JavaScript Object Notation (JSON) Path
draft-ietf-jsonpath-base-00

JSONPath WG                                           G. Normington, Ed.
Internet-Draft                                              VMware, Inc.
Intended status: Standards Track                           E. Surov, Ed.
Expires: 8 September 2021                        TheSoul Publishing Ltd.
                                                       M. Mikulicic, Ed.
                                                        InfluxData, Inc.
                                                              S. Gössner
                                                 Fachhochschule Dortmund
                                                            7 March 2021

                 JavaScript Object Notation (JSON) Path
                      draft-ietf-jsonpath-base-00

Abstract

   JSONPath defines a string syntax for identifying values within a
   JavaScript Object Notation (JSON) document.

Contributing

   This document picks up the popular JSONPath specification dated
   2007-02-21 and provides a normative definition for it.  In its
   current state, it is a strawman document showing what needs to be
   covered.

   Comments and issues may be directed to this document's github
   repository (https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath-
   jsonpath).

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 September 2021.

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Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Inspired by XPath . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Overview of JSONPath Expressions  . . . . . . . . . . . .   5
   2.  JSONPath Examples . . . . . . . . . . . . . . . . . . . . . .   8
   3.  JSONPath Syntax and Semantics . . . . . . . . . . . . . . . .  11
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  Implementation  . . . . . . . . . . . . . . . . . . . . .  11
     3.4.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Semantics . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.6.  Selectors . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.6.1.  Dot Child Selector  . . . . . . . . . . . . . . . . .  13
       3.6.2.  Union Selector  . . . . . . . . . . . . . . . . . . .  14
         3.6.2.1.  Syntax  . . . . . . . . . . . . . . . . . . . . .  14
         3.6.2.2.  Semantics . . . . . . . . . . . . . . . . . . . .  14
         3.6.2.3.  Child . . . . . . . . . . . . . . . . . . . . . .  14
         3.6.2.4.  Array Selector  . . . . . . . . . . . . . . . . .  16
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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1.  Introduction

   This document picks up the popular JSONPath specification dated
   2007-02-21 [JSONPath-orig] and provides a normative definition for
   it.  In its current state, it is a strawman document showing what
   needs to be covered.

   JSON is defined by [RFC8259].

   JSONPath is not intended as a replacement, but as a more powerful
   companion, to JSON Pointer [RFC6901]. [insert reference to section
   where the relationship is detailed.  The purposes of the two syntaxes
   are different.  Pointer is for isolating a single location within a
   document.  Path is a query syntax that can also be used to pull
   multiple locations.]

1.1.  Terminology

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

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

   The terminology of [RFC8259] applies.

   Data Item:  A structure complying to the generic data model of JSON,
      i.e., composed of containers, namely JSON objects and arrays, and
      of atomic data, namely null, true, false, numbers, and text
      strings.  Also called a JSON value.

   Object:  A JSON object as defined in [RFC8259].  Never used in its
      generic sense, e.g., for programming language objects.

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

   Name:  The name in a name/value pair constituting a member.  (Also
      known as "key", "tag", or "label".)

   Element:  An item in an array.  (Also used with a distinct meaning in
      XML context for XML elements.)

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   Query:  Short name for JSONPath expression.

   Argument:  Short name for the JSON data item a JSONPath expression is
      applied to.

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

   Position:  A JSON data item identical to or nested within the JSON
      data item to which the query is applied to, expressed either by
      the value of that data item or by providing a Normalized Path
      Expression as a JSONPath Output Path.

   Normalized Path Expression:  A query in a normalized form that
      identifies exactly one Position in an Argument; see Section 1.3.

1.2.  Inspired by XPath

   A frequently emphasized advantage of XML is the availability of
   powerful tools to analyse, transform and selectively extract data
   from XML documents.  [XPath] is one of these tools.

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

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

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

   So what does such a tool look like for JSON?  When defining a
   JSONPath, how should expressions look?

   The XPath expression

   /store/book[1]/title

   looks like

   x.store.book[0].title

   or

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

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   in popular programming languages such as JavaScript, Python and PHP,
   with a variable x holding the JSON data item.  Here we observe that
   such languages already have a fundamentally XPath-like feature built
   in.

   The JSONPath tool in question should:

   *  be naturally based on those language characteristics.

   *  cover only essential parts of XPath 1.0.

   *  be lightweight in code size and memory consumption.

   *  be runtime efficient.

1.3.  Overview of JSONPath Expressions

   JSONPath expressions always apply to a JSON data item in the same way
   as XPath expressions are used in combination with an XML document.
   Since a JSON data item is anonymous, JSONPath uses the abstract name
   "$" to refer to the top level data item of the argument.

   JSONPath expressions can use the _dot-notation_

   $.store.book[0].title

   or the _bracket-notation_

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

   for paths input to a JSONPath processor. [1] Where a JSONPath
   processor uses JSONPath expressions as output paths, these will
   always be converted to normalized JSONPath expressions which employ
   the more general _bracket-notation_. [2] Bracket notation is more
   general than dot notation and can serve as a canonical form when a
   JSONPath processor uses JSONPath expressions as output paths.

   JSONPath allows the wildcard symbol "*" for member names and array
   indices.  It borrows the descendant operator ".." from [E4X] and the
   array slice syntax proposal "[start:end:step]" [SLICE] from
   ECMASCRIPT 4.

   JSONPath was originally designed to employ an _underlying scripting
   language_ for computing expressions.  The present specification
   defines a simple expression language that is independent from any
   scripting language in use on the platform.

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   JSONPath can use expressions, written in parentheses: "(<expr>)", as
   an alternative to explicit names or indices as in:

   $.store.book[(@.length-1)].title

   The symbol "@" is used for the current item.  Filter expressions are
   supported via the syntax "?(<boolean expr>)" as in

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

   Here is a complete overview and a side by side comparison of the
   JSONPath syntax elements with their XPath counterparts.

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    +=======+==================+=====================================+
    | XPath | JSONPath         | Description                         |
    +=======+==================+=====================================+
    | /     | $                | the root element/item               |
    +-------+------------------+-------------------------------------+
    | .     | @                | the current element/item            |
    +-------+------------------+-------------------------------------+
    | /     | "." or "[]"      | child operator                      |
    +-------+------------------+-------------------------------------+
    | ..    | n/a              | parent operator                     |
    +-------+------------------+-------------------------------------+
    | //    | ..               | nested descendants (JSONPath        |
    |       |                  | borrows this syntax from E4X)       |
    +-------+------------------+-------------------------------------+
    | *     | *                | wildcard: All elements/items        |
    |       |                  | regardless of their names           |
    +-------+------------------+-------------------------------------+
    | @     | n/a              | attribute access: JSON data items   |
    |       |                  | do not have attributes              |
    +-------+------------------+-------------------------------------+
    | []    | []               | subscript operator: XPath uses it   |
    |       |                  | to iterate over element collections |
    |       |                  | and for predicates; native array    |
    |       |                  | indexing as in JavaScript here      |
    +-------+------------------+-------------------------------------+
    | |     | [,]              | Union operator in XPath (results in |
    |       |                  | a combination of node sets);        |
    |       |                  | JSONPath allows alternate names or  |
    |       |                  | array indices as a set              |
    +-------+------------------+-------------------------------------+
    | n/a   | [start:end:step] | array slice operator borrowed from  |
    |       |                  | ES4                                 |
    +-------+------------------+-------------------------------------+
    | []    | ?()              | applies a filter (script)           |
    |       |                  | expression                          |
    +-------+------------------+-------------------------------------+
    | n/a   | ()               | expression engine                   |
    +-------+------------------+-------------------------------------+
    | ()    | n/a              | grouping in Xpath                   |
    +-------+------------------+-------------------------------------+

           Table 1: Overview over JSONPath, comparing to XPath

   XPath has a lot more to offer (location paths in unabbreviated
   syntax, operators and functions) than listed here.  Moreover there is
   a significant difference how the subscript operator works in Xpath
   and JSONPath:

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

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

2.  JSONPath Examples

   This section provides some more examples for JSONPath expressions.
   The examples are based on a simple JSON data item patterned after a
   typical XML example representing a bookstore (that also has
   bicycles):

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

                      Figure 1: Example JSON data item

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

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   +======================+=========================+==================+
   | XPath                | JSONPath                | Result           |
   +======================+=========================+==================+
   | /store/book/author   | $.store.book[*].author  | the authors of   |
   |                      |                         | all books in     |
   |                      |                         | the store        |
   +----------------------+-------------------------+------------------+
   | //author             | $..author               | all authors      |
   +----------------------+-------------------------+------------------+
   | /store/*             | $.store.*               | all things in    |
   |                      |                         | store, which     |
   |                      |                         | are some books   |
   |                      |                         | and a red        |
   |                      |                         | bicycle          |
   +----------------------+-------------------------+------------------+
   | /store//price        | $.store..price          | the prices of    |
   |                      |                         | everything in    |
   |                      |                         | the store        |
   +----------------------+-------------------------+------------------+
   | //book[3]            | $..book[2]              | the third book   |
   +----------------------+-------------------------+------------------+
   | //book[last()]       | "$..book[(@.length-1)]" | the last book    |
   |                      | "$..book[-1]"           | in order         |
   +----------------------+-------------------------+------------------+
   | //book[position()<3] | "$..book[0,1]"          | the first two    |
   |                      | "$..book[:2]"           | books            |
   +----------------------+-------------------------+------------------+
   | //book[isbn]         | $..book[?(@.isbn)]      | filter all       |
   |                      |                         | books with       |
   |                      |                         | isbn number      |
   +----------------------+-------------------------+------------------+
   | //book[price<10]     | $..book[?(@.price<10)]  | filter all       |
   |                      |                         | books cheaper    |
   |                      |                         | than 10          |
   +----------------------+-------------------------+------------------+
   | //*                  | $..*                    | all elements     |
   |                      |                         | in XML           |
   |                      |                         | document; all    |
   |                      |                         | members of       |
   |                      |                         | JSON data item   |
   +----------------------+-------------------------+------------------+

     Table 2: Example JSONPath expressions applied to the example JSON
                                 data item

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3.  JSONPath Syntax and Semantics

3.1.  Overview

   A JSONPath is a string which selects zero or more nodes of a piece of
   JSON.  A valid JSONPath conforms to the ABNF syntax defined by this
   document.

   A JSONPath MUST be encoded using UTF-8.  To parse a JSONPath
   according to the grammar in this document, its UTF-8 form SHOULD
   first be decoded into Unicode code points as described in [RFC3629].

3.2.  Terminology

   A JSON value is logically a tree of nodes.

   Each node holds a JSON value (as defined by [RFC8259]) of one of the
   types object, array, number, string, or one of the literals "true",
   "false", or "null".  The type of the JSON value held by a node is
   sometimes referred to as the type of the node.

3.3.  Implementation

   An implementation of this specification, from now on referred to
   simply as "an implementation", SHOULD takes two inputs, a JSONPath
   and a JSON value, and produce a possibly empty list of nodes of the
   JSON value which are selected by the JSONPath or an error (but not
   both).

   If no node is selected and no error has occurred, an implementation
   MUST return an empty list of nodes.

   Syntax errors in the JSONPath SHOULD be detected before selection is
   attempted since these errors do not depend on the JSON value.
   Therefore, an implementation SHOULD take a JSONPath and produce an
   optional syntax error and then, if and only if an error was not
   produced, SHOULD take a JSON value and produce a list of nodes or an
   error (but not both).

   Alternatively, an implementation MAY take a JSONPath and a JSON value
   and produce a list of nodes or an optional error (but not both).

   For any implementation, if a syntactically invalid JSONPath is
   provided, the implementation MUST return an error.

   If a syntactially invalid JSON value is provided, any implementation
   SHOULD return an error.

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3.4.  Syntax

   Syntactically, a JSONPath consists of a root selector ("$"), which
   selects the root node of a JSON value, followed by a possibly empty
   sequence of _selectors_.

   json-path = root-selector *selector
   root-selector = "$"               ; $ selects document root node

   The syntax and semantics of each selector is defined below.

3.5.  Semantics

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

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

   But a selector doesn't just act on a single node: each selector acts
   on a list of nodes and produces a list of nodes, as follows.

   After the root selector, the remainder of the JSONPath is processed
   by passing lists of nodes from one selector to the next ending up
   with a list of nodes which is the result of applying the JSONPath to
   the input JSON value.

   Each selector acts on its input list of nodes as follows.  For each
   node in the list, the selector selects zero or more nodes, each of
   which is a descendant of the node or the node itself.  The output
   list of nodes of a selector is the concatenation of the lists of
   selected nodes for each input node.

   A specific, non-normative example will make this clearer.  Suppose
   the input document is: "{"a":[{"b":0},{"b":1},{"c":2}]}".  As we will
   see later, the JSONPath "$.a[*].b" selects the following list of
   nodes: "0", "1".  Let's walk through this in detail.

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

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

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

   Next, "[*]" selects from any input node which is an array and selects
   all the elements of the input node.  The result is a list of three
   nodes: "{"b":0}", "{"b":1}", and "{"c":2}".

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

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

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

3.6.  Selectors

3.6.1.  Dot Child Selector

Syntax

   A dot child selector has a member name known as a dot child name or a
   single asterisk ("*").

   A dot child name corresponds to a name in a object.

   selector = dot-child              ; see below for alternatives
   dot-child = "." dot-child-name / ; .<dot-child-name>
               "." "*"             ; .*
   dot-child-name = 1*(
                      "-" /         ; -
                      DIGIT /
                      ALPHA /
                      "_" /         ; _
                      %x80-10FFFF    ; any non-ASCII Unicode character
                    )
   DIGIT =  %x30-39                  ; 0-9
   ALPHA = %x41-5A / %x61-7A         ; A-Z / a-z

   More general child names, such as the empty string, are supported by
   "Union Child" (Section 3.6.2.3).

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   Note that the "dot-child-name" rule follows the philosophy of JSON
   strings and is allowed to contain bit sequences that cannot encode
   Unicode characters (a single unpaired UTF-16 surrogate, for example).
   The behaviour of an implementation is undefined for child names which
   do not encode Unicode characters.

Semantics

   A dot child name which is not a single asterisk ("*") is considered
   to have a member name.  It selects the value corresponding to the
   name from any object node.  It selects no nodes from a node which is
   not a object.

   The member name of a dot child name is the sequence of Unicode
   characters contained in that name.

   A dot child name consisting of a single asterisk is a wild card.  It
   selects all the values of any object node.  It also selects all the
   elements of any array node.  It selects no nodes from number, string,
   or literal nodes.

3.6.2.  Union Selector

3.6.2.1.  Syntax

   A union selector consists of one or more union elements.

   selector =/ union
   union = "[" ws union-elements ws "]" ; [...]
   ws = *" "                             ; zero or more spaces
   union-elements = union-element /
                    union-element ws "," ws union-elements
                                          ; ,-separated list

3.6.2.2.  Semantics

   A union selects any node which is selected by at least one of the
   union selectors and selects the concatenation of the lists (in the
   order of the selectors) of nodes selected by the union elements.

3.6.2.3.  Child

Syntax

   A child is a quoted string.

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   union-element = child ; see below for more alternatives
   child = %x22 *double-quoted %x22 / ; "string"
           "'" *single-quoted "'"   ; 'string'

   double-quoted = dq-unescaped /
             escape (
                 %x22 /         ; "    quotation mark  U+0022
                 "/" /          ; /    solidus         U+002F
                 "\" /          ; \    reverse solidus U+005C
                 "b" /          ; b    backspace       U+0008
                 "f" /          ; f    form feed       U+000C
                 "n" /          ; n    line feed       U+000A
                 "r" /          ; r    carriage return U+000D
                 "t" /          ; t    tab             U+0009
                 "u" 4HEXDIG )  ; uXXXX                U+XXXX

   dq-unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

   single-quoted = sq-unescaped /
             escape (
                 "'" /          ; '    apostrophe      U+0027
                 "/" /          ; /    solidus         U+002F
                 "\" /          ; \    reverse solidus U+005C
                 "b" /          ; b    backspace       U+0008
                 "f" /          ; f    form feed       U+000C
                 "n" /          ; n    line feed       U+000A
                 "r" /          ; r    carriage return U+000D
                 "t" /          ; t    tab             U+0009
                 "u" 4HEXDIG )  ; uXXXX                U+XXXX

   sq-unescaped = %x20-26 / %x28-5B / %x5D-10FFFF

   escape = "\"                 ; \

   HEXDIG =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
                                 ; case insensitive hex digit

   Notes: 1. double-quoted strings follow JSON in [RFC8259].  Single-
   quoted strings follow an analogous pattern. 2.  "HEXDIG" includes A-F
   and a-f.

Semantics

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

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                  +=================+===================+
                  | Escape Sequence | Unicode Character |
                  +=================+===================+
                  |     "" %x22     |       U+0022      |
                  +-----------------+-------------------+
                  |      "" "'"     |       U+0027      |
                  +-----------------+-------------------+
                  |      "" "/"     |       U+002F      |
                  +-----------------+-------------------+
                  |      "" ""      |       U+005C      |
                  +-----------------+-------------------+
                  |      "" "b"     |       U+0008      |
                  +-----------------+-------------------+
                  |      "" "f"     |       U+000C      |
                  +-----------------+-------------------+
                  |      "" "n"     |       U+000A      |
                  +-----------------+-------------------+
                  |      "" "r"     |       U+000D      |
                  +-----------------+-------------------+
                  |      "" "t"     |       U+0009      |
                  +-----------------+-------------------+
                  |     "" uXXXX    |       U+XXXX      |
                  +-----------------+-------------------+

                   Table 3: Escape Sequence Replacements

   The child selects the value corresponding to the member name from any
   object node that has a member with that name.  It selects no nodes
   from a node which is not a object.

3.6.2.4.  Array Selector

Syntax

   An array selector selects zero or more elements of an array node.  An
   array selector takes the form of an index, which selects at most one
   element, or a slice, which selects zero or more elements.

   union-element =/ array-index / array-slice

   An array index is an integer (in base 10).

   array-index = integer

   integer = ["-"] ("0" / (DIGIT1 *DIGIT))
                               ; optional - followed by 0 or
                               ; sequence of digits with no leading zero
   DIGIT1 = %x31-39            ; non-zero digit

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   Note: the syntax does not allow integers with leading zeros such as
   "01" and "-01".

   An array slice consists of three optional integers (in base 10)
   separated by colons.

   array-slice = [ start ] ws ":" ws [ end ]
                      [ ws ":" ws [ step ] ]
   start = integer
   end = integer
   step = integer

   Note: the array slices ":" and "::" are both syntactically valid, as
   are ":2:2", "2::2", and "2:4:".

Semantics

Informal Introduction

   This section is non-normative.

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

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

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

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

   When "step" is negative, elements are selected in reverse order.
   Thus, for example, "[5:1:-2]" selects elements with indices "5" and
   "3", in that order and "[::-1]" selects all the elements of an array
   in reverse order.

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   When "step" is "0", no elements are selected.  This is the one case
   which differs from the behaviour of Python, which raises an error in
   this case.

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

Detailed Semantics

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

   A slice expression selects a subset of the elements of the input
   array, in the same order as the array or the reverse order, depending
   on the sign of the "step" parameter.  It selects no nodes from a node
   which is not an array.

   A slice is defined by the two slice parameters, "start" and "end",
   and an iteration delta, "step".  Each of these parameters is
   optional. "len" is the length of the input array.

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

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

                       Table 4: Default array slice
                           start and end values

   Slice expression parameters "start" and "end" are not directly usable
   as slice bounds and must first be normalized.  Normalization is
   defined as:

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

   The result of the array indexing expression "[i]" is defined to be
   the result of the array slicing expression "[i:Normalize(i)+1:1]".

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   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)
     n_end = Normalize(end)

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

     RETURN (lower, upper)

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

   IF step > 0 THEN

     i = lower
     WHILE i < upper:
       SELECT a(i)
       i = i + step
     END WHILE

   ELSE if step < 0 THEN

     i = upper
     WHILE lower < i:
       SELECT a(i)
       i = i + step
     END WHILE

   END IF

   When "step = 0", no elements are selected and the result array is
   empty.

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   An implementation MUST raise an error if any of the slice expression
   parameters does not fit in the implementation's representation of an
   integer.  If a successfully parsed slice expression is evaluated
   against an array whose size doesn't fit in the implementation's
   representation of an integer, the implementation MUST raise an error.

4.  IANA Considerations

   TBD: Define a media type for JSON Path expressions.

5.  Security Considerations

   This section gives security considerations, as required by [RFC3552].

6.  References

6.1.  Normative References

   [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>.

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

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

6.2.  Informative References

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

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

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

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

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

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

Acknowledgements

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

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

Contributors

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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Authors' Addresses

   Glyn Normington (editor)
   VMware, Inc.
   Winchester
   United Kingdom

   Email: glyn.normington@gmail.com

   Edward Surov (editor)
   TheSoul Publishing Ltd.
   Limassol
   Cyprus

   Email: esurov.tsp@gmail.com

   Marko Mikulicic (editor)
   InfluxData, Inc.
   Pisa
   Italy

   Email: mmikulicic@gmail.com

   Stefan Gössner
   Fachhochschule Dortmund
   Sonnenstraße 96
   D-44139 Dortmund
   Germany

   Email: stefan.goessner@fh-dortmund.de

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