$re = '/-{3}.*[\s\S]*?(?=\s*(?:-{3}|$))/';
$str = '---Agent 1: Wednesday 08/16/2017 | 11:43 AM ---
Message 1
--- Agent 1: Friday 06/09/2017 | 9:02 AM ---
Message 2
--- Agent 1: Friday 04/14/2017 | 10:35 AM ---
Message 3
--- Agent 1: Monday 02/13/2017 | 12:07 AM ---
This
is
message
3
--- Agent 1: Monday 12/19/2016 | 1:31 PM ---
Message 4
--- Agent 1: Monday 10/24/2016 | 10:48 AM ---
Message 5';
preg_match_all($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