Data Validation of exact 2 character list

Hello

In Data Validation / custom I need a formula that looks for an exact match to an alpha numeric.
I want the person entering data to only be able to input the following:

0a 0b 0c 1a 1b 1c 2a 2b 2c 3a 3b 3c

I can get a formula to partially work but it will also allow a ‘0’ or ‘b’ or ‘1A’ or ‘B’ to be entered. I need a formula that only allows the exact 2 characters to be entered and the letter must be lower case.

I am happy to use a Data Validation List but the cell needs to stop anyone trying to manually type in the 2 characters and using a capital letter.

Your help is appreciated.
Thanks
Pete

That's great. Many thanks for the quick reply - works perfectly when the Source is entered as you have done.

I originally entered the alpha/numeric as a list form in a column. When asked for the source I dragged down the column cells and inserted this range. Doing this worked but allowed me to manually enter an upper case which is what IO wanted to restrict.

Don't know why as not that good with Excel.

Really appreciate your fast response, thanks again.

Happy to have helped ...