$re = '/^(\/[^\/]*\/[^\/]*\/[^\/]*\/[^\/]*\/[^\/]*\/)[^\/]*(\/[^\/]*)/m';
$str = '/v1/cp/members/XXXX/benefits/XXXXBG-XXXX-XXXX/benefitlist
/v1/cp/members/XXXX/benefits/XXXXBG-XXXX-XXXX-/excluded
/v1/cp/members/XXXX/benefits/XXXXUF-XXXX-XXXX-/benefitlist
/v1/cp/members/XXXX/benefits/XXXXUF-XXXX-XXXX-/excluded
/v1/cp/members/XXXX/benefits/XXXXUU-XXXX-XXXX-/benefitlist
/v1/cp/members/XXXX/benefits/XXXXUU-XXXX-XXXX-/excluded
/v1/cp/members/XXXX/benefits/XXXXXXXX-XXXX-XXXX-/benefitlist
/v1/cp/members/XXXX/benefits/XXXXXXXX-XXXX-XXXX-/excluded';
$subst = "\1XXXX\2";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php