title: Username only lowercase and numbers in WP multi?
body: <p>We are using wordpress in multi sub-domains at work.
  So we have administrator for each domain who can create users for subscribing editing contributing.
  The problem is, our company edit the rule that all logins/usernames have to be like this "first-name.family-name" ex "barack.obama"... :-)
  But the administrator can not do it, only super admin can do it !
  In sub-domain administrator mode, there is a message that says : "Only the lowercase letters a-z and numbers allowed".</p>
  
  <p>So we want to make it possible to admin to create users with first-name.family-name, but we try to search how to change it in the code, without finding it.</p>
  
  <p>If someone can help, thanks !</p>