public Optional<String> getIdentityValidationExpression() {
        return Optional.ofNullable(identityValidationExpression);
    }
Gets the regular expression for validating the token as the incoming
identity. For example, {@code "^x-[a-z]+"}.

@return Returns the identity regular expression.
smithy-aws-apigateway-traits/src/main/java/software/amazon/smithy/aws/apigateway/traits/AuthorizerDefinition.java