hi,
use this javascript function on KeyDown event
function ValidateTextBox() {
var txt = document.getElementById('txtTest').value;
if (txt.indexOf('>') > 0 || txt.indexOf('<') > 0) {
alert('Invalid data.');
document.getElementById('txtTest').value = '';
}
}
<asp:textbox id="txtTest" runat="server" onkeydown="ValidateTextBox();" xmlns:asp="#unknown"></asp:textbox>
you can also use HTML element
<input type="text" id="txtTest" önkeydown="ValidateTextBox();" />