type Digit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';

type ExpiryDate<E extends string> = NumberString<E> | '20380119';

type NumberString<T extends string, V = T> = T extends Digit ? V : T extends `${Digit}${infer R}` ? NumberString<R, V> : `0`;

export type { Digit as D, ExpiryDate as E, NumberString as N };