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function search_filter($query) {
    // Categories
    if ( $query->is_search && ! is_admin() ) {
        $query->set('cat','8,15'); 
    }
    // Posts
    if ($query->is_search) {
        $query->set('post_type', 'post');
    }
    return $query; 
}
add_filter('pre_get_posts','search_filter');