How to process shortcode on post save and update only

If you want to modify content when the user saves a post, you’ll want to hook in a function using the save_post hook.

Within that function, you can process registered shortcodes using get_shortcode_regex which will give you the shortcodes and attributes used in that post.

(However, if you want to process the content when the user saves, not when the content is displayed, then you probably don’t need to register the shortcode to begin with. Just use the save_post hook and process the shortcode with your own regular expression.)