ES#sec-regexpinitialize Runtime Semantics: RegExpInitialize ( obj, pattern, flags )
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;
}