$re = '/<span class=\'indent\'>.*?(<br>).*?<\/span>/';
$str = '<span class=\'together\'>line one,<br><span class=\'indent\'>line two.</span><br>Line three,<br><span class=\'indent\'>line four,<br>line five,<br>line six,<br>line seven;<br>line eight.<br>Line nine;<br>line ten,<br>line eleven,<br>line twelve.</span><br>Line thriteen,<br><span class=\'indent\'>line fourteen,<br>line fifteen,<br>line sixteen,<br>line seventeen,<br>line eighteen.</span></span>';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php