RFC 9535 | JSONPath | February 2024
Gössner, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9535
Category:
  Standards Track
Published:
  February 2024
ISSN:
  2070-1721
Authors:
  S. Gössner, Ed.
  Fachhochschule Dortmund
  G. Normington, Ed.
  C. Bormann, Ed.
  Universität Bremen TZI

# RFC 9535

# 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.¶

## Copyright Notice

Copyright (c) 2024 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 described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

## 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.¶

Table 1: 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

### 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.¶

Table 2: Example JSONPath Expressions and Their Intended Results When Applied to the Example JSON Value
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
`$..book[0,1]` `$..book[:2]` | the first two books
`$..book[?@.isbn]` | all books with an ISBN number
`$..book[?@.price<10]` | all books cheaper than 10
`$..*` | all member values and array elements contained in the input value

## 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 [-(253)+1, (253)-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:¶

Table 3: Root Identifier Example
Query | Result | Result Path | Comment
--- | --- | --- | ---
`$` | `{"k": "v"}` | `$` | Root node

### 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:¶

Table 4: Escape Sequence Replacements
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 Section 2.3.1.1 | hexadecimal escape

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.¶

Table 5: Name Selector Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$.o['j j']` | `{"k.k": 3}` | `$['o']['j j']` | Named 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

#### 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.¶

Table 6: Wildcard Selector Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$[*]` | `{"j": 1, "k": 2}` `[5, 3]` | `$['o']` `$['a']` | Object values
`$.o[*]` | `1` `2` | `$['o']['j']` `$['o']['k']` | Object values
`$.o[*]` | `2` `1` | `$['o']['k']` `$['o']['j']` | Alternative result
`$.o[*, *]` | `1` `2` `2` `1` | `$['o']['j']` `$['o']['k']` `$['o']['k']` `$['o']['j']` | Non-deterministic ordering
`$.a[*]` | `5` `3` | `$['a'][0]` `$['a'][1]` | Array members

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.¶

Table 7: Index Selector Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$[1]` | `"b"` | `$[1]` | Element of array
`$[-2]` | `"a"` | `$[0]` | Element of array, from the end

#### 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.¶

Table 8: Default Array Slice start and end Values
Condition | start | end
--- | --- | ---
step >= 0 | 0 | len
step < 0 | len - 1 | -len - 1

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+1`th 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.¶

Table 9: Array Slice Selector Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$[1:3]` | `"b"` `"c"` | `$[1]` `$[2]` | Slice with default step
`$[5:]` | `"f"` `"g"` | `$[5]` `$[6]` | Slice with no end index
`$[1:5:2]` | `"b"` `"d"` | `$[1]` `$[3]` | Slice with step 2
`$[5:1:-2]` | `"f"` `"d"` | `$[5]` `$[3]` | Slice with negative step
`$[::-1]` | `"g"` `"f"` `"e"` `"d"` `"c"` `"b"` `"a"` | `$[6]` `$[5]` `$[4]` `$[3]` `$[2]` `$[1]` `$[0]` | Slice in reverse order

#### 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          = literal /
                          singular-query / ; singular query value
                          function-expr    ; ValueType
    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).¶

Table 10: Filter Expression Operator Precedence
Precedence | Operator type | Syntax
--- | --- | ---
5 | Grouping Function Expressions | `(...)` name`(...)`
4 | Logical NOT | `!`
3 | Relations | `==` `!=` `<` `<=` `>` `>=`
2 | Logical AND | `&&`
1 | Logical OR | `||`

##### 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:¶

    - both objects have the same collection of names (with no duplicates) and¶
    - 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:¶

Table 11: Comparison Examples
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

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.¶

Table 12: Filter Selector Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$.a[?@.b == 'kilo']` | `{"b": "kilo"}` | `$['a'][9]` | Member value comparison
`$.a[?(@.b == 'kilo')]` | `{"b": "kilo"}` | `$['a'][9]` | Equivalent query with enclosing parentheses
`$.a[?@>3.5]` | `5` `4` `6` | `$['a'][1]` `$['a'][4]` `$['a'][5]` | Array value comparison
`$.a[?@.b]` | `{"b": "j"}` `{"b": "k"}` `{"b": {}}` `{"b": "kilo"}` | `$['a'][6]` `$['a'][7]` `$['a'][8]` `$['a'][9]` | Array value existence
`$[?@.*]` | `[3, 5, 1, 2, 4, 6, {"b": "j"}, {"b": "k"}, {"b": {}}, {"b": "kilo"}]` `{"p": 1, "q": 2, "r": 3, "s": 5, "t": {"u": 6}}` | `$['a']` `$['o']` | Existence of non-singular queries
`$[?@[?@.b]]` | `[3, 5, 1, 2, 4, 6, {"b": "j"}, {"b": "k"}, {"b": {}}, {"b": "kilo"}]` | `$['a']` | Nested filters
`$.o[?@<3, ?@<3]` | `1` `2` `2` `1` | `$['o']['p']` `$['o']['q']` `$['o']['q']` `$['o']['p']` | Non-deterministic ordering
`$.a[?@<2 || @.b == "k"]` | `1` `{"b": "k"}` | `$['a'][2]` `$['a'][7]` | Array value logical OR
`$.a[?match(@.b, "[jk]")]` | `{"b": "j"}` `{"b": "k"}` | `$['a'][6]` `$['a'][7]` | Array value regular expression match
`$.a[?search(@.b, "[jk]")]` | `{"b": "j"}` `{"b": "k"}` `{"b": "kilo"}` | `$['a'][6]` `$['a'][7]` `$['a'][9]` | Array value regular expression search
`$.o[?@>1 && @<4]` | `2` `3` | `$['o']['q']` `$['o']['r']` | Object value logical AND
`$.o[?@>1 && @<4]` | `3` `2` | `$['o']['r']` `$['o']['q']` | Alternative result
`$.o[?@.u || @.x]` | `{"u": 6}` | `$['o']['t']` | Object value logical OR
`$.a[?@.b == $.x]` | `3` `5` `1` `2` `4` `6` | `$['a'][0]` `$['a'][1]` `$['a'][2]` `$['a'][3]` `$['a'][4]` `$['a'][5]` | Comparison of queries with no values
`$.a[?@ == @]` | `3` `5` `1` `2` `4` `6` `{"b": "j"}` `{"b": "k"}` `{"b": {}}` `{"b": "kilo"}` | `$['a'][0]` `$['a'][1]` `$['a'][2]` `$['a'][3]` `$['a'][4]` `$['a'][5]` `$['a'][6]` `$['a'][7]` `$['a'][8]` `$['a'][9]` | Comparisons of primitive and of structured values

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   = literal /
                          filter-query / ; (includes singular-query)
                          logical-expr /
                          function-expr

¶

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.¶

Table 13: Function Extension Type System
Type | Instances
--- | ---
`ValueType` | JSON values or `Nothing`
`LogicalType` | `LogicalTrue` or `LogicalFalse`
`NodesType` | Nodelists

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:¶

       - If the query results in a nodelist consisting of a single node, the argument is the value of the node.¶
       - 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

Table 14: Function Expression 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
`$[?count(foo(@.*)) == 1]` | well-typed, where `foo()` is a function extension with a parameter of type `NodesType` and result type `NodesType`
`$[?match(@.timezone, 'Europe/.*')]` | well-typed
`$[?match(@.timezone, 'Europe/.*') == true]` | not well-typed as `LogicalType` may not be used in comparisons
`$[?value(@..color) == "red"]` | well-typed
`$[?value(@..color)]` | not well-typed as `ValueType` may not be used in a test expression
`$[?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`

### 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:¶

Table 15: Child Segment Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$[0, 3]` | `"a"` `"d"` | `$[0]` `$[3]` | Indices
`$[0:2, 5]` | `"a"` `"b"` `"f"` | `$[0]` `$[1]` `$[5]` | Slice and index
`$[0, 0]` | `"a"` `"a"` | `$[0]` `$[0]` | Duplicated entries

#### 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.)¶

Table 16: Descendant Segment Examples
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$..j` | `1` `4` | `$['o']['j']` `$['a'][2][0]['j']` | Object values
`$..j` | `4` `1` | `$['a'][2][0]['j']` `$['o']['j']` | Alternative result
`$..[0]` | `5` `{"j": 4}` | `$['a'][0]` `$['a'][2][0]` | Array values
`$..[*]` or `$..*` | `{"j": 1, "k": 2}` `[5, 3, [{"j": 4}, {"k": 6}]]` `1` `2` `5` `3` `[{"j": 4}, {"k": 6}]` `{"j": 4}` `{"k": 6}` `4` `6` | `$['o']` `$['a']` `$['o']['j']` `$['o']['k']` `$['a'][0]` `$['a'][1]` `$['a'][2]` `$['a'][2][0]` `$['a'][2][1]` `$['a'][2][0]['j']` `$['a'][2][1]['k']` | All values
`$..o` | `{"j": 1, "k": 2}` | `$['o']` | Input value is visited
`$.o..[*, *]` | `1` `2` `2` `1` | `$['o']['j']` `$['o']['k']` `$['o']['k']` `$['o']['j']` | Non-deterministic ordering
`$.a..[0, 1]` | `5` `3` `{"j": 4}` `{"k": 6}` | `$['a'][0]` `$['a'][1]` `$['a'][2][0]` `$['a'][2][1]` | Multiple segments

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:¶

Table 17: Examples Involving (or Not Involving) `null`
Query | Result | Result Paths | Comment
--- | --- | --- | ---
`$.a` | `null` | `$['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

### 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

Table 18: Normalized Path 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

## 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":¶

Table 19: Initial Entries in the Function Extensions Subregistry
Function Name | Brief Description | Parameters | Result
--- | --- | --- | ---
length | length of string, array, or object | `ValueType` | `ValueType`
count | size of nodelist | `NodesType` | `ValueType`
match | regular expression full match | `ValueType`, `ValueType` | `LogicalType`
search | regular expression substring match | `ValueType`, `ValueType` | `LogicalType`
value | value of the single node in nodelist | `NodesType` | `ValueType`

## 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          = literal /
                          singular-query / ; singular query value
                          function-expr    ; ValueType
    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 "]"
    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
    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   = literal /
                          filter-query / ; (includes singular-query)
                          logical-expr /
                          function-expr
    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.¶

Table 20: XPath Syntax Compared to JSONPath
XPath | JSONPath | Description
--- | --- | ---
`/` | `$` | the root XML element
`.` | `@` | the current XML element
`/` | `.` or `[]` | child operator
`..` | n/a | parent operator
`//` | `..name`, `..⁠[index]`, `..*`, or `..[*]` | 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

For further illustration, Table 21 shows some XPath expressions and their JSONPath equivalents.¶

Table 21: Example 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]` | `$..book[0,1]` `$..book[:2]` | the first two 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

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

## Authors' Addresses