Check value against the named format. Returns true for unknown formats.
RFC 3339 full-date with real calendar checking.
RFC 3339 date-time ("date" "T" "time").
ISO 8601 duration as defined by RFC 3339 appendix A.
RFC 5321 mailbox, without the obsolete forms: dot-string or quoted-string local part, hostname or address-literal domain.
RFC 1123 hostname (255 octets total, labels of 1-63 alnum/hyphen).
Dotted-quad IPv4 (no leading zeros, each octet 0-255).
RFC 4291 IPv6 text form, including :: compression and an embedded IPv4 tail.
RFC 6901 JSON Pointer.
A string that is a valid ECMA-262 regular expression: it must compile, and every backslash escape of an ASCII letter must be one ECMA defines (\a, for example, is not a control escape and not a permitted identity escape).
Relative JSON Pointer (draft-handrews-relative-json-pointer).
RFC 3339 full-time. A leap second (ss == 60) is accepted only when the time corresponds to 23:59:60 UTC after offset adjustment.
RFC 3986 URI (absolute, scheme required) or URI reference.
RFC 6570 URI Template (syntax check of literals and expressions).
RFC 4122 UUID (8-4-4-4-12 hex digits).
Implementations of the JSON Schema 2020-12 defined formats, used when format assertion is enabled (FormatMode.assertion or a dialect declaring the format-assertion vocabulary). Formats apply only to strings; unknown format names always pass (they are annotations for some other consumer).
Not implemented (always pass, documented in the README): idn-email, idn-hostname, iri, iri-reference.