Application development atop a Complex Event Processing (CEP) system requires extensive knowledge of how the runtime system operates, and specification-language dependencies create barriers for developers. This paper presents CEPLESS, a serverless system for CEP that decouples the specification language from the runtime system: developers can write CEP applications in any specification language while the complexity of the runtime is abstracted away. Novel in-memory processing and batching mechanisms support stateful operator processing at high event rates, achieving performance comparable to Apache Flink while enabling dynamic operator updates in around 238 milliseconds.