compare field validator

I have a text box <asp:textbox, in this text box, I would like people to enter 8 digits and (1 letter or no letter) ( like 10034867B or 10034867), if not, an error message will come up. How should I set up the field validator? Thanks for help.
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Just do a Regular Expressions Validator and fill in this expression: ^\d{8}[\d,a-zA-Z]{0,1}$
This will validate exactly 8 digits and then none or one letter or digit

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