code: public int decide(final LoggingEvent event) {
          if (expressionRule.evaluate(event, null)) {
              if (acceptOnMatch) {
                  return Filter.ACCEPT;
              } else {
                  return Filter.DENY;
              }
          }
          return Filter.NEUTRAL;
      }
docs: Determines if event matches the filter.
  @param event logging event;
  @return {@link Filter#NEUTRAL} is there is no string match.