Need a solution to edit numbers

Here's my problem: Several months ago I had edited an excel price list file for importing into a inventory management program. This price list has a lot of numbers that begin with a 0 (zero), and Excel gives the error "Number stored as text". Well, I didn't know at the time, and I'm very inexperienced with Excel, but I must have converted those numbers stored as text to numbers, and it eliminated all the leading zeros. Now, it came time to update the price list, and I have all these incomplete item numbers, so I can't update the program properly. I'm hoping to find a way to add zeros in front of these incomplete numbers, in bulk (there's several thousand rows of them). But I don't know how. The problem is that the incomplete numbers are mixed right in with the correct numbers. I know how to add something to all the numbers, but I can't figure out how to isolate the bad numbers. There'd have to be a way, because they are unique. The item numbers are supposed to be 6 digits (there are a few 8 digit numbers also). The numbers that had their zeros cut off are 5 digits or less (Just a handful had more than one leading zero). So what I'm wondering is if there's any way to isolate all the five digit numbers. Then I could easily do what I want with them. I've tried sorting the item number column, but it doesn't group the five digit numbers. Here's an example of the numbers:

Correct item number (btw, all the numbers have the same two character letter prefix; the zeroes were deleted before I added the prefix):

AB123456

Incorrect:

AB12345 (should have been AB012345)


This is putting me in a real pinch and I'm hoping there's a way to do it other than scrolling through the whole thing and manually adding zeros before the item numbers that only have 5 digit numbers