FIM String manipulation with declartive rules

I thought this would be easy, but apparently it isn't.  I have an attribute called HR_DISPLAYNAME fed from a database.  I need to split the sting into first name last name.  However, sometimes there is a middle initial, and sometimes a full middle name.  I tried this, but it fails:

IIF(IsPresent(Word(HR_DISPLAYNAME,3," ")),Word(HR_DISPLAYNAME , 3, " "),Word(HR_DISPLAYNAME , 2, " "))

Attribute HR_DISPLAYNAME

FirstName LastName
FirstNam MiddleName LastName
FirstNam MI LastName