In response to this question you must mention that regret which is not related to your professional or study life. HR will give no consideration to such regret and it will be a positive sign. But if you start mentioning your professional shortcomings, it will give an opportunity to the HR to question your professional credibility and he will not hire you. The whole crux of the matter is you should be positive and not mention your negative qualities which call lead to loss of opportunity for you.