Search by word, category, tag, author

You’ll want to add some radio buttons inside your search form. Then add a filter to your search:

function filter_search( $query ) {
if( $query->is_search ) {
            if ( isset($_GET['tag']) )
            // alter your search query here.
}
return $query;
}
add_filter( 'pre_get_posts' , 'filter_search' );

Influenced by http://wordpress.org/support/topic/how-to-add-search-filter-by-custom-values#post-1463329