code: def format_spec_to_regex(field_name, format_spec):
          
          # NOTE: remove escaped backslashes so regex matches
          regex_match = fmt_spec_regex.match(format_spec.replace('\\', ''))
          if regex_match is None:
              raise ValueError("Invalid format specification: '{}'".format(format_spec))
          regex_dict = regex_match.groupdict()
          fill = regex_dict['fill']
          ftype = regex_dict['type']
          width = regex_dict['width']
          align = regex_dict['align']
          # NOTE: does not properly handle `=` alignment
          if fill is None:
              if width is not None and width[0] == '0':
                  fill = '0'
              elif ftype in ['s', 'd']:
                  fill = ' '
  
          char_type = spec_regexes[ftype]
          if ftype == 's' and align and align.endswith('='):
              raise ValueError("Invalid format specification: '{}'".format(format_spec))
          final_regex = char_type
          if ftype in allow_multiple and (not width or width == '0'):
              final_regex += r'*'
          elif width and width != '0':
              if not fill:
                  # we know we have exactly this many characters
                  final_regex += r'{{{}}}'.format(int(width))
              elif fill:
                  # we don't know how many fill characters we have compared to
                  # field characters so just match all characters and sort it out
                  # later during type conversion.
                  final_regex = r'.{{{}}}'.format(int(width))
              elif ftype in allow_multiple:
                  final_regex += r'*'
  
          return r'(?P<{}>{})'.format(field_name, final_regex)
docs: Make an attempt at converting a format spec to a regular expression.