I'd like to understand it.. ;=)

Is this a lex parser as noted in perlre ?

It just support the test cases? or can be extended to support more regex operators?

I was trying a raw approach but when i saw your i gave up.

Brilliant regex hacker!

UPDATE: my attempt with the simplicistic data sample proposed in OP (show representative data as corollary to Know your data principle), was something like:

use strict;
use warnings;

while(<DATA>){
      chomp;
      my $rx;
      eval{ $rx = qr($_) };
      if($@){print "-->$_<-- INVALID: ",lc $@=~s/$0 line \d+,//r; next
+}
      print "$rx\t=>\t";
      s/[\(\)]//g;
      foreach my $part (split /\|/){
          if ($part=~/(\d+)?\[\d+\]/){
              print join ' ', grep{/$rx/}$1.0 .. $1.9;
              print ' ';
          }
          else{print "$part "}
      }
      print "\n";
}
__DATA__
(65|70)
(3999[678]|4[1678])
5[45]
(6[4569]|7[01])
(((broken
(65|70)

# out

(?^:(65|70))    =>      65 70
(?^:(3999[678]|4[1678]))        =>      39996 39997 39998 41 46 47 48
(?^:5[45])      =>      54 55
(?^:(6[4569]|7[01]))    =>      64 65 66 69 70 71
-->(((broken<-- INVALID: unmatched ( in regex; marked by <-- here in m
+/((( <-- here broken/ at <data> line 5.
(?^:(65|70))    =>      65 70
[download]

L*