Regular expression weirdness (Asp.net validator control)

^(?=.*[a-z].*[a-z])(?=.*[A-Z].*[A-Z])(?=.*\d.*\d)[a-zA-Z0-9]{8,}$

This checks that a string is at least 8 characters long, has 2 lower case character, 2 uppercase characters and two numeric characters. The above regex seems to be valid, at least it works in code. But when I apply it to a regex validator control it fails all the time. Does anybody have an idea why its not working? Suppose I'd better include the markup for the control ...

[html]
<asp:TextBox ID="txtDemo" runat="server"></asp:TextBox>
<asp:RegularExpressionValidator ID="revDemo" runat="server" ErrorMessage="Failed"
ValidationExpression="^(?=.*[a-z].*[a-z])(?=.*[A-Z].*[A-Z])(?=.*[0-9].*[0-9])[a-zA-Z0-9]{8,}$"
ControlToValidate="txtDemo" EnableClientScript="true"></asp:RegularExpressionValidator>
<asp:Button ID="btnSubmit" runat="server" Text="Submit" CausesValidation="true" />
[/html]