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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 check(self, diff):
 """Check that the name of the subpackage within contrib is valid The package name must match ``user_[a-zA-Z0-9_]+``. """

        relative_path = relative_to_contrib(diff, self.project)
        subpackage_name = relative_path.parts[0]
        assert re_test(SUBPACKAGE_NAME_REGEX, subpackage_name)