code: public
    int decide(LoggingEvent event) {
      String msg = event.getRenderedMessage();
  
      if(msg == null ||  stringToMatch == null)
        return Filter.NEUTRAL;
      
  
      if( msg.indexOf(stringToMatch) == -1 ) {
        return Filter.NEUTRAL;
      } else { // we've got a match
        if(acceptOnMatch) {
  	return Filter.ACCEPT;
        } else {
  	return Filter.DENY;
        }
      }
    }
docs: Returns {@link Filter#NEUTRAL} is there is no string match.