Sar* wrote...
I want to create a validation that when i enter a mixed numeric
and alpha reference that i can prevent changing its format.
....

The most general approach is something like

=AND(COUNT(SEARCH(MID(X99,ROW(1:3),1),"ABCDEFGHIJK LMNOPQRSTUVWXYZ"),
SEARCH(MID(X99,ROW(4:7),1),"0123456789"))=7,LEN(X9 9)=7)

Two of the other suggestions checked that the 4 through 7 characters
were numeric using --ISNUMBER(A1,4). First, the numeric conversion
requires only one unary minus, and second, this would blissfully
accept such strings as

1E12
-300
28.5
-2.5
33.%
7/12

which is probably not what you want. There are times when simple beats
clever.