Try using the RegularExpressionValidator, which will validate the user's
input. If you want to restrict the characters that the user can type, you
can use JavaScript (there is a function I wrote that adds this JavaScript if
you would like me to send it to you). You can also use something from the
ASP.NET AJAX Control Toolkit called MaskedEdit, which you can see a
demonstration of at:
http://ajax.asp.net/ajaxtoolkit/MaskedEdit/MaskedEdit.aspx
However, if you do not have the ASP.NET AJAX Control Toolkit already
installed, that may be more work than it's worth (my function is much
simpler, and you do not need to install anything or change any configuration
settings, but is not as fancy). Hopefully one of these options is what you
are looking for, or at least helps you find a solution. Good Luck!