Variable length lookbehind not implemented in regex...

##</code><code>##

my $pattern = "(?<!ss)abc"; 	 # 'ss' in lookbehind
qr/$pattern/i; 			 # throws the error

my $pattern = "(?<!st)abc";	 # 'st' in lookbehind 
qr/$pattern/i; 			 # throws the error

my $pattern =  "(?<!st)abc";	# 'st' in lookbehind
qr/$pattern/;  			# this works fine since modifier 'i' is not used

my $pattern =  "(?<!s)abc";	# 's' in lookbehind
qr/$pattern/i; 			# this works fine

my $pattern =  "(?<!s[st])abc";	# 's[st]' in lookbehind
qr/$pattern/i;                  # this works, whereas these fail: (?<!s[s]),(?<!s[t])