How to validate alpha-numeric along-with space will be allowed in a Textbox?

<asp:TextBox runat="server" Id="txt_policy_code" />



<asp:RegularExpressionValidator runat="server" id="rex_policy_code" ControlToValidate="txt_policy_code" ValidationExpression="^[a-zA-Z0-9 ]$" ErrorMessage="Only Alpha-Numeric and Space Allowed" />