Variable PLAIN_DATE_PATTERN`Const`

PLAIN_DATE_PATTERN: "^\\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\\d|3[01])$"

Regex pattern for Temporal.PlainDate ISO 8601 strings (e.g. 2023-01-15). Validates month (01–12) and day (01–31).