old_contents: import socket
  import argparse
  import re
  
  
  class RegexMatch(object):
      """
      Performs regular expression match on value.
      If the regular expression pattern matches it will it will return an error
      message that will work with argparse.
      """
  
      def __init__(self, pattern, statement=None):
          self.string_pattern = pattern
          self.pattern = re.compile(pattern)
          self.statement = statement
          if not self.statement:
              self.statement = "must match pattern %s" % self.string_pattern
  
      def __call__(self, string):
          match = self.pattern.search(string)
          if match:
              raise argparse.ArgumentError(None, self.statement)
          return string
  
  
  class Hostname(object):
      """
      Checks wether a given hostname is resolvable in DNS, otherwise raising and
      argparse error.
      """
  
      def __init__(self, _socket=None):
          self.socket = _socket or socket  # just used for testing
  
      def __call__(self, string):
          parts = string.split(':')
          name = parts[0]
          host = parts[-1]
          try:
              self.socket.gethostbyname(host)
          except self.socket.gaierror:
              msg = "hostname: %s is not resolvable" % host
              raise argparse.ArgumentError(None, msg)
  
          try:
              self.socket.inet_aton(name)
          except self.socket.error:
              return string  # not an IP
          else:
              msg = '%s must be a hostname not an IP' % name
              raise argparse.ArgumentError(None, msg)
  
          return string
  
new_contents: import socket
  import argparse
  import re
  
  
  class RegexMatch(object):
      """
      Performs regular expression match on value.
      If the regular expression pattern matches it will it will return an error
      message that will work with argparse.
      """
  
      def __init__(self, pattern, statement=None):
          self.string_pattern = pattern
          self.pattern = re.compile(pattern)
          self.statement = statement
          if not self.statement:
              self.statement = "must match pattern %s" % self.string_pattern
  
      def __call__(self, string):
          match = self.pattern.search(string)
          if match:
              raise argparse.ArgumentError(None, self.statement)
          return string
  
  
  class Hostname(object):
      """
      Checks wether a given hostname is resolvable in DNS, otherwise raising and
      argparse error.
      """
  
      def __init__(self, _socket=None):
          self.socket = _socket or socket  # just used for testing
  
      def __call__(self, string):
          parts = string.split(':')
          name = parts[0]
          host = parts[-1]
          try:
              self.socket.getaddrinfo(host, 0)
          except self.socket.gaierror:
              msg = "hostname: %s is not resolvable" % host
              raise argparse.ArgumentError(None, msg)
  
          try:
              self.socket.inet_aton(name)
          except self.socket.error:
              return string  # not an IP
          else:
              msg = '%s must be a hostname not an IP' % name
              raise argparse.ArgumentError(None, msg)
  
          return string
  
subject: Support hostname that resolve to IPv6-only address