Computer vision, Coupon code detection, sugested algorithms
You may try an OCR detector like Teseract with OpenCV:

https://github.com/tesseract-ocr

Train the Teseract algorithm with text samples obtained from the numbers you want to detect and it will work.

EDIT: If you want a similar implementation of what you want you may check this:

http://answers.opencv.org/question/10985/how-can-i-determine-the-location-of-lcdled-display-in-an-image-with-opencv-and-perl/