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JSON Content Rules Validator

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JSON Content Rules (JCR) is a language for specifying and testing the interchange of data in JSON format used by computer protocols and processes. The syntax of JCR is a superset of JSON possessing the conciseness and utility that has made JSON popular. It was created by the American Registry for Internet Numbers (ARIN) in an effort to better describe the JSON structures in protocols such as RDAP.

A First Example: Specifying Content

The following JSON data describes a JSON object with two members, "line-count" and "word-count", each containing an integer.

  { "line-count" : 3426, "word-count" : 27886 }

This is also JCR that describes a JSON object with a member named "line-count" that is an integer that is exactly 3426 and a member named "word-count" that is an integer that is exactly 27886.

For a protocol specification, it is probably more useful to specify that each member is any integer and not specific, exact integers:

  { "line-count" : integer, "word-count" : integer }

Since line counts and word counts should be either zero or a positive integer, the specification may be further narrowed:

  { "line-count" : 0.. , "word-count" : 0.. }

A Second Example: Testing Content

Building on the first example, this second example describes the same object but with the addition of another member, "file-name".

    "file-name"  : "rfc7159.txt",
    "line-count" : 3426,
    "word-count" : 27886

The following JCR describes objects like it.

    "file-name"  : string,
    "line-count" : 0..,
    "word-count" : 0..

For the purposes of writing a protocol specification, JCR may be broken down into named rules to reduce complexity and to enable re-use. The following example takes the JCR from above and rewrites the members as named rules.


  $fn = "file-name"  : string
  $lc = "line-count" : 0..
  $wc = "word-count" : 0..

With each member specified as a named rule, software testers can override them locally for specific test cases. In the following example, the named rules are locally overridden for the test case where the file name is "rfc4627.txt".

  $fn = "file-name"  : "rfc4627.txt"
  $lc = "line-count" : 2102
  $wc = "word-count" : 16714

In this example, the protocol specification describes the JSON object in general and an implementation overrides the rules for testing specific cases.

More Information on JCR

More information on JCR can be found at The current published specification is an IETF Internet Draft (I-D) versioned as -09, This software closely tracks the -09 version, which can be found here

Version History

The version history can be seen in the CHANGE LOG on the project wiki.

The current version of the JCR specification can be found here


This JCR Validator can be used by other Ruby code directly, or it may be invoked on the command line using the jcr command.

The command line utility can be given specific override rulesets for the purposes of local testing. If no root rule is given, it will test against all roots.

The library has all the features of the command line utility, and also has the ability to allow for custom validation of rules using Ruby code.


To install the JCR Validator:

gem install jcrvalidator

This code was written and tested on Ruby 2.1, 2.3, and 2.4.

Command Line Usage

You can find a bunch of command line examples in examples/

Here are some quick nibbles:

$ echo "[ 1, 2]" | bin/jcr -v -R "[ integer * ]"

$ echo "[ 1, 2]" | bin/jcr -v -R "[ string * ]"
Failure: ....

$ bin/jcr -v -r example.jcr example.json

$ bin/jcr -h


Evaluates JSON against JSON Content Rules (JCR).

If -J is not specified, JSON_FILES is used.
If JSON_FILES is not specified, standard input (STDIN) is used.

Use -v to see results, otherwise check the exit code.

    -r FILE                          file containing ruleset
    -R STRING                        string containing ruleset. Should probably be quoted
        --test-jcr                   parse and test the JCR only
        --process-parts [DIRECTORY]  creates smaller files for specification writing
    -S STRING                        name of root rule. All roots will be tried if none is specified
    -o FILE                          file containing overide ruleset (option can be repeated)
    -O STRING                        string containing overide rule (option can be repeated)
    -J STRING                        string containing JSON to evaluate. Should probably be quoted
    -v                               verbose
    -q                               quiet
    -h                               display help

Return codes:
 0 = success
 1 = bad JCR parsing or other bad condition
 2 = invalid option or bad use of command
 3 = unsuccessful evaluation of JSON

JCR Version 0.8.1

Usage as a Library

It is easy to call the JCR Validator from Ruby programs. The examples directory contains some good examples:

  • simple.rb is a simple and basic example
  • override.rb shows how to override specific rules in a ruleset.
  • callback.rb demonstrates how to do custom validation with callbacks
  • trace_failures.rb demonstrates how to access validation failure information

Custom Validation Using Callbacks

The callback.rb demonstrates the usage of custom code for evaluation of rules. There are a few important things to note about how callbacks work:

  1. The validator will first evaluate a rule with internal validation before calling the callback code. This means child rules are evaluated by the validators own internal logic before a callback is invoked, and also that a callback for a child rule is called before the callback for its parent.
  2. Depending on the internal evaluation, the callback is either invoked at the rule_eval_true or rule_eval_false methods.
  3. The callback can return a JCR::Evaluation object to signify if the evaluation passed or not.
  4. If the callback simply returns true, this is turned into a JCR::Evaluation signifying a passed evaluation.
  5. If the callback returns false or a string, this is turned into a JCR::Evaluation signifying a failed evaluation. In cases where a string is returned, the string is used as the reason for failing the evaluation.
  6. For validation of rules inside arrays and objects, a failed evaluation will usually result in the terminating the evaluation of the rest of the sibling rules of the containing array or object.

Using the --process-parts Option

The --process-parts option extracts parts of a JCR file into multiple files based on comments in the file. It can also create a new file without the comments. This is useful for rulesets going into specification documents where it is nice to break the rulesets up for illustrative purposes in the specification but to also have one JCR file for programmatic testing purposes.

The file parts are extracted using the comments

; start_part FILENAME


; end_part

The comments must also be the only thing present on the line though leading whitespace is allowed if desired.

To get a new file with all parts but these comments, use this

; all_parts FILENAME

The --process-parts parameter will also take an optional directory name where it will write the files.


Use bundler to install all the dependencies.

If you do not have bundler, it is simple to install:

$ gem install bundler

From there, tell bundler to go get the rest of the gems:

$ bundle install

To run the unit tests on Linux or Unix-like OS (including Windows Subsystem for Linux):

$ bundle exec rake test

To run the unit tests on native Windows (Ruby installed via RubyInstaller):

$ bundle exec rake win_test

Implementation Notes

The parslet produces a tree of hashes and arrays. The hashes are accessed using identifiers such as rule[:member_rule]. During validation, additional hash members may be added. To avoid conflict with names that maybe included future versions of the ABNF, all additional hash values start with an underscore. e.g. rule[:_is_choice].


JSON Content Rules Validator




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