Well, there is no simple solution unfortunately, because of the irregular patterns of the singularization – pluralization forms of english language. In many cases adding/removing the “s” from the word ending is not enough.
I’ve done some research, and looks like there might be a pre-created code that covers most of these cases as well. I’ve made a few changes to integrate it with the search, and it looks like it’s working as a temporary solution.
Try putting the code on this url to your theme’s functions.php file: http://pastebin.com/raw.php?i=m4q0VF6K
This will go through all search words inputted and detects if in singular/plural form, then it will append (invisibly to the user) the opposing forms as well to the search phrase.Best,
If you like my products, don't forget to rate them on codecanyon :)