How to upload Only Excel file using RegularExpressionValidator?

i need to control Only ExcelFile uploading with RegularExpressionValidator How can i do that? i need to write a regex pattern in ValidationExpression...

 <h3>FileUpload Test</h3>
    <div>
        <asp:FileUpload ID="FileUpload1" runat="server" />
        <asp:Button id="UploadBtn" Text="Upload File" OnClick="UploadBtn_Click"开发者_JAVA技巧 runat="server" Width="105px" />
        <asp:Label ID="Label1" runat="server" Text=""></asp:Label>
        <asp:RegularExpressionValidator
id="FileUpLoadValidator" runat="server"
ErrorMessage="Upload Excel only."
ValidationExpression="([a-z]\w*)(.xlsx|.xlsm|.xls)$"
ControlToValidate="FileUpload1">
</asp:RegularExpressionValidator>
    </div>

I don't see why you'd validate the file name part - what's wrong about 123.abcd.xls? Just use

^.*\.xls[xm]?$

Above given solutions might not work for extensions in upper cases. Enhancing "Penguen"'s solution:

Regex regx = new Regex(@"([a-zA-Z0_9].*\bxyz)|([a-zA-Z0_9].*\bXYZ)\b");

Replace "xyz" with xlsx and "XYZ" with "XLSX". Please give me alternate of this expression, if it is a bulky one.

var reg = /^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))+(.xls|.xlsx|.XLS|.XLSX)$/;

if (reg.test($("[id$=FileUpload1]").val()))
{
    return true;
}

ValidationExpression="[a-zA-Z0_9].*\bxlsx\b" is solution...

 var ext = this.value.match(/\.(.+)$/)[1];
         var cext = ext.toLowerCase();
    switch (cext) {
        case 'xlsx':

n code goes here. this checks if nly .xlsx is uploaded