Detect   in the input file

Hi all,

I have input XHTML that uses   to lay out the page. I've searched and
found discussions of using and generating   in the output document but,
how do I detect it in the input?

I have them in sets of three before the expand/collapse images in a book as
follows:

<img>
&nbsp;&nbsp;&nbsp;<img>
&nbsp;&nbsp;&nbsp;<img>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img>
<img>

I'd like to detect each group of three and convert it to a spacer image.

Thanks in advance,
Richard