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here an experimental code and result. any comment to get better result is welcome. i believe %90 accuracy is possible this way by some improvements

    #include <opencv2/core/utility.hpp>
    #include "opencv2/imgproc.hpp"
    #include "opencv2/imgcodecs.hpp"
    #include "opencv2/highgui.hpp"
    #include <iostream>

    using namespace cv;
    using namespace std;

    Mat src,gray,small,srcclone;
    int threshval = 25;

    static void on_trackbar(int, void*)
    {

        Mat bw = threshval < 128 ? (gray < threshval) : (gray > threshval);
        //imshow( "threshold", bw );

        vector<Vec4i> hierarchy;
        vector<vector<Point> > contours;

        findContours(bw, contours, hierarchy, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);
        srcclone=src.clone();
        Rect minRect;

        for (size_t i = 0; i < contours.size(); ++i)
        {
            // Calculate the area of each contour
            double area = contourArea(contours[i]);
            // Ignore contours that are too small or too large
           // if (area < 1e2 || 1e5 < area) continue;

            minRect = boundingRect( Mat(contours[i]) );

            if (minRect.height < 20 ) continue;

            Scalar color= Scalar(0,255,0);

            if (minRect.height < 50 | minRect.height > minRect.width*2) color= Scalar(0,0,255);

            rectangle( srcclone, minRect,color , 2, 8 );

            // Draw each contour only for visualisation purposes
            //  drawContours(srcclone, contours, static_cast<int>(i), Scalar(0, 0, 255), 2, 8, hierarchy, 0);
            // Find the orientation of each shape
        }

        imwrite("result.jpg",srcclone);
        resize(srcclone,small,Size(src.cols/2,src.rows/2));


        imshow( "Connected Components", small );
    }


    int main( int argc, const char** argv )
    {

        src=imread("14357464885366057.png");
        if(src.empty())
        {
            cout << "Could not read input image file: " << endl;
            return -1;
        }


        cvtColor( src, gray, COLOR_BGR2GRAY );


// you can try different values below
        GaussianBlur(gray,gray,Size(5,5),1,1);  
        Mat kernel = Mat::ones(3, 3, CV_8UC1);
        dilate(gray, gray, kernel);


        namedWindow( "Connected Components", 1 );
        createTrackbar( "Threshold", "Connected Components", &threshval, 127, on_trackbar );
        on_trackbar(threshval, 0);

        waitKey(0);
        return 0;
    }