EDIT: original and not for stackoverflow simplified code to replace the string is:

$entry=preg_replace('/[^-a-z0-9_ßüÜöÖäÄßÜüÖöÄä_@:\.\, ]/i', '', $entry); 

and in my question i used this simplified code:

$entry = preg_replace( '/[^a-z0-9 ]/i', '', $entry );

That should do it, as gre_gor commented. So the problem is the not-simplified preg_replace above. Solved. Thank you.