Description:
------------
The regular expression /\Q$z\E/ matches (as far as I can tell) every 
string.  It certainly matches some strings it shouldn't match:

preg_match("/\Q$z\E/", "") -> MATCH (WRONG!)
preg_match("/\Q$z\E/", "qqqq") -> MATCH (WRONG!)

preg_match("/\Q $z\E/", "qqqq") -> NO MATCH (CORRECT)
preg_match("/\Q $z\E/", " $z") -> MATCH (CORRECT)
preg_match("/\Q\$z\E/", "\$z") -> MATCH (CORRECT)
preg_match("/\Q\$z\E/", "$z") -> NO MATCH (CORRECT)

The problem seems to occur if the regular expression begins with the 
string "\Q$" followed by at least one character.