$re = '/(15:58:45[\s\S]*?15:58:46)/';
$str = '2017-02-20 15:58:45.442 - [XYZ] 155 RADIO_TRAIN_TO_TRACK_INITIATION_COM_SESSION
.VAR1
.VAR2
.VAR3
.VAR4
2017-02-20 15:58:46.432 - [XYZ] 32 RADIO_TRACK_TO_TRAIN_CONFIGURATION_DETERMINATION
.VAR1
.VAR2
.VAR3
.VAR4
.VAR5
.VAR6
2017-02-20 15:58:48.541 - [XYZ] 155 RADIO_TRAIN_TO_TRACK_INITIATION_COM_SESSION
';
preg_match_all($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