The http_client_body keyword is a content modifier that restricts the search to the body of an HTTP client request.
As this keyword is a modifier to the previous content keyword, there must be a content in the rule before 'http_client_body' is specified.
The amount of data that is inspected with this option depends on the post_depth config option of HttpInspect. Pattern matches with this keyword wont work when post_depth is set to -1.