How to match non-ASCII (German, Spanish, etc.) letters in regular expression?

I was unable to find or create a regular expression that matches only letters, spaces, accented letters, and Spanish and German letters.

I am using this now:

var reg = new RegExp("^[a-z _]*$");

I tried:

^[:alpha: _]*$   
^[a-zA-Z0-9äöüÄÖÜ]*$  
^[-\p{L}]*$   

Any idea? Or is the regex supported by javascript engines limited?