<?php
$results = array();
$string = 'asdfasdf {_LINK_2_} asdf {_LINK_1_}{_LINK_3_} asdf{_LINK_8_}';
$exp = '/(?<=\{_LINK_)[0-9]+(?=_\})/';
$find = preg_match_all($exp, $string, $results);
print_r($results);
?>