$re = '/((?:(?:https?|ftp):\/{2})(?:(?:[0-9a-z_@-]+\.)+(?:[0-9a-z]){2,4})?(?:(?:\/(?:[~0-9a-z\#\+\%\@\.\/_-]+))?\/)*(?=\s|\"))(\/)?(index\.html?)?/i';
$str = 'there is some text with <a href=" http://www.Something.com/ "> but without index.html; then there is another one that is <a href = "https://www.other.com/more/stuff/index.html"> which should be unaltered.
Finally does it work for href = "http://www.com/stuff/"
http://www.index.com/
http://ex.com/a/b/"
http://www.example.com add /index.html
http://www.example.com/something do nothing
http://www.example.com/something/ add /index.html
http://www.example.com/something/index.html do nothing
';
$subst = "\1/index.html";
$result = preg_replace($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