Creating your own Pro Search Filter
Creating your own filter is easy. Pro Search boasts a simple custom filter API that you can use to create custom filters for your searching needs. These filters can be stand-alone, or you can add them to your own add-ons. Pro Search uses this API itself for its native filters.
All filters are executed one after another and each filter passes its search results — consisting of Entry IDs — on to the next. As soon as a filter ends up with an empty set, No Results is triggered, skipping all next filters.
File structure
Filters live in a filters folder inside your add-on package. So, say you’ve created a filter called foo, Pro Search will be looking for /my_addon/filters/foo/lsf.foo.php. That name should be unique, so be aware of already existing filters. You can have multiple custom filters in your filters folder.
Naming convention
Inside the lsf.foo.php file should be a class called Pro_search_filter_foo which extends Pro_search_filter:
class Pro_search_filter_foo extends Pro_search_filter {}