Are there criteria for knowing how the middle characters appear?

"bdwood" wrote:

We start with a cell(s) containing 13 to 15 characters, with no spaces:
1234567100AAAAA, 123456710AAAAA, 12345671AAAAA. We want the format to be
1234567-100-AAAAA, 1234567-10-AAAAA, 1234567-1-AAAAA. The first 7 characters
always belong together, and the last 5 characters always belong together. It
is the middle 1 -3 that varies. How can this be done? Thank you.