import { TReferenceProps } from '../../..';
import { IStringProps, TStringValidatorResult } from '../_types';
export interface IIsOptionalProps {
}
/**
 * Allow a `string` to be `undefined`.
 */
export declare const isOptional: (props?: TReferenceProps<IIsOptionalProps> & Omit<IStringProps, 'message'>) => TStringValidatorResult;