When extracting configuration data from managed network gear such as Juniper or Cisco switches, consecutive vlans are combined into ranges "from-to". Given a $list of vlans such as "2-4,10-12" I needed an @array of every single vlan. An easy way to accomplish this is to make use of Perl's ".." range operator like so: 
my $list = "2-4,10-12";

$list =~ s/\-/../g;
my @array = eval( $list );

# @array is now (2,3,4,10,11,12)

[download]

So far so good. What I'm looking for is an equally simple way to reverse the process. I mashed together the following code to get the job done but it's just painful to look at: 

my @array = (2,3,4,10,11,12);

my @vlans = ();
my $span = undef;
my $last = undef;
foreach my $vlan (sort @array) {
  if (defined $last && $vlan == $last+1) {
    unless (defined $span) {
      $span = $last;
    }
    $vlans[$#vlans] = $span.'-'.$vlan;
  } else {
    push @vlans, $vlan;
    $span = undef;
  }
  $last = $vlan;
}
my $list = join(',', @vlans);

# $list is now "2-4,10-12" again

[download]

Surely there must be a better way?!

Edit: Single elements should appear like "2-4,7,10-12,20". Sorry for not pointing this out.

-- Time flies when you don't know what you're doing

In reply to [Solved] Converting a list of numbers to use a range operator by FloydATC

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