GREP Style Inquiry: Adding a period to an abbreviation that doesn't already have one?

Hey there, 

I'm currently in the throws of creating an in-depth directory for the US Forest Service- which has resulted in my having to utilize GREP Styles- which has been fabulously effective. 

The only thing I was given was a very poorly executed (disorganized, poor punctuation, that kind of thing) spreadsheet. So, yea- Scripts and GREP have really saved me here. 

Anyway, I have stumbled across something I'm sure will prove to be very simple to all you Adobe Wizards; I'm hoping I may tap into your wisdom.

I need to add a period to the abbreviation "Ext" so that it reads "Ext."

I've only been successful so far in adding a period to all instances of "Ext", even if that already have a period- which results in many of the instances reading "Ext.." - How do I avoid that? 

Thank you in advance,

 - Vitto