Validate web form textbox on client side

[Jim M]

As shown below, I can use onKeyPress for the html text box control, but
onKeyPress (client side) is not available to me using a web form text box to
do my validation on the client side. So how do I do client side validation
with a web form text box?

<asp:TextBox id="TextBox1" onKeyPress="javascript:Validatephone(this)"
style="Z-INDEX: 101; LEFT: 232px; POSITION: absolute; TOP: 56px"
runat="server" Width="216px" Height="24px"></asp:TextBox>

<INPUT id="txtPhone" onKeyPress="javascript:Validatephone(this)"
style="Z-INDEX: 102; LEFT: 248px; POSITION: absolute; TOP: 160px"
type="text">