How to set URL rewriting rules?

Question

PHP

Somehow it so happened that I'm not strong in regular expressions, as well as in creating CNC (human-readable URLs for WordPress) :-)

Actually the problem:

In WordPress, in the theme we use, there is a custom post type tovar. By default, this post type has a URL structure:

https://oursite.ru/catalog/%tovar_category%/%tovar_name%

where:
%tovar_category%- product category slug - product
%tovar_name%slug It is

required to make a URL like:
https://oursite.ru/%city%/%tovar_name%
where:
%city%- city name (can change dynamically, can be obtained through the corresponding function)
%tovar_name%- product slug

To solve the problem, when registering this custom post type, I try to set the slug
'slug' => '%city%'

and rewrite the value of the variable %city%with the following code:

function show_city_permalinks( $post_link, $post ) {
  if ( is_object( $post ) && $post->post_type == 'tovar' ) {
    return str_replace( '%city%', strtolower( IC_Geo::sc_current_city_lat() ), $post_link );
  }

  return $post_link;

}
add_filter( 'post_type_link', 'show_city_permalinks', 1, 2 );

At the same time, for some reason, the URLs of ordinary pages (page) break.

But if you set it
'slug' => 'city/%city%'
(by adding another city element to the URL ), then everything works fine.

The problem is that we don't need this element.

How can this problem be solved?