Allowing periods in usernames

I have a multi-site installation of wordpress currently running, and I am having some problems with user registration.

Currently, I use an external authentication plugin (similar to LDAP i guess) and I use this to validate usernames/passwords. For this to work, every user is first added to the WordPress list of users, and the plugin checks against this list.

The problem is that my organization allows periods inside of a persons username (eg, john.smith). WordPress is not allowing me to enter john.smith as a username into the system (only allows a-z characters).

Is there any type of workaround for this? I would rather not edit the wordpress core as thats a horrible practice

Solutions Collecting From Web of "Allowing periods in usernames"

There is a plugin for this. It is Network Username Restrictions Override. Please remember to check out the FAQ #2 to know how to allow periods in usernames. Let me quote it here…

You’ll have to update the regular expressions in your .htaccess file
if you use email addresses or periods in site URLs. The default
WordPress .htaccess configuration does not allow them, nor will this
plugin edit your .htaccess automatically due to the risk involved.