Validate userform data

[Bidwin]

I have a userform with a textbox that's used as a search argument against a large worksheet. Data in the textbox must be 8 characters long and only contain numbers and letters. I want to validate data as it's being entered, and give relevant error messages to the user if an invalid character is entered. Error conditions are: enter a return before entering 8 characters; enter a character other than an alpha, upper or lower case, or a numeric; attempting to enter more than 8 characters. Please tell me how to do that.
Thanks,
Bidwin