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lsjonschema: JSON schema validator

This library provides a JSON schema draft 4 validator for Lua. Note that even though it uses the JSON Schema semantics, it is neither bound or limited to JSON. It can be used to validate saner key/value data formats as well (Lua tables, msgpack, bencode, ...).

It has been designed to validate incoming data for HTTP APIs so it is decently fast: it works by transforming the given schema into a pure Lua function on-the-fly. Work is currently in progress to make it as JIT-friendly as possible.


This module is pure Lua and does not depend on any particular JSON library (cjson.null will be used for null tokens, but you can override that if necessary, see Advanced usage)

The preferred way to install this library is to use Luarocks:

luarocks install ljsonschema

Running the tests also requires the cjson library and the Telescope test runner:

git submodule update --init --recrusive
luarocks install net-url
luarocks install lua-cjson
luarocks install
tsc ./spec/suite.lua


Getting started

local jsonschema = require 'jsonschema'

-- Note: do cache the result of schema compilation as this is a quite
-- expensive process
local myvalidator = jsonschema.generate_validator {
  type = 'object',
  properties = {
    foo = { type = 'string' },
    bar = { type = 'number' },

print(myvalidator{ foo='hello', bar=42 })

Advanced usage

Some advanced features of JSON Schema are not possible to implement using the standard library and require third party libraries to be work.

In order to not force one particular library, and not bloat this library for the simple schemas, extension points are provided: the generate_validator takes a second table argument that can be used to customise the generated parser.

local v = jsonschema.generate_validator(schema, {
    -- a value used to check null elements in the validated documents
    -- defaults to `cjson.null` (if available) or `nil`
    null = null_token,

    -- function called to match patterns, defaults to string.find.
    -- The JSON schema specification mentions that the validator should obey
    -- the ECMA-262 specification but Lua pattern matching library is much more
    -- primitive than that. Users might want to use PCRE or other more powerful
    -- libraries here
    match_pattern = function(string, patt)
        return ... -- boolean value

    -- function called to resolve external schemas. It is called with the full
    -- url to fetch (without the fragment part) and must return the
    -- corresponding schema as a Lua table.
    -- There is no default implementation: this function must be provided if
    -- resolving external schemas is required.
    external_resolver = function(url)
        return ... -- Lua table

    -- name when generating the validator function, it might ease debugging as
    -- as it will appear in stack traces.
    name = "myschema",

Differences with JSONSchema

Due to the nature of the Lua language, the full JSON schema support is difficult to reach. Some of the limitations can be solved using the advanced options detailed previously, but some features are not supported (correctly) at this time:

  • Empty tables and empty arrays are the same from Lua point of view
  • Unicode strings are considered as a stream of bytes (so length checks might not behave as expected)

On the other hand, some extra features are supported:

  • The type table can be used to match arrays or objects, it is also much faster than array or object as it does not involve walking the table to find out if it's a sequence or a hash
  • The type function can be used to check for functions


Pure Lua JSON schema validator







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