[Plugin: Search Regex] Replacing URL characters in WordPress Search

I have a site where I want the user to be able to select from a list of categories that they’d like to search (via checkboxes). I’m running into an issue where the URLs of the search look something like:

https://www.test.com/test_index.php?s=Keywords&cat=15&cat=14

I want to use Search Regex to try and make that URL into:

https://www.test.com/test_index.php?s=Keywords&cat=15,14

I haven’t had any luck so far. Is there a way to accomplish something like that?