$a="abcdefghijklm";
1 print "match" if ( $a =~ /(?<!cde).*?jkl/ );	# match because a does not match cde, then bcdefghi follows, then jkl matches?
2 print "match" if ( $a =~ /(?<=cde).*?jkl/ );	# match because cde matches then fgh follws, then jkl matches

$a="xyzfghijklm";
1 print "match" if ( $a =~ /(?<!cde).*?jkl/ );	# match because cde is not before jkl, then jkl matches
2 print "match" if ( $a =~ /(?<=cde).*?jkl/ );	# no match because cde is not before jkl