$re = '/BEGIN:VEVENT.*?END:VEVENT/ms';
$str = 'BEGIN:VEVENT
DTSTAMP:20220301T023567Z
UID: xxx
DTSTART;TZID=Asia/Tokyo:20210630T110000
DTEND;TZID=Asia/Tokyo:20210630T160000
DESCRIPTION:\\n--------\\n同日入替 : false\\n前回ゲスト数 : 1\\n次
SUMMARY: xxxxx (xxx / xxx xxx
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20221105T072143Z
UID: xxx
DTSTART;TZID=Asia/Tokyo:20210501T110000
DTEND;TZID=Asia/Tokyo:20210701T160000
DESCRIPTION:\\n--------\\n同日入替 : false\\n前回ゲスト数 : 1\\n次
SUMMARY: xxx (xxx / xxx xxx)
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20220905T023143Z
UID: xxx
DTSTART;TZID=Asia/Tokyo:20220227T110000
DTEND;TZID=Asia/Tokyo:20220227T160000
DESCRIPTION:\\n--------\\n同日入替 : true\\n前回ゲスト数 : 2\\n次回
SUMMARY:★ xxx (xxx / xxx xxx)
END:VEVENT';
preg_match_all($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