<script language ="javascript" type ="text/javascript" >
 function validate()

    
     var b = /^([0-9]*)$/;
 var c= document.getElementById('<%=txtPhoneNo.ClientID %>').value;
 if(document.getElementById('<%=txtFirstName.ClientID %>').value =="")
         {
         alert('Enter the firstname');
         document.getElementById('<%=txtFirstName.ClientID %>').focus();
         return false;
         }
 else if (!b.test(c))
         {
         alert(' Enter only numeric value in phone number');
         document.getElementById('<%=txtPhoneNo.ClientID %>').focus();
         document.getElementById('<%=txtPhoneNo.ClientID %>').select();
         return false;
         }
  else if(document.getElementById('<%=txtEmail.ClientID %>').value !="")
       {
         validRegExp = /^[^@]+@[^@]+.[a-z]{2,}$/i;
         strEmail = document.getElementById('<%=txtEmail.ClientID %>').value;
       if (strEmail.search(validRegExp) == -1) 
        {
      
          alert(" A valid e-mail address is required");
           document.getElementById('<%=txtEmail.ClientID %>').focus();
           document.getElementById('<%=txtEmail.ClientID %>').select();
          return false;
        }
        } 
         return true;
         }
         
 </script>