<?php

$re = '~<a\s[^>]+\.zip[^"]*">.*?</a>$~';
$str = '<a title="" href="http://s...content-available-to-author-only...e.ru/downloads/soft.zip">bestsoft</a>';
$subst = '$0<hr />';
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;