c:\@Work\Perl\monks>perl -wMstrict -le
"for my $pn (qw(
   :1 :23 :456 :999 :1000 :1023
   :1024 :9999 :10000 :65535  :1024x :9999x :10000x :65535x
   :65536 :99999 :123456
   :x :foo :
   ), '', ' ', @ARGV) {
   my $pn_ok =
     $pn =~ m{ : (\d{4,5}) (?! \d) }xms && $1 >= 1024 && $1 <= 65535;
   print qq{'$pn' }, $pn_ok ? 'in range' : 'BAD';
   }
"
':1' BAD
':23' BAD
':456' BAD
':999' BAD
':1000' BAD
':1023' BAD
':1024' in range
':9999' in range
':10000' in range
':65535' in range
':1024x' in range
':9999x' in range
':10000x' in range
':65535x' in range
':65536' BAD
':99999' BAD
':123456' BAD
':x' BAD
':foo' BAD
':' BAD
'' BAD
' ' BAD