Similar to How replace all spaces inside HTML elements with using preg_replace?
Except I only want to modify spaces found between PRE tags. For example:
<table atrr="zxzx"><tr>
<td>adfa a adfadfaf></td><td><br /> dfa dfa</td>
</tr></table>
<pre class="abc" id="abc">abc abc</pre>
<pre>123 123</pre>
would be converted to (note the pre tag may contain attributes, or may not):
<table atrr="zxzx"><tr>
<td>adfa a adfadfaf></td><td><br /> dfa dfa</td>
</tr></table>
<pre class="abc" id="abc">abc abc</pre>
<pre>123 123</pre>