<?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, $matches, PREG_SET_ORDER);
var_dump($matches);
foreach($matches as $m) {
    echo $m[1] . "\n";
}