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