$re = '/(^\d{4}(-\d{2}){2}([^|]+\|){3})([\s\S]*?)\n*(?=^\d{4}.*?(?:[^|\n]+\|){3}|\z)/m';
$str = '2015-03-03 19:30:47.2725|INFO|MyApp|This is a single line log message.
2015-03-03 19:31:29.1209|INFO|MyApp|This log message has multiple
lines with
2015-03-03
a date in it.
2015-03-03 19:32:50.1106|INFO|MyApp|This log message has
multiple lines
but just text only.
2015-03-03 19:33:20.2683|ERROR|MyApp|This log message has multiple lines but
also some confusing text like
2015-03-03 19:33:20.2683|ERROR| which should
still be a valid log message.';
preg_match_all($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