<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _preprocess_format(self):
 """ Preprocess the format_string attribute. Splits the format string on each placeholder and returns a list of tuples containing substring, placeholder name, and function retrieving content for placeholder (getter). Relevant placeholder functions (getters) are taken from 'placeholders' attribute which is a dict. If no matching placeholder is found in 'placeholders' getter is set to None. Getter and placeholder are also always set to None in first element of the returned list, because it never contain a real placeholder (read re.split documentation for further information). """

        format_split = re.split(r'(?<!\\)%', self.format_string)
        preprocessed_format = []

        for idx, substr in enumerate(format_split):
            if idx == 0:
                getter = None
                placeholder = None
            else:
                pattern = MAIN_PATTERN.format(ph=r'\S')
                try:
                    placeholder = re.match(pattern, substr).group('placeholder').strip('[]')
                except AttributeError:
                    placeholder = None

                if placeholder == 'S':
                    self.one_line = True

                try:
                    getter = self.placeholders[placeholder]
                except KeyError:
                    getter = None
                    substr = re.sub(pattern, '', substr)

            format_elem = (substr, placeholder, getter)
            preprocessed_format.append(format_elem)

        return preprocessed_format