asp:RegularExpressionValidator to prevent consecutive repeated characters

I have a regular expression validator control that currently only validates for characters and length. I'm looking for the simplest way to modify the validation expression below to add the ability to disallow consecutive repeated characters of any of the allowed characters. For instance: W#d3aZhT would pass validation, but W#d3ZZhT, or W#d33ZhT, or W##d3ZhT would not. It would be best if the expression was case-sensitive, meaning that, for example, W#d3zZhT would pass validation, but W#d3ZZhT would not, but this is not essential in this first iteration. Here's the current control definition:
<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" ControlToValidate="NewPassword" Display="Dynamic" ErrorMessage="New password is not valid. Please check it against the rules above and try again." SetFocusOnError="true" ValidationExpression="[0-9a-zA-Z@#$]{8}" />
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