$str = "http://aaa.bbb/wafwaowja/c=d1e2f3G4H5/ijk/l=m6n7/";
if (preg_match($regex = "%http://aaa.bbb/.*?c=([[:alnum:]]{1,})/.*?$%", $str, $matchs)) {
$str = preg_replace($regex, "http://eee/f={$matchs[1]}/ggg", $str);
}
echo $str;