/**
 * @file Spec-compliant fallback for the TC39 `RegExp.escape`
 *   (https://tc39.es/ecma262/#sec-regexp.escape). Implements
 *   `escapeRegExpFallback` plus the `isSpecHexEscapeCp` classifier used by both
 *   the fallback and downstream callers. The fallback handles every branch of
 *   the spec `EncodeForRegExpEscape` algorithm: leading [0-9A-Za-z] are always
 *   `\xHH`-escaped (guards against `\0..\9` / `\c` merging in a larger
 *   pattern), syntax characters (and `/`) get a plain backslash prefix, the
 *   `ControlEscape` set maps to its `\t \n \v \f \r` names, the other
 *   punctuators and the whitespace / line-terminator / lone-surrogate code
 *   points route through `\xHH` or `\uHHHH`. Every other code point is emitted
 *   verbatim.
 */
export declare function escapeRegExpFallback(str: string): string;
export declare function isSpecHexEscapeCp(cp: number): boolean;
