NSString *string = @"abavavasdsvx,as.dsf/,.[abcdefghijklmn]dgdfg";

　　　　　　NSRange start = [string rangeOfString:@"["];
                     NSRange end = [string rangeOfString:@"]"];
                     NSString *sub = [string substringWithRange:NSMakeRange(start.location, end.location-start.location+1)];

　　　　　　　NSLog(@"sub=%@",sub);