$a = "fweffeawf";
preg_match('/.{2}(.{4})/', $a, $r);
echo $r[1]; //effe$b = "www.baidu.com/aa/bb.php";
preg_match('#/([^/]+)#', $b, $r);
echo $r[1]; //aa
preg_match('#/([^/]+)$#', $b, $r);
echo $r[1]; //bb.php
preg_match('/.{2}(.{4})/', $a, $r);
echo $r[1]; //effe$b = "www.baidu.com/aa/bb.php";
preg_match('#/([^/]+)#', $b, $r);
echo $r[1]; //aa
preg_match('#/([^/]+)$#', $b, $r);
echo $r[1]; //bb.php
substr($a,2,4);basename($b);