WordPress Category Permastructure

category-permastructure.php

<?php  
function wpa85084_category_permastruct(){
     global $wp_rewrite;
     $wp_rewrite->extra_permastructs['category']['struct'] = '/blog/%category%/';
 }
 add_action( 'init', 'wpa85084_category_permastruct' );

 add_action( 'init', 'wpa58471_category_base' );
 function wpa58471_category_base() {
     // Remember to flush the rules once manually after you added this code!
     add_rewrite_rule(
         // The regex to match the incoming URL
         'blog/([^/]+)/([^/]+)/([^/]+)(/[0-9]+)?/?$',
         // The resulting internal URL
         'index.php?category_name=$matches[1]/$matches[2]&name=$matches[3]&paged=$matches[4]',
         // Add the rule to the top of the rewrite list
         'top' );
 }