Can't get asp.net regular expression validator to work for blank passwords?

I do not want to allow blank passwords, but if I leave the textbox blank, the text property 开发者_开发百科from the regular expression does not show up until I enter at least 1 character. How do I allow it not to have a blank password? I tried using the + sign instead of the * sign, but it still did not work.

<asp:RegularExpressionValidator 
                                ControlToValidate="txtPassword" 
                                Display="Dynamic"
                                runat="server" 
                                Text="Invalid" 
                                ValidationExpression="^(?=.*\d{1})(?=.*[a-zA-Z]{2}).{7,}$"
                                ValidationGroup="UserRegistrationValidation">
</asp:RegularExpressionValidator>

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A RegularExpressionValidator doesnt validate empty text by design. You need also a RequiredFieldValidator or have a look at the following link: RegularExpressionValidator not firing on white-space entry

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That regex does not match an empty string, so i can only assume that asp.net isn't invoke the validator, try adding a required validator as well.