Disallow non-alphanumeric characters in password

Hi,
I have a user creation page but I do not want the password to contain any non-alphanumeric character.
I tried to set this under the web.config under the membership section
passwordStrengthRegularExpression="(?=(.*\W){,0})"/>
However, it ended up disallowing even password without non-alphanumeric character.
Also I would like to know how to customize the error message under the user creation, when invalid password (non-alphanumeric character exists) is entered.
I tried the following,
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="Password" ErrorMessage="Cannot use non-alphanumeric character" ValidationGroup="CreateUserWizard1">
       </asp:RegularExpressionValidator>
 
Hope someone out there can help me out.
Thank you!
...