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Lesson 2: String Validation

Schema Editor
Data Editor
Validation Result

Type in the editors to see validation results.

minLengthmaxLengthpatternformatenum

JSON Schema provides several keywords specifically for validating string values. "minLength" and "maxLength" set lower and upper bounds on the number of characters in the string. The "pattern" keyword accepts a regular expression and requires the string to match it — useful for enforcing formats like usernames or phone numbers. "format" checks a semantic format such as "email", "uri", or "date-time", while "enum" restricts the value to an explicit list of allowed strings.

Why This Matters

Strings are the most common primitive in real APIs — usernames, emails, IDs, status codes — and they're also the easiest place for silently-bad data to sneak in, since any string is technically a valid string. Bounding length prevents abuse (a 50,000-character "username"), "pattern" enforces exact shapes like slugs or ZIP codes, and "enum" turns a loosely-typed string field into a closed set of known values — effectively a lightweight, cross-language alternative to an enum type, which is especially valuable in JSON, a format with no native enum.

Common Mistakes

  • ✕ Assuming "format": "email" is always strictly enforced.Per the JSON Schema spec, "format" is technically an annotation unless a validator opts into assertion behavior — some validators (like plain Ajv without ajv-formats) ignore it entirely. This playground's validator does check email, uri, date, date-time, ipv4 and uuid as real assertions, but always confirm your production validator does the same.
  • ✕ Forgetting anchors (^ and $) in "pattern".JSON Schema's "pattern" matches if the regex is found anywhere in the string, not the whole string, unless you anchor it. "[a-z]+" matches "123abc456" too — use "^[a-z]+$" to require the entire string to match.
  • ✕ Using "enum" for a list of values that changes frequently (like a country's list of active promo codes).Reserve "enum" for genuinely fixed, small vocabularies (roles, statuses, currency codes). For dynamic sets, validate against a lookup table in application code instead of hardcoding it into the schema.
  • ✕ Counting bytes instead of characters when reasoning about minLength/maxLength.JSON Schema counts Unicode code points, not bytes — a string with emoji or multi-byte characters can hit maxLength sooner than a byte-counting mental model would predict.

Challenge

Change the username to 'ab' (too short) and see the error. Then fix it.

Frequently Asked Questions

Not necessarily — the spec defines format as an annotation by default, so enforcement depends on the validator. This lesson's playground does enforce common formats (email, uri, date, date-time, ipv4, uuid) as real assertions, but double-check the behavior of whichever validator library you use in production.

JSON Schema specifies ECMA-262 (JavaScript) regular expression syntax for "pattern", without the ^ and $ implicitly added — you must add your own anchors if you want a full-string match rather than a partial match.

Simply don't add it to the "required" array, but still define its schema under "properties". JSON Schema only validates a property's schema when that property is present in the data — omitting an optional field never fails validation.

Yes — "enum" accepts an array of any JSON values, including numbers, booleans, null, or a mix of types, e.g. "enum": [1, 2, 3] or "enum": ["low", "medium", "high", null].