Post url rewriting for posts with certain category

Question

Attempting to create a custom rewrite rule for any post that has a category of Shop. The posts use /%postname%/ but I want /shop/%postname%/ to appear in the url. Below is what I have but I can't get it working.

add_filter( 'post_link', 'custom_permalink', 10, 3 );

function custom_permalink( $permalink, $post, $leavename ) {
    // Get the categories for the post
    $category = get_the_category($post->ID); 
    if (  !empty($category) && $category[0]->cat_name == "Shop" ) {
        $permalink = trailingslashit( home_url('/' . $post->post_name ) );
    }
    return $permalink;
}

add_action('generate_rewrite_rules', 'custom_rewrite_rules');

function custom_rewrite_rules( $wp_rewrite ) {
    $new_rules['^shop/([^/]*)-([0-9]+)/?'] = 'index.php?postname=$matches[1]';
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
    return $wp_rewrite;
}

Answer 1

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;
}