Wordpress search only in specific categories.

wordpress-search-only-in-category.php

<?php
wp_reset_query();
//http://wordpress.org/support/topic/search-one-category-only
global $wp_query;

$category_sub_1 = get_category_by_path('main-category/sub-category-1');
$category_sub_2 = get_category_by_path('main-category/sub-category-2');

if(is_search()):
	$cat = array(
		'category__in' => array(
			$category_sub_1->cat_ID,
			$category_sub_2->cat_ID,
		),
	);

	$args = array_merge($wp_query->query, $cat);
	query_posts($args);
endif;