function rescape( str ) {
	var len;
	var s;
	var i;

	if ( !isString( str ) ) {
		throw new TypeError( format( 'invalid argument. Must provide a regular expression string. Value: `%s`.', str ) );
	}
	// Check if the string starts with a forward slash...
	if ( str[ 0 ] === '/' ) {
		// Find the last forward slash...
		len = str.length;
		for ( i = len-1; i >= 0; i-- ) {
			if ( str[ i ] === '/' ) {
				break;
			}
		}
	}
	// If we searched the string to no avail or if the first letter is not `/`, assume that the string is not of the form `/[...]/[guimy]`:
	if ( i === void 0 || i <= 0 ) {
		return str.replace( RE_CHARS, '\\$&' );
	}
	// We need to de-construct the string...
	s = str.substring( 1, i );

	// Only escape the characters between the `/`:
	s = s.replace( RE_CHARS, '\\$&' );

	// Reassemble:
	str = str[ 0 ] + s + str.substring( i );

	return str;
}
Escapes a regular expression string.

@param {string} str - regular expression string
@throws {TypeError} first argument must be a string
@returns {string} escaped string

@example
var str = rescape( '[A-Z]*' );
// returns '\\[A\\-Z\\]\\*'