When can SWQL refer to an Alias

I'm trying to parse strings and am trying to refer to an alias in the SWQL query.

In the following query, I can refer to the alias IP without issue.  However, the alias from the CHARINDEX expression gives an exception: Cannot resolve property P1.

SELECT N.IPAddress AS IP

     , N.DnsBackward

     , CHARINDEX('.',IP,1)-1 AS P1

     , ToString(P1)

FROM IPAM.IPNode AS N

Is this a bug?  If not, what determines when an alias can be referenced?  Is there another way to accomplish this?