Custom validation for textbox

<asp:TextBox ID="txt_userid" runat="server" />
    <asp:CustomValidator ID="CustomValidator" runat="server"     EnableClientScript="true"
        ErrorMessage="Userid should start with letter u followed with 7 digits"
        ClientValidationFunction="validateuserid"
        ControlToValidate="txt_userid" Display="Dynamic" >
    </asp:CustomValidator>

<script type="text/javascript">

function validateuserid(sender, args) {
var Userid = document.getElementById('txt_userid').value;

if (Userid.length == 8) {
if (Userid.substring(0, 1) == 'u') {
args.IsValid = true;
}
else {
var regex = '^[0-9]{7}$';
var str = Userid.substring(1, 8)
if (str.search(regex) == -1) {
args.IsValid = false;
}
else {
args.IsValid = true;
}
}
}
else {
args.IsValid = false;
}
}

Hi Abhijit, try this;

<asp:customvalidator id="CustomValidator1" controltovalidate="<b" xmlns:asp="#unknown">"Your ControlID" OnServerValidate="cusCustom_ServerValidate" ErrorMessage="Your Msg!" runat="server"></asp:customvalidator>

use this functions;

protected void cusCustom_ServerValidate(object sender, ServerValidateEventArgs e)
   {
       if (e.Value.Length == 8)
       {
           if (e.Value[0] == 'U')
           {
               System.Globalization.NumberStyles ns = System.Globalization.NumberStyles.Integer;

               string str = e.Value.Substring(1, e.Value.Length - 1).ToString();
               bool bol = isNumeric(str, ns);
               if (bol == true)
               {
                   e.IsValid = true;
               }
               else
               {
                   e.IsValid = false;
               }
           }
           else
           {
               e.IsValid = false;
           }
       }
       else
       {
           e.IsValid = false;
       }
   }

   public bool isNumeric(string val, System.Globalization.NumberStyles NumberStyle)
   {
       Double result;
       return Double.TryParse(val, NumberStyle,
           System.Globalization.CultureInfo.CurrentCulture, out result);
   }

I hope this will resolve your problem.

Thank's
Mohan G

if(document.getElementById('<%= textbox.ClientID%>').value=="")

{
alert("Please Enter Number");
document.getElementById('<%= textbox.ClientID%>').focus();
return false;
}

var y = document.getElementById("<%=textbox.ClientID %>").value;

if(document.getElementById('<%= txtmobileno.ClientID %>').value != "")
{
if (y.length > 7 || y.length < 7)
{
alert("Please Enter 7 characters");
document.getElementById("<%=textbox.ClientID %>").focus();
return false;
}


Assign U as Static in TextBox