しまった、ちゃった

This literally means 'to complete, finish' but can (and usually does) involve a regret over having done something. Also it can be used sarcastically to mean the speaker really wanted to do something, but gives a halfhearted apology. For example, 最後のクーキーを食べてしまった。 I unfortunately ate the last cookie. Of course there really wasn't anything unfortunate about it.

The construction is usually after the ~te form of any verb

全部のお金を使ってしまいました
zenbu no okane o tsukatte shimaimashita.

Unfortunately,
I spent all my money.

私は完全に日本語を忘れてしまった
watashi wa kanzen ni nihongo o wasurete shimatta.

Unfortunately, I have completely forgotten Japanese.

Another very useful variation is ~chatta. This is informal and is used by both male and female speakers. chau is made by combining te shimau -> chau

試験に落ちちゃった
shiken ni ochichatta.

I flunked the test unfortunately.

or in the present tense

ケーキを全部食べちゃう。 
ke