<?php
$str = "FORT/310117/200826/12979898000170";
//$str = "FORT/310117/200826";
//$str = "FORT/12979898000170";
preg_match_all('/(?|(\b\d{6}\b)(?=.+?\b\d{14}\b)|(?<=\b\d{14}\b).+?(\b\d{6}\b)|(\b\d{14}\b)(?=.+?\b\d{6}\b)|(?<=\b\d{6}\b).+?(\b\d{14}\b))/', $str, $matches, PREG_SET_ORDER
); foreach($matches as $m) {
echo $m[1] . "\n";
}
PD9waHAKCiRzdHIgPSAiRk9SVC8zMTAxMTcvMjAwODI2LzEyOTc5ODk4MDAwMTcwIjsKLy8kc3RyID0gIkZPUlQvMzEwMTE3LzIwMDgyNiI7Ci8vJHN0ciA9ICJGT1JULzEyOTc5ODk4MDAwMTcwIjsKcHJlZ19tYXRjaF9hbGwoJy8oP3woXGJcZHs2fVxiKSg/PS4rP1xiXGR7MTR9XGIpfCg/PD1cYlxkezE0fVxiKS4rPyhcYlxkezZ9XGIpfChcYlxkezE0fVxiKSg/PS4rP1xiXGR7Nn1cYil8KD88PVxiXGR7Nn1cYikuKz8oXGJcZHsxNH1cYikpLycsICRzdHIsICRtYXRjaGVzLCBQUkVHX1NFVF9PUkRFUik7CnZhcl9kdW1wKCRtYXRjaGVzKTsKZm9yZWFjaCgkbWF0Y2hlcyBhcyAkbSkgewogICAgZWNobyAkbVsxXSAuICJcbiI7Cn0=