$re = '/(\d+/\d+/\d+, \d+:\d+\d+) - (.+?): (.*)/';
$str = '20/01/18, 08:11 - Peter: Good morning
How are you?
Peter
20/01/18, 09:00 - Caroline: I am fine thanks.
You?
20/01/18, 09:01 - Peter: Good
I had some problems few days ago.
Now I am happy
Are you working?
20/01/18, 09:02 - Caroline: No I have to go to the supermarket to buy vegetables
20/01/18, 09:12 - Peter: Nice!
Where are you now?';
preg_match_all($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