/**
 * Converts a string to title case.
 *
 * Capitalizes the first letter of each word, except for small words
 * (like "of", "and", "the") unless they appear at the beginning.
 * Preserves words already containing uppercase letters (e.g., acronyms).
 *
 * @example
 * ```ts
 * toTitleCase("the quick brown fox"); // "The Quick Brown Fox"
 * ```
 *
 * @example
 * ```ts
 * toTitleCase("a tale of two cities"); // "A Tale of Two Cities"
 * ```
 *
 * @example
 * ```ts
 * toTitleCase("in the heart of the sea"); // "In the Heart of the Sea"
 * ```
 *
 * @example
 * ```ts
 * toTitleCase("API reference guide"); // "API Reference Guide"
 * ```
 *
 * @example
 * ```ts
 * toTitleCase("   war and peace   "); // "War and Peace"
 * ```
 *
 * @param input The string to be converted to title case.
 * @returns The input string formatted in title case.
 */
export declare function toTitleCase(input: string): string;