Deleting Starting And Trailing Characters Scanned Into Cell?

I'm scanning data into a spreadsheet using a barcode scanner, but I want the starting character and trailing characters to be removed after I hit enter.

For example...let's say I'm scanning the following text in cell A1:

=W05281212345600

When I hit enter (or the barcode scanner does auto-enter), I want cell A1 to read:

W052812123456

I will always want the "=" and the last two zeros removed from the 16 character number, leaving the 13-character number in its place.

I tried using Excel's various truncate functions (LEFT,RIGHT,MID, etc.), but that only places the corrected text in another cell. That would be fine if I could copy that truncated text back over the original text...but that created a loop problem.