Suppress OnBlur Validation Alert On Window Close

Using Javascript, ASP.net with C# code behind. I am validating a textbox using the onblur event. It is being validated in that it must have an alphanumeric entry before the user tabs off of it to the next box. The validation part works, the part that doesn't work is that I get the validation alert textbox if I close the window. I have a workaround in that I am assigning that textbox a character using windo.onberforeunload but I know that isn't right.

Here is what I have so far...
Code behind on Page_Load...
txtCLBRTNWC.Attributes.Add("onblur", "return reqVLD(this)");

Here is my textbox field...
<asp:TextBox ID="txtCLBRTNWC" runat="server" Width="75" MaxLength="4"
ontextchanged="txtCLBRTNWC_TextChanged" TabIndex="1"
onkeyup="clbwclngCHK(this);" ></asp:TextBox>

And here are the 2 functions being used...
function reqVLD(alphanumericChar){
var chk = /^[a-zA-Z0-9]+$/;
// var boxid = document.getElementById("<%=txtCLBRTNWC.ClientID%>").value;
// var matchArray = boxid.match(chk);
if(document.getElementById("<%=txtCLBRTNWC.ClientID%>").value.match(chk)){
return true;
} else {
document.getElementById("<%=txtCLBRTNWC.ClientID%>").focus();
document.getElementById("<%=txtCLBRTNWC.ClientID%>").value = "";
alert("Please enter a Work Center.");
return false;
}}
function clbwclngCHK(obj_in)//When txtCLBRTNWC entry hits the max of 4 auto focuses to next box. {
if (obj_in.value.length == 4) document.getElementById("<%=txtSTRTDT.ClientID%>").focus();
}

Here is my work around...
window.onbeforeunload = function (){
document.getElementById("<%=txtCLBRTNWC.ClientID%>").value = "1";
}

User has to enter something in box before they can move on. This is checked and validating using an onblur event. Problem is, the onblur event fires if closingexiting the window.