<?php
$re = '/(?<!\bTN: )\d{30}/';
$str = 'Item/s shipped:
Mug 15oz
TN: 720315589361269903507180163568
URL: https://w...content-available-to-author-only...l.com/global-en/home/tracking/tracking-ecommerce.html?submit=1&tracking-id=720315589361269903507180163568';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE
, 0);
?>
PD9waHAKJHJlID0gJy8oPzwhXGJUTjogKVxkezMwfS8nOwokc3RyID0gJ0l0ZW0vcyBzaGlwcGVkOgpNdWcgMTVvegpUTjogNzIwMzE1NTg5MzYxMjY5OTAzNTA3MTgwMTYzNTY4ClVSTDogaHR0cHM6Ly93Li4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5sLmNvbS9nbG9iYWwtZW4vaG9tZS90cmFja2luZy90cmFja2luZy1lY29tbWVyY2UuaHRtbD9zdWJtaXQ9MSZ0cmFja2luZy1pZD03MjAzMTU1ODkzNjEyNjk5MDM1MDcxODAxNjM1NjgnOwoKcHJlZ19tYXRjaCgkcmUsICRzdHIsICRtYXRjaGVzLCBQUkVHX09GRlNFVF9DQVBUVVJFLCAwKTsKCnZhcl9kdW1wKCRtYXRjaGVzWzBdWzBdKTsKCj8+