$re = '/^(?=[\d.:]+[ \t]+DEBUG[^\n]*$)(.+?)(?=^[\d.:]+[ \t]+DEBUG|\z)/ms';
$str = '01:15:21.882 DEBUG [SampleProcess]
Sample Message
01:15:21.882 DEBUG [SampleProcess1]
Summary Report
Sample Text1: 126
Sample Text2: 2330
Sample Text3: 2331
Sample Text4: 0
01:15:21.883 DEBUG [SampleProcess2]';
preg_match_all($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