Multilanguage validation with German "Umlaute"

Hello!
Today I was working in more details with the valitaion. I choose the model to add the rules and messages. But while testing, I go always an error, when a German "Umlaut" was part of the text. I don't know how to solve this, I didn't find anything  

Here a short examle of the validation:

PHP Code:

'street_no' => 'required|alpha_numeric_space|min_length[5]', 


And the text in this filed is

PHP Code:

Düsseldorfer Straße 


The Ü (and maybe the ß) will throw a error message regarding the alpha_numeric_space rule.
Has anyone an Idea hot to upgrade the validation? Or is there an other solution?
Thank you!