Fileupload Regular expression validation fails for certain file name in asp.net

I am using fileupload control to upload file and at same time using regular expression for validating file name. I want following file extension to be uploaded for `.doc, .docx, .pdf` i use following command to valid file name `ValidationExpression="[a-zA-Z\\].*(.doc|.DOC|.docx|.DOCX|.pdf|.PDF)$"` ``` <asp:FileUpload ID="FileUpload1" runat="server" CssClass="fileUpload" /> <asp:RequiredFieldValidator ID="ValidateF1" runat="server" ErrorMessage="*" CssClass="row-validate" ControlToValidate="FileUpload1"></asp:RequiredFieldValidator> <asp:RegularExpressionValidator ID="ValidateEx" runat="server" ValidationExpression="[a-zA-Z\\].*(.doc|.DOC|.docx|.DOCX|.pdf|.PDF)$" ControlToValidate="FileUpload1" ValidationGroup="Careers" ErrorMessage="*"></asp:RegularExpressionValidator> ``` It fails to validate following file name `(K)+J01461+abced+high+En+(HR)(1).pdf` I am not sure why it fails while it works for `ABC_COMPANY_Privacy_v4.0_123456(5).pdf` Am i using the wrong validation expression. I want to allow any file name with extension as mentioned above.