If you've cut off all the foliage the plant can't photosynthasise, so it is unlikely to recover. The way to keep supermarket plants going is to only take so much leaf at a time, giving new growth time to come through, though to be honest supermarkets don't sell many things that will last for ever, do they? Your best bet is to get some seeds and sow a few pots on the windowsill. Parsley's quite easy. That way, when the plants are established you can take a bit off each one at a time so they all keep going. In the meantime I think you'll just have to go back to Tesco.