$re = '/^/([^/]*)/([^/]*)/([^/]*)/([^/]*)/(.*)/';
$str = 'C:/eclipsews/irpages2/report_clients/$1/$2/$3/$4/docs/$5';
$subst = "C:/eclipsews/irpages2/report_clients/\1/\2/\3/\4/docs/\5";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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