ruefully

To smile in a way that shows you are sad or regretful about something.

To move your head from side to side to show sadness or regret.

To laugh in a way that shows you are sad or regretful about something.

To grin in a way that shows you are sad or regretful about something.

To reflect on something with sadness or regret.

To think deeply or carefully about something with sadness or regret.

To say something as a comment, expressing sadness or regret.

To mention something, expressing sadness or regret.