Regular Expression Validators

I'm having a problem with a regular expression validator - code below. It's supposed to check for a 6-digit number beginning with a 1 or 2. What's happening is that when I enter a valid number (ie, "123456") it's throwing up the error, until I make a selection with another control that does an autopostback. THEN it clears the error. I can't figure out why - it really shouldn't be kicking up an error at all.
Thanks!
Kaiti
<asp:TextBox ID="textID" runat="server"></asp:TextBox> <asp:RegularExpressionValidator ID="RegularExpressionValidator1" Display="dynamic" ControlToValidate="textID" ValidationExpression="^(1/d{5}|2/d{5})$" runat="server" ErrorMessage="Invalid Employee ID."></asp:RegularExpressionValidator>
 
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