/**
 * Replaces newlines with HTML line breaks.
 *
 * @param {string} str The string to convert.
 * @param {boolean} [is_xhtml=false] Whether to output XHTML-style line breaks.
 * @returns {string} The string with newlines replaced with HTML line breaks.
 */
export declare function nl2br(str: string, is_xhtml?: boolean): string;