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JSONPath: Query expressions for JSON
draft-ietf-jsonpath-base-05

Document Type Active Internet-Draft (jsonpath WG)
Authors Stefan Gössner , Glyn Normington , Carsten Bormann
Last updated 2022-04-25
Replaces draft-normington-jsonpath
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draft-ietf-jsonpath-base-05
JSONPath WG                                              S. Gössner, Ed.
Internet-Draft                                   Fachhochschule Dortmund
Intended status: Standards Track                      G. Normington, Ed.
Expires: 27 October 2022                                                
                                                         C. Bormann, Ed.
                                                  Universität Bremen TZI
                                                           25 April 2022

                  JSONPath: Query expressions for JSON
                      draft-ietf-jsonpath-base-05

Abstract

   JSONPath defines a string syntax for selecting and extracting values
   within a JavaScript Object Notation (JSON, RFC 8259) value.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-jsonpath-base/.

   Discussion of this document takes place on the JSON Path Working
   Group mailing list (mailto:jsonpath@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/jsonpath/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath-base.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 October 2022.

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  History . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Overview of JSONPath Expressions  . . . . . . . . . . . .   5
   2.  JSONPath Examples . . . . . . . . . . . . . . . . . . . . . .   7
   3.  JSONPath Syntax and Semantics . . . . . . . . . . . . . . . .   9
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Processing Model  . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  Semantics . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.5.  Selectors . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.5.1.  Root Selector . . . . . . . . . . . . . . . . . . . .  13
       3.5.2.  Dot Selector  . . . . . . . . . . . . . . . . . . . .  14
       3.5.3.  Dot Wildcard Selector . . . . . . . . . . . . . . . .  15
       3.5.4.  Index Selector  . . . . . . . . . . . . . . . . . . .  15
       3.5.5.  Index Wildcard Selector . . . . . . . . . . . . . . .  19
       3.5.6.  Array Slice Selector  . . . . . . . . . . . . . . . .  20
       3.5.7.  Descendant Selector . . . . . . . . . . . . . . . . .  25
       3.5.8.  Filter Selector . . . . . . . . . . . . . . . . . . .  28
       3.5.9.  List Selector . . . . . . . . . . . . . . . . . . . .  31
     3.6.  Semantics of null . . . . . . . . . . . . . . . . . . . .  33
     3.7.  Normalized Paths  . . . . . . . . . . . . . . . . . . . .  34
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
     4.1.  Registration of Media Type application/jsonpath . . . . .  36
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  37
     5.1.  Attack vectors on JSONPath Implementations  . . . . . . .  37
     5.2.  Attacks on Security Mechanisms that Employ JSONPath . . .  38
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  38
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  39
   Appendix A.  Inspired by XPath  . . . . . . . . . . . . . . . . .  39
     A.1.  JSONPath and XPath  . . . . . . . . . . . . . . . . . . .  41

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   Appendix B.  JSON Pointer . . . . . . . . . . . . . . . . . . . .  44
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  44
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

1.  Introduction

   JavaScript Object Notation (JSON, [RFC8259]) is a popular
   representation format for structured data values.  JSONPath defines a
   string syntax for identifying values within a JSON value.

   JSONPath is not intended as a replacement for, but as a more powerful
   companion to, JSON Pointer [RFC6901].  See Appendix B.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The grammatical rules in this document are to be interpreted as ABNF,
   as described in [RFC5234].  ABNF terminal values in this document
   define Unicode code points rather than their UTF-8 encoding.  For
   example, the Unicode PLACE OF INTEREST SIGN (U+2318) would be defined
   in ABNF as %x2318.

   The terminology of [RFC8259] applies except where clarified below.
   The terms "Primitive" and "Structured" are used to group the types as
   in Section 1 of [RFC8259].  Definitions for "Object", "Array",
   "Number", and "String" remain unchanged.  Importantly "object" and
   "array" in particular do not take on a generic meaning, such as they
   would in a general programming context.

   Additional terms used in this specification are defined below.

   Value:  As per [RFC8259], a structure complying to the generic data
      model of JSON, i.e., composed of components such as structured
      values, namely JSON objects and arrays, and primitive data, namely
      numbers and text strings as well as the special values null, true,
      and false.

   Member:  A name/value pair in an object.  (Not itself a value.)

   Name:  The name in a name/value pair constituting a member.  (Also

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      known as "key", "tag", or "label".)  This is also used in
      [RFC8259], but that specification does not formally define it.  It
      is included here for completeness.

   Element:  A value in an array.  (Not to be confused with XML
      element.)

   Index:  A non-negative integer that identifies a specific element in
      an array.  Note that the term _indexing_ is also used for
      accessing elements using negative integers (Section "Semantics"),
      and for accessing member values in an object using their member
      name.

   Query:  Short name for JSONPath expression.

   Argument:  Short name for the value a JSONPath expression is applied
      to.

   Node:  The pair of a value along with its location within the
      argument.

   Root Node:  The unique node whose value is the entire argument.

   Children (of a node):  If the node is an array, each of its elements,
      or if the node is an object, each its member values (but not its
      member names).  If the node is neither an array nor an object, it
      has no descendants.

   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.

   Nodelist:  A list of nodes.  The output of applying a query to an
      argument is manifested as a list of nodes.  While this list can be
      represented in JSON, e.g. as an array, the nodelist is an abstract
      concept unrelated to JSON values.

   Normalized Path:  A simple form of JSONPath expression that
      identifies a node by providing a query that results in exactly
      that node.  Similar to, but syntactically different from, a JSON
      Pointer [RFC6901].

   Singular Path:  A JSONPath expression built from selectors which each
      select at most one node.

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   For the purposes of this specification, a value as defined by
   [RFC8259] is also viewed as a tree of nodes.  Each node, in turn,
   holds a value.  Further nodes within each value are the elements of
   arrays and the member values of objects and are themselves values.
   (The type of the value held by a node may also be referred to as the
   type of the node.)

   A query is applied to an argument, and the output is a nodelist.

1.2.  History

   This document picks up Stefan Goessner's popular JSONPath proposal
   dated 2007-02-21 [JSONPath-orig] and provides a normative definition
   for it.

   Appendix A describes how JSONPath was inspired by XML's XPath
   [XPath].

   JSONPath was intended as a light-weight companion to JSON
   implementations on platforms such as PHP and JavaScript, so instead
   of defining its own expression language like XPath did, JSONPath
   delegated this to the expression language of the platform.  While the
   languages in which JSONPath is used do have significant
   commonalities, over time this caused non-portability of JSONPath
   expressions between the ensuing platform-specific dialects.

   The present specification intends to remove platform dependencies and
   serve as a common JSONPath specification that can be used across
   platforms.  Obviously, this means that backwards compatibility could
   not always be achieved; a design principle of this specification is
   to go with a "consensus" between implementations even if it is rough,
   as long as that does not jeopardize the objective of obtaining a
   usable, stable JSON query language.

1.3.  Overview of JSONPath Expressions

   JSONPath expressions are applied to a JSON value, the _argument_.
   Within the JSONPath expression, the abstract name $ is used to refer
   to the _root node_ of the argument, i.e., to the argument as a whole.

   JSONPath expressions can use the _dot notation_

   $.store.book[0].title

   or the more general _bracket notation_

   $['store']['book'][0]['title']

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   to build paths that are input to a JSONPath implementation.

   JSONPath allows the wildcard symbol * to select any member of an
   object or any element of an array (Section 3.5.3).  The descendant
   operator .. selects all the descendants (Section 3.5.7) of a node.
   The array slice syntax [start:end:step] allows selecting a regular
   selection of an element from an array, giving a start position, an
   end position, and possibly a step value that moves the position from
   the start to the end (Section 3.5.6).

   Filter expressions are supported via the syntax ?(<boolean expr>) as
   in

   $.store.book[?(@.price < 10)].title

   Table 1 provides a quick overview of the JSONPath syntax elements.

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        +===========+=============================================+
        | JSONPath  | Description                                 |
        +===========+=============================================+
        | $         | the root node (Section 3.5.1)               |
        +-----------+---------------------------------------------+
        | @         | the current node: within filter selectors   |
        |           | (Section 3.5.8)                             |
        +-----------+---------------------------------------------+
        | .name     | child selectors for JSON objects: dot       |
        |           | selector (Section 3.5.2)                    |
        +-----------+---------------------------------------------+
        | ['name']  | child selectors for JSON objects: index     |
        |           | selector (Section 3.5.4)                    |
        +-----------+---------------------------------------------+
        | ..        | descendants: descendant selector            |
        |           | (Section 3.5.7)                             |
        +-----------+---------------------------------------------+
        | *         | all child member values and array elements: |
        |           | dot wildcard selector (Section 3.5.3),      |
        |           | index wildcard selector (Section 3.5.5)     |
        +-----------+---------------------------------------------+
        | [3]       | index (subscript) selector (Section 3.5.4): |
        |           | index current node as an array (from 0)     |
        +-----------+---------------------------------------------+
        | [..,..]   | list selector (Section 3.5.9): allow        |
        |           | combining selector styles                   |
        +-----------+---------------------------------------------+
        | [0:100:5] | array slice selector (Section 3.5.6):       |
        |           | start:end:step                              |
        +-----------+---------------------------------------------+
        | ?...      | filter selector (Section 3.5.8)             |
        +-----------+---------------------------------------------+
        | ()        | expression: within filter selectors         |
        |           | (Section 3.5.8)                             |
        +-----------+---------------------------------------------+

                       Table 1: Overview of JSONPath

2.  JSONPath Examples

   This section provides some more examples for JSONPath expressions.
   The examples are based on the simple JSON value shown in Figure 1,
   which was patterned after a typical XML example representing a
   bookstore (that also has bicycles).

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   { "store": {
       "book": [
         { "category": "reference",
           "author": "Nigel Rees",
           "title": "Sayings of the Century",
           "price": 8.95
         },
         { "category": "fiction",
           "author": "Evelyn Waugh",
           "title": "Sword of Honour",
           "price": 12.99
         },
         { "category": "fiction",
           "author": "Herman Melville",
           "title": "Moby Dick",
           "isbn": "0-553-21311-3",
           "price": 8.99
         },
         { "category": "fiction",
           "author": "J. R. R. Tolkien",
           "title": "The Lord of the Rings",
           "isbn": "0-395-19395-8",
           "price": 22.99
         }
       ],
       "bicycle": {
         "color": "red",
         "price": 19.95
       }
     }
   }

                        Figure 1: Example JSON value

   The examples in Table 2 use the expression mechanism to obtain the
   number of elements in an array, to test for the presence of a member
   in a object, and to perform numeric comparisons of member values with
   a constant.

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      +========================+===================================+
      | JSONPath               | Result                            |
      +========================+===================================+
      | $.store.book[*].author | the authors of all books in the   |
      |                        | store                             |
      +------------------------+-----------------------------------+
      | $..author              | all authors                       |
      +------------------------+-----------------------------------+
      | $.store.*              | all things in store, which are    |
      |                        | some books and a red bicycle      |
      +------------------------+-----------------------------------+
      | $.store..price         | the prices of everything in the   |
      |                        | store                             |
      +------------------------+-----------------------------------+
      | $..book[2]             | the third book                    |
      +------------------------+-----------------------------------+
      | $..book[-1]            | the last book in order            |
      +------------------------+-----------------------------------+
      | $..book[0,1]           | the first two books               |
      | $..book[:2]            |                                   |
      +------------------------+-----------------------------------+
      | $..book[?(@.isbn)]     | filter all books with isbn number |
      +------------------------+-----------------------------------+
      | $..book[?(@.price<10)] | filter all books cheaper than 10  |
      +------------------------+-----------------------------------+
      | $..*                   | all elements in XML document; all |
      |                        | member values and array elements  |
      |                        | contained in input value          |
      +------------------------+-----------------------------------+

           Table 2: Example JSONPath expressions applied to the
                            example JSON value

3.  JSONPath Syntax and Semantics

3.1.  Overview

   A JSONPath query is a string which selects zero or more nodes of a
   piece of JSON.

   A query MUST be encoded using UTF-8.  The grammar for queries given
   in this document assumes that its UTF-8 form is first decoded into
   Unicode code points as described in [RFC3629]; implementation
   approaches that lead to an equivalent result are possible.

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   A string to be used as a JSONPath query needs to be _well-formed_ and
   _valid_. A string is a well-formed JSONPath query if it conforms to
   the ABNF syntax in this document.  A well-formed JSONPath query is
   valid if it also fulfills all semantic requirements posed by this
   document.

   To be valid, integer numbers in the JSONPath query that are relevant
   to the JSONPath processing (e.g., index values and steps) MUST be
   within the range of exact values defined in I-JSON [RFC7493], namely
   within the interval [-(2^53)+1, (2^53)-1]).

   To be valid, strings on the right hand side of the =~ regex matching
   operator need to conform to [I-D.draft-bormann-jsonpath-iregexp].

   The well-formedness and the validity of JSONPath queries are
   independent of the JSON value the query is applied to; no further
   errors can be raised during application of the query to a value.

   Obviously, an implementation can still fail when executing a JSONPath
   query, e.g., because of resource depletion, but this is not modeled
   in the present specification.  However, the implementation MUST NOT
   silently malfunction.  Specifically, if a valid JSONPath query is
   evaluated against a structured value whose size doesn't fit in the
   range of exact values, interfering with the correct interpretation of
   the query, the implementation MUST provide an indication of overflow.

   (Readers familiar with the HTTP error model may be reminded of 400
   type errors when pondering well-formedness and validity, while
   resource depletion and related errors are comparable to 500 type
   errors.)

3.2.  Processing Model

   In this specification, the semantics of a JSONPath query are defined
   in terms of a _processing model_.  That model is not prescriptive of
   the internal workings of an implementation: Implementations may wish
   (or need) to design a different process that yields results that are
   consistent with this model.

   In the processing model, a valid query is executed against a value,
   the _argument_, and produces a list of zero or more nodes of the
   value.

   The query is a sequence of zero or more _selectors_, each of which is
   applied to the result of the previous selector and provides input to
   the next selector.  These results and inputs take the form of a
   _nodelist_, i.e., a sequence of zero or more nodes.

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   The nodelist going into the first selector contains a single node,
   the argument.  The nodelist resulting from the last selector is
   presented as the result of the query; depending on the specific API,
   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 that the
   API must be capable of presenting an empty nodelist as the result of
   the query.

   A selector performs its function on each of the nodes in its input
   nodelist, during such a function execution, such a node is referred
   to as the "current node".  Each of these function executions produces
   a nodelist, which are then concatenated into the result of the
   selector.

   The processing within a selector may execute nested queries, which
   are in turn handled with the processing model defined here.
   Typically, the argument to that query will be the current node of the
   selector or a set of nodes subordinate to that current node.

3.3.  Syntax

   Syntactically, a JSONPath query consists of a root selector ($),
   which stands for a nodelist that contains the root node of the
   argument, followed by a possibly empty sequence of _selectors_.

   json-path = root-selector *(S (dot-selector        /
                                  dot-wild-selector   /
                                  index-selector      /
                                  index-wild-selector /
                                  slice-selector      /
                                  descendant-selector /
                                  filter-selector     /
                                  list-selector))

   The syntax and semantics of each selector is defined below.

3.4.  Semantics

   The root selector $ not only selects the root node of the argument,
   but it also produces as output a list consisting of one node: the
   argument itself.

   A selector may select zero or more nodes for further processing.  A
   syntactically valid selector MUST NOT produce errors.  This means
   that some operations that might be considered erroneous, such as
   indexing beyond the end of an array, simply result in fewer nodes
   being selected.

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   But a selector doesn't just act on a single node: a selector acts on
   each of the nodes in its input nodelist and concatenates the
   resultant nodelists to form the result nodelist of the selector.

   For each node in the list, the selector selects zero or more nodes,
   each of which is a descendant of the node or the node itself.

   For instance, with the argument {"a":[{"b":0},{"b":1},{"c":2}]}, the
   query $.a[*].b selects the following list of nodes: 0, 1 (denoted
   here by their value).  Let's walk through this in detail.

   The query consists of $ followed by three selectors: .a, [*], and .b.

   Firstly, $ selects the root node which is the argument.  So the
   result is a list consisting of just the root node.

   Next, .a selects from any input node of type object and selects the
   node of any member value of the input node corresponding to the
   member name "a".  The result is again a list of one node:
   [{"b":0},{"b":1},{"c":2}].

   Next, [*] selects from an input node of type array all its elements
   (if the input note were of type object, it would select all its
   member values, but not the member names).  The result is a list of
   three nodes: {"b":0}, {"b":1}, and {"c":2}.

   Finally, .b selects from any input node of type object with a member
   name b and selects the node of the member value of the input node
   corresponding to that name.  The result is a list containing 0, 1.
   This is the concatenation of three lists, two of length one
   containing 0, 1, respectively, and one of length zero.

   As a consequence of this approach, if any of the selectors selects no
   nodes, then the whole query selects no nodes.

   In what follows, the semantics of each selector are defined for each
   type of node.

3.5.  Selectors

   A JSONPath query consists of a sequence of selectors.  Valid
   selectors are

   *  Root selector $ (used at the start of a query and in expressions)

   *  Dot selector .<name>, used with object member names exclusively.

   *  Dot wildcard selector .*.

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   *  Index selector [<index>], where <index> is either a (possibly
      negative, see Section "Semantics") array index or an object member
      name.

   *  Index wildcard selector [*].

   *  Array slice selector [<start>:<end>:<step>], where the optional
      values <start>, <end>, and <step> are integer literals.

   *  Descendants selector ...

   *  List selector [<sel1>,<sel2>,...,<selN>], holding a comma
      separated list of index and slice selectors.

   *  Filter selector [?(<expr>)]

   *  Current item selector @ (used in expressions)

3.5.1.  Root Selector

Syntax

   Every valid JSONPath query MUST begin with the root selector $.

   root-selector  = "$"

Semantics

   The Argument -- the root JSON value -- becomes the root node, which
   is addressed by the root selector $.

Examples

   JSON document:

   {"k": "v"}

   Queries:

             +=======+============+=============+===========+
             | Query | Result     | Result Path | Comment   |
             +=======+============+=============+===========+
             |   $   | {"k": "v"} |      $      | Root node |
             +-------+------------+-------------+-----------+

                     Table 3: Root selector examples

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3.5.2.  Dot Selector

Syntax

   A dot selector starts with a dot . followed by an object's member
   name.

   dot-selector    = "." dot-member-name
   dot-member-name = name-first *name-char
   name-first      =
                         ALPHA /
                         "_"   /           ; _
                         %x80-10FFFF       ; any non-ASCII Unicode character
   name-char = DIGIT / name-first

   DIGIT           =  %x30-39              ; 0-9
   ALPHA           =  %x41-5A / %x61-7A    ; A-Z / a-z

   Member names containing characters other than allowed by dot-selector
   -- such as space ` , minus-, or dot.characters -- MUST NOT be used
   with thedot-selector.  (Such member names can be addressed by
   theindex-selector` instead.)

Semantics

   The dot-selector selects the node of the member value corresponding
   to the member name from any JSON object in its input nodelist.  It
   selects no nodes from any other JSON value.

Examples

   JSON document:

   {"j": {"k": 3}}

   Queries:

    +=======+==========+==============+==============================+
    | Query | Result   | Result Paths | Comment                      |
    +=======+==========+==============+==============================+
    |  $.j  | {"k": 3} |    $['j']    | Named value of an object     |
    +-------+----------+--------------+------------------------------+
    | $.j.k | 3        | $['j']['k']  | Named value in nested object |
    +-------+----------+--------------+------------------------------+

                      Table 4: Dot selector examples

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3.5.3.  Dot Wildcard Selector

Syntax

   The dot wildcard selector has the form .* as defined in the following
   syntax:

   dot-wild-selector    = "." "*"            ;  dot followed by asterisk

Semantics

   A dot-wild-selector acts as a wildcard by selecting the nodes of all
   member values of an object in its input nodelist as well as all
   element nodes of an array in its input nodelist.  Applying the dot-
   wild-selector to a primitive JSON value (number, string, or
   true/false/null) selects no node.

Examples

   JSON document:

   {
     "o": {"j": 1, "k": 2},
     "a": [5, 3]
   }

   Queries:

              +=======+========+==============+=============+
              | Query | Result | Result Paths | Comment     |
              +=======+========+==============+=============+
              | $.o.* | 1      | $['o']['j']  | Object      |
              |       | 2      | $['o']['k']  | values      |
              +-------+--------+--------------+-------------+
              | $.o.* | 2      | $['o']['k']  | Alternative |
              |       | 1      | $['o']['j']  | result      |
              +-------+--------+--------------+-------------+
              | $.a.* | 5      |  $['a'][0]   | Array       |
              |       | 3      |  $['a'][1]   | members     |
              +-------+--------+--------------+-------------+

                  Table 5: Dot wildcard selector examples

3.5.4.  Index Selector

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Syntax

   An index selector [<index>] addresses at most one object member value
   or at most one array element value.

   index-selector      = "[" S (quoted-member-name / element-index) S "]"

   Applying the index-selector to an object value in its input nodelist,
   a quoted-member-name string is required to select the corresponding
   member value.  In contrast to JSON, the JSONPath syntax allows
   strings to be enclosed in _single_ or _double_ quotes.

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   quoted-member-name  = string-literal

   string-literal      = %x22 *double-quoted %x22 /       ; "string"
                         %x27 *single-quoted %x27         ; 'string'

   double-quoted       = unescaped /
                         %x27      /                       ; '
                         ESC %x22  /                       ; \"
                         ESC escapable

   single-quoted       = unescaped /
                         %x22      /                       ; "
                         ESC %x27  /                       ; \'
                         ESC escapable

   ESC                 = %x5C                              ; \  backslash

   unescaped           = %x20-21 /                         ; s. RFC 8259
                         %x23-26 /                         ; omit "
                         %x28-5B /                         ; omit '
                         %x5D-10FFFF                       ; omit \

   escapable           = ( %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
                             ; b /         ;  BS backspace U+0008
                             ; t /         ;  HT horizontal tab U+0009
                             ; n /         ;  LF line feed U+000A
                             ; f /         ;  FF form feed U+000C
                             ; r /         ;  CR carriage return U+000D
                             "/" /          ;  /  slash (solidus) 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"

   ; Task from 2021-06-15 interim: update ABNF later

   Applying the index-selector to an array, a numerical element-index is
   required to select the corresponding element.  JSONPath allows it to
   be negative (see Section "Semantics").

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   element-index   = int                             ; decimal integer

   int             = ["-"] ( "0" / (DIGIT1 *DIGIT) ) ; -  optional
   DIGIT1          = %x31-39                         ; 1-9 non-zero digit

   Notes: 1. double-quoted strings follow the JSON string syntax
   (Section 7 of [RFC8259]); single-quoted strings follow an analogous
   pattern (Section "Syntax"). 2.  An element-index is an integer (in
   base 10, as in JSON numbers). 3.  As in JSON numbers, the syntax does
   not allow octal-like integers with leading zeros such as 01 or -01.

Semantics

   A quoted-member-name string MUST be converted to a member name by
   removing the surrounding quotes and replacing each escape sequence
   with its equivalent Unicode character, as in the table below:

   +=================+===================+=============================+
   | Escape Sequence | Unicode Character | Description                 |
   +=================+===================+=============================+
   |        \b       |       U+0008      | BS backspace                |
   +-----------------+-------------------+-----------------------------+
   |        \t       |       U+0009      | HT horizontal tab           |
   +-----------------+-------------------+-----------------------------+
   |        \n       |       U+000A      | LF line feed                |
   +-----------------+-------------------+-----------------------------+
   |        \f       |       U+000C      | FF form feed                |
   +-----------------+-------------------+-----------------------------+
   |        \r       |       U+000D      | CR carriage return          |
   +-----------------+-------------------+-----------------------------+
   |        \"       |       U+0022      | quotation mark              |
   +-----------------+-------------------+-----------------------------+
   |        \'       |       U+0027      | apostrophe                  |
   +-----------------+-------------------+-----------------------------+
   |        \/       |       U+002F      | slash (solidus)             |
   +-----------------+-------------------+-----------------------------+
   |        \\       |       U+005C      | backslash (reverse          |
   |                 |                   | solidus)                    |
   +-----------------+-------------------+-----------------------------+
   |      \uXXXX     |       U+XXXX      | unicode character           |
   +-----------------+-------------------+-----------------------------+

                   Table 6: Escape Sequence Replacements

   The index-selector applied with a quoted-member-name to an object
   selects the node of the corresponding member value from it, if and
   only if that object has a member with that name.  Nothing is selected
   from a value that is not a object.

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   Array indexing via element-index is a way of selecting a particular
   array element using a zero-based index.  For example, selector [0]
   selects the first and selector [4] the fifth element of a
   sufficiently long array.

   A negative element-index counts from the array end.  For example,
   selector [-1] selects the last and selector [-2] selects the
   penultimate element of an array with at least two elements.  As with
   non-negative indexes, it is not an error if such an element does not
   exist; this simply means that no element is selected.

Examples

   JSON document:

   {
     "o": {"j j": {"k.k": 3}},
     "a": ["a","b"]
   }

   Queries:

     +===================+========+==============+==================+
     |       Query       | Result | Result Paths | Comment          |
     +===================+========+==============+==================+
     | $.o['j j']['k.k'] | 3      |  $['o']['j   | Named value in   |
     |                   |        |  ']['k.k']   | nested object    |
     +-------------------+--------+--------------+------------------+
     | $.o["j j"]["k.k"] | 3      |  $['o']['j   | Named value in   |
     |                   |        |  ']['k.k']   | nested object    |
     +-------------------+--------+--------------+------------------+
     |       $.a[1]      | "b"    |  $['a'][1]   | Member of array  |
     +-------------------+--------+--------------+------------------+
     |      $.a[-2]      | "a"    |  $['a'][0]   | Member of array, |
     |                   |        |              | from the end     |
     +-------------------+--------+--------------+------------------+

                     Table 7: Index selector examples

3.5.5.  Index Wildcard Selector

Syntax

   The index wildcard selector has the form [*].

   index-wild-selector    = "[" "*" "]"  ;  asterisk enclosed by brackets

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Semantics

   An index-wild-selector selects the nodes of all member values of an
   object as well as of all elements of an array.  Applying the index-
   wild-selector to a primitive JSON value (such as a number, string, or
   true/false/null) selects no node.

   The index-wild-selector behaves identically to the dot-wild-selector.

Examples

   JSON document:

   {
     "o": {"j": 1, "k": 2},
     "a": [5, 3]
   }

   Queries:

             +=========+========+==============+=============+
             |  Query  | Result | Result Paths | Comment     |
             +=========+========+==============+=============+
             | $.o.[*] | 1      | $['o']['j']  | Object      |
             |         | 2      | $['o']['k']  | values      |
             +---------+--------+--------------+-------------+
             | $.o.[*] | 2      | $['o']['k']  | Alternative |
             |         | 1      | $['o']['j']  | result      |
             +---------+--------+--------------+-------------+
             | $.a.[*] | 5      |  $['a'][0]   | Array       |
             |         | 3      |  $['a'][1]   | members     |
             +---------+--------+--------------+-------------+

                 Table 8: Index wildcard selector examples

3.5.6.  Array Slice Selector

Syntax

   The array slice selector has the form [<start>:<end>:<step>].  It
   selects elements starting at index <start>, ending at -- but not
   including -- <end>, while incrementing by step.

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   slice-selector = "[" S slice-index S "]"

   slice-index    =  [start S] ":" S [end S] [":" [S step ]]

   start          = int       ; included in selection
   end            = int       ; not included in selection
   step           = int       ; default: 1

   B              =    %x20 / ; Space
                       %x09 / ; Horizontal tab
                       %x0A / ; Line feed or New line
                       %x0D   ; Carriage return
   S              = *B        ; optional blank space
   RS             = 1*B       ; required blank space

   The slice-selector consists of three optional decimal integers
   separated by colons.

Semantics

   The slice-selector was inspired by the slice operator of ECMAScript 4
   (ES4), which was deprecated in 2014, and that of Python.

Informal Introduction

   This section is non-normative.

   Array indexing is a way of selecting a particular element of an array
   using a 0-based index.  For example, the expression [0] selects the
   first element of a non-empty array.

   Negative indices index from the end of an array.  For example, the
   expression [-2] selects the last but one element of an array with at
   least two elements.

   Array slicing is inspired by the behaviour of the
   Array.prototype.slice method of the JavaScript language as defined by
   the ECMA-262 standard [ECMA-262], with the addition of the step
   parameter, which is inspired by the Python slice expression.

   The array slice expression [start:end:step] selects elements at
   indices starting at start, incrementing by step, and ending with end
   (which is itself excluded).  So, for example, the expression [1:3]
   (where step defaults to 1) selects elements with indices 1 and 2 (in
   that order) whereas [1:5:2] selects elements with indices 1 and 3.

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   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 behaviour of Python, which raises an error in this
   case.)

   The following section specifies the behaviour fully, without
   depending on JavaScript or Python behaviour.

Detailed Semantics

   An array selector is either an array slice or an array index, which
   is defined in terms of an array slice.

   A slice expression selects a subset of the elements of the input
   array, in the same order as the array or the reverse order, depending
   on the sign of the step parameter.  It selects no nodes from a node
   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. len is
   the length of the input array.

   The default value for step is 1.  The default values for start and
   end depend on the sign of step, as follows:

                    +===========+=========+==========+
                    | Condition | start   | end      |
                    +===========+=========+==========+
                    | step >= 0 | 0       | len      |
                    +-----------+---------+----------+
                    | step < 0  | len - 1 | -len - 1 |
                    +-----------+---------+----------+

                       Table 9: 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:

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   FUNCTION Normalize(i, len):
     IF i >= 0 THEN
       RETURN i
     ELSE
       RETURN len + i
     END IF

   The result of the array indexing expression [i] applied to an array
   of length len is defined to be the result of the array slicing
   expression [i:Normalize(i, len)+1:1].

   Slice expression parameters start and end are used to derive slice
   bounds lower and upper.  The direction of the iteration, defined by
   the sign of step, determines which of the parameters is the lower
   bound and which is the upper bound:

   FUNCTION Bounds(start, end, step, len):
     n_start = Normalize(start, len)
     n_end = Normalize(end, len)

     IF step >= 0 THEN
       lower = MIN(MAX(n_start, 0), len)
       upper = MIN(MAX(n_end, 0), len)
     ELSE
       upper = MIN(MAX(n_start, -1), len-1)
       lower = MIN(MAX(n_end, -1), len-1)
     END IF

     RETURN (lower, upper)

   The slice expression selects elements with indices between the lower
   and upper bounds.  In the following pseudocode, the a(i) construct
   expresses the 0-based indexing operation on the underlying array.

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   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.

   To be valid, the slice expression parameters MUST be in the I-JSON
   range of exact values, see Section 3.1.

Examples

   JSON document:

   ["a", "b", "c", "d", "e", "f", "g"]

   Queries:

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                +===========+========+========+==========+
                |   Query   | Result | Result | Comment  |
                |           |        | Paths  |          |
                +===========+========+========+==========+
                |   $[1:3]  | "b"    |  $[1]  | Slice    |
                |           | "c"    |  $[2]  | with     |
                |           |        |        | default  |
                |           |        |        | step     |
                +-----------+--------+--------+----------+
                |  $[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 10: Array slice selector examples

3.5.7.  Descendant Selector

Syntax

   The descendant selector starts with a double dot .. and can be
   followed by an object member name (similar to the dot-selector), by
   an index-selector acting on objects or arrays, or by a wildcard.

   descendant-selector = ".." ( dot-member-name      /  ; ..<name>
                                index-selector       /  ; ..[<index>]
                                index-wild-selector  /  ; ..[*]
                                "*"                     ; ..*
                              )

Semantics

   The descendant-selector selects certain descendants of a node:

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   *  the ..<name> form (and the ..[<index>] form where <index> is a
      quoted-member-name) selects those descendants of the node that are
      member values of an object with the given member name.

   *  the ..[<index>] where <index> is an element-index selects those
      descendants of the node that are array elements with the given
      index.

   *  the ..[*] and ..* forms select all the descendants of the node.

   In the resultant nodelist:

   *  nodes occur before their children, and

   *  nodes of an array occur in array order.

   Children of an object may occur in any order, since JSON objects are
   unordered.

Examples

   JSON document:

   {
     "o": {"j": 1, "k": 2},
     "a": [5, 3, [{"j": 4}]]
   }

   Queries:

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     +========+====================+===================+=============+
     | Query  | Result             |    Result Paths   | Comment     |
     +========+====================+===================+=============+
     |  $..j  | 1                  |    $['o']['j']    | Object      |
     |        | 4                  | $['a'][2][0]['j'] | values      |
     +--------+--------------------+-------------------+-------------+
     |  $..j  | 4                  | $['a'][2][0]['j'] | Alternative |
     |        | 1                  |    $['o']['j']    | result      |
     +--------+--------------------+-------------------+-------------+
     | $..[0] | 5                  |     $['a'][0]     | Array       |
     |        | {"j": 4}           |    $['a'][2][0]   | values      |
     +--------+--------------------+-------------------+-------------+
     | $..[0] | {"j": 4}           |    $['a'][2][0]   | Alternative |
     |        | 5                  |     $['a'][0]     | result      |
     +--------+--------------------+-------------------+-------------+
     | $..[*] | {"j": 1, "k" : 2}  |       $['o']      | All values  |
     |        | [5, 3, [{"j": 4}]] |       $['a']      |             |
     |        | 1                  |    $['o']['j']    |             |
     |        | 2                  |    $['o']['k']    |             |
     |        | 5                  |     $['a'][0]     |             |
     |        | 3                  |     $['a'][1]     |             |
     |        | [{"j": 4}]         |     $['a'][2]     |             |
     |        | {"j": 4}           |    $['a'][2][0]   |             |
     |        | 4                  | $['a'][2][0]['j'] |             |
     +--------+--------------------+-------------------+-------------+
     |  $..*  | [5, 3, [{"j": 4}]] |       $['a']      | All values  |
     |        | {"j": 1, "k" : 2}  |       $['o']      |             |
     |        | 2                  |    $['o']['k']    |             |
     |        | 1                  |    $['o']['j']    |             |
     |        | 5                  |     $['a'][0]     |             |
     |        | 3                  |     $['a'][1]     |             |
     |        | [{"j": 4}]         |     $['a'][2]     |             |
     |        | {"j": 4}           |    $['a'][2][0]   |             |
     |        | 4                  | $['a'][2][0]['j'] |             |
     +--------+--------------------+-------------------+-------------+

                   Table 11: Descendant selector 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}]] must appear before 5, 3, and [{"j": 4}],

   *  5 must appear before 3 which must appear before [{"j": 4}],

   *  [{"j": 4}] must appear before {"j": 4}, and

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   *  {"j": 4} must appear before 4.

3.5.8.  Filter Selector

Syntax

   The filter selector has the form [?<expr>].  It works via iterating
   over structured values, i.e. arrays and objects.

   filter-selector    = "[" S filter S "]"
   filter             = "?" S boolean-expr

   During iteration process each array element or object member is
   visited and its value -- accessible via symbol @ -- or one of its
   descendants -- uniquely defined by a relative path -- is tested
   against a boolean expression boolean-expr.

   The current item is selected if and only if the result is true.

   boolean-expr     = logical-or-expr
   logical-or-expr  = logical-and-expr *(S "||" S logical-and-expr)
                                                         ; disjunction
                                                         ; binds less tightly than conjunction
   logical-and-expr = basic-expr *(S "&&" S basic-expr)  ; conjunction
                                                         ; binds more tightly than disjunction

   basic-expr        = exist-expr /
                       paren-expr /
                       relation-expr
   exist-expr        = [neg-op S] singular-path          ; path existence or non-existence

   Paths in filter expressions are Singular Paths, each of which selects
   at most one node.

   singular-path     = rel-singular-path / abs-singular-path
   rel-singular-path = "@" *(S (dot-selector / index-selector))
   abs-singular-path = root-selector *(S (dot-selector / index-selector))

   Parentheses can be used with boolean-expr for grouping.  So filter
   selection syntax in the original proposal [?(<expr>)] is naturally
   contained in the current lean syntax [?<expr>] as a special case.

   paren-expr        = [neg-op S] "(" S boolean-expr S ")" ; parenthesized expression
   neg-op            = "!"                               ; not operator

   relation-expr = comp-expr /                           ; comparison test
                   regex-expr                            ; regular expression test

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   Comparisons are restricted to Singular Path values and primitive
   values (such as number, string, true, false, null).

   Comparisons with complex values will fail, i.e. no selection occurs.

   Data types are not implicitly converted in comparisons.  So "13 ==
   '13'" selects no node.

   A member or element value by itself in a Boolean context is
   interpreted as false only if it does not exist.  Otherwise it is
   interpreted as true.  To be more specific about the actual value,
   explicit comparisons are necessary.  This existence test -- as an
   exception to the general rule -- also works with structured values.

   comp-expr    = comparable S comp-op S comparable
   comparable   = number / string-literal /              ; primitive ...
                  true / false / null /                  ; values only
                  singular-path                          ; Singular Path value
   comp-op      = "==" / "!=" /                          ; comparison ...
                  "<"  / ">"  /                          ; operators
                  "<=" / ">="

   Alphabetic characters in ABNF are case-insensitive, so "e" can be
   either "e" or "E".

   true, false, and null are lower-case only (case-sensitive).

   number       = int [ frac ] [ exp ]                   ; decimal number
   frac         = "." 1*DIGIT                            ; decimal fraction
   exp          = "e" [ "-" / "+" ] 1*DIGIT              ; decimal exponent
   true         = %x74.72.75.65                          ; true
   false        = %x66.61.6c.73.65                       ; false
   null         = %x6e.75.6c.6c                          ; null

   Regular expression tests can be applied to JSON string values
   (Section 7 of [RFC8259]) only (on the left-hand side of =~); they
   yield false otherwise.

   The syntax of regular expressions in the string-literals on the
   right-hand side of =~ is as defined in
   [I-D.draft-bormann-jsonpath-iregexp].

   regex-expr   = (singular-path / string-literal) S regex-op S regex
   regex-op     = "=~"                                   ; regular expression match
   regex        = string-literal                         ; I-Regexp

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   The following table lists filter expression operators in order of
   precedence from highest (binds most tightly) to lowest (binds least
   tightly).

                  +============+===========+===========+
                  | Precedence |  Operator |   Syntax  |
                  |            |    type   |           |
                  +============+===========+===========+
                  |     5      |  Grouping |   (...)   |
                  +------------+-----------+-----------+
                  |     4      |  Logical  |     !     |
                  |            |    NOT    |           |
                  +------------+-----------+-----------+
                  |     3      | Relations |   == !=   |
                  |            |           | < <= > >= |
                  |            |           |     =~    |
                  +------------+-----------+-----------+
                  |     2      |  Logical  |     &&    |
                  |            |    AND    |           |
                  +------------+-----------+-----------+
                  |     1      |  Logical  |     ||    |
                  |            |     OR    |           |
                  +------------+-----------+-----------+

                       Table 12: Filter expression
                           operator precedence

Semantics

   The filter-selector works with arrays and objects exclusively.  Its
   result is a list of _zero_, _one_, _multiple_ or _all_ of their array
   elements or member values, respectively.  Applied to other value
   types, it will select nothing.

   A relative path, beginning with @, refers to the current array
   element or member value as the filter selector iterates over the
   array or object.

   Comparisons using one of the operators <, <=, >, and >= are between
   numeric values only.  Using these operators to compare other types of
   values produces a "false" comparison result.

   The semantics of regular expressions are as defined in
   [I-D.draft-bormann-jsonpath-iregexp].

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Examples

   JSON document:

   {
     "a": [3, 5, 1, 2, 4, 6, {"b": "ij"}, {"b": "ik"}],
     "o": {"p": 1, "q": 2, "r": 3, "s": 5, "t": {"u": 6}}
   }

   Queries:

         +=============+=============+=============+=============+
         |    Query    | Result      |    Result   | Comment     |
         |             |             |    Paths    |             |
         +=============+=============+=============+=============+
         | $.a[?@>3.5] | 5           |  $['a'][1]  | Array value |
         |             | 4           |  $['a'][4]  | comparison  |
         |             | 6           |  $['a'][5]  |             |
         +-------------+-------------+-------------+-------------+
         |  $.a[?@.b]  | {"b": "ij"} |  $['a'][6]  | Array value |
         |             | {"b": "ik"} |  $['a'][7]  | existence   |
         +-------------+-------------+-------------+-------------+
         | $.a[?@<2 || | 1           |  $['a'][2]  | Array value |
         |    @.b ==   | {"b": "ik"} |  $['a'][7]  | logical OR  |
         |    "ik"]    |             |             |             |
         +-------------+-------------+-------------+-------------+
         | $.a[?@.b =~ | {"b": "ij"} |  $['a'][6]  | Array value |
         |    "i.*"]   | {"b": "ik"} |  $['a'][7]  | regular     |
         |             |             |             | expression  |
         +-------------+-------------+-------------+-------------+
         | $.o[?@>1 && | 2           | $['o']['q'] | Object      |
         |     @<4]    | 3           | $['o']['r'] | value       |
         |             |             |             | logical AND |
         +-------------+-------------+-------------+-------------+
         | $.o[?@>1 && | 3           | $['o']['r'] | Alternative |
         |     @<4]    | 2           | $['o']['q'] | result      |
         +-------------+-------------+-------------+-------------+
         | $.o[?@.u || | {"u": 6}    | $['o']['t'] | Object      |
         |     @.x]    |             |             | value       |
         |             |             |             | logical OR  |
         +-------------+-------------+-------------+-------------+

                     Table 13: Filter selector examples

3.5.9.  List Selector

   The list selector allows combining member names, array indices, and
   slices in a single selector.

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   Note: The list selector was called "union selector" in
   [JSONPath-orig], as it was intended to solve use cases addressed by
   the union selector in XPath.  However, the term "union" has the
   connotation of a set operation that involves merging input sets while
   avoiding duplicates, so the concept was renamed into "list selector".

Syntax

   The list selector is syntactically related to the index-selector and
   the slice-selector.  It contains two or more entries, separated by
   commas.

   list-selector  = "[" S list-entry 1*(S "," S list-entry) S "]"

   list-entry     =  ( quoted-member-name /
                       element-index      /
                       slice-index /
                       filter
                     )

Semantics

   A list selector selects the nodes that are selected by at least one
   of the selector entries in the list and yields the concatenation of
   the lists (in the order of the selector entries) of nodes selected by
   the selector entries.  Note that any node selected in more than one
   of the selector entries is kept as many times in the node list.

   To be valid, integer values in the element-index and slice-index
   components MUST be in the I-JSON range of exact values, see
   Section 3.1.

Examples

   JSON document:

   ["a", "b", "c", "d", "e", "f", "g"]

   Queries:

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                 +========+========+========+============+
                 | 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 14: List selector examples

3.6.  Semantics of null

   Note that JSON null is treated the same as any other JSON value: it
   is not taken to mean "undefined" or "missing".

Examples

   JSON document:

   {"a": null, "b": [null], "c": [{}], "null": 1}

   Queries:

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      +===================+========+===========+====================+
      |       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 string |
      |                   |        |           | as object key      |
      +-------------------+--------+-----------+--------------------+

            Table 15: Examples involving (or not involving) null

3.7.  Normalized Paths

   A Normalized Path is a JSONPath with restricted syntax that
   identifies a node by providing a query that results in exactly that
   node.  For example, the JSONPath expression $.book[?(@.price<10)]
   could select two values with Normalized Paths $['book'][3] and
   $['book'][5].  For a given JSON document, there is a one to one
   correspondence between the document's nodes and the Normalized Paths
   that identify these nodes.

   A JSONPath implementation may output Normalized Paths instead of, or
   in addition to, the values identified by these paths.

   Since bracket notation is more general than dot notation, it is used
   to construct Normalized Paths.  Single quotes are used to delimit
   string member names.  This reduces the number of characters that need
   escaping when Normalized Paths appear as strings (which are delimited
   with double quotes) in JSON documents.

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   The syntax of Normalized Paths is restricted so that there is one and
   only one way of representing any given Normalized Path.  Putting this
   another way, for any two distinct Normalized Paths, a JSON document
   exists that will yield distinct results when the Normalized Paths are
   applied to it.

   Certain characters are escaped, in one and only one way; all other
   characters are unescaped.

   Normalized Paths are Singular Paths.  Not all Singular Paths are
   Normalized Paths: $[-3], for example, is a Singular Path, but not a
   Normalized Path.

   normalized-path           = root-selector *(normal-index-selector)
   normal-index-selector     = "[" (normal-quoted-member-name / normal-element-index) "]"
   normal-quoted-member-name = %x27 *normal-single-quoted %x27 ; 'string'
   normal-single-quoted      = normal-unescaped /
                               ESC normal-escapable
   normal-unescaped          = %x20-26 /                       ; omit control codes
                               %x28-5B /                       ; omit '
                               %x5D-10FFFF                     ; omit \
   normal-escapable          = ( %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
                                   ; b /         ;  BS backspace U+0008
                                   ; t /         ;  HT horizontal tab U+0009
                                   ; n /         ;  LF line feed U+000A
                                   ; f /         ;  FF form feed U+000C
                                   ; r /         ;  CR carriage return U+000D
                                   "'" /         ;  ' 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"
                                 ("0" %x62) /    ; "0b"      ; omit U+0008-U+000A
                                 ("0" %x65-66) /  ; "0e"-"0f" ; omit U+000C-U+000D
                                 ("1" normal-HEXDIG)
                               )
   normal-HEXDIG             = DIGIT / %x61-66   ; "0"-"9", "a"-"f"
   normal-element-index      = "0" / (DIGIT1 *DIGIT) ; non-negative decimal integer

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Examples

            +============+=================+==================+
            |    Path    | Normalized Path | Comment          |
            +============+=================+==================+
            |    $.a     |      $['a']     | Object value     |
            +------------+-----------------+------------------+
            |    $[1]    |       $[1]      | Array index      |
            +------------+-----------------+------------------+
            | $.a.b[1:2] |  $['a']['b'][1] | Nested structure |
            +------------+-----------------+------------------+

                     Table 16: Normalized Path examples

4.  IANA Considerations

4.1.  Registration of Media Type application/jsonpath

   IANA is requested to register 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
      RFCXXXX.

   Interoperability considerations:  N/A

   Published specification:  RFCXXXX

   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

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                            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:  IESG

   Provisional registration? (standards tree only):  no

5.  Security Considerations

   Security considerations for JSONPath can stem from

   *  attack vectors on JSONPath implementations, and

   *  the way JSONPath is used in security-relevant mechanisms.

5.1.  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.  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
   [I-D.draft-bormann-jsonpath-iregexp] 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 that
   trigger surprisingly high, possibly exponential, CPU usage.

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5.2.  Attacks on Security Mechanisms that Employ JSONPath

   Where JSONPath is used as a part of a security mechanism, attackers
   can attempt to evoke unexpected behavior, or take advantage of
   differences in behavior between JSONPath implementations.

   This also applies to underlying technologies such as UTF-8 (see
   Section 10 of [RFC3629]), the Unicode character set, and JSON.  A
   characteristic of JSON that can lead to varying results is the fact
   that JSON objects are unordered; therefore, the order in which
   results of a JSONPath query reflect the presence of JSON object
   members can vary with implementations.

6.  References

6.1.  Normative References

   [I-D.draft-bormann-jsonpath-iregexp]
              Bormann, C. and T. Bray, "I-Regexp: An Interoperable
              Regexp Format", Work in Progress, Internet-Draft, draft-
              bormann-jsonpath-iregexp-03, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-bormann-jsonpath-
              iregexp-03.txt>.

   [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>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

6.2.  Informative References

   [E4X]      ISO, "Information technology — ECMAScript for XML (E4X)
              specification", ISO/IEC 22537:2006 , 2006.

   [ECMA-262] Ecma International, "ECMAScript Language Specification,
              Standard ECMA-262, Third Edition", December 1999,
              <http://www.ecma-international.org/publications/files/
              ECMA-ST-ARCH/ECMA-
              262,%203rd%20edition,%20December%201999.pdf>.

   [JSONPath-orig]
              Gössner, S., "JSONPath — XPath for JSON", 21 February
              2007, <https://goessner.net/articles/JsonPath/>.

   [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", n.d.,
              <https://github.com/tc39/proposal-slice-notation>.

   [XPath]    Berglund, A., Boag, S., Chamberlin, D., Fernandez, M.,
              Kay, M., Robie, J., and J. Simeon, "XML Path Language
              (XPath) 2.0 (Second Edition)", World Wide Web Consortium
              Recommendation REC-xpath20-20101214, 14 December 2010,
              <https://www.w3.org/TR/2010/REC-xpath20-20101214>.

Appendix A.  Inspired by XPath

   This appendix is informative.

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   At the time JSONPath was invented, XML was noted for the availability
   of powerful tools to analyse, transform and selectively extract data
   from XML documents.  [XPath] is one of these tools.

   In 2007, the need for something solving the same class of problems
   for the emerging JSON community became apparent, specifically for:

   *  Finding data interactively and extracting them out of [RFC8259]
      JSON values without special scripting.

   *  Specifying the relevant parts of the JSON data in a request by a
      client, so the server can reduce the amount of data in its
      response, minimizing bandwidth usage.

   (Note that XPath has evolved since 2007, and recent versions even
   nominally support operating inside JSON values.  This appendix only
   discusses the more widely used version of XPath that was available in
   2007.)

   JSONPath picks up the overall feeling of XPath, but maps the concepts
   to syntax (and partially semantics) that would be familiar to someone
   using JSON in a dynamic language.

   E.g., in popular dynamic programming languages such as JavaScript,
   Python and PHP, the semantics of the XPath expression

   /store/book[1]/title

   can be realized in the expression

   x.store.book[0].title

   or, in bracket notation,

   x['store']['book'][0]['title']

   with the variable x holding the argument.

   The JSONPath language was designed to:

   *  be naturally based on those language characteristics;

   *  cover only the most essential parts of XPath 1.0;

   *  be lightweight in code size and memory consumption;

   *  be runtime efficient.

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A.1.  JSONPath and XPath

   JSONPath expressions apply to JSON values in the same way as XPath
   expressions are used in combination with an XML document.  JSONPath
   uses $ to refer to the root node of the argument, similar to XPath's
   / at the front.

   JSONPath expressions move further down the hierarchy using _dot
   notation_ ($.store.book[0].title) or the _bracket notation_
   ($['store']['book'][0]['title']), a lightweight/limited, and a more
   heavyweight syntax replacing XPath's / within query expressions.

   Both JSONPath and XPath use * for a wildcard.  The descendant
   operator .., borrowed from [E4X], is similar to XPath's //. The array
   slicing construct [start:end:step] is unique to JSONPath, inspired by
   [SLICE] from ECMASCRIPT 4.

   Filter expressions are supported via the syntax ?(<boolean expr>) as
   in

   $.store.book[?(@.price < 10)].title

   Table 17 extends Table 1 by providing a comparison with similar XPath
   concepts.

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    +==========+==================+===================================+
    | XPath    | JSONPath         | Description                       |
    +==========+==================+===================================+
    | /        | $                | the root XML element              |
    +----------+------------------+-----------------------------------+
    | .        | @                | the current XML element           |
    +----------+------------------+-----------------------------------+
    | /        | . or []          | child operator                    |
    +----------+------------------+-----------------------------------+
    | ..       | n/a              | parent operator                   |
    +----------+------------------+-----------------------------------+
    | //       | ..               | descendants (JSONPath borrows     |
    |          |                  | this syntax from E4X)             |
    +----------+------------------+-----------------------------------+
    | *        | *                | wildcard: All XML elements        |
    |          |                  | regardless of their names         |
    +----------+------------------+-----------------------------------+
    | @        | n/a              | attribute access: JSON values do  |
    |          |                  | not have attributes               |
    +----------+------------------+-----------------------------------+
    | []       | []               | subscript operator used to        |
    |          |                  | iterate over XML element          |
    |          |                  | collections and for predicates    |
    +----------+------------------+-----------------------------------+
    | |        | [,]              | Union operator (results in a      |
    |          |                  | combination of node sets); 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 17: XPath syntax compared to JSONPath

   For further illustration, Table 18 shows some XPath expressions and
   their JSONPath equivalents.

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   +======================+========================+===================+
   | XPath                | JSONPath               | Result            |
   +======================+========================+===================+
   | /store/book/author   | $.store.book[*].author | the authors of    |
   |                      |                        | all books in      |
   |                      |                        | the store         |
   +----------------------+------------------------+-------------------+
   | //author             | $..author              | all authors       |
   +----------------------+------------------------+-------------------+
   | /store/*             | $.store.*              | all things in     |
   |                      |                        | store, which      |
   |                      |                        | are some books    |
   |                      |                        | and a red         |
   |                      |                        | bicycle           |
   +----------------------+------------------------+-------------------+
   | /store//price        | $.store..price         | the prices of     |
   |                      |                        | everything in     |
   |                      |                        | the store         |
   +----------------------+------------------------+-------------------+
   | //book[3]            | $..book[2]             | the third book    |
   +----------------------+------------------------+-------------------+
   | //book[last()]       | $..book[-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 isbn   |
   |                      |                        | number            |
   +----------------------+------------------------+-------------------+
   | //book[price<10]     | $..book[?(@.price<10)] | filter all        |
   |                      |                        | books cheaper     |
   |                      |                        | than 10           |
   +----------------------+------------------------+-------------------+
   | //*                  | $..*                   | all elements in   |
   |                      |                        | XML document;     |
   |                      |                        | all member        |
   |                      |                        | values and        |
   |                      |                        | array elements    |
   |                      |                        | contained in      |
   |                      |                        | input value       |
   +----------------------+------------------------+-------------------+

     Table 18: Example XPath expressions and their JSONPath equivalents

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   XPath has a lot more functionality (location paths in unabbreviated
   syntax, operators and functions) than listed in this comparison.
   Moreover, there are significant differences in how the subscript
   operator works in XPath and JSONPath:

   *  Square brackets in XPath expressions always operate on the _node
      set_ resulting from the previous path fragment.  Indices always
      start at 1.

   *  With JSONPath, square brackets operate on the _object_ or _array_
      addressed by the previous path fragment.  Array indices always
      start at 0.

Appendix B.  JSON Pointer

   This appendix is informative.

   JSONPath is not intended as a replacement for, but as a more powerful
   companion to, JSON Pointer [RFC6901].  The purposes of the two
   standards are different.

   JSON Pointer is for identifying a single value within a JSON document
   whose structure is known.

   JSONPath can identify a single value within a JSON document, 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 documents 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 document.  The
   inverse is not generally true: a numeric path component in a JSON
   Pointer may identify a member of a JSON object or may index an array.
   For conversion to a JSONPath query, knowledge of the structure of the
   JSON document is needed to distinguish these cases.

Acknowledgements

   This specification is based on Stefan Gössner's original online
   article defining JSONPath [JSONPath-orig].

   The books example was taken from http://coli.lili.uni-
   bielefeld.de/~andreas/Seminare/sommer02/books.xml -- a dead link now.

Contributors

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   Marko Mikulicic
   InfluxData, Inc.
   Pisa
   Italy
   Email: mmikulicic@gmail.com

   Edward Surov
   TheSoul Publishing Ltd.
   Limassol
   Cyprus
   Email: esurov.tsp@gmail.com

Authors' Addresses

   Stefan Gössner (editor)
   Fachhochschule Dortmund
   Sonnenstraße 96
   D-44139 Dortmund
   Germany
   Email: stefan.goessner@fh-dortmund.de

   Glyn Normington (editor)
   Winchester
   United Kingdom
   Email: glyn.normington@gmail.com

   Carsten Bormann (editor)
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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