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There's no rule without exception.
Literal
Exception [of-の] not-exist [attributive] rule [topic-は] does-not-exist.
A proverb built from nested negatives. 例外のない ('exception-less,' i.e., 'rule without exception') is a relative clause modifying 規則 ('rule'). The whole subject then negates existence via ありません ('does not exist'). Two negatives stacking: rules-without-exceptions + don't-exist.