Firepass -- User certificate (regex extraction) for login username

I am having difficulty writing a regex extraction expression for the F5 Firepass (v. 5.5.1) to extract a part of the certificate CN and use it to match up with an Active Directory user object for logon. The information I have is listed below.

 

 

CN=firstname.lastname.MI.10digitserial

 

 

Active Directory User object:

 

employeeID = 10digitserial

 

sAMAccountName = firstname.lastname

 

userPrincipalName = 10digitserial@domainname

 

 

I can not figure out how to write a proper client certificate regex extraction statement to user the 10digitserial out of the certificate CN and match it up against the users AD employeeID. Does anyone know how this can be done, or if another object in Active Directory would need to be modified to get client certificate based logon to work?

 

 

Any help would be greatly appreciated.

 

 

-Jake