Use a custom validator:
<asp:CustomValidator id="CustomValidator1"
ControlToValidate="LimitAmount"
OnServerValidate="ServerValidation"
ErrorMessage="This field requires a number"
ForeColor="red"
runat="server"/>
In the code behind:
void ServerValidation(object source, ServerValidateEventArgs args)
{
double tmp;
if(double.TryParse(args.Value, out tmp))
{
args.IsValid = true;
}
else
{
args.IsValid = false;
LimitAmount.text = string.empty;
}
}
If you prefer, you can also validate this in Javascript with the ClientValidationFunction