$re = '/(?m)(?:^####(.*)\R)?^(OVERVIEW|INTRODUCTION|DETAILS)\R(?s:(.*?))(?=^####|^(?:OVERVIEW|INTRODUCTION|DETAILS)$|\z)/';
$str = '<Irrelevant line>
...
<Irrelevant line>
####<section_title>
OVERVIEW
...
...
INTRODUCTION
...
...
DETAILS
...
...
####<section_title>
OVERVIEW
...
...
INTRODUCTION
...
...
DETAILS
...
...';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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