old_contents: # casrn.py - functions for handling CAS Registry Numbers
  #
  # Copyright (C) 2017 Arthur de Jong
  #
  # This library is free software; you can redistribute it and/or
  # modify it under the terms of the GNU Lesser General Public
  # License as published by the Free Software Foundation; either
  # version 2.1 of the License, or (at your option) any later version.
  #
  # This library is distributed in the hope that it will be useful,
  # but WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  # Lesser General Public License for more details.
  #
  # You should have received a copy of the GNU Lesser General Public
  # License along with this library; if not, write to the Free Software
  # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  # 02110-1301 USA
  
  """CAS RN (Chemical Abstracts Service Registry Number).
  
  The CAS Registry Number is a unique identifier assigned by the Chemical
  Abstracts Service (CAS) to a chemical substance.
  
  More information:
  
  * https://en.wikipedia.org/wiki/CAS_Registry_Number
  
  >>> validate('87-86-5')
  '87-86-5'
  >>> validate('87-86-6')
  Traceback (most recent call last):
      ...
  InvalidChecksum: ...
  """
  
  from stdnum.exceptions import *
  from stdnum.util import clean, isdigits
  
  
  def compact(number):
      """Convert the number to the minimal representation."""
      number = clean(number, ' ').strip()
      if '-' not in number:
          number = '-'.join((number[:-3], number[-3:-1], number[-1:]))
      return number
  
  
  def calc_check_digit(number):
      """Calculate the check digit for the number. The passed number should not
      have the check digit included."""
      number = number.replace('-', '')
      return str(
          sum((i + 1) * int(n) for i, n in enumerate(reversed(number))) % 10)
  
  
  def validate(number):
      """Check if the number provided is a valid CAS RN."""
      number = compact(number)
      if not 7 <= len(number) <= 12:
          raise InvalidLength()
      if not isdigits(number[:-5]) or not isdigits(number[-4:-2]):
          raise InvalidFormat()
      if number[-2] != '-' or number[-5] != '-':
          raise InvalidFormat()
      if number[-1] != calc_check_digit(number[:-1]):
          raise InvalidChecksum()
      return number
  
  
  def is_valid(number):
      """Check if the number provided is a valid CAS RN."""
      try:
          return bool(validate(number))
      except ValidationError:
          return False
  
new_contents: # casrn.py - functions for handling CAS Registry Numbers
  #
  # Copyright (C) 2017-2022 Arthur de Jong
  #
  # This library is free software; you can redistribute it and/or
  # modify it under the terms of the GNU Lesser General Public
  # License as published by the Free Software Foundation; either
  # version 2.1 of the License, or (at your option) any later version.
  #
  # This library is distributed in the hope that it will be useful,
  # but WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  # Lesser General Public License for more details.
  #
  # You should have received a copy of the GNU Lesser General Public
  # License along with this library; if not, write to the Free Software
  # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  # 02110-1301 USA
  
  """CAS RN (Chemical Abstracts Service Registry Number).
  
  The CAS Registry Number is a unique identifier assigned by the Chemical
  Abstracts Service (CAS) to a chemical substance.
  
  More information:
  
  * https://en.wikipedia.org/wiki/CAS_Registry_Number
  
  >>> validate('87-86-5')
  '87-86-5'
  >>> validate('87-86-6')
  Traceback (most recent call last):
      ...
  InvalidChecksum: ...
  >>> validate('012770-26-2')
  Traceback (most recent call last):
      ...
  InvalidFormat: ...
  """
  
  import re
  
  from stdnum.exceptions import *
  from stdnum.util import clean, isdigits
  
  
  _cas_re = re.compile(r'^[1-9][0-9]{1,6}-[0-9]{2}-[0-9]$')
  
  
  def compact(number):
      """Convert the number to the minimal representation."""
      number = clean(number, ' ').strip()
      if '-' not in number:
          number = '-'.join((number[:-3], number[-3:-1], number[-1:]))
      return number
  
  
  def calc_check_digit(number):
      """Calculate the check digit for the number. The passed number should not
      have the check digit included."""
      number = number.replace('-', '')
      return str(
          sum((i + 1) * int(n) for i, n in enumerate(reversed(number))) % 10)
  
  
  def validate(number):
      """Check if the number provided is a valid CAS RN."""
      number = compact(number)
      if not 7 <= len(number) <= 12:
          raise InvalidLength()
      if not _cas_re.match(number):
          raise InvalidFormat()
      if number[-1] != calc_check_digit(number[:-1]):
          raise InvalidChecksum()
      return number
  
  
  def is_valid(number):
      """Check if the number provided is a valid CAS RN."""
      try:
          return bool(validate(number))
      except ValidationError:
          return False
  
subject: Improve validation of CAS Registry Number