The ugly way is to do something like:

$num =~ m{
  \A
  5
  (?:
    2 (?: 7[89] | [89][0-9] )
    |
    3 (?: [0-8][0-9] | 9[01] )
  )
  \z
}x;
[download]

You can see why I say it's ugly. But it works. I think I could even come up with a range-to-regex converter, for people in such a sad predicament. ;)

Originally posted as a Categorized Answer.