This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A090150 #8 Aug 25 2016 20:09:47
%S A090150 2,23,29,211,233,277,2111,2333,2777,2999,23333,2999999,29999999,
%T A090150 2777777777,23333333333,2111111111111,2777777777777777,
%U A090150 23333333333333333,2111111111111111111,2777777777777777777
%N A090150 Primes of the form a string of identical digits preceded by a 2.
%H A090150 Vincenzo Librandi, <a href="/A090150/b090150.txt">Table of n, a(n) for n = 1..47</a>
%t A090150 Select[ FromDigits /@ Flatten[ Table[ PadRight[{2}, i, # ] & /@ {1, 2, 3, 4, 5, 6, 7, 8, 9}, {i, 2, 23}], 1], PrimeQ[ # ] &]
%t A090150 Select[Union[Flatten[Table[FromDigits[PadRight[{2},n,d]],{n,20},{d,{1,3,7,9}}]]],PrimeQ] (* _Harvey P. Dale_, Aug 25 2016 *)
%K A090150 nonn,base
%O A090150 1,1
%A A090150 _Robert G. Wilson v_, Nov 22 2003