old_contents: import ipaddress
  
  second = 1
  minute = 60*second
  hour = 60*minute
  day = 24*hour
  
  class Address:
      def __init__(self, IPv4 = None, IPv6 = None):
          self._IPv4 = None if IPv4 is None else ipaddress.IPv4Address(IPv4)
          self._IPv6 = None if IPv6 is None else ipaddress.IPv6Address(IPv6)
      
      def IPv4(self):
          return Address(IPv4 = self._IPv4)
      
      def IPv6(self):
          return Address(IPv6 = self._IPv6)
  
  class Name:
      def __init__(self, address = None, MX = None, TCP = None, UDP = None):
          self._address = address
  
  class Service:
      def __init__(self, SRV = None, TLSA=None):
          self._SRV = SRV
          self._TLSA = TLSA
  
  class CName:
      def __init__(self, name):
          self._name = name
  
  class Delegation():
      def __init__(self, NS, DS = None):
          pass
  
  class Zone:
      def __init__(self, name, mail, NS,
                   secondary_refresh, secondary_retry, secondary_discard,
                   NX_TTL = None, A_TTL = None, other_TTL = None,
                   domains = []):
          assert other_TTL is not None
          self._NX_TTL = other_TTL if NX_TTL is None else NX_TTL
          self._A_TTL = other_TTL if A_TTL is None else A_TTL
          self._other_TTL = other_TTL
      
      def write(self, file):
          raise NotImplementedError()
  
new_contents: import ipaddress, re
  
  second = 1
  minute = 60*second
  hour = 60*minute
  day = 24*hour
  
  def hostname(name):
      # check hostname for validity
      label = r'[a-zA-Z90-9]([a-zA-Z90-9-]{0,61}[a-zA-Z90-9])?' # must not start or end with hyphen
      pattern = r'^{0}(\.{0})*\.?'.format(label)
      print(pattern)
      if re.match(pattern, name):
          return name
      raise Exception(name+" is not a valid hostname")
  
  class Address:
      def __init__(self, IPv4 = None, IPv6 = None):
          self._IPv4 = None if IPv4 is None else ipaddress.IPv4Address(IPv4)
          self._IPv6 = None if IPv6 is None else ipaddress.IPv6Address(IPv6)
      
      def IPv4(self):
          return Address(IPv4 = self._IPv4)
      
      def IPv6(self):
          return Address(IPv6 = self._IPv6)
  
  class Name:
      def __init__(self, address = None, MX = None, TCP = None, UDP = None):
          self._address = address
  
  class Service:
      def __init__(self, SRV = None, TLSA=None):
          self._SRV = SRV
          self._TLSA = TLSA
  
  class CName:
      def __init__(self, name):
          self._name = name
  
  class Delegation():
      def __init__(self, NS, DS = None):
          pass
  
  class Zone:
      def __init__(self, name, mail, NS,
                   secondary_refresh, secondary_retry, secondary_discard,
                   NX_TTL = None, A_TTL = None, other_TTL = None,
                   domains = []):
          self._name = hostname(name)
          assert mail.endswith('.'), "Mail must be absolute, end with a dot"
          atpos = mail.find('@')
          assert atpos >= 0 and atpos < mail.find('.'), "Mail must contain an @ before the first dot"
          self._mail = hostname(mail.replace('@', '.', 1))
          self._NS = list(map(hostname, NS))
          
          self._secondary_refresh = secondary_refresh
          self._secondary_retry = secondary_retry
          self._secondary_discard = secondary_discard
          
          assert other_TTL is not None
          self._NX_TTL = other_TTL if NX_TTL is None else NX_TTL
          self._A_TTL = other_TTL if A_TTL is None else A_TTL
          self._other_TTL = other_TTL
      
      def write(self, file):
          raise NotImplementedError()
  
subject: validate hostnames