[Zope-CMF] just what is a valid username?

[David (Hamish) Harvey]

I've just discovered that the CMF registration tool (and this the CMF and 
thus Plone) seems to reject a range of usernames which the acl_users folder 
is happy with. Further, if you add these names manually, you can log into 
the site using them. Names with spaces in them are an example, and there 
are a few good reasons for not allowing these. Names with . in them are 
also blocked (although once the registration mechanism is bypassed they 
seem to work fine) and I have a lot of (potential) users wanting to use 
part of their email address as an easy to remember user name.

Can anyone tell me how deep the restriction on usernames goes in Zope/CMF? 
If I change the code which tests for valid names at registration times and 
loosen the restrictions, will I hit problems later?

Cheers,
Hamish