appent text to input field

Is there any way to append a text to input field?

So lets say instead of:

<input type="text" id="val" name="val" value="http://" style="width: 140px;" />
 

with "http://" already written, have a "http://" added automatically when someone types an address, and if someone types full url "http://www.****.com" the http should not be added so it's not doubled.

Any one has any ideas? I can't use php.

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