I would like to add the string "1212" (no quotes) to some part numbers.

Essentially, I have two versions of part numbers to modify:
xxxxxxx(space)(space)xxxxxxxxxx(space)xxxxx(space) xxxx
or
xxxxxxxxxx(space)xxxxx(space)(space)xxxxxxx(space) xxxx(space)xxxx

The length of the xxxxxx strings varies and represents alphanumeric
characters that must remain unchanged. The string "1212" (no quotes) must
always be appended to the first xxxxx string of alphanumeric characters.

Thus the end result of the modification should be as follows:
xxxxxxx1212(space)(space)xxxxxxxxxx(space)xxxxx(sp ace)xxxx
or
xxxxxxxxxx1212(space)xxxxx(space)(space)xxxxxxx(sp ace)xxxx(space)xxxx

All the part numbers that I want to modify will be listed in Column A of my
spreadsheet. (There are other part numbers in other columns but I do not
want to modify them.)

Would somebody please suggest how to do it? This is beyond my knowledge of
Excel.

Thanks in advance.
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Tiziano