export interface MaskOptions {
    /** Number of leading characters to keep visible. Default: 1 */
    start?: number;
    /** Number of trailing characters to keep visible. Default: 1 */
    end?: number;
    /** Mask character. Default: "*" */
    char?: string;
}
/**
 * Masks the middle portion of a string with a fill character, preserving the
 * leading and trailing visible counts. Surrogate-pair safe.
 *
 * @param {string} string_ - Input string
 * @param {MaskOptions} [options] - Visible-character counts and mask char
 * @returns {string} Masked string
 * @example
 * mask("1234567890", { start: 2, end: 4 }); // "12****7890"
 * mask("secret"); // "s****t"
 */
export declare const mask: (string_: string, options?: MaskOptions) => string;