Why not just use the OnClick event of the LinkButton? Or, better yet, look into the CustomValidator control, since it looks like all you're trying to do is validate a zip code and that's exactly what a CustomValidator can do (you'll need to look at the ClientValidationFunction, which is where you want to put your regex test).
Here's an old discussion thread where I listed the main differences and the conditions in which you should use each of these methods. I think you may find it useful to go through the discussion.
To explain the differences as relevant to your posted example:
a. When you use RegisterStartupScript, it will render your script after all the elements in the page (right before the form's end tag). This enables the script to call or reference page elements without the possibility of it not finding them in the Page's DOM.
Here is the rendered source of the page when you invoke the RegisterStartupScript method:
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1"><title></title></head>
<body>
<form name="form1" method="post" action="StartupScript.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="someViewstategibberish" />
</div>
<div> <span id="lblDisplayDate">Label</span>
<br />
<input type="submit" name="btnPostback" value="Register Startup Script" id="btnPostback" />
<br />
<input type="submit" name="btnPostBack2" value="Register" id="btnPostBack2" />
</div>
<div>
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="someViewstategibberish" />
</div>
<!-- Note this part -->
<script language='javascript'>
var lbl = document.getElementById('lblDisplayDate');
lbl.style.color = 'red';
</script>
</form>
<!-- Note this part -->
</body>
</html>
b. When you use RegisterClientScriptBlock, the script is rendered right after the Viewstate tag, but before any of the page elements. Since this is a direct script (not a function that can be called, it will immediately be executed by the browser. But the browser does not find the label in the Page's DOM at this stage and hence you should receive an "Object not found" error.
Here is the rendered source of the page when you invoke the RegisterClientScriptBlock method:
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1"><title></title></head>
<body>
<form name="form1" method="post" action="StartupScript.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="someViewstategibberish" />
</div>
<script language='javascript'>
var lbl = document.getElementById('lblDisplayDate');
// Error is thrown in the next line because lbl is null.
lbl.style.color = 'green';
Therefore, to summarize, you should call the latter method if you intend to render a function definition. You can then render the call to that function using the former method (or add a client side attribute).
Edit after comments:
For instance, the following function would work:
protected void btnPostBack2_Click(object sender, EventArgs e)
{
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append("<script language='javascript'>function ChangeColor() {");
sb.Append("var lbl = document.getElementById('lblDisplayDate');");
sb.Append("lbl.style.color='green';");
sb.Append("}</script>");
//Render the function definition.
if (!ClientScript.IsClientScriptBlockRegistered("JSScriptBlock"))
{
ClientScript.RegisterClientScriptBlock(this.GetType(), "JSScriptBlock", sb.ToString());
}
//Render the function invocation.
string funcCall = "<script language='javascript'>ChangeColor();</script>";
if (!ClientScript.IsStartupScriptRegistered("JSScript"))
{
ClientScript.RegisterStartupScript(this.GetType(), "JSScript", funcCall);
}
}
Best Answer
So the page load events flow as:
At step 2 you are reading a variable from the viewstate and setting a label. At step 3 you're setting the value in the viewstate.
As you've observed you're using the events in incorrect order. There are two ways you can solve this: Place your label setter code in Render/Unload event. The other is to refactor your code such that the label is set via the Button Click Event instead of in the Page_Load event; which I would recommend as the proper course of action.
You may want to checkout What is the ‘page lifecycle’ of an ASP.NET WebForm? to get a better handle on page lifecycle.
In response to the comment: If there's multiple labels to set you can look at inline code to set your labels. The code would look like this:
CodePage
CodeBehind
This way, you set your text via the Label1Text property and it gets transfered to the form when it's rendered. You can try this Tutorial here if you need more information on inlining code.