Laravel / Php regular expression

I want to create a regular which checks if an input field has ü,ä or ö.

If the input field has for instance the letter ü I want to switch it to ue.

So far I have this:

public static $rules = [
    'email' => 'required|max:30'
    'filename' => 'required|max:30|regex:/'
    ];

But I do not know how to further go on.

via Chebli Mohamed