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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 validate_k8s_version(namespace):
 """Validates a string as a possible Kubernetes version. An empty string is also valid, which tells the server to use its default version."""

    if namespace.kubernetes_version:
        k8s_release_regex = re.compile(r'^[v|V]?(\d+\.\d+\.\d+.*)$')
        found = k8s_release_regex.findall(namespace.kubernetes_version)
        if found:
            namespace.kubernetes_version = found[0]
        else:
            raise CLIError('--kubernetes-version should be the full version number, '
                           'such as "1.7.12" or "1.8.7"')