Make a div and a input width to be 100% of the parent

Consider a parent <div> which is position:relative; width:100%. It contains a <div> and a <input>. I want them to have together a 100% width.

The <div> ideally should be fixed (it's a custom button) and the <input> should fill the rest of the width (think about window resizing).

Is it possible doing it without involving JS code?

Also, it's not a "must-have" but I wish to support as for as I can with IE7+.

CODE:

<div id="parent" style="position:relative; width:100%">
  <div id="button">Click</div>
  <input type="text" id="txt_input" />
</div>