regular expression question

[Guest]

In the following regular express
$_="BCADeFGHIJK L";
if (/\L[\w]\E/) { print "true"} else { print "false\n" };
if (/\U[\w]\E/) { print "true"} else { print "false\n" };

why the first one print true while the second one print false?
Can someone explain \L \U?