Using Highlight Module with Default Search Module
I think, looking at the examples, that something like this expression should work if you have clean URLs turned on and searches have URLs like http://example.com/search/node/keywords.

\/search\/node\/([^?#]+)


But I have never used this module and it looks a bit ropey. You can test your own regular expressions at https://regex101.com/ (use JavaScript mode). Contrary to the examples you should not need to include the domain of your site in the search patterns.