Using PowerShell to find Active Directory logon names with a certain format.

We recently changed our companies Active Directory logon name format to: first letter of first name + last name

Previously, the format was: first 7 letters of last name (or less for shorter last names) plus first letter of first name

I would like to use PowerShell to give me a list of users whose logon names are in the older format but my PowerShell skills are lacking.  Can someone point me in the right direction?  Thanks!