public Builder identityValidationExpression(String identityValidationExpression) {
            this.identityValidationExpression = identityValidationExpression;
            return this;
        }
Sets the regular expression for validating the token as the incoming
identity. For example, {@code "^x-[a-z]+"}.

@param identityValidationExpression Expression to set.
@return Returns the builder.
smithy-aws-apigateway-traits/src/main/java/software/amazon/smithy/aws/apigateway/traits/AuthorizerDefinition.java