validation of hyphen in the beginning or at the end

Question

how to validate if hyphen is present in the beginning or at the end? I've done:

<asp:RegularExpressionValidator ID="Regex1" runat="server" ControlToValidate="txtPANnu"

Display="Dynamic" SetFocusOnError="true" ErrorMessage="*Enter Only Alphanumeric characters" ValidationExpression="^[0-9a-zA-Z-]+$">

</asp:RegularExpressionValidator>

this checks only for hyphen but I've to check whether the hypen is present in the beginning or at the end, if so generates error.

so correct inputs are ::

1243-adfsda

fjld-303-ljsdl-3o32

invalid inputs ::

-dfds-322 (present at the beginning)

jlsd-8080- (present at the end)