JSONPath: Query expressions for JSON
draft-ietf-jsonpath-base-03
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9535.
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Authors | Stefan Gössner , Glyn Normington , Carsten Bormann | ||
Last updated | 2022-01-16 | ||
Replaces | draft-normington-jsonpath | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
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IESG | IESG state | Became RFC 9535 (Proposed Standard) | |
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draft-ietf-jsonpath-base-03
JSONPath WG S. Gössner, Ed. Internet-Draft Fachhochschule Dortmund Intended status: Standards Track G. Normington, Ed. Expires: 20 July 2022 C. Bormann, Ed. Universität Bremen TZI 16 January 2022 JSONPath: Query expressions for JSON draft-ietf-jsonpath-base-03 Abstract JSONPath defines a string syntax for selecting and extracting values within a JavaScript Object Notation (JSON, RFC 8259) value. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-jsonpath-base/. Discussion of this document takes place on the JSON Path Working Group mailing list (mailto:jsonpath@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/jsonpath/. Source for this draft and an issue tracker can be found at https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath-base. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 20 July 2022. Gössner, et al. Expires 20 July 2022 [Page 1] Internet-Draft JSONPath January 2022 Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. History . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Overview of JSONPath Expressions . . . . . . . . . . . . 5 2. JSONPath Examples . . . . . . . . . . . . . . . . . . . . . . 6 3. JSONPath Syntax and Semantics . . . . . . . . . . . . . . . . 8 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2. Processing Model . . . . . . . . . . . . . . . . . . . . 9 3.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 10 3.5. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 11 3.5.1. Root Selector . . . . . . . . . . . . . . . . . . . . 12 3.5.2. Dot Selector . . . . . . . . . . . . . . . . . . . . 12 3.5.3. Dot Wild Card Selector . . . . . . . . . . . . . . . 13 3.5.4. Index Selector . . . . . . . . . . . . . . . . . . . 13 3.5.5. Index Wild Card Selector . . . . . . . . . . . . . . 16 3.5.6. Array Slice Selector . . . . . . . . . . . . . . . . 16 3.5.7. Descendant Selector . . . . . . . . . . . . . . . . . 20 3.5.8. Union Selector . . . . . . . . . . . . . . . . . . . 20 3.5.9. Filter Selector . . . . . . . . . . . . . . . . . . . 21 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 5. Security Considerations . . . . . . . . . . . . . . . . . . . 24 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.1. Normative References . . . . . . . . . . . . . . . . . . 24 6.2. Informative References . . . . . . . . . . . . . . . . . 25 Appendix A. Inspired by XPath . . . . . . . . . . . . . . . . . 26 A.1. JSONPath and XPath . . . . . . . . . . . . . . . . . . . 27 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 30 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 Gössner, et al. Expires 20 July 2022 [Page 2] Internet-Draft JSONPath January 2022 1. Introduction JavaScript Object Notation (JSON, [RFC8259]) is a popular representation format for structured data values. JSONPath defines a string syntax for identifying values within a JSON value. JSONPath is not intended as a replacement, but as a more powerful companion, to JSON Pointer [RFC6901]. // Insert reference to section where the relationship is detailed. // The purposes of the two syntaxes are different. Pointer is for // isolating a single location within a JSON document. Path is a // query syntax that can also be used to pull multiple locations. 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The grammatical rules in this document are to be interpreted as ABNF, as described in [RFC5234]. ABNF terminal values in this document define Unicode code points rather than their UTF-8 encoding. For example, the Unicode PLACE OF INTEREST SIGN (U+2318) would be defined in ABNF as %x2318. The terminology of [RFC8259] applies except where clarified below. The terms "Primitive" and "Structured" are used to group the types as in Section 1 of [RFC8259]. Definitions for "Object", "Array", "Number", and "String" remain unchanged. Importantly "object" and "array" in particular do not take on a generic meaning, such as they would in a general programming context. Additional terms used in this specification are defined below. Value: As per [RFC8259], a structure complying to the generic data model of JSON, i.e., composed of components such as structured values, namely JSON objects and arrays, and primitive data, namely numbers and text strings as well as the special values null, true, and false. Member: A name/value pair in an object. (Not itself a value.) Name: The name in a name/value pair constituting a member. (Also known as "key", "tag", or "label".) This is also used in [RFC8259], but that specification does not formally define it. It is included here for completeness. Gössner, et al. Expires 20 July 2022 [Page 3] Internet-Draft JSONPath January 2022 Element: A value in an array. (Not to be confused with XML element.) Index: A non-negative integer that identifies a specific element in an array. Note that the term _indexing_ is also used for accessing elements using negative integers (Section "Semantics"), and for accessing member values in an object using their member name. Query: Short name for JSONPath expression. Argument: Short name for the value a JSONPath expression is applied to. Node: The pair of a value along with its location within the argument. Root Node: The unique node whose value is the entire argument. Children (of a node): If the node is an array, each of its elements, or if the node is an object, each its member values (but not its member names). Descendants (of a node): The node itself, plus the descendants of each of its children. // Note that this is often more selectively called descendant-or- // self. Should we define descendants non-inclusive of the node // itself? We do have the language to say "node + descendants" in // several places. Nodelist: A list of nodes. The output of applying a query to an argument is manifested as a list of nodes. While this list can be represented in JSON, e.g. as an array, the nodelist is an abstract concept unrelated to JSON values. Normalized Path: A simple form of JSONPath expression that identifies a node by providing a query that results in exactly that node. Similar to, but syntactically different from, a JSON Pointer [RFC6901]. For the purposes of this specification, a value as defined by [RFC8259] is also viewed as a tree of nodes. Each node, in turn, holds a value. Further nodes within each value are the elements of arrays and the member values of objects and are themselves values. (The type of the value held by a node may also be referred to as the type of the node.) A query is applied to an argument, and the output is a nodelist. Gössner, et al. Expires 20 July 2022 [Page 4] Internet-Draft JSONPath January 2022 1.2. History This document picks up Stefan Goessner's popular JSONPath proposal dated 2007-02-21 [JSONPath-orig] and provides a normative definition for it. Appendix A describes how JSONPath was inspired by XML's XPath [XPath]. JSONPath was intended as a light-weight companion to JSON implementations on platforms such as PHP and JavaScript, so instead of defining its own expression language like XPath did, JSONPath delegated this to the expression language of the platform. While the languages in which JSONPath is used do have significant commonalities, over time this caused non-portability of JSONPath expressions between the ensuing platform-specific dialects. The present specification intends to remove platform dependencies and serve as a common JSONPath specification that can be used across platforms. Obviously, this means that backwards compatibility could not always be achieved; a design principle of this specification is to go with a "consensus" between implementations even if it is rough, as long as that does not jeopardize the objective of obtaining a usable, stable JSON query language. 1.3. Overview of JSONPath Expressions JSONPath expressions are applied to a JSON value, the _argument_. Within the JSONPath expression, the abstract name $ is used to refer to the _root node_ of the argument, i.e., to the argument as a whole. JSONPath expressions can use the _dot notation_ $.store.book[0].title or the _bracket notation_ $['store']['book'][0]['title'] to build paths that are input to a JSONPath processor. Bracket notation is more general than dot notation and can serve as a canonical form (for instance, when a JSONPath processor uses JSONPath expressions as output paths). JSONPath allows the wildcard symbol * to select any member of an object or any element of an array (Section 3.5.3). The descendant operator .. selects the node and all its descendants (Section 3.5.7). The array slice syntax [start:end:step] allows selecting a regular Gössner, et al. Expires 20 July 2022 [Page 5] Internet-Draft JSONPath January 2022 selection of an element from an array, giving a start position, an end position, and possibly a step value that moves the position from the start to the end (Section 3.5.6). Filter expressions are supported via the syntax ?(<boolean expr>) as in $.store.book[?(@.price < 10)].title Table 1 provides a quick overview of the JSONPath syntax elements. +==================+==============================================+ | JSONPath | Description | +==================+==============================================+ | $ | the root node | +------------------+----------------------------------------------+ | @ | the current node | +------------------+----------------------------------------------+ | . or [] | child operator | +------------------+----------------------------------------------+ | n/a | parent operator | +------------------+----------------------------------------------+ | .. | nested descendants | +------------------+----------------------------------------------+ | * | wildcard: all member values/array elements | | | regardless of their names/indices | +------------------+----------------------------------------------+ | [] | subscript operator: index current node as an | | | array (from 0) | +------------------+----------------------------------------------+ | [,] | Union operator JSONPath allows alternate(??) | | | names or array indices as a set | +------------------+----------------------------------------------+ | [start:end:step] | array slice operator | +------------------+----------------------------------------------+ | ?() | applies a filter expression | +------------------+----------------------------------------------+ | () | expression, e.g., for indexing | +------------------+----------------------------------------------+ Table 1: Overview over JSONPath 2. JSONPath Examples This section provides some more examples for JSONPath expressions. The examples are based on the simple JSON value shown in Figure 1, which was patterned after a typical XML example representing a bookstore (that also has bicycles). Gössner, et al. Expires 20 July 2022 [Page 6] Internet-Draft JSONPath January 2022 { "store": { "book": [ { "category": "reference", "author": "Nigel Rees", "title": "Sayings of the Century", "price": 8.95 }, { "category": "fiction", "author": "Evelyn Waugh", "title": "Sword of Honour", "price": 12.99 }, { "category": "fiction", "author": "Herman Melville", "title": "Moby Dick", "isbn": "0-553-21311-3", "price": 8.99 }, { "category": "fiction", "author": "J. R. R. Tolkien", "title": "The Lord of the Rings", "isbn": "0-395-19395-8", "price": 22.99 } ], "bicycle": { "color": "red", "price": 19.95 } } } Figure 1: Example JSON value The examples in Table 2 use the expression mechanism to obtain the number of elements in an array, to test for the presence of a member in a object, and to perform numeric comparisons of member values with a constant. Gössner, et al. Expires 20 July 2022 [Page 7] Internet-Draft JSONPath January 2022 +========================+===================================+ | JSONPath | Result | +========================+===================================+ | $.store.book[*].author | the authors of all books in the | | | store | +------------------------+-----------------------------------+ | $..author | all authors | +------------------------+-----------------------------------+ | $.store.* | all things in store, which are | | | some books and a red bicycle | +------------------------+-----------------------------------+ | $.store..price | the prices of everything in the | | | store | +------------------------+-----------------------------------+ | $..book[2] | the third book | +------------------------+-----------------------------------+ | $..book[(@.length-1)] | the last book in order | | $..book[-1] | | +------------------------+-----------------------------------+ | $..book[0,1] | the first two books | | $..book[:2] | | +------------------------+-----------------------------------+ | $..book[?(@.isbn)] | filter all books with isbn number | +------------------------+-----------------------------------+ | $..book[?(@.price<10)] | filter all books cheaper than 10 | +------------------------+-----------------------------------+ | $..* | all elements in XML document; all | | | member values and array elements | | | contained in input value | +------------------------+-----------------------------------+ Table 2: Example JSONPath expressions applied to the example JSON value 3. JSONPath Syntax and Semantics 3.1. Overview A JSONPath query is a string which selects zero or more nodes of a piece of JSON. A query MUST be encoded using UTF-8. The grammar for queries given in this document assumes that its UTF-8 form is first decoded into Unicode code points as described in [RFC3629]; implementation approaches that lead to an equivalent result are possible. Gössner, et al. Expires 20 July 2022 [Page 8] Internet-Draft JSONPath January 2022 A string to be used as a JSONPath query needs to be _well-formed_ and _valid_. A string is a well-formed JSONPath query if it conforms to the ABNF syntax in this document. A well-formed JSONPath query is valid if it also fulfills all semantic requirements posed by this document. To be valid, integer numbers in the JSONPath query that are relevant to the JSONPath processing (e.g., index values and steps) MUST be within the range of exact values defined in I-JSON [RFC7493], namely within the interval [-(2^53)+1, (2^53)-1]). The well-formedness and the validity of JSONPath queries are independent of the JSON value the query is applied to; no further errors can be raised during application of the query to a value. Obviously, an implementation can still fail when executing a JSONPath query, e.g., because of resource depletion, but this is not modeled in the present specification. However, the implementation MUST NOT silently malfunction. Specifically, if a valid JSONPath query is evaluated against a structured value whose size doesn't fit in the range of exact values, interfering with the correct interpretation of the query, the implementation MUST provide an indication of overflow. (Readers familiar with the HTTP error model may be reminded of 400 type errors when pondering well-formedness and validity, while resource depletion and related errors are comparable to 500 type errors.) 3.2. Processing Model In this specification, the semantics of a JSONPath query are defined in terms of a _processing model_. That model is not prescriptive of the internal workings of an implementation: Implementations may wish (or need) to design a different process that yields results that are consistent with this model. In the processing model, a valid query is executed against a value, the _argument_, and produces a list of zero or more nodes of the value. The query is a sequence of zero or more _selectors_, each of which is applied to the result of the previous selector and provides input to the next selector. These results and inputs take the form of a _nodelist_, i.e., a sequence of zero or more nodes. The nodelist going into the first selector contains a single node, the argument. The nodelist resulting from the last selector is presented as the result of the query; depending on the specific API, Gössner, et al. Expires 20 July 2022 [Page 9] Internet-Draft JSONPath January 2022 it might be presented as an array of the JSON values at the nodes, an array of Output Paths referencing the nodes, or both -- or some other representation as desired by the implementation. Note that the API must be capable of presenting an empty nodelist as the result of the query. A selector performs its function on each of the nodes in its input nodelist, during such a function execution, such a node is referred to as the "current node". Each of these function executions produces a nodelist, which are then concatenated into the result of the selector. The processing within a selector may execute nested queries, which are in turn handled with the processing model defined here. Typically, the argument to that query will be the current node of the selector or a set of nodes subordinate to that current node. 3.3. Syntax Syntactically, a JSONPath query consists of a root selector ($), which stands for a nodelist that contains the root node of the argument, followed by a possibly empty sequence of _selectors_. json-path = root-selector *(S (dot-selector / dot-wild-selector / index-selector / index-wild-selector / union-selector / slice-selector / descendant-selector / filter-selector)) The syntax and semantics of each selector is defined below. 3.4. Semantics The root selector $ not only selects the root node of the argument, but it also produces as output a list consisting of one node: the argument itself. A selector may select zero or more nodes for further processing. A syntactically valid selector MUST NOT produce errors. This means that some operations which might be considered erroneous, such as indexing beyond the end of an array, simply result in fewer nodes being selected. Gössner, et al. Expires 20 July 2022 [Page 10] Internet-Draft JSONPath January 2022 But a selector doesn't just act on a single node: a selector acts on each of the nodes in its input nodelist and concatenates the resultant nodelists to form the result nodelist of the selector. For each node in the list, the selector selects zero or more nodes, each of which is a descendant of the node or the node itself. For instance, with the argument {"a":[{"b":0},{"b":1},{"c":2}]}, the query $.a[*].b selects the following list of nodes: 0, 1 (denoted here by their value). Let's walk through this in detail. The query consists of $ followed by three selectors: .a, [*], and .b. Firstly, $ selects the root node which is the argument. So the result is a list consisting of just the root node. Next, .a selects from any input node of type object and selects the node of any member value of the input node corresponding to the member name "a". The result is again a list of one node: [{"b":0},{"b":1},{"c":2}]. Next, [*] selects from an input node of type array all its elements (if the input note were of type object, it would select all its member values, but not the member names). The result is a list of three nodes: {"b":0}, {"b":1}, and {"c":2}. Finally, .b selects from any input node of type object with a member name b and selects the node of the member value of the input node corresponding to that name. The result is a list containing 0, 1. This is the concatenation of three lists, two of length one containing 0, 1, respectively, and one of length zero. As a consequence of this approach, if any of the selectors selects no nodes, then the whole query selects no nodes. In what follows, the semantics of each selector are defined for each type of node. 3.5. Selectors A JSONPath query consists of a sequence of selectors. Valid selectors are * Root selector $ (used at the start of a query and in expressions) * Dot selector .<name>, used with object member names exclusively. * Dot wild card selector .*. Gössner, et al. Expires 20 July 2022 [Page 11] Internet-Draft JSONPath January 2022 * Index selector [<index>], where <index> is either a (possibly negative, see Section "Semantics") array index or an object member name. * Index wild card selector [*]. * Array slice selector [<start>:<end>:<step>], where the optional values <start>, <end>, and <step> are integer literals. * Nested descendants selector ... * Union selector [<sel1>,<sel2>,...,<selN>], holding a comma delimited list of index, index wild card, array slice, and filter selectors. * Filter selector [?(<expr>)] * Current item selector @ (used in expressions) 3.5.1. Root Selector Syntax Every valid JSONPath query MUST begin with the root selector $. root-selector = "$" Semantics The Argument -- the root JSON value -- becomes the root node, which is addressed by the root selector $. 3.5.2. Dot Selector Syntax A dot selector starts with a dot . followed by an object's member name. Gössner, et al. Expires 20 July 2022 [Page 12] Internet-Draft JSONPath January 2022 dot-selector = "." dot-member-name dot-member-name = name-first *name-char name-first = ALPHA / "_" / ; _ %x80-10FFFF ; any non-ASCII Unicode character name-char = DIGIT / name-first DIGIT = %x30-39 ; 0-9 ALPHA = %x41-5A / %x61-7A ; A-Z / a-z Member names containing characters other than allowed by dot-selector -- such as space ` , minus-, or dot.characters -- MUST NOT be used with thedot-selector. (Such member names can be addressed by theindex-selector` instead.) Semantics The dot-selector selects the node of the member value corresponding to the member name from any JSON object in its input nodelist. It selects no nodes from any other JSON value. 3.5.3. Dot Wild Card Selector Syntax The dot wild card selector has the form .* as defined in the following syntax: dot-wild-selector = "." "*" ; dot followed by asterisk Semantics A dot-wild-selector acts as a wild card by selecting the nodes of all member values of an object in its input nodelist as well as all element nodes of an array in its input nodelist. Applying the dot- wild-selector to a primitive JSON value (number, string, or true/false/null) selects no node. 3.5.4. Index Selector Syntax An index selector [<index>] addresses at most one object member value or at most one array element value. index-selector = "[" S (quoted-member-name / element-index) S "]" Gössner, et al. Expires 20 July 2022 [Page 13] Internet-Draft JSONPath January 2022 Applying the index-selector to an object value in its input nodelist, a quoted-member-name string is required to select the corresponding member value. In contrast to JSON, the JSONPath syntax allows strings to be enclosed in _single_ or _double_ quotes. quoted-member-name = string-literal string-literal = %x22 *double-quoted %x22 / ; "string" %x27 *single-quoted %x27 ; 'string' double-quoted = unescaped / %x27 / ; ' ESC %x22 / ; \" ESC escapable single-quoted = unescaped / %x22 / ; " ESC %x27 / ; \' ESC escapable ESC = %x5C ; \ backslash unescaped = %x20-21 / ; s. RFC 8259 %x23-26 / ; omit " %x28-5B / ; omit ' %x5D-10FFFF ; omit \ escapable = ( %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t ; b / ; BS backspace U+0008 ; t / ; HT horizontal tab U+0009 ; n / ; LF line feed U+000A ; f / ; FF form feed U+000C ; r / ; CR carriage return U+000D "/" / ; / slash (solidus) "\" / ; \ backslash (reverse solidus) (%x75 hexchar) ; uXXXX U+XXXX ) hexchar = non-surrogate / (high-surrogate "\" %x75 low-surrogate) non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" %x30-37 2HEXDIG ) high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" ; Task from 2021-06-15 interim: update ABNF later Gössner, et al. Expires 20 July 2022 [Page 14] Internet-Draft JSONPath January 2022 Applying the index-selector to an array, a numerical element-index is required to select the corresponding element. JSONPath allows it to be negative (see Section "Semantics"). element-index = int ; decimal integer int = ["-"] ( "0" / (DIGIT1 *DIGIT) ) ; - optional DIGIT1 = %x31-39 ; 1-9 non-zero digit Notes: 1. double-quoted strings follow the JSON string syntax (Section 7 of [RFC8259]); single-quoted strings follow an analogous pattern (Section "Syntax"). 2. An element-index is an integer (in base 10, as in JSON numbers). 3. As in JSON numbers, the syntax does not allow octal-like integers with leading zeros such as 01 or -01. Semantics A quoted-member-name string MUST be converted to a member name by removing the surrounding quotes and replacing each escape sequence with its equivalent Unicode character, as in the table below: +=================+===================+=============================+ | Escape Sequence | Unicode Character | Description | +=================+===================+=============================+ | \b | U+0008 | BS backspace | +-----------------+-------------------+-----------------------------+ | \t | U+0009 | HT horizontal tab | +-----------------+-------------------+-----------------------------+ | \n | U+000A | LF line feed | +-----------------+-------------------+-----------------------------+ | \f | U+000C | FF form feed | +-----------------+-------------------+-----------------------------+ | \r | U+000D | CR carriage return | +-----------------+-------------------+-----------------------------+ | \" | U+0022 | quotation mark | +-----------------+-------------------+-----------------------------+ | \' | U+0027 | apostrophe | +-----------------+-------------------+-----------------------------+ | \/ | U+002F | slash (solidus) | +-----------------+-------------------+-----------------------------+ | \\ | U+005C | backslash (reverse | | | | solidus) | +-----------------+-------------------+-----------------------------+ | \uXXXX | U+XXXX | unicode character | +-----------------+-------------------+-----------------------------+ Table 3: Escape Sequence Replacements Gössner, et al. Expires 20 July 2022 [Page 15] Internet-Draft JSONPath January 2022 The index-selector applied with a quoted-member-name to an object selects the node of the corresponding member value from it, if and only if that object has a member with that name. Nothing is selected from a value which is not a object. Array indexing via element-index is a way of selecting a particular array element using a zero-based index. For example, selector [0] selects the first and selector [4] the fifth element of a sufficiently long array. A negative element-index counts from the array end. For example, selector [-1] selects the last and selector [-2] selects the penultimate element of an array with at least two elements. As with non-negative indexes, it is not an error if such an element does not exist; this simply means that no element is selected. 3.5.5. Index Wild Card Selector Syntax The index wild card selector has the form [*]. index-wild-selector = "[" "*" "]" ; asterisk enclosed by brackets Semantics An index-wild-selector selects the nodes of all member values of an object as well as of all elements of an array. Applying the index- wild-selector to a primitive JSON value (such as a number, string, or true/false/null) selects no node. The index-wild-selector behaves identically to the dot-wild-selector. 3.5.6. Array Slice Selector Syntax The array slice selector has the form [<start>:<end>:<step>]. It selects elements starting at index <start>, ending at -- but not including -- <end>, while incrementing by step. Gössner, et al. Expires 20 July 2022 [Page 16] Internet-Draft JSONPath January 2022 slice-selector = "[" S [start S] ":" S [end S] [":" S [step S]] "]" start = int ; included in selection end = int ; not included in selection step = int ; default: 1 B = %x20 / ; Space %x09 / ; Horizontal tab %x0A / ; Line feed or New line %x0D ; Carriage return S = *B ; optional blank space RS = 1*B ; required blank space The slice-selector consists of three optional decimal integers separated by colons. Semantics The slice-selector was inspired by the slice operator of ECMAScript 4 (ES4), which was deprecated in 2014, and that of Python. Informal Introduction This section is non-normative. Array indexing is a way of selecting a particular element of an array using a 0-based index. For example, the expression [0] selects the first element of a non-empty array. Negative indices index from the end of an array. For example, the expression [-2] selects the last but one element of an array with at least two elements. Array slicing is inspired by the behaviour of the Array.prototype.slice method of the JavaScript language as defined by the ECMA-262 standard [ECMA-262], with the addition of the step parameter, which is inspired by the Python slice expression. The array slice expression [start:end:step] selects elements at indices starting at start, incrementing by step, and ending with end (which is itself excluded). So, for example, the expression [1:3] (where step defaults to 1) selects elements with indices 1 and 2 (in that order) whereas [1:5:2] selects elements with indices 1 and 3. When step is negative, elements are selected in reverse order. Thus, for example, [5:1:-2] selects elements with indices 5 and 3, in that order and [::-1] selects all the elements of an array in reverse order. Gössner, et al. Expires 20 July 2022 [Page 17] Internet-Draft JSONPath January 2022 When step is 0, no elements are selected. (This is the one case which differs from the behaviour of Python, which raises an error in this case.) The following section specifies the behaviour fully, without depending on JavaScript or Python behaviour. Detailed Semantics An array selector is either an array slice or an array index, which is defined in terms of an array slice. A slice expression selects a subset of the elements of the input array, in the same order as the array or the reverse order, depending on the sign of the step parameter. It selects no nodes from a node which is not an array. A slice is defined by the two slice parameters, start and end, and an iteration delta, step. Each of these parameters is optional. len is the length of the input array. The default value for step is 1. The default values for start and end depend on the sign of step, as follows: +===========+=========+==========+ | Condition | start | end | +===========+=========+==========+ | step >= 0 | 0 | len | +-----------+---------+----------+ | step < 0 | len - 1 | -len - 1 | +-----------+---------+----------+ Table 4: Default array slice start and end values Slice expression parameters start and end are not directly usable as slice bounds and must first be normalized. Normalization for this purpose is defined as: FUNCTION Normalize(i, len): IF i >= 0 THEN RETURN i ELSE RETURN len + i END IF Gössner, et al. Expires 20 July 2022 [Page 18] Internet-Draft JSONPath January 2022 The result of the array indexing expression [i] applied to an array of length len is defined to be the result of the array slicing expression [i:Normalize(i, len)+1:1]. Slice expression parameters start and end are used to derive slice bounds lower and upper. The direction of the iteration, defined by the sign of step, determines which of the parameters is the lower bound and which is the upper bound: FUNCTION Bounds(start, end, step, len): n_start = Normalize(start, len) n_end = Normalize(end, len) IF step >= 0 THEN lower = MIN(MAX(n_start, 0), len) upper = MIN(MAX(n_end, 0), len) ELSE upper = MIN(MAX(n_start, -1), len-1) lower = MIN(MAX(n_end, -1), len-1) END IF RETURN (lower, upper) The slice expression selects elements with indices between the lower and upper bounds. In the following pseudocode, the a(i) construct expresses the 0-based indexing operation on the underlying array. IF step > 0 THEN i = lower WHILE i < upper: SELECT a(i) i = i + step END WHILE ELSE if step < 0 THEN i = upper WHILE lower < i: SELECT a(i) i = i + step END WHILE END IF When step = 0, no elements are selected and the result array is empty. Gössner, et al. Expires 20 July 2022 [Page 19] Internet-Draft JSONPath January 2022 To be valid, the slice expression parameters MUST be in the I-JSON range of exact values, see Section 3.1. 3.5.7. Descendant Selector Syntax The descendant selector starts with a double dot .. and can be followed by an object member name (similar to the dot-selector), by an index-selector acting on objects or arrays, or by a wild card. descendant-selector = ".." ( dot-member-name / ; ..<name> index-selector / ; ..[<index>] index-wild-selector / ; ..[*] "*" ; ..* ) Semantics The descendant-selector selects the node and all its descendants. In the resultant nodelist: * nodes occur before their children, and * nodes of an array occur in array order. Children of an object may occur in any order, since JSON objects are unordered. 3.5.8. Union Selector Syntax The union selector is syntactically related to the index-selector. It contains two or more entries, separated by commas. union-selector = "[" S union-entry 1*(S "," S union-entry) S "]" union-entry = ( quoted-member-name / element-index / slice-index ) Semantics A union selects any node which is selected by at least one of the union selectors and selects the concatenation of the lists (in the order of the selectors) of nodes selected by the union elements. Note that any node selected in more than one of the union selectors is kept as many times in the node list. Gössner, et al. Expires 20 July 2022 [Page 20] Internet-Draft JSONPath January 2022 To be valid, integer values in the element-index and slice-index components MUST be in the I-JSON range of exact values, see Section 3.1. 3.5.9. Filter Selector Syntax The filter selector has the form [?<expr>]. It works via iterating over structured values, i.e. arrays and objects. filter-selector = "[" S "?" S boolean-expr S "]" During iteration process each array element or object member is visited and its value -- accessible via symbol @ -- or one of its descendants -- uniquely defined by a relative path -- is tested against a boolean expression boolean-expr. The current item is selected if and only if the result is true. Gössner, et al. Expires 20 July 2022 [Page 21] Internet-Draft JSONPath January 2022 boolean-expr = logical-or-expr logical-or-expr = logical-and-expr *(S "||" S logical-and-expr) ; disjunction ; binds less tightly than conjunction logical-and-expr = basic-expr *(S "&&" S basic-expr) ; conjunction ; binds more tightly than disjunction basic-expr = exist-expr / paren-expr / relation-expr exist-expr = [neg-op S] path ; path existence or non-existence path = rel-path / json-path rel-path = "@" *(S (dot-selector / index-selector)) paren-expr = [neg-op S] "(" S boolean-expr S ")" ; parenthesized expression neg-op = "!" ; not operator relation-expr = comp-expr / ; comparison test regex-expr ; regular expression test comp-expr = comparable S comp-op S comparable comparable = number / string-literal / ; primitive ... true / false / null / ; values only path ; path value comp-op = "==" / "!=" / ; comparison ... "<" / ">" / ; operators "<=" / ">=" number = int [ frac ] [ exp ] ; decimal number frac = "." 1*DIGIT ; decimal fraction exp = "e" [ "-" / "+" ] 1*DIGIT ; decimal exponent true = %x74.72.75.65 ; true false = %x66.61.6c.73.65 ; false null = %x6e.75.6c.6c ; null regex-expr = (path / string-literal) S regex-op S regex regex-op = "=~" ; regular expression match regex = <TO BE DEFINED> Notes: * Parentheses can be used with boolean-expr for grouping. So filter selection syntax in the original proposal [?(<expr>)] is naturally contained in the current lean syntax [?<expr>] as a special case. * Comparisons are restricted to primitive values (such as number, string, true, false, null). Comparisons with complex values will fail, i.e. no selection occurs. Gössner, et al. Expires 20 July 2022 [Page 22] Internet-Draft JSONPath January 2022 * Types are not implicitly converted in comparisons. So "13 == '13'" selects no node. * A member or element value by itself in a Boolean context is interpreted as false only if it does not exist. Otherwise it is interpreted as true. To be more specific about the actual value, explicit comparisons are necessary. This existence test -- as an exception to the general rule -- also works with structured values. * Regular expression tests can be applied to string values only. * Alphabetic characters in ABNF are case-insensitive, so "e" can be either "e" or "E". * false, null, true are lower-case only (case-sensitive). The following table lists filter expression operators in order of precedence from highest (binds most tightly) to lowest (binds least tightly). +============+===========+===========+ | Precedence | Operator | Syntax | | | type | | +============+===========+===========+ | 5 | Grouping | (...) | +------------+-----------+-----------+ | 4 | Logical | ! | | | NOT | | +------------+-----------+-----------+ | 3 | Relations | == != | | | | < <= > >= | | | | =~ | | | | in | +------------+-----------+-----------+ | 2 | Logical | && | | | AND | | +------------+-----------+-----------+ | 1 | Logical | || | | | OR | | +------------+-----------+-----------+ Table 5: Filter expression operator precedence Gössner, et al. Expires 20 July 2022 [Page 23] Internet-Draft JSONPath January 2022 Semantics The filter-selector works with arrays and objects exclusively. Its result might be a list of _zero_, _one_, _multiple_ or _all_ of their element or member values then. Applied to other value types, it will select nothing. Some examples: +===================+=====================+===============+=========+ |JSON | Query | Result |Comment | +===================+=====================+===============+=========+ |{"a":1,"b":2} | $[?@] | [1,2] |Same as | |[2,3,4] | | [2,3,4] |$.* or | | | | |$[*] | +-------------------+---------------------+---------------+---------+ |./. | $[?@==2] | [2] |Select by| | | | [2] |value. | +-------------------+---------------------+---------------+---------+ |{"a":{"b":{"c":{}}}| $[?@.b] |[{"b":{"c":{}}]|Existence| | | $[?@.b.c] | | | +-------------------+---------------------+---------------+---------+ |{"key":false} | $[?index(@)=='key'] | [false] |Select | | | $[?index(@)==0] | [] |object | | | | |member | +-------------------+---------------------+---------------+---------+ |[3,4,5] | $[?index(@)==2] | [5] |Select | | | $[?index(@)==17] | [] |array | | | | |element | +-------------------+---------------------+---------------+---------+ |{"a":{"b":{5},c:0}}| $[?@.b==5 && !@.c] |[{"b":{5},c:0}]|Existence| +-------------------+---------------------+---------------+---------+ Table 6 4. IANA Considerations TBD: Define a media type for JSONPath expressions. 5. Security Considerations This section gives security considerations, as required by [RFC3552]. 6. References 6.1. Normative References Gössner, et al. Expires 20 July 2022 [Page 24] Internet-Draft JSONPath January 2022 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, <https://www.rfc-editor.org/info/rfc7493>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/info/rfc8259>. 6.2. Informative References [E4X] ISO, "Information technology — ECMAScript for XML (E4X) specification", ISO/IEC 22537:2006 , 2006. [ECMA-262] Ecma International, "ECMAScript Language Specification, Standard ECMA-262, Third Edition", December 1999, <http://www.ecma-international.org/publications/files/ ECMA-ST-ARCH/ECMA- 262,%203rd%20edition,%20December%201999.pdf>. [JSONPath-orig] Gössner, S., "JSONPath — XPath for JSON", 21 February 2007, <https://goessner.net/articles/JsonPath/>. [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, <https://www.rfc-editor.org/info/rfc3552>. Gössner, et al. Expires 20 July 2022 [Page 25] Internet-Draft JSONPath January 2022 [RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed., "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013, <https://www.rfc-editor.org/info/rfc6901>. [SLICE] "Slice notation", n.d., <https://github.com/tc39/proposal-slice-notation>. [XPath] Berglund, A., Boag, S., Chamberlin, D., Fernandez, M., Kay, M., Robie, J., and J. Simeon, "XML Path Language (XPath) 2.0 (Second Edition)", World Wide Web Consortium Recommendation REC-xpath20-20101214, 14 December 2010, <https://www.w3.org/TR/2010/REC-xpath20-20101214>. Appendix A. Inspired by XPath This appendix is informative. At the time JSONPath was invented, XML was noted for the availability of powerful tools to analyse, transform and selectively extract data from XML documents. [XPath] is one of these tools. In 2007, the need for something solving the same class of problems for the emerging JSON community became apparent, specifically for: * Finding data interactively and extracting them out of [RFC8259] JSON values without special scripting. * Specifying the relevant parts of the JSON data in a request by a client, so the server can reduce the amount of data in its response, minimizing bandwidth usage. (Note that XPath has evolved since 2007, and recent versions even nominally support operating inside JSON values. This appendix only discusses the more widely used version of XPath that was available in 2007.) JSONPath picks up the overall feeling of XPath, but maps the concepts to syntax (and partially semantics) that would be familiar to someone using JSON in a dynamic language. E.g., in popular dynamic programming languages such as JavaScript, Python and PHP, the semantics of the XPath expression /store/book[1]/title can be realized in the expression Gössner, et al. Expires 20 July 2022 [Page 26] Internet-Draft JSONPath January 2022 x.store.book[0].title or, in bracket notation, x['store']['book'][0]['title'] with the variable x holding the argument. The JSONPath language was designed to: * be naturally based on those language characteristics; * cover only the most essential parts of XPath 1.0; * be lightweight in code size and memory consumption; * be runtime efficient. A.1. JSONPath and XPath JSONPath expressions apply to JSON values in the same way as XPath expressions are used in combination with an XML document. JSONPath uses $ to refer to the root node of the argument, similar to XPath's / at the front. JSONPath expressions move further down the hierarchy using _dot notation_ ($.store.book[0].title) or the _bracket notation_ ($['store']['book'][0]['title']), a lightweight/limited, and a more heavyweight syntax replacing XPath's / within query expressions. Both JSONPath and XPath use * for a wildcard. The descendant operator .., borrowed from [E4X], is similar to XPath's //. The array slicing construct [start:end:step] is unique to JSONPath, inspired by [SLICE] from ECMASCRIPT 4. Filter expressions are supported via the syntax ?(<boolean expr>) as in $.store.book[?(@.price < 10)].title Table 7 extends Table 1 by providing a comparison with similar XPath concepts. Gössner, et al. Expires 20 July 2022 [Page 27] Internet-Draft JSONPath January 2022 +==========+==================+=================================+ | XPath | JSONPath | Description | +==========+==================+=================================+ | / | $ | the root XML element | +----------+------------------+---------------------------------+ | . | @ | the current XML element | +----------+------------------+---------------------------------+ | / | . or [] | child operator | +----------+------------------+---------------------------------+ | .. | n/a | parent operator | +----------+------------------+---------------------------------+ | // | .. | nested descendants (JSONPath | | | | borrows this syntax from E4X) | +----------+------------------+---------------------------------+ | * | * | wildcard: All XML elements | | | | regardless of their names | +----------+------------------+---------------------------------+ | @ | n/a | attribute access: JSON values | | | | do not have attributes | +----------+------------------+---------------------------------+ | [] | [] | subscript operator used to | | | | iterate over XML element | | | | collections and for predicates | +----------+------------------+---------------------------------+ | | | [,] | Union operator (results in a | | | | combination of node sets); | | | | JSONPath allows alternate names | | | | or array indices as a set | +----------+------------------+---------------------------------+ | n/a | [start:end:step] | array slice operator borrowed | | | | from ES4 | +----------+------------------+---------------------------------+ | [] | ?() | applies a filter (script) | | | | expression | +----------+------------------+---------------------------------+ | seamless | n/a | expression engine | +----------+------------------+---------------------------------+ | () | n/a | grouping | +----------+------------------+---------------------------------+ Table 7: XPath syntax compared to JSONPath For further illustration, Table 8 shows some XPath expressions and their JSONPath equivalents. Gössner, et al. Expires 20 July 2022 [Page 28] Internet-Draft JSONPath January 2022 +======================+========================+===================+ | XPath | JSONPath | Result | +======================+========================+===================+ | /store/book/author | $.store.book[*].author | the authors of | | | | all books in | | | | the store | +----------------------+------------------------+-------------------+ | //author | $..author | all authors | +----------------------+------------------------+-------------------+ | /store/* | $.store.* | all things in | | | | store, which | | | | are some books | | | | and a red | | | | bicycle | +----------------------+------------------------+-------------------+ | /store//price | $.store..price | the prices of | | | | everything in | | | | the store | +----------------------+------------------------+-------------------+ | //book[3] | $..book[2] | the third book | +----------------------+------------------------+-------------------+ | //book[last()] | $..book[(@.length-1)] | the last book | | | $..book[-1] | in order | +----------------------+------------------------+-------------------+ | //book[position()<3] | $..book[0,1] | the first two | | | $..book[:2] | books | +----------------------+------------------------+-------------------+ | //book[isbn] | $..book[?(@.isbn)] | filter all | | | | books with isbn | | | | number | +----------------------+------------------------+-------------------+ | //book[price<10] | $..book[?(@.price<10)] | filter all | | | | books cheaper | | | | than 10 | +----------------------+------------------------+-------------------+ | //* | $..* | all elements in | | | | XML document; | | | | all member | | | | values and | | | | array elements | | | | contained in | | | | input value | +----------------------+------------------------+-------------------+ Table 8: Example XPath expressions and their JSONPath equivalents Gössner, et al. Expires 20 July 2022 [Page 29] Internet-Draft JSONPath January 2022 XPath has a lot more functionality (location paths in unabbreviated syntax, operators and functions) than listed in this comparison. Moreover, there are significant differences in how the subscript operator works in XPath and JSONPath: * Square brackets in XPath expressions always operate on the _node set_ resulting from the previous path fragment. Indices always start at 1. * With JSONPath, square brackets operate on the _object_ or _array_ addressed by the previous path fragment. Array indices always start at 0. Acknowledgements This specification is based on Stefan Gössner's original online article defining JSONPath [JSONPath-orig]. The books example was taken from http://coli.lili.uni- bielefeld.de/~andreas/Seminare/sommer02/books.xml -- a dead link now. Contributors Marko Mikulicic InfluxData, Inc. Pisa Italy Email: mmikulicic@gmail.com Edward Surov TheSoul Publishing Ltd. Limassol Cyprus Email: esurov.tsp@gmail.com Authors' Addresses Stefan Gössner (editor) Fachhochschule Dortmund Sonnenstraße 96 D-44139 Dortmund Germany Email: stefan.goessner@fh-dortmund.de Gössner, et al. Expires 20 July 2022 [Page 30] Internet-Draft JSONPath January 2022 Glyn Normington (editor) Winchester United Kingdom Email: glyn.normington@gmail.com Carsten Bormann (editor) Universität Bremen TZI Postfach 330440 D-28359 Bremen Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Gössner, et al. Expires 20 July 2022 [Page 31]