<?php
$string = 'asdfasdf {_LINK_2_} asdf {_LINK_1_}{_LINK_3_} asdf{_LINK_8_}';
$exp = '/(?<=\{_LINK_)[0-9]+(?=_\})/';
?>
PD9waHAKJHJlc3VsdHMgPSBhcnJheSgpOwokc3RyaW5nID0gJ2FzZGZhc2RmIHtfTElOS18yX30gYXNkZiB7X0xJTktfMV99e19MSU5LXzNffSBhc2Rme19MSU5LXzhffSc7CiRleHAgPSAnLyg/PD1ce19MSU5LXylbMC05XSsoPz1fXH0pLyc7CiRmaW5kID0gcHJlZ19tYXRjaF9hbGwoJGV4cCwgJHN0cmluZywgJHJlc3VsdHMpOwpwcmludF9yKCRyZXN1bHRzKTsKPz4=