text: ##############################################################################
  # Copyright 2009, Gerhard Weis
  # All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without
  # modification, are permitted provided that the following conditions are met:
  #
  #  * Redistributions of source code must retain the above copyright notice,
  #    this list of conditions and the following disclaimer.
  #  * Redistributions in binary form must reproduce the above copyright notice,
  #    this list of conditions and the following disclaimer in the documentation
  #    and/or other materials provided with the distribution.
  #  * Neither the name of the authors nor the names of its contributors
  #    may be used to endorse or promote products derived from this software
  #    without specific prior written permission.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  # AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  # IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  # ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  # LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  # CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  # SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  # INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  # CONTRACT, STRICT LIABILITY, OR TORT
  ##############################################################################
  """
  This module provides an ISO 8601:2004 duration parser.
  
  It also provides a wrapper to strftime. This wrapper makes it easier to
  format timedelta or Duration instances as ISO conforming strings.
  """
  import re
  from datetime import timedelta
  from decimal import Decimal
  
  from isodate.duration import Duration
  from isodate.isodatetime import parse_datetime
  from isodate.isoerror import ISO8601Error
  from isodate.isostrf import D_DEFAULT, strftime
  
  ISO8601_PERIOD_REGEX = re.compile(
      r"^(?P<sign>[+-])?"
      r"P(?!\b)"
      r"(?P<years>[0-9]+([,.][0-9]+)?Y)?"
      r"(?P<months>[0-9]+([,.][0-9]+)?M)?"
      r"(?P<weeks>[0-9]+([,.][0-9]+)?W)?"
      r"(?P<days>[0-9]+([,.][0-9]+)?D)?"
      r"((?P<separator>T)(?P<hours>[0-9]+([,.][0-9]+)?H)?"
      r"(?P<minutes>[0-9]+([,.][0-9]+)?M)?"
      r"(?P<seconds>[0-9]+([,.][0-9]+)?S)?)?$"
  )
  # regular expression to parse ISO duration strings.
  
  
  def parse_duration(datestring):
      """
      Parses an ISO 8601 durations into datetime.timedelta or Duration objects.
  
      If the ISO date string does not contain years or months, a timedelta
      instance is returned, else a Duration instance is returned.
  
      The following duration formats are supported:
        -PnnW                  duration in weeks
        -PnnYnnMnnDTnnHnnMnnS  complete duration specification
        -PYYYYMMDDThhmmss      basic alternative complete date format
        -PYYYY-MM-DDThh:mm:ss  extended alternative complete date format
        -PYYYYDDDThhmmss       basic alternative ordinal date format
        -PYYYY-DDDThh:mm:ss    extended alternative ordinal date format
  
      The '-' is optional.
  
      Limitations:  ISO standard defines some restrictions about where to use
        fractional numbers and which component and format combinations are
        allowed. This parser implementation ignores all those restrictions and
        returns something when it is able to find all necessary components.
        In detail:
          it does not check, whether only the last component has fractions.
          it allows weeks specified with all other combinations
  
        The alternative format does not support durations with years, months or
        days set to 0.
      """
      if not isinstance(datestring, str):
          raise TypeError("Expecting a string %r" % datestring)
      match = ISO8601_PERIOD_REGEX.match(datestring)
      if not match:
          # try alternative format:
          if datestring.startswith("P"):
              durdt = parse_datetime(datestring[1:])
              if durdt.year != 0 or durdt.month != 0:
                  # create Duration
                  ret = Duration(
                      days=durdt.day,
                      seconds=durdt.second,
                      microseconds=durdt.microsecond,
                      minutes=durdt.minute,
                      hours=durdt.hour,
                      months=durdt.month,
                      years=durdt.year,
                  )
              else:  # FIXME: currently not possible in alternative format
                  # create timedelta
                  ret = timedelta(
                      days=durdt.day,
                      seconds=durdt.second,
                      microseconds=durdt.microsecond,
                      minutes=durdt.minute,
                      hours=durdt.hour,
                  )
              return ret
          raise ISO8601Error("Unable to parse duration string %r" % datestring)
      groups = match.groupdict()
      for key, val in groups.items():
          if key not in ("separator", "sign"):
              if val is None:
                  groups[key] = "0n"
              # print groups[key]
              if key in ("years", "months"):
                  groups[key] = Decimal(groups[key][:-1].replace(",", "."))
              else:
                  # these values are passed into a timedelta object,
                  # which works with floats.
                  groups[key] = float(groups[key][:-1].replace(",", "."))
      if groups["years"] == 0 and groups["months"] == 0:
          ret = timedelta(
              days=groups["days"],
              hours=groups["hours"],
              minutes=groups["minutes"],
              seconds=groups["seconds"],
              weeks=groups["weeks"],
          )
          if groups["sign"] == "-":
              ret = timedelta(0) - ret
      else:
          ret = Duration(
              years=groups["years"],
              months=groups["months"],
              days=groups["days"],
              hours=groups["hours"],
              minutes=groups["minutes"],
              seconds=groups["seconds"],
              weeks=groups["weeks"],
          )
          if groups["sign"] == "-":
              ret = Duration(0) - ret
      return ret
  
  
  def duration_isoformat(tduration, format=D_DEFAULT):
      """
      Format duration strings.
  
      This method is just a wrapper around isodate.isostrf.strftime and uses
      P%P (D_DEFAULT) as default format.
      """
      # TODO: implement better decision for negative Durations.
      #       should be done in Duration class in consistent way with timedelta.
      if (
          isinstance(tduration, Duration)
          and (
              tduration.years < 0
              or tduration.months < 0
              or tduration.tdelta < timedelta(0)
          )
      ) or (isinstance(tduration, timedelta) and (tduration < timedelta(0))):
          ret = "-"
      else:
          ret = ""
      ret += strftime(tduration, format)
      return ret
  