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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 sed(filename, before, after, flags='g'):
 """ Search and replaces the given pattern on filename. :param filename: relative or absolute file path. :param before: expression to be replaced (see 'man sed') :param after: expression to replace with (see 'man sed') :param flags: sed-compatible regex flags in example, to make the search and replace case insensitive, specify ``flags="i"``. The ``g`` flag is always specified regardless, so you do not need to remember to include it when overriding this parameter. :returns: If the sed command exit code was zero then return, otherwise raise CalledProcessError. """

    expression = r's/{0}/{1}/{2}'.format(before,
                                         after, flags)

    return subprocess.check_call(["sed", "-i", "-r", "-e",
                                  expression,
                                  os.path.expanduser(filename)])