generate_rewrite_rules didn’t work for me at all and I found posts on forums where others were having the same issue. Adding to the rewrite_rules_array did work. Below is the solution.
add_filter( 'post_link', 'custom_permalink', 10, 3 );
add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_filter('init','flushRules');
function custom_permalink( $permalink, $post, $leavename ) {
$category = get_the_category($post->ID);
if ( !empty($category) && $category[0]->cat_name == "Shop" )
{
$permalink = trailingslashit( home_url('shop/' . $post->post_name ) );
}
return $permalink;
}
function flushRules(){
global $wp_rewrite;
$wp_rewrite->flush_rules();
}
function wp_insertMyRewriteRules($rules)
{
$newrules = array();
$newrules['^shop/(.*)$'] = 'index.php?name=$matches[1]';
return $newrules + $rules;
}