>> This works surprisingly well.  Maybe there are more corner cases,
>> but something already works with quick tests:
>>
>> diff --git a/lisp/dired-aux.el b/lisp/dired-aux.el
>
> Yes, not bad indeed.  Did you find any corner cases?

Using a temporary buffer like you proposed works fine,
and handles all possible regexps including corner cases.
However, wouldn't this make the search too inefficient?