How to only return blog posts for the default WordPress search function.

search_posts_only.php

<?php
// Don't copy opening PHP tag above
// Add below code to your theme's functions.php file.

// Only show blog posts in search results
add_filter( 'pre_get_posts','wsm_post_only_search_filter' );
function wsm_post_only_search_filter( $query ) {
	if ( $query->is_search ) {
		$query->set( 'post_type', 'post' );
	}
return $query;
}