arrepentirse

The everyday verb for "to regret" or "to repent," always reflexive and always paired with de before what's regretted: arrepentirse de algo / de haber hecho algo. It covers both the secular sense (regretting a decision) and the religious sense (repenting of sin).

• Me arrepiento de haberle mentido. — I regret having lied to him.
• Nunca te arrepentirás de aprender otro idioma. — You'll never regret learning another language.
• Se arrepintió y pidió perdón. — He repented and asked for forgiveness.

Dropping de before an infinitive or noun — *me arrepiento haberlo hecho* is wrong; Spanish requires me arrepiento de haberlo hecho, unlike English "I regret doing it," which needs no preposition.