$re = '/(\d+:\d{4}) (\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{3}) ([A-Za-z\n]+) (.*?)(?=\n\d+:|$)/s';
$str = '5376:0084 2015-08-07 13:51:29.103 Error ### Error Message ###
5376:0084 2015-08-07 13:51:35.545 Error Discarding invalid session
System.Web.Services.Protocols.SoapException: Verify Session ID failed
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
5376:0084 2015-08-07 13:51:36.013 Error ### Error Message ###';
preg_match_all($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