Using ASP.NET
<asp:textbox id="TextBox1" runat="server"></asp:textbox>
<asp:regularexpressionvalidator id="RegularExpressionValidator1" controltovalidate="TextBox1" runat="server" errormessage="Only Numbers allowed" validationexpression="\d+"></asp:regularexpressionvalidator>
Using Javascript
<input type="text" id="txtID" />
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
return true;
}