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  1. or download this
      !/[^01]/ && !/10/ && ($_ ^ reverse $_) !~ /[^\001]/
  2. or download this
      1 while s/(?<!1)01(?!0)//g;
  length($_) == 0;
  3. or download this
      our $re;
  $re = qr{(?:0(??{$re})1)?};
  /^$re\z/;