<script type="text/javascript">
function Validation()
{
var a=document.getElementById('TextBox1').value;

var regex1=/^[A-Z]{5}\d{4}[A-Z]{1}$/;  //this is the pattern of regular expersion
if(regex1.test(a)== false)
{
  alert('Please enter valid pan number');
  return false;
}
}
</script>

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="javascript:return Validation();" />