REGEXP 15 DIGITS V2

<?php
//That what i've understood in the begging :P
$str = 'aaaaaaaaaa83744484855443543bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb162539847192837ccccccccccc54354ddddddddddd304948374658903eeeeeeeeeeeeee32532532g543ferfewfaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
$regExp = '#(.)\1{14}#';
preg_match_all($regExp, $str, $match);

var_dump($match);

//That what you need:

$str = '162539847192837ccccccccccc54354ddddddddddd304948374658903eeeeeeeeeeeeee32532532g543ferfewfaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';

//with subpattrens will work with all PCRE libraries i think. use this if you get regexp compile error
//$regExp = '#([^0-9]|^)([1-9][0-9]{14})[^0-9]([^0-9]|$)#';

//With assertions, more Clean
$regExp = '#(?<![0-9])([1-9][0-9]{14})(?=[^0-9])#';
preg_match_all($regExp, $str, $match);

var_dump($match);

?>