the regular expression is simple, you just need to do 
^[0-9a-zA-Z]{12}$but I provided an alternative way to enforce the input<form runat="server">
 text1:<asp:TextBox id="txt" runat="server"/>
 <asp:RequiredFieldValidator id="rfv" runat="server" ControlToValidate="txt" ErrorMessage="Required" Display="Dynamic" />
 <asp:RegularExpressionValidator id="rev" runat="server" ControlToValidate="txt" ValidationExpression="^[0-9a-zA-Z]{12}$" ErrorMessage="输入范围26个字母大小写和数字(A--Z,a--z,0--9), 长度正好12个字符" Display="Dynamic" /> <asp:Button id="btn" runat="server" Text="Submit"/> text2:<asp:TextBox id="txt2" runat="server" MaxLength="12" Onkeypress="javascript:if (!/^[0-9a-zA-Z]$/.test(String.fromCharCode(event.keyCode))) event.keyCode = 0;" OnPaste="return false;"/>
</form>