/**
 * Masks a string by replacing characters with a masking character, leaving leading and trailing characters visible.
 *
 * @example
 * mask("1234567890123456", { start: 4, end: 4, char: "*" }) // "1234********3456"
 * mask("password123") // "p*********3"
 *
 * @param str The string to mask.
 * @param options Configuration for start/end unmasked count and mask char.
 * @returns Masked string.
 */
export declare function mask(str: string, options?: {
    start?: number;
    end?: number;
    char?: string;
}): string;