File Upload Validation asp

I try to validate FileUpload control to check the extension of files by this regular expression

<asp:RegularExpressionValidator ID="RegularExpressionValidator5" ValidationGroup="PersonalGroup" 
           ControlToValidate="FileUpload1" Runat="Server"
            ErrorMessage="Only .pdf & .doc files are allowed" ValidationExpression="[^\.]\.pdf\.docx\.doc$" />*</asp:RegularExpressionValidator>

my problem to check the FileUpload already has file or not before i click the button submit because i got NullReferenceException if don't have file i try to make this

if (FileUpload1.HasFile)
                {
                    book.Book_File = System.IO.Path.GetPathRoot(FileUpload1.PostedFile.FileName);
                }

but the problem still exist if the user didn't uploaded any file. How to check the FileUpload is required filed before submit?