- Output for 5.6.38, 7.1.0 - 7.1.24, 7.2.0 - 7.2.33, 7.3.16 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.33, 8.2.0 - 8.2.29, 8.3.0 - 8.3.25, 8.4.1 - 8.4.12
- 12345
<?php
$subject = "New tour subscription [ALKJ-102] [ID:12345]";
preg_match("/(?<=\[ID:)\d+/", $subject, $mat);
echo $mat[0];