javascript or validation in textbox

Hi,
 
I have a text box:

' mce_style="BACKGROUND-COLOR: #fff" closure_hashcode_ukiqhg="225"><asp:TextBox ID="txtURL" runat="server" Text="http://" />

Want (Text = "http://") to remain. So it can not be erased from the textbox. And then it should not be possible to write "http://" again in the text box.

Is it javascript or some regular expression validation to be used in this case? - perhaps both?
Hope you can help me with some code!

thanks 
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