search for registered users

I don’t quite understand. Do you mean the search should be only available for registered users? Yes, you can do that with Relevanssi, but it requires some custom programming.

thanks for replying,
I just want, to search by name of registered users on my site,
and also search by title and content of the post.

I’d like to allow searches for the names of registered users, too–so that visitors can search for blog posts by particular users.

Nhale and a77a77, searching for author names is a default feature, just enable it in the settings. If you want the search to give user profiles as results, you need Relevanssi Premium.