add javascript event after the asp net valid

[24sharon]

I have a text box with RequiredFieldValidator and RegularExpressionValidator

I add to this text box another event "onchange"
<asp:TextBox onchange="check_Available(this.value)" ID="siteName" MaxLength="100" runat="server" Width="250px" ></asp:TextBox>


my problem is that the event run either the validtor not good.

how can I in javascript run the function just if the page valid.
(not all page just this text box)

thanks!