Regex-validated strings in TypeScript

regex-validated-strings.ts

const isDevMode = () => true;

type ValidatedString<T extends string> = string & {
    [K in keyof string] : K extends `as${string}String` ? never : string[K]
} & {
   readonly __format: T
};

/*
eslint-disable
@typescript-eslint/no-explicit-any, @typescript-eslint/ban-types,
@typescript-eslint/unbound-method, @typescript-eslint/no-unsafe-assignment,
@typescript-eslint/no-unsafe-member-access
*/
function addStringCheckMethod(
    checkMethod: keyof string & `as${string}String`,
    regExp: RegExp) {

    (String.prototype as any)[checkMethod] = String.prototype.valueOf;
    if (isDevMode()) {
        (String.prototype as any)[checkMethod] = function (this: String) {
            if (regExp.test(this as string)) {
                return this.valueOf();
            }
            throw Error(`'${this}' failed the ${checkMethod} check`);
        };
    }
}
/* eslint-enable */

/** UTC date string (2000-12-31T12:00:00.000Z) */
type UtcDateString = ValidatedString<'UTC Date'>;
declare interface String { asUtcDateString(): UtcDateString; }
addStringCheckMethod('asUtcDateString', /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:.\d{1,3})?Z$/);

/** Local date string (2000-12-31T08:00:00.000) */
type LocalDateString = ValidatedString<'Local Date'>;
declare interface String { asLocalDateString(): LocalDateString; }
addStringCheckMethod('asLocalDateString', /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:.\d{1,3})?$/);

/** HEX color string (#BB3355) */
type HexColorString = ValidatedString<'Hex Color'>;
declare interface String { asHexColorString(): HexColorString; }
addStringCheckMethod('asHexColorString', /^#[0-9a-f]{6}$/i);

// type: HexColorString extends string
let color = '#bbccdd'.asHexColorString();

// type: UtcDateString extends string
let utcDate = '2000-12-31T12:00:00.000Z'.asUtcDateString();

// type: LocalDateString extends string
let localDate = '2000-12-31T08:00:00.000'.asLocalDateString();

const s: string = localDate;

// tsc error: Type '"Local Date"' is not assignable to type '"UTC Date"'
utcDate = localDate;