Regex to verify a PO Number

OK, I have a bit of an issue.

I have a directory that I need to start filtering some files on.

I thought I had my script working perfectly, based on what they originally told me the requirements were, then they threw a wrench at me, I mean into it.

These file names are based on Purchase Order Numbers.  First they told me that the PO Numbers would be a minimum of 7 alphanumeric characters.  Simple

Now, for the PO part of the file they are telling me...
MUST be at least 7 characters - CAN be alphanumeric - CANNOT contain 3 or more consecutive letters

I'm not sure how to do the 2 letter max

123AB67 - would be a match

123ABC7 - would fail

Is this possible?  If so, can someone point me in the right direction?

Thanks and have a Great Weekend