/^.{10,}$/
use this one
or
/[0-9a-zA-Z]{10,}/
here is the example
<asp:TextBox id="txtUsername" runat="server" />
<asp:RegularExpressionValidator
id="RegularExpressionValidator1"
runat="server"
ErrorMessage="Field not valid!"
ControlToValidate="txtUsername"
ValidationExpression="[0-9a-zA-Z]{10,}" />
regards...:)