<?php
$sourcestring="test <br><br> <br> <br><br>test";
preg_match_all('/ <br><br> <br> <br><br>/',$sourcestring,$matches);
echo "<pre>".print_r($matches,true);
?>
or if you want to use a shorter version of the same pattern:
<?php
$sourcestring="your source string";
preg_match_all('/ {9}(<br>)\1( ){4} \1\2{8} \1\1/',$sourcestring,$matches);
echo "<pre>".print_r($matches,true);
?>