$re = '/.+?\/([^\/]+?\).+)$/m';
$str = 'url(../img/repeat-elements/tpl-action-section_bg-1200.png) no-repeat 50%/1200px;
url(../img/pages/home/what-is_bg-1400.png) no-repeat 100% 50%/500px;
url(../img/blog/action/action_1-1400.png) no-repeat 100% 50%/500px;';
$subst = "ВАЛЕНОК$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