Email replies to cases in workflow

Hello,

In CRM 2011 I have a workflow that creates a case from an email, but for the life of me I can't figure out how to handle replies to the case.

We send out an email thanking the client for submitting a case request, and we put the case number in the subject and set the regarding: field to regarding: case.

I have tried setting the conditions to look for "If email: regarding - contains data", "If email: subject - contains Regarding: Case Case Number" and similar but none work.

Can anyone point me in the right direction?

Thanks.