not displaying search results correctly from querystring keyword - asp.net

Question

I had a basicSearch page working perfectly with the keyword searching (highlighting the keyword on the search result page) but when I modified it to work with a querystring, it's displaying ALL the records with the keyword highlighted.
aspx page:
<form id="form1" runat="server">
<div id="mainContent">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<script type="text/javascript">
function updated() {
// close the popup
tb_remove();
// refresh the update panel so we can view the changes
$('#<%= me.btnRefreshResources.ClientID %>').click();
}
function pageLoad(sender, args) {
if (args.get_isPartialLoad()) {
// reapply the thick box stuff
tb_init('a.thickbox');
}
}
</script>
<asp:AccessDataSource ID="AccessDataSource" runat="server"
DataFile="~/Dir/search.mdb"
SelectCommand="SELECT * FROM [searches] ORDER BY [Title]"
FilterExpression="Title like '%{0}%' or LastName like '%{1}%' or FirstName like '%{2}%' or Description like '%{3}%' ">
<FilterParameters>
<asp:ControlParameter Name="Title" ControlID="txtSearch" PropertyName="Text" />
<asp:ControlParameter Name="LastName" ControlID="txtSearch" PropertyName="Text" />
<asp:ControlParameter Name="FirstName" ControlID="txtSearch" PropertyName="Text" />
<asp:ControlParameter Name="Description" ControlID="txtSearch" PropertyName="Text" />
</FilterParameters>
</asp:AccessDataSource>
<div id="basicSearch" align="left">
<b style="font-family: Arial, Helvetica, sans-serif; font-size: 18px">Enter a keyword: </b><asp:TextBox ID="txtSearch" runat="server" Width="300px" Font-Size="18px" />
<asp:Button ID="btnBasicSearch" Text="Search" Runat="Server"/>
<asp:Button ID="btnBasicClear" Text="Clear" Runat="Server"/><br /><br />
</div>
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Button ID="btnRefreshResources" runat="server" style="display:none" OnClick="Refresh_Click" />
<asp:GridView ID="gvResources" runat="server" DataSourceID="AccessDataSource" CssClass="datagrid" GridLines="None" AutoGenerateColumns="false" AllowSorting="True" PageSize="50" AllowPaging="True" Width="100%" OnPageIndexChanging="gvResources_PageIndexChanging"> <PagerSettings Position="TopAndBottom" />
<Columns>
<asp:BoundField DataField="ID" HeaderText="ID" InsertVisible="False" ReadOnly="True" />
<asp:TemplateField HeaderText="Type" HeaderStyle-HorizontalAlign="Right" SortExpression="Title">
<ItemTemplate>
<%# DisplayType(Eval("Book"))%>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Title" SortExpression="Title">
<ItemTemplate>
<asp:Label ID="lblTitle" Text='<%# HighlightText(Eval("Title").ToString()) %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Last Name" SortExpression="LastName">
<ItemTemplate>
<asp:Label ID="lblLastName" Text='<%# HighlightText(Eval("LastName").ToString()) %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="First Name" SortExpression="FirstName">
<ItemTemplate>
<asp:Label ID="lblFirstName" Text='<%# HighlightText(Eval("FirstName").ToString()) %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Description">
<ItemTemplate>
<asp:Label ID="lblDescription" Text='<%# HighlightText(Eval("Description").ToString()) %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<ItemTemplate>
<a id="btnShowPopup" runat="server" class="thickbox" title='<%# Eval("ID", "Request Resource ID: {0}") %>' href='<%# Eval("ID", "RequestResource.aspx?ID={0}&TB_iframe=true&height=350&width=500&modal=true") %>'>Request This Item</a> </ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
</div>
</form>
Code behind:
Partial Class BasicSearch
Inherits System.Web.UI.Page
Dim SearchString As String = ""
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
SearchString = Request.QueryString("Keyword")
End Sub
Protected Sub Refresh_Click(ByVal sender As Object, ByVal args As EventArgs)
' update the grids contents
Me.gvResources.DataBind()
End Sub
Function HighlightText(ByVal InputTxt As String) As String
If SearchString = "" Then
Return InputTxt
Else
Dim ResultStr As Regex
ResultStr = New Regex(SearchString.Replace(" ", "|"), RegexOptions.IgnoreCase)
Return ResultStr.Replace(InputTxt, New MatchEvaluator(AddressOf ReplaceWords))
End If
End Function
Public Function ReplaceWords(ByVal m As Match) As String
Return "<span class=highlight>" + m.ToString + "</span>"
End Function
Protected Sub gvResources_PageIndexChanging(ByVal sender As Object, ByVal e As GridViewPageEventArgs)
gvResources.PageIndex = e.NewPageIndex
SearchString = txtSearch.Text
gvResources.DataBind()
End Sub
Protected Sub btnBasicSearch_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnBasicSearch.Click
SearchString = txtSearch.Text
Response.Redirect("BasicSearch.aspx?Keyword=" & SearchString)
End Sub
Protected Sub btnBasicClear_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnBasicClear.Click
txtSearch.Text = ""
SearchString = ""
End Sub
Protected Function DisplayType(ByVal Book As Boolean) As String
If Book Then
Return "Book"
Else
Return "Other"
End If
End Function
End Class

Answer 1

Just a guess - what is the value of SearchString when the HighlightText function is executing? You are storing the querystring in a local page variable, which only exists while the page request is executing.
If HighlightText gets called without Page_Load being called (in the same request), SearchString will be empty.