$re = '/\d+(?=\D*$)/m';
$str = 'http://forums.scamadviser.com/site-feedback-issues-feature-requests/20/
https://forums.questionablecontent.net/index.php/board,1.50.html
https://forums.comodo.com/how-can-i-help-comodo-please-we-need-you-b39.30/';
$subst = "";
$result = preg_replace($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