=IF(AND(
LEN(B1)=7,
CODE(MID("AAA0000",{1,2,3,4,5,6,7},1))<=CODE(MID(B1,{1,2,3,4,5,6,7},1)),
CODE(MID(B1,{1,2,3,4,5,6,7},1))<=CODE(MID("ZZZ9999",{1,2,3,4,5,6,7},1))
),"○","×")

=IF(IFERROR(AND(
LEN(B1)=7,
FIND(MID(B1,{1,2,3},1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ"),
FIND(MID(B1,{4,5,6,7},1),"0123456789")
),),"○","×")