$ perl -wE 'say "aaaab" =~ /(?<=a{1,20})b/'
Variable length lookbehind not implemented in regex m/(?<=a{1,20})b/ at -e line 1.
[$?: 255]
$ blead perl -wE 'say "aaaab" =~ /(?<=a{1,20})b/'
Variable length lookbehind is experimental in regex; marked by <-- HERE in m/(?<=a{1,20})b <-- HERE / at -e line 1.
1