$re = '/\[.+?\]\s*/';
$str = '[Block 1] Thisfile.txt
[Block 1] Thisfile1.txt
[Block 1] Thisfile2.txt
[Backup001] Thatfile1.doc
[Backup001] Thatfile2.doc
[Backup001] Thatfile3.doc
[Explode] Thisplace.xls
[Explode] Thisplace1.xls
[Explode] Thisplace2.xls';
preg_match_all($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