Allowing certain things in a cell

Hi All,
Thanks for all those who helped on other threads.

I have a new problem.

In column A i would the cells to only be to contain a certain kind of text...
e.g

A Letter, A Letter, A Number, A Number, A Number, A Number, A Number & A Number
(AA123456).

This is what will be entered in most cases but there may be the odd one that needs to be different. Ideally i would like a warning message to appear and say "this is not the standard...do you want to continue" so different enteries are allowed but the inputter is made aware its not the same.

Im guessing its going to be done under Data>Validation but not sure exactly what.

Any help would be great.

Thanks in advance,

Emma x

can you use column B for the message?

if so you could check if LEFT(A1,1) is between A and Z and the same check for MID(A1,2,1)

MID(A1,3,1) and so on would be checked for values between 0 and 9

OR

RIGHT(A1,6) is between 0 and 999999

robert111 : thanks for the help but column B is already in use and I cant hide columns or things like that in this sheet. Thank you anyway

pjoaquin : thanks for this. Ill go and try it now but no doubt it will work. Thanks again

pjoaquin : this works fine...but how things can change in a matter of moments.

Now i need it to be

AA123456A

(adding the extra letter at the end)...

Any easy amendments? I did fiddle with the formula but couldnt get it working.

Thanks

Emma x