Required Apache,Network Administration freelancer for Writing .htaccess file rule for redirection job

I need htaccess file rules for redirecting urls for one specific case
If a user types 'example/categories' it should be redirected to 'example/public/categories'
if a user types 'example/public/categories' the url should not be changed.

So basically the htaccess should search for '/public' after the site name or other specified string and if it does not exist then it should add '/public' after the site name.

Will need explanation so that I can edit it in the future for other cases

Skills & Expertise Required

Apache Network Administration