$re = '/(?:\G(?!^)|!)(?:[^:\n]*:)+?\K\d{6}/';
$str = ':123456
23/06/2021!06:31:00!872701!4125:063100,4143,101,207,225,229,303,325,329,441:252222
:123456
abcdef:123456
23/06/2021!06:31:00!872701!4125:063100,4143,101,207,225,229,303,325,329,441:252222
';
preg_match_all($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