/**
 * Converts a string to snake_case.
 *
 * Replaces spaces, dashes, and camelCase transitions with underscores,
 * removes special characters, preserves embedded version tokens (like "v2"),
 * and lowercases the result.
 *
 * @example
 * ```ts
 * toSnakeCase("hello world"); // "hello_world"
 * ```
 *
 * @example
 * ```ts
 * toSnakeCase("UserProfileV2"); // "user_profile_v2"
 * ```
 *
 * @example
 * ```ts
 * toSnakeCase("HTML5 Parser"); // "html_5_parser"
 * ```
 *
 * @example
 * ```ts
 * toSnakeCase("  messy-input_string  "); // "messy_input_string"
 * ```
 *
 * @example
 * ```ts
 * toSnakeCase("ReportV3Final"); // "report_v3_final"
 * ```
 *
 * @param input A string to be converted to snake_case.
 * @returns The snake_cased version of the input.
 */
export declare function toSnakeCase(input: string): string;