/**
 * Returns the singular form of a plural word.
 *
 * Handles common English pluralization patterns such as:
 * - ies → y
 * - es → base (for s, x, z, ch, sh endings)
 * - s → base (general fallback)
 * - known irregulars (e.g., children → child)
 *
 * Preserves the casing of the original input, including acronyms or title case.
 *
 * @example
 * ```ts
 * toSingular("cities"); // "city"
 * ```
 *
 * @example
 * ```ts
 * toSingular("boxes"); // "box"
 * ```
 *
 * @example
 * ```ts
 * toSingular("children"); // "child"
 * ```
 *
 * @example
 * ```ts
 * toSingular("teeth"); // "tooth"
 * ```
 *
 * @example
 * ```ts
 * toSingular("dogs"); // "dog"
 * ```
 *
 * @example
 * ```ts
 * toSingular("BUZZES"); // "BUZZ"
 * ```
 *
 * @example
 * ```ts
 * toSingular("IDs"); // "ID"
 * ```
 *
 * @param word Plural word to be converted to singular.
 * @returns The singular form of the input word.
 */
export declare function toSingular(word: string): string;