At the moment one is able to limit the advanced search in the Moodle forums to the subject of a post.
However, this will also list all of the replies to a certain topic as most people don't change the subject line when replying to a post.
It would be helpful to be able to limit your search to only the first post of such a thread, that is when someone introduces a new discussion topic.
I just thought I found a clever workaround by putting "re:" in the field "These words should NOT be included", but the result wasn't as expected (see screen shots). It seems that you can't combine both restrictions. (If that's the case, that would be another issue.)