Conditional Formatting - offset ?

Hello everyone
I am using a worksheet and am trying to highlight differences in cell values between 2 columns,
column A and B.
So I highlight Column A, Format, Conditional Formatting,
Cell Value, is not equal to , =B1 , Format, Pattern, Colour.

Differences in cell values in Column A and Colmn B are thereby highlighted in the cells in Column A.

This works well, but what I would ideally like to do is refine, and use the Left offset function ( I think ! ) to ONLY identify where there is a difference between the first three characters in the cells in the two columns.
Example
Column A Column B
AB1 2AY AB1 2AZ
BB1 3AZ AB1 3AZ This line would be highlighted
AB3 3AT AB3 5TY

In other words, what syntax do I need to use to "offset"
Any answers would be most gratefully receiveed !
Pete

Thankyou Portuga, I have tried the formula, but I cannot get it to work, using either ; or , in case I have an different version of Excel. The error box comes up every time
with "The formula you typed contains an error" Thanks anyway, Pete

HUMBERTDING,

Did you try substituting the semi-colon [;] with a comma (,)?

Ed

Hello, me again,
Here is an example of data on a spreadsheet where I need
data identified using Conditional Formatting, if you look at the example, there are two cells that I would like to highlight... could anyone help with the formula, please ?
( I have had to paste a screenshot into .doc as the system will not upload .xls files )
Pete

Thanks Portuga,
I made two mistakes
1) I was Inserting your formula in the "Cell Value Is" ( Not equal to ) box rather than "Formula is" box.
2) ( ; ) does not work on my version of excel I had to use ( , ) instead.

All sorted now, and thanks very much for your posts !
Pete

I have another problem, related to the first . Now what I would like to do is the reverse ;-
to ONLY identify where there is a MATCH between the first three characters in the cells in the two columns.
Can anyone help, please, with a Formula ?
MANY THANKS !
Pete