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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def extensions(self):
 """ Generate the regular expression to match all the known extensions. @return: the regular expression. @rtype: regular expression object """

        _tmp_extensions = self.mimes.encodings_map.keys() + \
            self.mimes.suffix_map.keys() + \
            self.mimes.types_map[1].keys() + \
            cfg['CFG_BIBDOCFILE_ADDITIONAL_KNOWN_FILE_EXTENSIONS']
        extensions = []
        for ext in _tmp_extensions:
            if ext.startswith('.'):
                extensions.append(ext)
            else:
                extensions.append('.' + ext)
        extensions.sort()
        extensions.reverse()
        extensions = set([ext.lower() for ext in extensions])
        extensions = '\\' + '$|\\'.join(extensions) + '$'
        extensions = extensions.replace('+', '\\+')
        return re.compile(extensions, re.I)