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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 get_signature_names(self):
 """ Return a list of the signature file names. """

        signature_expr = re.compile("^(META-INF/)(.*)(\.RSA|\.EC|\.DSA)$")
        signatures = []

        for i in self.get_files():
            if signature_expr.search(i):
                signatures.append(i)

        if len(signatures) > 0:
            return signatures

        return None