<?php
$data = 'foo <DelayEvent>123</DelayEvent> bar';
$new_data = preg_replace('|(?<=<DelayEvent>)\w+(?=</DelayEvent>)|', 'X', $data); echo $new_data;
?>
PD9waHAKJGRhdGEgPSAnZm9vIDxEZWxheUV2ZW50PjEyMzwvRGVsYXlFdmVudD4gYmFyJzsKJG5ld19kYXRhID0gcHJlZ19yZXBsYWNlKCd8KD88PTxEZWxheUV2ZW50PilcdysoPz08L0RlbGF5RXZlbnQ+KXwnLCAnWCcsICRkYXRhKTsJCmVjaG8gJG5ld19kYXRhOwo/Pg==