struct_parser.ts

type StringFlags = "c" | "b" | "B";
type NumberFlags = "n"

type Resolve<T extends string> = (
  T extends StringFlags ? [string] :
  T extends NumberFlags ? [number] :
  [] // ignore stuff like ">"
);

type Repeat<
  T extends unknown[],
  Count extends string,
  Result extends unknown[] = [],
> = (
    `${Result["length"]}` extends Count
    ? Result
    : Repeat<T, Count, [...T, ...Result]>
)

type Parse<
  Input extends string,
  Result extends unknown[] = [],
  CurrentNumber extends string = "",
> = (
  Input extends ""
  ? Result
  : Input extends `${infer Digit extends number}${infer Rest}`
  ? Parse<Rest, Result, `${CurrentNumber}${Digit}`>
  : Input extends `${infer Next}${infer Rest}`
  ? CurrentNumber extends ""
    ? Parse<Rest, [...Result, ...Resolve<Next>]>
    : Parse<Rest, [...Result, ...Repeat<Resolve<Next>, CurrentNumber>]>
  : never
);

type Foo = Parse<">5c22n">