Human’s emotions are complex, life is complex as well, one has to make big decision for a once-in-life choice or small decisions in the daily basis, to quote Skyfall (2012 movie) : Regret is not professional, do you agree, why or why not?
If a regretful mind drives the positive motivation for oneself or a team, then it is not unprofessional. Regret is a dwelling in the past, a fixation on the already-occurred consequence of one or more defects among assumptions, data collection, analyses, the interpolation of unknowns from what was known, or execution that is part of the professional record. If regret is directed, and even mined as a positive motivator for oneself, or a team, in those efforts to fix the process, then it is not unprofessional. Regret occurs as a direct consequence of understanding and empathizing with the consequences of something. One must not dwell upon it, but learn from it and improve.
A regretful mind can be unprofessional if it distracts your energy and decrease your EQ. In most cases decisions are made on the level of information, knowledge and existing situation. Hence decisions made in the past based on the knowledge, information and situation at that time are correct. When you pass that time you may gain better knowledge and better information that enables you make better decision than the past. BUT the decision you passed to the level of your knowledge was a better decision. So why should you regret. Regretting may come if only you make the decisions deliberately without searching for information. What is needed is focus of energies on the revelation of the root causes of the defects, and the engineering of compensating measures may be possible with available or obtainable resources. Any ways you have to learn from your mistakes and move forward.
The regretful mind shall go with reflective thinking, not just the reflexive thinking. Be concerned if you can't move on after a disappointment. You should be alarmed if a regretful emotion quagmires you, destroying in turn, your self-confidence. let us be evidenced based, where we learn from mistakes, and build a better safer more rational world. If a regretful mind can also do the self-reflective thinking, be proactively learning from the past, and then, it is professional. With that being said, it is true that too much regret, or not letting go of regrets after one has done something about them, can be unprofessional.
As with any things, regret itself is not the problem, it's what you do with it. Wallowing in regret to the point where it distracts from getting on with life is a problem, but using regret as a motivator to avoid the same mistakes is beneficial. To regret sometimes, a decision in the past, is quite natural. Such occasions only remind us that we are not perfect, all the time. Such occasions are teaching situations. and then there is nothing unprofessional about that.
“Nothing helps a man to reform like thinking of the past with regret.” ― Fyodor Dostoyevsky, The Idiot