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Variable length lookbehind not implemented in regex...

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

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