Disable button and validate data in javascript

I have this function that works great :
function disableBtn(btnID, newText) {
    var btn = document.getElementById(btnID);
    //setTimeout("setImage('"+btnID+"')", 10);
    btn.style.background = "background-image: url('../images/indicator.gif') no-repeat";
    btn.disabled = true;
    btn.value = newText;
}
I associate to a buton tihs way:
 <asp:Button ID="Bt_save" runat="server" onclick="Bt_save_Click" onclientclick="disableBtn(this.id, 'Submitting...')" UseSubmitBehavior="false"  Text="Save Changes" />

So far everything is good, the only issue is that I need to validate data from my form in JAvascript.
So if I add something like :
 <asp:Button ID="Bt_save" runat="server" onclick="Bt_save_Click" onclientclick="return confirm('Do you want to save?');disableBtn(this.id, 'Submitting...')" UseSubmitBehavior="false"  Text="Save Changes" />, it does not work anymore.

Thanks