Feeling sorry about something — usually something you did, or failed to do, and now wish you had handled differently. A regretful person carries a quiet sadness about the past. It is a fairly formal, written-feeling word; in speech people are far more likely to say 'I feel bad about it' or 'I wish I hadn't'.

Most often it describes a look, a voice, a smile, or a shake of the head — the outside signs of the feeling rather than a big statement of it. 'He gave a regretful smile' means the smile had sadness in it, the smile of someone saying no when they wanted to say yes. That is where you will meet the word most.

Here is the part worth slowing down for: regretful and regrettable look like twins, but they sit on opposite sides. Regretful is about the PERSON who feels — only people (and their faces and voices) can be regretful. Regrettable is about the THING that happened — the event nobody is happy about. 'He was regretful' means he felt sorry. 'It was regrettable' means it was a bad situation. Say 'a regretful mistake' and you have accidentally given feelings to the mistake itself.

Also, regretful is milder than it looks. It does not mean tortured by guilt. It is closer to 'wishing things had gone otherwise' — a soft, adult, slightly resigned feeling. For real guilt English reaches for 'remorseful' or 'ashamed'.