String formats

For type: string, the format keyword applies additional checks for common serialized forms. Formats follow JSON Schema conventions (RFC 3339 dates and times, ISO 8601 duration, etc.).

Combine format with other string keywords such as minLength, pattern, and maxLength; all applicable constraints must pass.

Supported formats

format value	Validates
date	Full-date per RFC 3339
date-time	Date-time per RFC 3339
time	Partial-time with optional timezone per RFC 3339
duration	ISO 8601 duration
email	Common email address shape
hostname	RFC 1123 hostname
ipv4	IPv4 address
ipv6	IPv6 address
uri	Absolute URI reference
uuid	UUID string
json-pointer	JSON Pointer (RFC 6901)
regex	A valid regular expression string

Examples

Date (RFC 3339 full-date)

# Schema
type: string
format: date

Valid: "2024-01-15", "2000-02-29"
Invalid: "not-a-date", invalid months/days.

Date-time

type: string
format: date-time

Valid: "2024-01-15T12:00:00Z", with offsets or fractional seconds.
Invalid: a bare date without time where a full date-time is required.

Time

type: string
format: time

Valid: "12:00:00Z", "23:59:59+00:00". Invalid hour ranges, malformed strings.

Duration (ISO 8601)

type: string
format: duration

Valid: "P1Y2M3D", "PT1H30M". Invalid: "P" alone, arbitrary text.

Email, hostname, IPv4, IPv6, URI, UUID follow the usual shapes (e.g. "user@example.com" for email, "::1" for ipv6).

JSON Pointer

type: string
format: json-pointer

Valid: "", "/foo/bar", "/foo/0". Invalid: missing leading / where required.

Regex

The value must be a syntactically valid regular expression string (as used by the implementation), not arbitrary text.

Combining format with other keywords

Other string constraints apply in addition to format. For example, format: email with minLength: 10 rejects short but syntactically valid local parts.

type: string
format: email
minLength: 10

Unknown formats (annotation-only)

If format names a value that the implementation does not recognize, it is treated as an annotation only: validation does not fail based on that keyword. This allows custom format labels for tools while keeping instances valid.

type: string
format: my-custom-format

Any string value is accepted for validation purposes.

See also

• Types - String type for pattern, minLength, and maxLength.