A string that is a valid ECMA-262 regular expression: it must compile, and every backslash escape of an ASCII letter must be one ECMA defines (\a, for example, is not a control escape and not a permitted identity escape).
A string that is a valid ECMA-262 regular expression: it must compile, and every backslash escape of an ASCII letter must be one ECMA defines (\a, for example, is not a control escape and not a permitted identity escape).