<?php
$xml = '<set1>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set1>
<set2>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set2>';
$xml = preg_replace('/<uid1>#([\n])<\/uid1>/', 'dee', $xml);
//preg_match('/<uid1>#([\n])<\/uid1>/', $file, $match);
echo $xml;
<set1>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set1>
<set2>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set2>