$l .= $_ foreach <DATA>;

while($l =~s/(\w):\s?(\d+)-(\d+)//){
	$h{$1}{$_} = 1 for ($2 .. $3);
}

for $k (sort keys %h){
	print "\n$k: ";
	for (sort {$a <=> $b} keys %{$h{$k}}){
		print "$_ " if $h{$k}{$_-1} == 0;
		print "- $_ " if $h{$k}{$_+1} == 0;
	}
}

__DATA__
a:10-34 b:9-12 c:12-24 e: 1-9
a:1-8 a: 19-24 b:2-6
a:7-11 d:9-23 e: 12-23