#!/usr/bin/env ruby 

require 'ipaddr'
def dec2ip(dec) return IPAddr.new(dec, Socket::AF_INET).to_s end
def ip2dec(ip) return IPAddr.new(ip).to_i end
file=ARGV[0]
insertline=ARGV[1]
data=File.read(file) # read the whole file
# store all ip address
sortedip=data.scan(/(\d+\.\d+\.\d+\.\d+)/).flatten.map {|x| ip2dec(x)}
# get the ip address to be inserted
insertip=ip2dec(insertline.scan(/\d+\.\d+\.\d+\.\d+/)[0])
sortedip<<insertip
sortedip.sort!
# get the ip before the one being inserted
beforeip=dec2ip(sortedip[sortedip.index(insertip)-1])
data.split("\n").each do |x|
  x="#{x}\n#{insertline}\n"  if x.match(beforeip)
  puts x
end

$ echo $insert
testmachine4 IN A 192.189.1.4

# ruby test.rb file "$insert"
;zone file for test001.btcuk
$TTL 2d
@      IN     SOA    testserver.servers009.btcuk.    root.servers009.btcuk (
                        2010092401            ;               serial number with no of editings
                        24h                       ;               refresh
                        5m                        ;               update entry
                        1w                        ;               expirary
                        2h )                      ;                minimum
                        NS        testserver.servers009.btcuk
testmachine1      IN         A          192.189.1.1
testmachine2      IN         A          192.189.1.2
testmachine3      IN         A          192.189.1.3
testmachine4      IN         A          192.189.1.4
wibble                IN         A          192.189.1.42
tesmachineend    IN         A           192.189.1.255


$ insert="testmachine20      IN         A          192.190.1.5"
$ ruby test.rb file "$insert"
;zone file for test001.btcuk
$TTL 2d
@      IN     SOA    testserver.servers009.btcuk.    root.servers009.btcuk (
                        2010092401            ;               serial number with no of editings
                        24h                       ;               refresh
                        5m                        ;               update entry
                        1w                        ;               expirary
                        2h )                      ;                minimum
                        NS        testserver.servers009.btcuk
testmachine1      IN         A          192.189.1.1
testmachine2      IN         A          192.189.1.2
testmachine3      IN         A          192.189.1.3
wibble                IN         A          192.189.1.42
tesmachineend    IN         A           192.189.1.255
testmachine20      IN         A          192.190.1.5