Concatenate two primes is also prime

var upto = 100
var arr  = upto.primes
var base = 10

arr = arr.map {|p|
    var d = p.digits(base);
    arr.map {|q| [q.digits(base)..., d...].digits2num(base) }.grep { .is_prime }
}.flat.uniq.sort

say "Concatenated primes from primes p,q <= #{upto}:"

arr.each_slice(10, {|*a|
    say a.map { '%6s' % _ }.join(' ')
})

say "\nFound #{arr.len} such concatenated primes."

Output:

Concatenated primes from primes p,q <= 100:
    23     37     53     73    113    137    173    193    197    211
   223    229    233    241    271    283    293    311    313    317
   331    337    347    353    359    367    373    379    383    389
   397    433    523    541    547    571    593    613    617    673
   677    719    733    743    761    773    797    977   1117   1123
  1129   1153   1171   1319   1361   1367   1373   1723   1741   1747
  1753   1759   1783   1789   1913   1931   1973   1979   1997   2311
  2341   2347   2371   2383   2389   2917   2953   2971   3119   3137
  3167   3719   3761   3767   3779   3797   4111   4129   4153   4159
  4337   4373   4397   4723   4729   4759   4783   4789   5323   5347
  5923   5953   6113   6131   6143   6173   6197   6719   6737   6761
  6779   7129   7159   7331   7919   7937   8311   8317   8329   8353
  8389   8923   8929   8941   8971   9719   9743   9767

Found 128 such concatenated primes.