$re = '~([^/\n]+)/(?=[^/\n]+$)~m';
$str = 'https://d2ynliea65eb6o.cloudfront.net/6100052500-STXMLOPEN/sub_1.m3u8
https://d2ynliea65eb6o.cloudfront.net/6100052499-STXMLOPEN/sub_1.m3u8
https://d2ynliea65eb6o.cloudfront.net/6100052498-STXMLOPEN/sub_1.m3u8';
$subst = "$1/$1";
$result = preg_replace($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