In the Edit template, add a RequiredFieldValidator
like this:
<EditItemTemplate>
<asp:TextBox ID="txtvid"
runat="server" Width="150px"
Text='<%# Bind("vid") %>'>
</asp:TextBox>
<asp:RequiredFieldValidator
ControlToValidate="txtvid"
runat="server"
ErrorMessage="Please enter a 'vid' number"
Text="*"/>
</EditItemTemplate>
Here is the reference for the RequiredFieldValidator on MSDN.
UPDATE:
If you wanted a regular expression validator, its pretty much the same, but with the RegularExpressionValidator
control:
<asp:RegularExpressionValidator
ControlToValidate="txtvid"
ValidationExpression="\d{10}"
runat="server"
ErrorMessage="Please enter a 'vid' of 10 digits"
Text="*"/>
Here is a complete list of the functionality for the RegularExpressionValidator on MSDN.