Variable PLAIN_MONTH_DAY_PATTERN`Const`

PLAIN_MONTH_DAY_PATTERN: "^(--)?(0[1-9]|1[0-2])-(0[1-9]|[12]\\d|3[01])$"

Regex pattern for Temporal.PlainMonthDay ISO 8601 strings (e.g. --01-15 or 01-15). Validates month (01–12) and day (01–31). The -- prefix is optional per ISO 8601.