code: function RegExpInitialize(object, pattern, flags) {
    pattern = IS_UNDEFINED(pattern) ? '' : TO_STRING(pattern);
    flags = IS_UNDEFINED(flags) ? '' : TO_STRING(flags);
    %RegExpInitializeAndCompile(object, pattern, flags);
    return object;
  }
docs: ES#sec-regexpinitialize Runtime Semantics: RegExpInitialize ( obj, pattern, flags )