JSON Schema validator

A clear answer. Here’s how.

Checks a JSON document against a JSON Schema with a validator written by hand for a subset of draft 2020-12; nothing is generated or evaluated from your schema. Enforced keywords: type (including lists of types, and 1.0 counts as an integer), enum, const, properties, required, additionalProperties, patternProperties, propertyNames, minProperties, maxProperties, items, prefixItems, minItems, maxItems, uniqueItems, contains with minContains and maxContains, minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf, minLength and maxLength (counted in code points), pattern, allOf, anyOf, oneOf, not, if, then, else, dependentRequired, dependentSchemas, unevaluatedProperties, unevaluatedItems, and $ref, $dynamicRef, $defs, $id, $anchor and $dynamicAnchor within the pasted schema. Numbers keep their written digits and are compared as exact decimals, so 19.99 is a multiple of 0.01, 1 equals 1.0, and objects are equal whatever their key order. A pattern is run by a matcher that parses the expression itself and cannot backtrack catastrophically; it reads the subset the specification recommends plus escapes, classes and \p{…}, and a pattern that uses lookahead, lookbehind or a backreference is listed as not checked. Remote references are never fetched and are listed as not checked. Formats are checked only if you ask: date, date-time, time, email, hostname, ipv4, ipv6, uuid and uri (international host names with xn-- labels are checked only as ASCII, not against IDNA rules). Every keyword the tool does not enforce is listed in the table as Not checked: content keywords, leftovers from older drafts such as dependencies and additionalItems, formats it does not know, unknown keywords (often a spelling mistake) and a $schema that names another dialect; the headline then says the document is valid against the checked keywords instead of simply valid. The validator passes every test it is given from the draft 2020-12 JSON Schema Test Suite: 1,864 tests in 430 groups (the keyword files, the reference tests with the suite’s own remote documents supplied by the test, the nine optional format files and eleven optional files on numbers, regular expressions, ids and anchors). Not run: the two files and one reference group that validate against the meta-schema, the 38 internationalised host name tests, and two optional files about draft-07 keywords and custom meta-schemas. It does not check the schema against the meta-schema, so a wrong keyword value is reported only where this tool reads it. Limits: 1,000,000 characters per input, 100 levels of nesting, 6,000,000 evaluation steps (a schema whose branches fan out into the same definitions can multiply the work, and is stopped with a message), 1,000 errors collected and 200 shown.