behavior that becomes unpredictable when
the JSON value for one of the objects under consideration was
constructed out of JSON text that exhibits multiple members for a
single object that share the same member name ("duplicate names"; see
Section 4 of [RFC8259]). Also, when selecting a child by name
(Section 2.3.1) and comparing strings (Section 2.3.5.2.2), it is
assumed these strings are sequences of Unicode scalar values; the
behavior becomes unpredictable if they are not (Section 8.2 of
[RFC8259]).
1.4. Overview of JSONPath Expressions
A JSONPath expression is applied to a JSON value, known as the query
argument. The output is a nodelist.
A JSONPath expression consists of an identifier followed by a series
of zero or more segments, each of which contains one or more
selectors.
1.4.1. Identifiers
The root node identifier $ refers to the root node of the query
argument, i.e., to the argument as a whole.
The current node identifier @ refers to the current node in the
context of the evaluation of a filter expression (Section 2.3.5).
1.4.2. Segments
Segments select children ([<selectors>]) or descendants
(..[<selectors>]) of an input value.
Segments can use _bracket notation_, for example:
$['store']['book'][0]['title']
or the more compact _dot notation_, for example:
$.store.book[0].title
Bracket notation contains one or more (comma-separated) selectors of
any kind. Selectors are detailed in the next section.
A JSONPath expression may use a combination of bracket and dot
notations.
This document treats the bracket notations as canonical and defines
the shorthand dot notation in terms of bracket notation. Examples
and descriptions use shorthand where convenient.
1.4.3. Selectors
A name selector, e.g., 'name', selects a named child of an object.
An index selector, e.g., 3, selects an indexed child of an array.
In the expression [*], a wildcard * (Section 2.3.2) selects all
children of a node, and in the expression ..[*], it selects all
descendants of a node.
An array slice start:end:step (Section 2.3.4) selects a series of
elements from an array, giving a start position, an end position, and
an optional step value that moves the position from the start to the
end.
A filter expression ?<logical-expr> selects certain children of an
object or array, as in:
$.store.book[?@.price < 10].title
1.4.4. Summary
Table 1 provides a brief overview of JSONPath syntax.
+==================+================================================+
| Syntax Element | Description |
+==================+================================================+
| $ | root node identifier (Section 2.2) |
+------------------+------------------------------------------------+
| @ | current node identifier (Section 2.3.5) |
| | (valid only within filter selectors) |
+------------------+------------------------------------------------+
| [<selectors>] | child segment (Section 2.5.1): selects |
| | zero or more children of a node |
+------------------+------------------------------------------------+
| .name | shorthand for ['name'] |
+------------------+------------------------------------------------+
| .* | shorthand for [*] |
+------------------+------------------------------------------------+
| ..[<selectors>] | descendant segment (Section 2.5.2): |
| | selects zero or more descendants of a node |
+------------------+------------------------------------------------+
| ..name | shorthand for ..['name'] |
+------------------+------------------------------------------------+
| ..* | shorthand for ..[*] |
+------------------+------------------------------------------------+
| 'name' | name selector (Section 2.3.1): selects a |
| | named child of an object |
+------------------+------------------------------------------------+
| * | wildcard selector (Section 2.3.2): selects |
| | all children of a node |
+------------------+------------------------------------------------+
| 3 | index selector (Section 2.3.3): selects an |
| | indexed child of an array (from 0) |
+------------------+------------------------------------------------+
| 0:100:5 | array slice selector (Section 2.3.4): |
| | start:end:step for arrays |
+------------------+------------------------------------------------+
| ?<logical-expr> | filter selector (Section 2.3.5): selects |
| | particular children using a logical |
| | expression |
+------------------+------------------------------------------------+
| length(@.foo) | function extension (Section 2.4): invokes |
| | a function in a filter expression |
+------------------+------------------------------------------------+
Table 1: Overview of JSONPath Syntax
1.5. JSONPath Examples
This section is informative. It provides examples of JSONPath
expressions.
The examples are based on the simple JSON value shown in Figure 1,
representing a bookstore (which also has a bicycle).
{ "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": 399
}
}
}
Figure 1: Example JSON Value
Table 2 shows some JSONPath queries that might be applied to this
example and their intended results.
+========================+=======================================+
| JSONPath | Intended Result |
+========================+=======================================+
| $.store.book[*].author | the authors of all books in the store |
+------------------------+---------------------------------------+
| $..author | all authors |
+------------------------+---------------------------------------+
| $.store.* | all things in the store, which are |
| | some books and a red bicycle |
+------------------------+---------------------------------------+
| $.store..price | the prices of everything in the store |
+------------------------+---------------------------------------+
| $..book[2] | the third book |
+------------------------+---------------------------------------+
| $..book[2].author | the third book's author |
+------------------------+---------------------------------------+
| $..book[2].publisher | empty result: the third book does not |
| | have a "publisher" member |
+------------------------+---------------------------------------+
| $..book[-1] | the last book in order |
+------------------------+---------------------------------------+
| $..book[0,1] | the first two books |
| $..book[:2] | |
+------------------------+---------------------------------------+
| $..book[?@.isbn] | all books with an ISBN number |
+------------------------+---------------------------------------+
| $..book[?@.price<10] | all books cheaper than 10 |
+------------------------+---------------------------------------+
| $..* | all member values and array elements |
| | contained in the input value |
+------------------------+---------------------------------------+
Table 2: Example JSONPath Expressions and Their Intended
Results When Applied to the Example JSON Value
2. JSONPath Syntax and Semantics
2.1. Overview
A JSONPath _expression_ is a string that, when applied to a JSON
value (the _query argument_), selects zero or more nodes of the
argument and outputs these nodes as a nodelist.
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 scalar values as described in [RFC3629]; implementation
approaches that lead to an equivalent result are possible.
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 both semantic requirements posed by this
document, which are as follows:
1. 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 integer values defined in Internet JSON
(I-JSON) (see Section 2.2 of [RFC7493]), namely within the
interval [-(2^53)+1, (2^53)-1].
2. Uses of function extensions MUST be _well-typed_, as described in
Section 2.4.3.
A JSONPath implementation MUST raise an error for any query that is
not well-formed and valid. The well-formedness and the validity of
JSONPath queries are independent of the JSON value the query is
applied to. No further errors relating to the well-formedness and
the validity of a JSONPath query can be raised during application of
the query to a value. This clearly separates well-formedness/
validity errors in the query from mismatches that may actually stem
from flaws in the data.
Mismatches between the structure expected by a valid query and the
structure found in the data can lead to empty query results, which
may be unexpected and indicate bugs in either. JSONPath
implementations might therefore want to provide diagnostics to the
application developer that aid in finding the cause of empty results.
Obviously, an implementation can still fail when executing a JSONPath
query, e.g., because of resource depletion, but this is not modeled
in this document. However, the implementation MUST NOT silently
malfunction. Specifically, if a valid JSONPath query is evaluated
against a structured value whose size is too large to process the
query correctly (for instance, requiring the processing of numbers
that fall outside the range of exact values), 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, and they may
recognize resource depletion and related errors as comparable to 500
type errors.)
2.1.1. Syntax
Syntactically, a JSONPath query consists of a root identifier ($),
which stands for a nodelist that contains the root node of the query
argument, followed by a possibly empty sequence of _segments_.
jsonpath-query = root-identifier segments
segments = *(S segment)
B = %x20 / ; Space
%x09 / ; Horizontal tab
%x0A / ; Line feed or New line
%x0D ; Carriage return
S = *B ; optional blank space
The syntax and semantics of segments are defined in Section 2.5.
2.1.2. Semantics
In this document, the semantics of a JSONPath query define the
required results and do not prescribe the internal workings of an
implementation. This document may describe semantics in a procedural
step-by-step fashion; however, such descriptions are normative only
in the sense that any implementation MUST produce an identical result
but not in the sense that implementers are required to use the same
algorithms.
The semantics are that a valid query is executed against a value (the
_query argument_) and produces a nodelist (i.e., a list of zero or
more nodes of the value).
The query is a root identifier followed by a sequence of zero or more
segments, each of which is applied to the result of the previous root
identifier or segment and provides input to the next segment. These
results and inputs take the form of nodelists.
The nodelist resulting from the root identifier contains a single
node (the query argument). The nodelist resulting from the last
segment is presented as the result of the query. Depending on the
specific API, it might be presented as an array of the JSON values at
the nodes, an array of Normalized Paths referencing the nodes, or
both -- or some other representation as desired by the
implementation. Note: An empty nodelist is a valid query result.
A segment operates on each of the nodes in its input nodelist in
turn, and the resultant nodelists are concatenated in the order of
the input nodelist they were derived from to produce the result of
the segment. A node may be selected more than once and appears that
number of times in the nodelist. Duplicate nodes are not removed.
A syntactically valid segment MUST NOT produce errors when executing
the query. This means that some operations that might be considered
erroneous, such as using an index lying outside the range of an
array, simply result in fewer nodes being selected. (Additional
discussion of this property can be found in the introduction of
Section 2.1.)
As a consequence of this approach, if any of the segments produces an
empty nodelist, then the whole query produces an empty nodelist.
If the semantics of a query give an implementation a choice of
producing multiple possible orderings, a particular implementation
may produce distinct orderings in successive runs of the query.
2.1.3. Example
Consider this example. With the query argument
{"a":[{"b":0},{"b":1},{"c":2}]}, the query $.a[*].b selects the
following list of nodes (denoted here by their values): 0, 1.
The query consists of $ followed by three segments: .a, [*], and .b.
First, $ produces a nodelist consisting of just the query argument.
Next, .a selects from any object input node and selects the node of
any member value of the input node corresponding to the member name
"a". The result is again a list containing a single node:
[{"b":0},{"b":1},{"c":2}].
Next, [*] selects all the elements from the input array node. The
result is a list of three nodes: {"b":0}, {"b":1}, and {"c":2}.
Finally, .b selects from any object input node 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.
2.2. Root Identifier
2.2.1. Syntax
Every JSONPath query (except those inside filter expressions; see
Section 2.3.5) MUST begin with the root identifier $.
root-identifier = "$"
2.2.2. Semantics
The root identifier $ represents the root node of the query argument
and produces a nodelist consisting of that root node.
2.2.3. Examples
| Note: In this example and the following examples in Sections
| 2.2 and 2.3, except for Table 11, we will present a JSON text
| to show the JSON value used as the query argument to the
| queries in the examples and then a table with the following
| columns:
|
| * Query: an example query to be applied to the query
| argument
|
| * Result: the query result as a list of JSON values that
| were located in the query argument
|
| * Result Path: the query result as a list of (normalized)
| paths into the query argument, giving locations of the
| JSON values in the previous column
|
| * Comment: descriptive information
JSON:
{"k": "v"}
Queries:
+=======+============+=============+===========+
| Query | Result | Result Path | Comment |
+=======+============+=============+===========+
| $ | {"k": "v"} | $ | Root node |
+-------+------------+-------------+-----------+
Table 3: Root Identifier Example
2.3. Selectors
Selectors appear only inside child segments (Section 2.5.1) and
descendant segments (Section 2.5.2).
A selector produces a nodelist consisting of zero or more children of
the input value.
There are various kinds of selectors that produce children of
objects, children of arrays, or children of either objects or arrays.
selector = name-selector /
wildcard-selector /
slice-selector /
index-selector /
filter-selector
The syntax and semantics of each kind of selector are defined below.
2.3.1. Name Selector
2.3.1.1. Syntax
A name selector '<name>' selects at most one object member value.
In contrast to JSON, the JSONPath syntax allows strings to be
enclosed in _single_ or _double_ quotes.
name-selector = 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 / ; see RFC 8259
; omit 0x22 "
%x23-26 /
; omit 0x27 '
%x28-5B /
; omit 0x5C \
%x5D-D7FF /
; skip surrogate code points
%xE000-10FFFF
escapable = %x62 / ; b BS backspace U+0008
%x66 / ; f FF form feed U+000C
%x6E / ; n LF line feed U+000A
%x72 / ; r CR carriage return U+000D
%x74 / ; t HT horizontal tab U+0009
"/" / ; / slash (solidus) U+002F
"\" / ; \ backslash (reverse solidus) U+005C
(%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"
Notes:
* Double-quoted strings follow the JSON string syntax (Section 7 of
[RFC8259]); single-quoted strings follow an analogous pattern. No
attempt was made to improve on this syntax, so if it is desired to
escape characters with scalar values above 0xFFFF, such as U+1F041
("🁁", DOMINO TILE HORIZONTAL-02-02), they need to be represented
by a pair of surrogate escapes ("\uD83C\uDC41" in this case).
* Alphabetic characters in quoted strings are case-insensitive in
ABNF, so each of the hexadecimal digits within \u escapes (as
specified in rules referenced by hexchar) can be either lowercase
or uppercase, while the u in \u needs to be lowercase (indicated
as %x75).
2.3.1.2. Semantics
A name-selector string MUST be converted to a member name M by
removing the surrounding quotes and replacing each escape sequence
with its equivalent Unicode character, as shown in Table 4:
+=================+===================+=============================+
| 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 | see | hexadecimal escape |
| | Section 2.3.1.1 | |
+-----------------+-------------------+-----------------------------+
Table 4: Escape Sequence Replacements
Applying the name-selector to an object node selects a member value
whose name equals the member name M or selects nothing if there is no
such member value. Nothing is selected from a value that is not an
object.
Note: Processing the name selector requires comparing the member name
string M with member name strings in the JSON to which the selector
is being applied. Two strings MUST be considered equal if and only
if they are identical sequences of Unicode scalar values. In other
words, normalization operations MUST NOT be applied to either the
member name string M from the JSONPath or the member name strings in
the JSON prior to comparison.
2.3.1.3. Examples
JSON:
{
"o": {"j j": {"k.k": 3}},
"'": {"@": 2}
}
Queries:
The examples in Table 5 show the name selector in use by child
segments.
+====================+=======+=======================+============+
| Query |Result | Result Paths | Comment |
+====================+=======+=======================+============+
| $.o['j j'] |{"k.k":| $['o']['j j'] | Named |
| |3} | | value in |
| | | | a nested |
| | | | object |
+--------------------+-------+-----------------------+------------+
| $.o['j j']['k.k'] |3 | $['o']['j j']['k.k'] | Nesting |
| | | | further |
| | | | down |
+--------------------+-------+-----------------------+------------+
| $.o["j j"]["k.k"] |3 | $['o']['j j']['k.k'] | Different |
| | | | delimiter |
| | | | in the |
| | | | query, |
| | | | unchanged |
| | | | Normalized |
| | | | Path |
+--------------------+-------+-----------------------+------------+
| $["'"]["@"] |2 | $['\'']['@'] | Unusual |
| | | | member |
| | | | names |
+--------------------+-------+-----------------------+------------+
Table 5: Name Selector Examples
2.3.2. Wildcard Selector
2.3.2.1. Syntax
The wildcard selector consists of an asterisk.
wildcard-selector = "*"
2.3.2.2. Semantics
A wildcard selector selects the nodes of all children of an object or
array. The order in which the children of an object appear in the
resultant nodelist is not stipulated, since JSON objects are
unordered. Children of an array appear in array order in the
resultant nodelist.
Note that the children of an object are its member values, not its
member names.
The wildcard selector selects nothing from a primitive JSON value
(that is, a number, a string, true, false, or null).
2.3.2.3. Examples
JSON:
{
"o": {"j": 1, "k": 2},
"a": [5, 3]
}
Queries:
The examples in Table 6 show the wildcard selector in use by a child
segment.
+========+==========+=============+===================+
| Query | Result | Result | Comment |
| | | Paths | |
+========+==========+=============+===================+
| $[*] | {"j": 1, | $['o'] | Object values |
| | "k": 2} | $['a'] | |
| | [5, 3] | | |
+--------+----------+-------------+-------------------+
| $.o[*] | 1 | $['o']['j'] | Object values |
| | 2 | $['o']['k'] | |
+--------+----------+-------------+-------------------+
| $.o[*] | 2 | $['o']['k'] | Alternative |
| | 1 | $['o']['j'] | result |
+--------+----------+-------------+-------------------+
| $.o[*, | 1 | $['o']['j'] | Non-deterministic |
| *] | 2 | $['o']['k'] | ordering |
| | 2 | $['o']['k'] | |
| | 1 | $['o']['j'] | |
+--------+----------+-------------+-------------------+
| $.a[*] | 5 | $['a'][0] | Array members |
| | 3 | $['a'][1] | |
+--------+----------+-------------+-------------------+
Table 6: Wildcard Selector Examples
The example above with the query $.o[*, *] shows that the wildcard
selector may produce nodelists in distinct orders each time it
appears in the child segment when it is applied to an object node
with two or more members (but not when it is applied to object nodes
with fewer than two members or to array nodes).
2.3.3. Index Selector
2.3.3.1. Syntax
An index selector <index> matches at most one array element value.
index-selector = int ; decimal integer
int = "0" /
(["-"] DIGIT1 *DIGIT) ; - optional
DIGIT1 = %x31-39 ; 1-9 non-zero digit
Applying the numerical index-selector selects the corresponding
element. JSONPath allows it to be negative (see Section 2.3.3.2).
To be valid, the index selector value MUST be in the I-JSON range of
exact values (see Section 2.1).
Notes:
* An index-selector is an integer (in base 10, as in JSON numbers).
* As in JSON numbers, the syntax does not allow octal-like integers
with leading zeros, such as 01 or -01.
2.3.3.2. Semantics
A non-negative index-selector applied to an array selects an array
element using a zero-based index. For example, the selector 0
selects the first, and the selector 4 selects the fifth element of a
sufficiently long array. Nothing is selected, and it is not an
error, if the index lies outside the range of the array. Nothing is
selected from a value that is not an array.
A negative index-selector counts from the array end backwards,
obtaining an equivalent non-negative index-selector by adding the
length of the array to the negative index. For example, the selector
-1 selects the last, and the 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.
2.3.3.3. Examples
JSON:
["a","b"]
Queries:
The examples in Table 7 show the index selector in use by a child
segment.
+=======+========+==============+================================+
| Query | Result | Result Paths | Comment |
+=======+========+==============+================================+
| $[1] | "b" | $[1] | Element of array |
+-------+--------+--------------+--------------------------------+
| $[-2] | "a" | $[0] | Element of array, from the end |
+-------+--------+--------------+--------------------------------+
Table 7: Index Selector Examples
2.3.4. Array Slice Selector
2.3.4.1. Syntax
The array slice selector has the form <start>:<end>:<step>. It
matches elements from arrays starting at index <start> and ending at
(but not including) <end>, while incrementing by step with a default
of 1.
slice-selector = [start S] ":" S [end S] [":" [S step ]]
start = int ; included in selection
end = int ; not included in selection
step = int ; default: 1
The slice selector consists of three optional decimal integers
separated by colons. The second colon can be omitted when the third
integer is omitted.
To be valid, the integers provided MUST be in the I-JSON range of
exact values (see Section 2.1).
2.3.4.2. Semantics
The slice selector was inspired by the slice operator that was
proposed for ECMAScript 4 (ES4), which was never released, and that
of Python.
2.3.4.2.1. Informal Introduction
This section is informative.
Array slicing is inspired by the behavior 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.
When step is 0, no elements are selected. (This is the one case that
differs from the behavior of Python, which raises an error in this
case.)
The following section specifies the behavior fully, without depending
on JavaScript or Python behavior.
2.3.4.2.2. Normative Semantics
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
that 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. In the
rest of this section, len denotes 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 shown in Table 8.
+===========+=========+==========+
| Condition | start | end |
+===========+=========+==========+
| step >= 0 | 0 | len |
+-----------+---------+----------+
| step < 0 | len - 1 | -len - 1 |
+-----------+---------+----------+
Table 8: 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
The result of the array index expression i applied to an array of
length len is the result of the array slicing expression Normalize(i,
len):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, a(i) is the i+1th
element of the array a (i.e., a(0) is the first element, a(1) the
second, and so forth).
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.
2.3.4.3. Examples
JSON:
["a", "b", "c", "d", "e", "f", "g"]
Queries:
The examples in Table 9 show the array slice selector in use by a
child segment.
+===========+========+========+==========+
| Query | Result | Result | Comment |
| | | Paths | |
+===========+========+========+==========+
| $[1:3] | "b" | $[1] | Slice |
| | "c" | $[2] | with |
| | | | default |
| | | | step |
+-----------+--------+--------+----------+
| $[5:] | "f" | $[5] | Slice |
| | "g" | $[6] | with no |
| | | | end |
| | | | index |
+-----------+--------+--------+----------+
| $[1:5:2] | "b" | $[1] | Slice |
| | "d" | $[3] | with |
| | | | step 2 |
+-----------+--------+--------+----------+
| $[5:1:-2] | "f" | $[5] | Slice |
| | "d" | $[3] | with |
| | | | negative |
| | | | step |
+-----------+--------+--------+----------+
| $[::-1] | "g" | $[6] | Slice in |
| | "f" | $[5] | reverse |
| | "e" | $[4] | order |
| | "d" | $[3] | |
| | "c" | $[2] | |
| | "b" | $[1] | |
| | "a" | $[0] | |
+-----------+--------+--------+----------+
Table 9: Array Slice Selector Examples
2.3.5. Filter Selector
Filter selectors are used to iterate over the elements or members of
structured values, i.e., JSON arrays and objects. The structured
values are identified in the nodelist offered by the child or
descendant segment using the filter selector.
For each iteration (element/member), a logical expression (the
_filter expression_) is evaluated, which decides whether the node of
the element/member is selected. (While a logical expression
evaluates to what mathematically is a Boolean value, this
specification uses the term _logical_ to maintain a distinction from
the Boolean values that JSON can represent.)
During the iteration process, the filter expression receives the node
of each array element or object member value of the structured value
being filtered; this element or member value is then known as the
_current node_.
The current node can be used as the start of one or more JSONPath
queries in subexpressions of the filter expression, notated via the
current-node-identifier @. Each JSONPath query can be used either for
testing existence of a result of the query, for obtaining a specific
JSON value resulting from that query that can then be used in a
comparison, or as a _function argument_.
Filter selectors may use function extensions, which are covered in
Section 2.4. Within the logical expression for a filter selector,
function expressions can be used to operate on nodelists and values.
The set of available functions is extensible, with a number of
functions predefined (see Section 2.4) and the ability to register
further functions provided by the "Function Extensions" subregistry
(Section 3.2). When a function is defined, it is given a unique
name, and its return value and each of its parameters are given a
_declared type_. The type system is limited in scope; its purpose is
to express restrictions that, without functions, are implicit in the
grammar of filter expressions. The type system also guides
conversions (Section 2.4.2) that mimic the way different kinds of
expressions are handled in the grammar when function expressions are
not in use.
2.3.5.1. Syntax
The filter selector has the form ?<logical-expr>.
filter-selector = "?" S logical-expr
As the filter expression is composed of constituents free of side
effects, the order of evaluation does not need to be (and is not)
defined. Similarly, for conjunction (&&) and disjunction (||)
(defined later), both a short-circuiting and a fully evaluating
implementation will lead to the same result; both implementation
strategies are therefore valid.
The current node is accessible via the current node identifier @.
This identifier addresses the current node of the filter-selector
that is directly enclosing the identifier. Note: Within nested
filter-selectors, there is no syntax to address the current node of
any other than the directly enclosing filter-selector (i.e., of
filter-selectors enclosing the filter-selector that is directly
enclosing the identifier).
Logical expressions offer the usual Boolean operators (|| for OR, &&
for AND, and ! for NOT). They have the normal semantics of Boolean
algebra and obey its laws (for example, see [BOOLEAN-LAWS]).
Parentheses MAY be used within logical-expr for grouping.
It is not required that logical-expr consist of a parenthesized
expression (which was required in [JSONPath-orig]), although it can
be, and the semantics are the same as without the parentheses.
logical-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 = paren-expr /
comparison-expr /
test-expr
paren-expr = [logical-not-op S] "(" S logical-expr S ")"
; parenthesized expression
logical-not-op = "!" ; logical NOT operator
A test expression either tests the existence of a node designated by
an embedded query (see Section 2.3.5.2.1) or tests the result of a
function expression (see Section 2.4). In the latter case, if the
function's declared result type is LogicalType (see Section 2.4.1),
it tests whether the result is LogicalTrue; if the function's
declared result type is NodesType, it tests whether the result is
non-empty. If the function's declared result type is ValueType, its
use in a test expression is not well-typed (see Section 2.4.3).
test-expr = [logical-not-op S]
(filter-query / ; existence/non-existence
function-expr) ; LogicalType or NodesType
filter-query = rel-query / jsonpath-query
rel-query = current-node-identifier segments
current-node-identifier = "@"
Comparison expressions are available for comparisons between
primitive values (that is, numbers, strings, true, false, and null).
These can be obtained via literal values; singular queries, each of
which selects at most one node, the value of which is then used; or
function expressions (see Section 2.4) of type ValueType.
comparison-expr = comparable S comparison-op S comparable
literal = number / string-literal /
true / false / null
comparable = singular-query / ; singular query value
function-expr / ; ValueType
literal
EID 8352 (Verified) is as follows:Section: 2.3.5.1
Original Text:
comparable = literal /
singular-query / ; singular query value
function-expr ; ValueType
Corrected Text:
comparable = singular-query / ; singular query value
function-expr / ; ValueType
literal
Notes:
The ABNF grammars in RFC 9535 were designed to be directly usable with PEG (Parsing Expression Grammar) parsers.
However, PEG parsers will fail to parse $[?blt(1==1)] or $[?true(1)==0] with the grammar as given, as they employ prioritized choice, where the order matters.
In the order given, they will try to match the `literal` rule in `function-argument` with the input `1==1`, and find that the `1` indeed matches a `number`, completing the match for `function-argument` and preempting the other choices. The intended rest of the `function-argument`, `==1` does not match anything, and the rule fails. By putting the more complex `logical-expr` first, the whole `1==1` matches, and the rule succeeds as intended.
Similary, the function name `true` matches as the literal `true` instead, and preempts parsing `true(1)` as the more complex `function-expr`. Putting the `literal` choice last prevents the preemptive match.
comparison-op = "==" / "!=" /
"<=" / ">=" /
"<" / ">"
singular-query = rel-singular-query / abs-singular-query
rel-singular-query = current-node-identifier singular-query-segments
abs-singular-query = root-identifier singular-query-segments
singular-query-segments = *(S (name-segment / index-segment))
name-segment = ("[" name-selector "]") /
("." member-name-shorthand)
index-segment = "[" index-selector "]"
Literals can be notated in the way that is usual for JSON (with the
extension that strings can use single-quote delimiters).
Note: Alphabetic characters in quoted strings are case-insensitive in
ABNF, so within a floating point number, the ABNF expression "e" can
be either the character 'e' or 'E'.
true, false, and null are lowercase only (case-sensitive).
number = (int / "-0") [ 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
Table 10 lists filter expression operators in order of precedence
from highest (binds most tightly) to lowest (binds least tightly).
+============+======================+=============+
| Precedence | Operator type | Syntax |
+============+======================+=============+
| 5 | Grouping | (...) |
| | Function Expressions | _name_(...) |
+------------+----------------------+-------------+
| 4 | Logical NOT | ! |
+------------+----------------------+-------------+
| 3 | Relations | == != |
| | | < <= > >= |
+------------+----------------------+-------------+
| 2 | Logical AND | && |
+------------+----------------------+-------------+
| 1 | Logical OR | || |
+------------+----------------------+-------------+
Table 10: Filter Expression Operator Precedence
2.3.5.2. Semantics
The filter selector works with arrays and objects exclusively. Its
result is a list of (_zero_, _one_, _multiple_, or _all_) their array
elements or member values, respectively. Applied to a primitive
value, it selects nothing (and therefore does not contribute to the
result of the filter selector).
In the resultant nodelist, children of an array are ordered by their
position in the array. The order in which the children of an object
(as opposed to an array) appear in the resultant nodelist is not
stipulated, since JSON objects are unordered.
2.3.5.2.1. Existence Tests
A query by itself in a logical context is an existence test that
yields true if the query selects at least one node and yields false
if the query does not select any nodes.
Existence tests differ from comparisons in that:
* They work with arbitrary relative or absolute queries (not just
singular queries).
* They work with queries that select structured values.
To examine the value of a node selected by a query, an explicit
comparison is necessary. For example, to test whether the node
selected by the query @.foo has the value null, use @.foo == null
(see Section 2.6) rather than the negated existence test !@.foo
(which yields false if @.foo selects a node, regardless of the node's
value). Similarly, @.foo == false yields true only if @.foo selects
a node and the value of that node is false.
2.3.5.2.2. Comparisons
The comparison operators == and < are defined first, and then these
are used to define !=, <=, >, and >=.
When either side of a comparison results in an empty nodelist or the
special result Nothing (see Section 2.4.1):
* A comparison using the operator == yields true if and only the
other side also results in an empty nodelist or the special result
Nothing.
* A comparison using the operator < yields false.
When any query or function expression on either side of a comparison
results in a nodelist consisting of a single node, that side is
replaced by the value of its node and then:
* A comparison using the operator == yields true if and only if the
comparison is between:
- numbers expected to interoperate, as per Section 2.2 of I-JSON
[RFC7493], that compare equal using normal mathematical
equality,
- numbers, at least one of which is not expected to interoperate
as per I-JSON, where the numbers compare equal using an
implementation-specific equality,
- equal primitive values that are not numbers,
- equal arrays, that is, arrays of the same length where each
element of the first array is equal to the corresponding
element of the second array, or
- equal objects with no duplicate names, that is, where:
o both objects have the same collection of names (with no
duplicates) and
o for each of those names, the values associated with the name
by the objects are equal.
* A comparison using the operator < yields true if and only if the
comparison is between values that are both numbers or both strings
and that satisfy the comparison:
- numbers expected to interoperate, as per Section 2.2 of I-JSON
[RFC7493], MUST compare using the normal mathematical ordering;
numbers not expected to interoperate, as per I-JSON, MAY
compare using an implementation-specific ordering,
- the empty string compares less than any non-empty string, and
- a non-empty string compares less than another non-empty string
if and only if the first string starts with a lower Unicode
scalar value than the second string or if both strings start
with the same Unicode scalar value and the remainder of the
first string compares less than the remainder of the second
string.
!=, <=, >, and >= are defined in terms of the other comparison
operators. For any a and b:
* The comparison a != b yields true if and only if a == b yields
false.
* The comparison a <= b yields true if and only if a < b yields true
or a == b yields true.
* The comparison a > b yields true if and only if b < a yields true.
* The comparison a >= b yields true if and only if b < a yields true
or a == b yields true.
2.3.5.3. Examples
The first set of examples shows some comparison expressions and their
result with a given JSON value as input.
JSON:
{
"obj": {"x": "y"},
"arr": [2, 3]
}
Comparisons:
+========================+========+========================+
| Comparison | Result | Comment |
+========================+========+========================+
| $.absent1 == $.absent2 | true | Empty nodelists |
+------------------------+--------+------------------------+
| $.absent1 <= $.absent2 | true | == implies <= |
+------------------------+--------+------------------------+
| $.absent == 'g' | false | Empty nodelist |
+------------------------+--------+------------------------+
| $.absent1 != $.absent2 | false | Empty nodelists |
+------------------------+--------+------------------------+
| $.absent != 'g' | true | Empty nodelist |
+------------------------+--------+------------------------+
| 1 <= 2 | true | Numeric comparison |
+------------------------+--------+------------------------+
| 1 > 2 | false | Numeric comparison |
+------------------------+--------+------------------------+
| 13 == '13' | false | Type mismatch |
+------------------------+--------+------------------------+
| 'a' <= 'b' | true | String comparison |
+------------------------+--------+------------------------+
| 'a' > 'b' | false | String comparison |
+------------------------+--------+------------------------+
| $.obj == $.arr | false | Type mismatch |
+------------------------+--------+------------------------+
| $.obj != $.arr | true | Type mismatch |
+------------------------+--------+------------------------+
| $.obj == $.obj | true | Object comparison |
+------------------------+--------+------------------------+
| $.obj != $.obj | false | Object comparison |
+------------------------+--------+------------------------+
| $.arr == $.arr | true | Array comparison |
+------------------------+--------+------------------------+
| $.arr != $.arr | false | Array comparison |
+------------------------+--------+------------------------+
| $.obj == 17 | false | Type mismatch |
+------------------------+--------+------------------------+
| $.obj != 17 | true | Type mismatch |
+------------------------+--------+------------------------+
| $.obj <= $.arr | false | Objects and arrays do |
| | | not offer < comparison |
+------------------------+--------+------------------------+
| $.obj < $.arr | false | Objects and arrays do |
| | | not offer < comparison |
+------------------------+--------+------------------------+
| $.obj <= $.obj | true | == implies <= |
+------------------------+--------+------------------------+
| $.arr <= $.arr | true | == implies <= |
+------------------------+--------+------------------------+
| 1 <= $.arr | false | Arrays do not offer < |
| | | comparison |
+------------------------+--------+------------------------+
| 1 >= $.arr | false | Arrays do not offer < |
| | | comparison |
+------------------------+--------+------------------------+
| 1 > $.arr | false | Arrays do not offer < |
| | | comparison |
+------------------------+--------+------------------------+
| 1 < $.arr | false | Arrays do not offer < |
| | | comparison |
+------------------------+--------+------------------------+
| true <= true | true | == implies <= |
+------------------------+--------+------------------------+
| true > true | false | Booleans do not offer |
| | | < comparison |
+------------------------+--------+------------------------+
Table 11: Comparison Examples
The second set of examples shows some complete JSONPath queries that
make use of filter selectors and the results of evaluating these
queries on a given JSON value as input. (Note: Two of the queries
employ function extensions; please see Sections 2.4.6 and 2.4.7 for
details about these.)
JSON:
{
"a": [3, 5, 1, 2, 4, 6,
{"b": "j"},
{"b": "k"},
{"b": {}},
{"b": "kilo"}
],
"o": {"p": 1, "q": 2, "r": 3, "s": 5, "t": {"u": 6}},
"e": "f"
}
Queries:
The examples in Table 12 show the filter selector in use by a child
segment.
+==================+==============+=============+===================+
| Query | Result | Result | Comment |
| | | Paths | |
+==================+==============+=============+===================+
| $.a[?@.b == | {"b": | $['a'][9] | Member value |
| 'kilo'] | "kilo"} | | comparison |
+------------------+--------------+-------------+-------------------+
| $.a[?(@.b == | {"b": | $['a'][9] | Equivalent query |
| 'kilo')] | "kilo"} | | with enclosing |
| | | | parentheses |
+------------------+--------------+-------------+-------------------+
| $.a[?@>3.5] | 5 | $['a'][1] | Array value |
| | 4 | $['a'][4] | comparison |
| | 6 | $['a'][5] | |
+------------------+--------------+-------------+-------------------+
| $.a[?@.b] | {"b": "j"} | $['a'][6] | Array value |
| | {"b": "k"} | $['a'][7] | existence |
| | {"b": {}} | $['a'][8] | |
| | {"b": | $['a'][9] | |
| | "kilo"} | | |
+------------------+--------------+-------------+-------------------+
| $[?@.*] | [3, 5, 1, | $['a'] | Existence of non- |
| | 2, 4, 6, | $['o'] | singular queries |
| | {"b": "j"}, | | |
| | {"b": "k"}, | | |
| | {"b": {}}, | | |
| | {"b": | | |
| | "kilo"}] | | |
| | {"p": 1, | | |
| | "q": 2, | | |
| | "r": 3, | | |
| | "s": 5, | | |
| | "t": {"u": | | |
| | 6}} | | |
+------------------+--------------+-------------+-------------------+
| $[?@[?@.b]] | [3, 5, 1, | $['a'] | Nested filters |
| | 2, 4, 6, | | |
| | {"b": "j"}, | | |
| | {"b": "k"}, | | |
| | {"b": {}}, | | |
| | {"b": | | |
| | "kilo"}] | | |
+------------------+--------------+-------------+-------------------+
| $.o[?@<3, ?@<3] | 1 | $['o']['p'] | Non-deterministic |
| | 2 | $['o']['q'] | ordering |
| | 2 | $['o']['q'] | |
| | 1 | $['o']['p'] | |
+------------------+--------------+-------------+-------------------+
| $.a[?@<2 || @.b | 1 | $['a'][2] | Array value |
| == "k"] | {"b": "k"} | $['a'][7] | logical OR |
+------------------+--------------+-------------+-------------------+
| $.a[?match(@.b, | {"b": "j"} | $['a'][6] | Array value |
| "[jk]")] | {"b": "k"} | $['a'][7] | regular |
| | | | expression match |
+------------------+--------------+-------------+-------------------+
| $.a[?search(@.b, | {"b": "j"} | $['a'][6] | Array value |
| "[jk]")] | {"b": "k"} | $['a'][7] | regular |
| | {"b": | $['a'][9] | expression search |
| | "kilo"} | | |
+------------------+--------------+-------------+-------------------+
| $.o[?@>1 && @<4] | 2 | $['o']['q'] | Object value |
| | 3 | $['o']['r'] | logical AND |
+------------------+--------------+-------------+-------------------+
| $.o[?@>1 && @<4] | 3 | $['o']['r'] | Alternative |
| | 2 | $['o']['q'] | result |
+------------------+--------------+-------------+-------------------+
| $.o[?@.u || @.x] | {"u": 6} | $['o']['t'] | Object value |
| | | | logical OR |
+------------------+--------------+-------------+-------------------+
| $.a[?@.b == $.x] | 3 | $['a'][0] | Comparison of |
| | 5 | $['a'][1] | queries with no |
| | 1 | $['a'][2] | values |
| | 2 | $['a'][3] | |
| | 4 | $['a'][4] | |
| | 6 | $['a'][5] | |
+------------------+--------------+-------------+-------------------+
| $.a[?@ == @] | 3 | $['a'][0] | Comparisons of |
| | 5 | $['a'][1] | primitive and of |
| | 1 | $['a'][2] | structured values |
| | 2 | $['a'][3] | |
| | 4 | $['a'][4] | |
| | 6 | $['a'][5] | |
| | {"b": "j"} | $['a'][6] | |
| | {"b": "k"} | $['a'][7] | |
| | {"b": {}} | $['a'][8] | |
| | {"b": | $['a'][9] | |
| | "kilo"} | | |
+------------------+--------------+-------------+-------------------+
Table 12: Filter Selector Examples
The example above with the query $.o[?@<3, ?@<3] shows that a filter
selector may produce nodelists in distinct orders each time it
appears in the child segment.
2.4. Function Extensions
Beyond the filter expression functionality defined in the preceding
subsections, JSONPath defines an extension point that can be used to
add filter expression functionality: "Function Extensions".
This section defines the extension point and some function extensions
that use this extension point. While these mechanisms are designed
to use the extension point, they are an integral part of the JSONPath
specification and are expected to be implemented like any other
integral part of this specification.
A function extension defines a registered name (see Section 3.2) that
can be applied to a sequence of zero or more arguments, producing a
result. Each registered function name is unique.
A function extension MUST be defined such that its evaluation is free
of side effects, i.e., all possible orders of evaluation and choices
of short-circuiting or full evaluation of an expression containing it
MUST lead to the same result. (Note: Memoization or logging are not
side effects in this sense as they are visible at the implementation
level only -- they do not influence the result of the evaluation.)
function-name = function-name-first *function-name-char
function-name-first = LCALPHA
function-name-char = function-name-first / "_" / DIGIT
LCALPHA = %x61-7A ; "a".."z"
function-expr = function-name "(" S [function-argument
*(S "," S function-argument)] S ")"
function-argument = logical-expr /
filter-query / ; (includes singular-query)
function-expr /
literal
Notes:
The ABNF grammars in RFC 9535 were designed to be directly usable with PEG (Parsing Expression Grammar) parsers.
However, PEG parsers will fail to parse $[?blt(1==1)] or $[?true(1)==0] with the grammar as given, as they employ prioritized choice, where the order matters.
In the order given, they will try to match the `literal` rule in `function-argument` with the input `1==1`, and find that the `1` indeed matches a `number`, completing the match for `function-argument` and preempting the other choices. The intended rest of the `function-argument`, `==1` does not match anything, and the rule fails. By putting the more complex `logical-expr` first, the whole `1==1` matches, and the rule succeeds as intended.
Similary, the function name `true` matches as the literal `true` instead, and preempts parsing `true(1)` as the more complex `function-expr`. Putting the `literal` choice last prevents the preemptive match.
Any function expressions in a query must be well-formed (by
conforming to the above ABNF) and well-typed; otherwise, the JSONPath
implementation MUST raise an error (see Section 2.1). To define
which function expressions are well-typed, a type system is first
introduced.
2.4.1. Type System for Function Expressions
Each parameter and the result of a function extension must have a
declared type.
Declared types enable checking a JSONPath query for well-typedness
independent of any query argument the JSONPath query is applied to.
Table 13 defines the available types in terms of the instances they
contain.
+=============+=============================+
| Type | Instances |
+=============+=============================+
| ValueType | JSON values or Nothing |
+-------------+-----------------------------+
| LogicalType | LogicalTrue or LogicalFalse |
+-------------+-----------------------------+
| NodesType | Nodelists |
+-------------+-----------------------------+
Table 13: Function Extension Type System
Notes:
* The only instances that can be directly represented in JSONPath
syntax are certain JSON values in ValueType expressed as literals
(which, in JSONPath, are limited to primitive values).
* The special result Nothing represents the absence of a JSON value
and is distinct from any JSON value, including null.
* LogicalTrue and LogicalFalse are unrelated to the JSON values
expressed by the literals true and false.
2.4.2. Type Conversion
Just as queries can be used in logical expressions by testing for the
existence of at least one node (Section 2.3.5.2.1), a function
expression of declared type NodesType can be used as a function
argument for a parameter of declared type LogicalType, with the
equivalent conversion rule:
* If the nodelist contains one or more nodes, the conversion result
is LogicalTrue.
* If the nodelist is empty, the conversion result is LogicalFalse.
Notes:
* Extraction of a value from a nodelist can be performed in several
ways, so an implicit conversion from NodesType to ValueType may be
surprising and has therefore not been defined.
* A function expression with a declared type of NodesType can
indirectly be used as an argument for a parameter of declared type
ValueType by wrapping the expression in a call to a function
extension, such as value() (see Section 2.4.8), that takes a
parameter of type NodesType and returns a result of type
ValueType.
The well-typedness of function expressions can now be defined in
terms of this type system.
2.4.3. Well-Typedness of Function Expressions
For a function expression to be well-typed:
1. Its declared type must be well-typed in the context in which it
occurs.
As per the grammar, a function expression can occur in three
different immediate contexts, which lead to the following
conditions for well-typedness:
As a test-expr in a logical expression:
The function's declared result type is LogicalType or (giving
rise to conversion as per Section 2.4.2) NodesType.
As a comparable in a comparison:
The function's declared result type is ValueType.
As a function-argument in another function expression:
The function's declared result type fulfills the following
rules for the corresponding parameter of the enclosing
function.
2. Its arguments must be well-typed for the declared type of the
corresponding parameters.
The arguments of the function expression are well-typed when each
argument of the function can be used for the declared type of the
corresponding parameter, according to one of the following
conditions:
* When the argument is a function expression with the same
declared result type as the declared type of the parameter.
* When the declared type of the parameter is LogicalType and the
argument is one of the following:
- A function expression with declared result type NodesType.
In this case, the argument is converted to LogicalType as
per Section 2.4.2.
- A logical-expr that is not a function expression.
* When the declared type of the parameter is NodesType and the
argument is a query (which includes singular query).
* When the declared type of the parameter is ValueType and the
argument is one of the following:
- A value expressed as a literal.
- A singular query. In this case:
o If the query results in a nodelist consisting of a
single node, the argument is the value of the node.
o If the query results in an empty nodelist, the argument
is the special result Nothing.
2.4.4. length() Function Extension
Parameters:
1. ValueType
Result: ValueType (unsigned integer or Nothing)
The length() function extension provides a way to compute the length
of a value and make that available for further processing in the
filter expression:
$[?length(@.authors) >= 5]
Its only argument is an instance of ValueType (possibly taken from a
singular query, as in the example above). The result is also an
instance of ValueType: an unsigned integer or the special result
Nothing.
* If the argument value is a string, the result is the number of
Unicode scalar values in the string.
* If the argument value is an array, the result is the number of
elements in the array.
* If the argument value is an object, the result is the number of
members in the object.
* For any other argument value, the result is the special result
Nothing.
2.4.5. count() Function Extension
Parameters:
1. NodesType
Result: ValueType (unsigned integer)
The count() function extension provides a way to obtain the number of
nodes in a nodelist and make that available for further processing in
the filter expression:
$[?count(@.*.author) >= 5]
Its only argument is a nodelist. The result is a value (an unsigned
integer) that gives the number of nodes in the nodelist.
Notes:
* There is no deduplication of the nodelist.
* The number of nodes in the nodelist is counted independent of
their values or any children they may have, e.g., the count of a
non-empty singular nodelist such as count(@) is always 1.
2.4.6. match() Function Extension
Parameters:
1. ValueType (string)
2. ValueType (string conforming to [RFC9485])
Result: LogicalType
The match() function extension provides a way to check whether (the
entirety of; see Section 2.4.7) a given string matches a given
regular expression, which is in the form described in [RFC9485].
$[?match(@.date, "1974-05-..")]
Its arguments are instances of ValueType (possibly taken from a
singular query, as for the first argument in the example above). If
the first argument is not a string or the second argument is not a
string conforming to [RFC9485], the result is LogicalFalse.
Otherwise, the string that is the first argument is matched against
the I-Regexp contained in the string that is the second argument; the
result is LogicalTrue if the string matches the I-Regexp and is
LogicalFalse otherwise.
2.4.7. search() Function Extension
Parameters:
1. ValueType (string)
2. ValueType (string conforming to [RFC9485])
Result: LogicalType
The search() function extension provides a way to check whether a
given string contains a substring that matches a given regular
expression, which is in the form described in [RFC9485].
$[?search(@.author, "[BR]ob")]
Its arguments are instances of ValueType (possibly taken from a
singular query, as for the first argument in the example above). If
the first argument is not a string or the second argument is not a
string conforming to [RFC9485], the result is LogicalFalse.
Otherwise, the string that is the first argument is searched for a
substring that matches the I-Regexp contained in the string that is
the second argument; the result is LogicalTrue if at least one such
substring exists and is LogicalFalse otherwise.
2.4.8. value() Function Extension
Parameters:
1. NodesType
Result: ValueType
The value() function extension provides a way to convert an instance
of NodesType to a value and make that available for further
processing in the filter expression:
$[?value(@..color) == "red"]
Its only argument is an instance of NodesType (possibly taken from a
filter-query, as in the example above). The result is an instance of
ValueType.
* If the argument contains a single node, the result is the value of
the node.
* If the argument is the empty nodelist or contains multiple nodes,
the result is Nothing.
Note: A singular query may be used anywhere where a ValueType is
expected, so there is no need to use the value() function extension
with a singular query.
2.4.9. Examples
+======================+==========================================+
| Query | Comment |
+======================+==========================================+
| $[?length(@) < 3] | well-typed |
+----------------------+------------------------------------------+
| $[?length(@.*) < 3] | not well-typed since @.* is a non- |
| | singular query |
+----------------------+------------------------------------------+
| $[?count(@.*) == 1] | well-typed |
+----------------------+------------------------------------------+
| $[?count(1) == 1] | not well-typed since 1 is not a query or |
| | function expression |
+----------------------+------------------------------------------+
| $[?count(foo(@.*)) | well-typed, where foo() is a function |
| == 1] | extension with a parameter of type |
| | NodesType and result type NodesType |
+----------------------+------------------------------------------+
| $[?match(@.timezone, | well-typed |
| 'Europe/.*')] | |
+----------------------+------------------------------------------+
| $[?match(@.timezone, | not well-typed as LogicalType may not be |
| 'Europe/.*') == | used in comparisons |
| true] | |
+----------------------+------------------------------------------+
| $[?value(@..color) | well-typed |
| == "red"] | |
+----------------------+------------------------------------------+
| $[?value(@..color)] | not well-typed as ValueType may not be |
| | used in a test expression |
+----------------------+------------------------------------------+
| $[?bar(@.a)] | well-typed for any function bar() with a |
| | parameter of any declared type and |
| | result type LogicalType |
+----------------------+------------------------------------------+
| $[?bnl(@.*)] | well-typed for any function bnl() with a |
| | parameter of declared type NodesType or |
| | LogicalType and result type LogicalType |
+----------------------+------------------------------------------+
| $[?blt(1==1)] | well-typed, where blt() is a function |
| | with a parameter of declared type |
| | LogicalType and result type LogicalType |
+----------------------+------------------------------------------+
| $[?blt(1)] | not well-typed for the same function |
| | blt(), as 1 is not a query, logical- |
| | expr, or function expression |
+----------------------+------------------------------------------+
| $[?bal(1)] | well-typed, where bal() is a function |
| | with a parameter of declared type |
| | ValueType and result type LogicalType |
+----------------------+------------------------------------------+
Table 14: Function Expression Examples
2.5. Segments
For each node in an input nodelist, segments apply one or more
selectors to the node and concatenate the results of each selector
into per-input-node nodelists, which are then concatenated in the
order of the input nodelist to form a single segment result nodelist.
It turns out that the more segments there are in a query, the greater
the depth in the input value of the nodes of the resultant nodelist:
* A query with N segments, where N >= 0, produces a nodelist
consisting of nodes at depth in the input value of N or greater.
* A query with N segments, where N >= 0, all of which are child
segments (Section 2.5.1), produces a nodelist consisting of nodes
precisely at depth N in the input value.
There are two kinds of segments: child segments and descendant
segments.
segment = child-segment / descendant-segment
The syntax and semantics of each kind of segment are defined below.
2.5.1. Child Segment
2.5.1.1. Syntax
The child segment consists of a non-empty, comma-separated sequence
of selectors enclosed in square brackets.
Shorthand notations are also provided for when there is a single
wildcard or name selector.
child-segment = bracketed-selection /
("."
(wildcard-selector /
member-name-shorthand))
bracketed-selection = "[" S selector *(S "," S selector) S "]"
member-name-shorthand = name-first *name-char
name-first = ALPHA /
"_" /
%x80-D7FF /
; skip surrogate code points
%xE000-10FFFF
name-char = name-first / DIGIT
DIGIT = %x30-39 ; 0-9
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
.*, a child-segment directly built from a wildcard-selector, is
shorthand for [*].
.<member-name>, a child-segment built from a member-name-shorthand,
is shorthand for ['<member-name>']. Note: This can only be used with
member names that are composed of certain characters, as specified in
the ABNF rule member-name-shorthand. Thus, for example, $.foo.bar is
shorthand for $['foo']['bar'] (but not for $['foo.bar']).
2.5.1.2. Semantics
A child segment contains a sequence of selectors, each of which
selects zero or more children of the input value.
Selectors of different kinds may be combined within a single child
segment.
For each node in the input nodelist, the resulting nodelist of a
child segment is the concatenation of the nodelists from each of its
selectors in the order that the selectors appear in the list. Note:
Any node matched by more than one selector is kept as many times in
the nodelist.
Where a selector can produce a nodelist in more than one possible
order, each occurrence of the selector in the child segment may
produce a nodelist in a distinct order.
In summary, a child segment drills down one more level into the
structure of the input value.
2.5.1.3. Examples
JSON:
["a", "b", "c", "d", "e", "f", "g"]
Queries:
+========+========+========+============+
| Query | Result | Result | Comment |
| | | Paths | |
+========+========+========+============+
| $[0, | "a" | $[0] | Indices |
| 3] | "d" | $[3] | |
+--------+--------+--------+------------+
| $[0:2, | "a" | $[0] | Slice and |
| 5] | "b" | $[1] | index |
| | "f" | $[5] | |
+--------+--------+--------+------------+
| $[0, | "a" | $[0] | Duplicated |
| 0] | "a" | $[0] | entries |
+--------+--------+--------+------------+
Table 15: Child Segment Examples
2.5.2. Descendant Segment
2.5.2.1. Syntax
The descendant segment consists of a double dot .. followed by a
child segment (using bracket notation).
Shorthand notations are also provided that correspond to the
shorthand forms of the child segment.
descendant-segment = ".." (bracketed-selection /
wildcard-selector /
member-name-shorthand)
..*, the descendant-segment directly built from a wildcard-selector,
is shorthand for ..[*].
..<member-name>, a descendant-segment built from a member-name-
shorthand, is shorthand for ..['<member-name>']. Note: As with the
similar shorthand of a child-segment, this can only be used with
member names that are composed of certain characters, as specified in
the ABNF rule member-name-shorthand.
Note: On its own, .. is not a valid segment.
2.5.2.2. Semantics
A descendant segment produces zero or more descendants of an input
value.
For each node in the input nodelist, a descendant selector visits the
input node and each of its descendants such that:
* nodes of any array are visited in array order, and
* nodes are visited before their descendants.
The order in which the children of an object are visited is not
stipulated, since JSON objects are unordered.
Suppose the descendant segment is of the form ..[<selectors>] (after
converting any shorthand form to bracket notation), and the nodes, in
the order visited, are D1, ..., Dn (where n >= 1). Note: D1 is the
input value.
For each i such that 1 <= i <= n, the nodelist Ri is defined to be a
result of applying the child segment [<selectors>] to the node Di.
For each node in the input nodelist, the result of the descendant
segment is the concatenation of R1, ..., Rn (in that order). These
results are then concatenated in input nodelist order to form the
result of the segment.
In summary, a descendant segment drills down one or more levels into
the structure of each input value.
2.5.2.3. Examples
JSON:
{
"o": {"j": 1, "k": 2},
"a": [5, 3, [{"j": 4}, {"k": 6}]]
}
Queries:
(Note that the fourth example can be expressed in two equivalent
queries, shown in Table 16 in one table row instead of two almost-
identical rows.)
+==========+================+===================+===================+
| Query | Result | Result Paths | Comment |
+==========+================+===================+===================+
| $..j | 1 | $['o']['j'] | Object values |
| | 4 | $['a'][2][0]['j'] | |
+----------+----------------+-------------------+-------------------+
| $..j | 4 | $['a'][2][0]['j'] | Alternative |
| | 1 | $['o']['j'] | result |
+----------+----------------+-------------------+-------------------+
| $..[0] | 5 | $['a'][0] | Array values |
| | {"j": 4} | $['a'][2][0] | |
+----------+----------------+-------------------+-------------------+
| $..[*] | {"j": 1, | $['o'] | All values |
| or | "k": 2} | $['a'] | |
| $..* | [5, 3, | $['o']['j'] | |
| | [{"j": 4}, | $['o']['k'] | |
| | {"k": 6}]] | $['a'][0] | |
| | 1 | $['a'][1] | |
| | 2 | $['a'][2] | |
| | 5 | $['a'][2][0] | |
| | 3 | $['a'][2][1] | |
| | [{"j": 4}, | $['a'][2][0]['j'] | |
| | {"k": 6}] | $['a'][2][1]['k'] | |
| | {"j": 4} | | |
| | {"k": 6} | | |
| | 4 | | |
| | 6 | | |
+----------+----------------+-------------------+-------------------+
| $..o | {"j": 1, | $['o'] | Input value is |
| | "k": 2} | | visited |
+----------+----------------+-------------------+-------------------+
| $.o..[*, | 1 | $['o']['j'] | Non-deterministic |
| *] | 2 | $['o']['k'] | ordering |
| | 2 | $['o']['k'] | |
| | 1 | $['o']['j'] | |
+----------+----------------+-------------------+-------------------+
| $.a..[0, | 5 | $['a'][0] | Multiple segments |
| 1] | 3 | $['a'][1] | |
| | {"j": 4} | $['a'][2][0] | |
| | {"k": 6} | $['a'][2][1] | |
+----------+----------------+-------------------+-------------------+
Table 16: Descendant Segment Examples
Note: The ordering of the results for the $..[*] and $..* examples
above is not guaranteed, except that:
* {"j": 1, "k": 2} must appear before 1 and 2,
* [5, 3, [{"j": 4}, {"k": 6}]] must appear before 5, 3, and [{"j":
4}, {"k": 6}],
* 5 must appear before 3, which must appear before [{"j": 4}, {"k":
6}],
* 5 and 3 must appear before {"j": 4}, 4, {"k": 6}, and 6,
* [{"j": 4}, {"k": 6}] must appear before {"j": 4} and {"k": 6},
* {"j": 4} must appear before {"k": 6},
* {"k": 6} must appear before 4, and
* 4 must appear before 6.
The example above with the query $.o..[*, *] shows that a selector
may produce nodelists in distinct orders each time it appears in the
descendant segment.
The example above with the query $.a..[0, 1] shows that the child
segment [0, 1] is applied to each node in turn (rather than the nodes
being visited once per selector, which is the case for some JSONPath
implementations that do not conform to this specification).
2.6. Semantics of null
Note: JSON null is treated the same as any other JSON value, i.e., it
is not taken to mean "undefined" or "missing".
2.6.1. Examples
JSON:
{"a": null, "b": [null], "c": [{}], "null": 1}
Queries:
+=================+========+===========+===========================+
| Query | Result | Result | Comment |
| | | Paths | |
+=================+========+===========+===========================+
| $.a | null | $['a'] | Object value |
+-----------------+--------+-----------+---------------------------+
| $.a[0] | | | null used as array |
+-----------------+--------+-----------+---------------------------+
| $.a.d | | | null used as object |
+-----------------+--------+-----------+---------------------------+
| $.b[0] | null | $['b'][0] | Array value |
+-----------------+--------+-----------+---------------------------+
| $.b[*] | null | $['b'][0] | Array value |
+-----------------+--------+-----------+---------------------------+
| $.b[?@] | null | $['b'][0] | Existence |
+-----------------+--------+-----------+---------------------------+
| $.b[?@==null] | null | $['b'][0] | Comparison |
+-----------------+--------+-----------+---------------------------+
| $.c[?@.d==null] | | | Comparison with "missing" |
| | | | value |
+-----------------+--------+-----------+---------------------------+
| $.null | 1 | $['null'] | Not JSON null at all, |
| | | | just a member name string |
+-----------------+--------+-----------+---------------------------+
Table 17: Examples Involving (or Not Involving) null
2.7. Normalized Paths
A Normalized Path is a unique representation of the location of a
node in a value that uniquely identifies the node in the value.
Specifically, a Normalized Path is a JSONPath query with restricted
syntax (defined below), e.g., $['book'][3], which when applied to the
value, results in a nodelist consisting of just the node identified
by the Normalized Path. Note: A Normalized Path represents the
identity of a node _in a specific value_. There is precisely one
Normalized Path identifying any particular node in a value.
A nodelist may be represented compactly in JSON as an array of
strings, where the strings are Normalized Paths.
Normalized Paths provide a predictable format that simplifies testing
and post-processing of nodelists, e.g., to remove duplicate nodes.
Normalized Paths are used in this document as result paths in
examples.
Normalized Paths use the canonical bracket notation, rather than dot
notation.
Single quotes are used in Normalized Paths to delimit string member
names. This reduces the number of characters that need escaping when
Normalized Paths appear in strings delimited by double quotes, e.g.,
in JSON texts.
Certain characters are escaped in Normalized Paths in one and only
one way; all other characters are unescaped.
| Note: Normalized Paths are singular queries, but not all
| singular queries are Normalized Paths. For example, $[-3] is a
| singular query but is not a Normalized Path. The Normalized
| Path equivalent to $[-3] would have an index equal to the array
| length minus 3. (The array length must be at least 3 if $[-3]
| is to identify a node.)
normalized-path = root-identifier *(normal-index-segment)
normal-index-segment = "[" normal-selector "]"
normal-selector = normal-name-selector / normal-index-selector
normal-name-selector = %x27 *normal-single-quoted %x27 ; 'string'
normal-single-quoted = normal-unescaped /
ESC normal-escapable
normal-unescaped = ; omit %x0-1F control codes
%x20-26 /
; omit 0x27 '
%x28-5B /
; omit 0x5C \
%x5D-D7FF /
; skip surrogate code points
%xE000-10FFFF
normal-escapable = %x62 / ; b BS backspace U+0008
%x66 / ; f FF form feed U+000C
%x6E / ; n LF line feed U+000A
%x72 / ; r CR carriage return U+000D
%x74 / ; t HT horizontal tab U+0009
"'" / ; ' apostrophe U+0027
"\" / ; \ backslash (reverse solidus) U+005C
(%x75 normal-hexchar)
; certain values u00xx U+00XX
normal-hexchar = "0" "0"
(
("0" %x30-37) / ; "00"-"07"
; omit U+0008-U+000A BS HT LF
("0" %x62) / ; "0b"
; omit U+000C-U+000D FF CR
("0" %x65-66) / ; "0e"-"0f"
("1" normal-HEXDIG)
)
normal-HEXDIG = DIGIT / %x61-66 ; "0"-"9", "a"-"f"
normal-index-selector = "0" / (DIGIT1 *DIGIT)
; non-negative decimal integer
Since there can only be one Normalized Path identifying a given node,
the syntax stipulates which characters are escaped and which are not.
So the definition of normal-hexchar is designed for hex escaping of
characters that are not straightforwardly printable, for example,
U+000B LINE TABULATION, but for which no standard JSON escape, such
as \n, is available.
2.7.1. Examples
+=============+=================+==========================+
| Path | Normalized Path | Comment |
+=============+=================+==========================+
| $.a | $['a'] | Object value |
+-------------+-----------------+--------------------------+
| $[1] | $[1] | Array index |
+-------------+-----------------+--------------------------+
| $[-3] | $[2] | Negative array index for |
| | | an array of length 5 |
+-------------+-----------------+--------------------------+
| $.a.b[1:2] | $['a']['b'][1] | Nested structure |
+-------------+-----------------+--------------------------+
| $["\u000B"] | $['\u000b'] | Unicode escape |
+-------------+-----------------+--------------------------+
| $["\u0061"] | $['a'] | Unicode character |
+-------------+-----------------+--------------------------+
Table 18: Normalized Path Examples
3. IANA Considerations
3.1. Registration of Media Type application/jsonpath
IANA has registered the following media type [RFC6838]:
Type name: application
Subtype name: jsonpath
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary (UTF-8)
Security considerations: See the Security Considerations section of
RFC 9535.
Interoperability considerations: N/A
Published specification: RFC 9535
Applications that use this media type: Applications that need to
convey queries in JSON data
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: JSONPath WG
Change controller: IETF
3.2. Function Extensions Subregistry
Per this specification, IANA has created a new "Function Extensions"
subregistry in a new "JSONPath" registry. The "Function Extensions"
subregistry has the policy "Expert Review" (Section 4.5 of
[RFC8126]).
The experts are instructed to be frugal in the allocation of function
extension names that are suggestive of generally applicable
semantics, keeping them in reserve for functions that are likely to
enjoy wide use and can make good use of their conciseness. The
expert is also instructed to direct the registrant to provide a
specification (Section 4.6 of [RFC8126]) but can make exceptions, for
instance, when a specification is not available at the time of
registration but is likely forthcoming. If the expert becomes aware
of function extensions that are deployed and in use, they may also
initiate a registration on their own if they deem such a registration
can avert potential future collisions.
Each entry in the subregistry must include the following:
Function Name:
A lowercase ASCII [RFC0020] string that starts with a letter and
can contain letters, digits, and underscore characters afterwards
([a-z][_a-z0-9]*). No other entry in the subregistry can have the
same function name.
Brief description:
A brief description
Parameters:
A comma-separated list of zero or more declared types, one for
each of the arguments expected for this function extension
Result:
The declared type of the result for this function extension
Change Controller:
See Section 2.3 of [RFC8126].
Reference:
A reference document that provides a description of the function
extension
The initial entries in this subregistry are listed in Table 19; the
entries in the "Change Controller" column all have the value "IETF",
and the entries in the "Reference" column all have the value
"Section 2.4 of RFC 9535":
+===============+=====================+============+=============+
| Function Name | Brief Description | Parameters | Result |
+===============+=====================+============+=============+
| length | length of string, | ValueType | ValueType |
| | array, or object | | |
+---------------+---------------------+------------+-------------+
| count | size of nodelist | NodesType | ValueType |
+---------------+---------------------+------------+-------------+
| match | regular expression | ValueType, | LogicalType |
| | full match | ValueType | |
+---------------+---------------------+------------+-------------+
| search | regular expression | ValueType, | LogicalType |
| | substring match | ValueType | |
+---------------+---------------------+------------+-------------+
| value | value of the single | NodesType | ValueType |
| | node in nodelist | | |
+---------------+---------------------+------------+-------------+
Table 19: Initial Entries in the Function Extensions Subregistry
4. Security Considerations
Security considerations for JSONPath can stem from:
* attack vectors on JSONPath implementations,
* attack vectors on how JSONPath queries are formed, and
* the way JSONPath is used in security-relevant mechanisms.
4.1. Attack Vectors on JSONPath Implementations
Historically, JSONPath has often been implemented by feeding parts of
the query to an underlying programming language engine, e.g.,
JavaScript's eval() function. This approach is well known to lead to
injection attacks and would require perfect input validation to
prevent these attacks (see Section 12 of [RFC8259] for similar
considerations for JSON itself). Instead, JSONPath implementations
need to implement the entire syntax of the query without relying on
the parsers of programming language engines.
Attacks on availability may attempt to trigger unusually expensive
runtime performance exhibited by certain implementations in certain
cases. (See Section 10 of [RFC8949] for issues in hash-table
implementations and Section 8 of [RFC9485] for performance issues in
regular expression implementations.) Implementers need to be aware
that good average performance is not sufficient as long as an
attacker can choose to submit specially crafted JSONPath queries or
query arguments that trigger surprisingly high, possibly exponential,
CPU usage or, for example, via a naive recursive implementation of
the descendant segment, stack overflow. Implementations need to have
appropriate resource management to mitigate these attacks.
4.2. Attack Vectors on How JSONPath Queries Are Formed
JSONPath queries are often not static but formed from variables that
provide index values, member names, or values to compare with in a
filter expression. These variables need to be validated (e.g., only
allowing specific constructs such as .name to be formed when the
given values allow that) and translated (e.g., by escaping string
delimiters). Not performing these validations and translations
correctly can lead to unexpected failures, which can lead to
availability, confidentiality, and integrity breaches, in particular,
if an adversary has control over the values (e.g., by entering them
into a web form). The resulting class of attacks, _injections_
(e.g., SQL injections), is consistently found among the top causes of
application security vulnerabilities and requires particular
attention.
4.3. Attacks on Security Mechanisms That Employ JSONPath
Where JSONPath is used as a part of a security mechanism, attackers
can attempt to provoke unexpected or unpredictable behavior or take
advantage of differences in behavior between JSONPath
implementations.
Unexpected or unpredictable behavior can arise from a query argument
with certain constructs described as unpredictable by [RFC8259].
Predictable behavior can be expected, except in relation to the
ordering of objects, for any query argument conforming with
[RFC7493].
Other attacks can target the behavior of underlying technologies,
such as UTF-8 (see Section 10 of [RFC3629]) and the Unicode character
set.
5. References
5.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[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>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC9485] Bormann, C. and T. Bray, "I-Regexp: An Interoperable
Regular Expression Format", RFC 9485,
DOI 10.17487/RFC9485, October 2023,
<https://www.rfc-editor.org/info/rfc9485>.
[UNICODE] The Unicode Consortium, "The Unicode® Standard",
<https://www.unicode.org/versions/latest/>. At the time
of writing,
<https://www.unicode.org/versions/Unicode15.0.0/
UnicodeStandard-15.0.pdf>.
5.2. Informative References
[BOOLEAN-LAWS]
"Boolean algebra: Laws", December 2023,
<https://en.wikipedia.org/w/
index.php?title=Boolean_algebra&oldid=1191386550#Laws>.
[COMPARISON]
Burgmer, C., "JSONPath Comparison",
<https://cburgmer.github.io/json-path-comparison/>.
[E4X] ISO, "Information technology - ECMAScript for XML (E4X)
specification", Withdrawn, ISO/IEC 22537:2006, February
2006, <https://www.iso.org/standard/41002.html>. An
equivalent specification, also withdrawn, is available
from <https://ecma-international.org/publications-and-
standards/standards/ecma-357>.
[ECMA-262] ECMA International, "ECMAScript Language Specification",
Standard ECMA-262, Third Edition, December 1999,
<https://www.ecma-international.org/wp-content/uploads/
ECMA-262_3rd_edition_december_1999.pdf>.
[JSONPath-orig]
Gössner, S., "JSONPath - XPath for JSON", February 2007,
<https://goessner.net/articles/JsonPath/>.
[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>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[SLICE] "Slice notation", commit 82f95b4, July 2022,
<https://github.com/tc39/proposal-slice-notation>.
[XPath] Berglund, A., Ed., Chamberlin, D., Ed., Simeon, J., Ed.,
Robie, J., Ed., Fernandez, M., Ed., Kay, M., Ed., and S.
Boag, Ed., "XML Path Language (XPath) 2.0 (Second
Edition)", W3C REC-xpath20-20101214, 14 December 2010,
<https://www.w3.org/TR/2010/REC-xpath20-20101214/>.
Appendix A. Collected ABNF Grammars
This appendix collects the ABNF grammar from the ABNF passages used
throughout the document.
Figure 2 contains the collected ABNF grammar that defines the syntax
of a JSONPath query.
jsonpath-query = root-identifier segments
segments = *(S segment)
B = %x20 / ; Space
%x09 / ; Horizontal tab
%x0A / ; Line feed or New line
%x0D ; Carriage return
S = *B ; optional blank space
root-identifier = "$"
selector = name-selector /
wildcard-selector /
slice-selector /
index-selector /
filter-selector
name-selector = 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 / ; see RFC 8259
; omit 0x22 "
%x23-26 /
; omit 0x27 '
%x28-5B /
; omit 0x5C \
%x5D-D7FF /
; skip surrogate code points
%xE000-10FFFF
escapable = %x62 / ; b BS backspace U+0008
%x66 / ; f FF form feed U+000C
%x6E / ; n LF line feed U+000A
%x72 / ; r CR carriage return U+000D
%x74 / ; t HT horizontal tab U+0009
"/" / ; / slash (solidus) U+002F
"\" / ; \ backslash (reverse solidus) U+005C
(%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"
wildcard-selector = "*"
index-selector = int ; decimal integer
int = "0" /
(["-"] DIGIT1 *DIGIT) ; - optional
DIGIT1 = %x31-39 ; 1-9 non-zero digit
slice-selector = [start S] ":" S [end S] [":" [S step ]]
start = int ; included in selection
end = int ; not included in selection
step = int ; default: 1
filter-selector = "?" S logical-expr
logical-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 = paren-expr /
comparison-expr /
test-expr
paren-expr = [logical-not-op S] "(" S logical-expr S ")"
; parenthesized expression
logical-not-op = "!" ; logical NOT operator
test-expr = [logical-not-op S]
(filter-query / ; existence/non-existence
function-expr) ; LogicalType or NodesType
filter-query = rel-query / jsonpath-query
rel-query = current-node-identifier segments
current-node-identifier = "@"
comparison-expr = comparable S comparison-op S comparable
literal = number / string-literal /
true / false / null
comparable = singular-query / ; singular query value
function-expr / ; ValueType
literal
EID 8353 (Verified) is as follows:Section: Appendix A says:
Original Text:
comparable = literal /
singular-query / ; singular query value
function-expr ; ValueType
Corrected Text:
comparable = singular-query / ; singular query value
function-expr / ; ValueType
literal
Notes:
The ABNF grammars in RFC 9535 were designed to be directly usable with PEG (Parsing Expression Grammar) parsers.
However, PEG parsers will fail to parse $[?blt(1==1)] or $[?true(1)==0] with the grammar as given, as they employ prioritized choice, where the order matters.
In the order given, they will try to match the `literal` rule in `function-argument` with the input `1==1`, and find that the `1` indeed matches a `number`, completing the match for `function-argument` and preempting the other choices. The intended rest of the `function-argument`, `==1` does not match anything, and the rule fails. By putting the more complex `logical-expr` first, the whole `1==1` matches, and the rule succeeds as intended.
Similary, the function name `true` matches as the literal `true` instead, and preempts parsing `true(1)` as the more complex `function-expr`. Putting the `literal` choice last prevents the preemptive match.
EID 8354 (Verified) is as follows:Section: Appendix A says:
Original Text:
function-argument = literal /
filter-query / ; (includes singular-query)
logical-expr /
function-expr
Corrected Text:
function-argument = logical-expr /
filter-query / ; (includes singular-query)
function-expr /
literal
Notes:
The ABNF grammars in RFC 9535 were designed to be directly usable with PEG (Parsing Expression Grammar) parsers.
However, PEG parsers will fail to parse $[?blt(1==1)] or $[?true(1)==0] with the grammar as given, as they employ prioritized choice, where the order matters.
In the order given, they will try to match the `literal` rule in `function-argument` with the input `1==1`, and find that the `1` indeed matches a `number`, completing the match for `function-argument` and preempting the other choices. The intended rest of the `function-argument`, `==1` does not match anything, and the rule fails. By putting the more complex `logical-expr` first, the whole `1==1` matches, and the rule succeeds as intended.
Similary, the function name `true` matches as the literal `true` instead, and preempts parsing `true(1)` as the more complex `function-expr`. Putting the `literal` choice last prevents the preemptive match.
segment = child-segment / descendant-segment
child-segment = bracketed-selection /
("."
(wildcard-selector /
member-name-shorthand))
bracketed-selection = "[" S selector *(S "," S selector) S "]"
member-name-shorthand = name-first *name-char
name-first = ALPHA /
"_" /
%x80-D7FF /
; skip surrogate code points
%xE000-10FFFF
name-char = name-first / DIGIT
DIGIT = %x30-39 ; 0-9
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
descendant-segment = ".." (bracketed-selection /
wildcard-selector /
member-name-shorthand)
Figure 2: Collected ABNF of JSONPath Queries
Figure 3 contains the collected ABNF grammar that defines the syntax
of a JSONPath Normalized Path while also using the rules root-
identifier, ESC, DIGIT, and DIGIT1 from Figure 2.
normalized-path = root-identifier *(normal-index-segment)
normal-index-segment = "[" normal-selector "]"
normal-selector = normal-name-selector / normal-index-selector
normal-name-selector = %x27 *normal-single-quoted %x27 ; 'string'
normal-single-quoted = normal-unescaped /
ESC normal-escapable
normal-unescaped = ; omit %x0-1F control codes
%x20-26 /
; omit 0x27 '
%x28-5B /
; omit 0x5C \
%x5D-D7FF /
; skip surrogate code points
%xE000-10FFFF
normal-escapable = %x62 / ; b BS backspace U+0008
%x66 / ; f FF form feed U+000C
%x6E / ; n LF line feed U+000A
%x72 / ; r CR carriage return U+000D
%x74 / ; t HT horizontal tab U+0009
"'" / ; ' apostrophe U+0027
"\" / ; \ backslash (reverse solidus) U+005C
(%x75 normal-hexchar)
; certain values u00xx U+00XX
normal-hexchar = "0" "0"
(
("0" %x30-37) / ; "00"-"07"
; omit U+0008-U+000A BS HT LF
("0" %x62) / ; "0b"
; omit U+000C-U+000D FF CR
("0" %x65-66) / ; "0e"-"0f"
("1" normal-HEXDIG)
)
normal-HEXDIG = DIGIT / %x61-66 ; "0"-"9", "a"-"f"
normal-index-selector = "0" / (DIGIT1 *DIGIT)
; non-negative decimal integer
Figure 3: Collected ABNF of JSONPath Normalized Paths
Appendix B. Inspired by XPath
This appendix is informative.
At the time JSONPath was invented, XML was noted for the availability
of powerful tools to analyze, 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 JSON values
[RFC8259] without special scripting and
* 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: 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.
For example, 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:
x.store.book[0].title
or in bracket notation:
x['store']['book'][0]['title']
with the variable x holding the query 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, and
* be runtime efficient.
B.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 query 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']); both replace XPath's / within query
expressions, where _dot notation_ serves as a lightweight but limited
syntax while _bracket notation_ is a heavyweight but more general
syntax.
Both JSONPath and XPath use * for a wildcard. JSONPath's descendant
segment notation, starting with .., 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 ?<logical-expr> as
in:
$.store.book[?@.price < 10].title
Table 20 extends Table 1 by providing a comparison with similar XPath
concepts.
+==========+==================+===================================+
| XPath | JSONPath | Description |
+==========+==================+===================================+
| / | $ | the root XML element |
+----------+------------------+-----------------------------------+
| . | @ | the current XML element |
+----------+------------------+-----------------------------------+
| / | . or [] | child operator |
+----------+------------------+-----------------------------------+
| .. | n/a | parent operator |
+----------+------------------+-----------------------------------+
| // | ..name, | descendants (JSONPath borrows |
| | ..[index], ..*, | this syntax from E4X) |
| | or ..[*] | |
+----------+------------------+-----------------------------------+
| * | * | 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); called |
| | | list operator in JSONPath, allows |
| | | combining member names, array |
| | | indices, and slices |
+----------+------------------+-----------------------------------+
| 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 20: XPath Syntax Compared to JSONPath
For further illustration, Table 21 shows some XPath expressions and
their JSONPath equivalents.
+=======================+========================+==================+
| 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[-1] | the last book |
| | | in order |
+-----------------------+------------------------+------------------+
| //book[position()<3] | $..book[0,1] | the first two |
| | $..book[:2] | books |
+-----------------------+------------------------+------------------+
| //book[isbn] | $..book[?@.isbn] | filter all |
| | | books with an |
| | | ISBN number |
+-----------------------+------------------------+------------------+
| //book[price<10] | $..book[?@.price<10] | filter all |
| | | books cheaper |
| | | than 10 |
+-----------------------+------------------------+------------------+
| //* | $..* | all elements |
| | | in an XML |
| | | document; all |
| | | member values |
| | | and array |
| | | elements |
| | | contained in |
| | | input value |
+-----------------------+------------------------+------------------+
Table 21: Example XPath Expressions and Their JSONPath Equivalents
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 each of the nodes in the
_nodelist_ resulting from the previous query segment. Array
indices always start at 0.
Appendix C. JSON Pointer
This appendix is informative.
In relation to JSON Pointer [RFC6901], JSONPath is not intended as a
replacement but as a more powerful companion. The purposes of the
two standards are different.
JSON Pointer is for identifying a single value within a JSON value
whose structure is known.
JSONPath can identify a single value within a JSON value, for
example, by using a Normalized Path. But JSONPath is also a query
syntax that can be used to search for and extract multiple values
from JSON values whose structure is known only in a general way.
A Normalized JSONPath can be converted into a JSON Pointer by
converting the syntax, without knowledge of any JSON value. The
inverse is not generally true, i.e., a numeric reference token (path
component) in a JSON Pointer may identify a member value of an object
or an element of an array. For conversion to a JSONPath query,
knowledge of the structure of the JSON value is needed to distinguish
these cases.
Acknowledgements
This document is based on Stefan Gössner's original online article
defining JSONPath [JSONPath-orig].
The books example was taken from course material that Bielefeld
University, Germany used in 2002.
This work is indebted to Christoph Burgmer for the superb JSONPath
comparison project [COMPARISON] that details the behavior of over
forty JSONPath implementations applied to numerous queries.
Contributors
Marko Mikulicic
InfluxData, Inc.
Pisa
Italy
Email: mmikulicic@gmail.com
Edward Surov
TheSoul Publishing Ltd.
Limassol
Cyprus
Email: esurov.tsp@gmail.com
Greg Dennis
Auckland
New Zealand
Email: gregsdennis@yahoo.com
URI: https://github.com/gregsdennis
Authors' Addresses
Stefan Gössner (editor)
Fachhochschule Dortmund
Sonnenstraße 96
D-44139 Dortmund
Germany
Email: stefan.goessner@fh-dortmund.de
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