How To Validate The Text Box And In That I Should Get Only Numbers And Should Not Accept Char

<asp:textbox id="TextBox1" runat="server"></asp:textbox>

<asp:regularexpressionvalidator id="RegularExpressionValidator1" controltovalidate="TextBox1" runat="server" errormessage="Only Numbers allowed" validationexpression="\d+"></asp:regularexpressionvalidator>

<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;
}