やっちゃった

やっちゃった is a casual contraction of やってしまった (did and regret it). The 〜ちゃった / 〜じゃった pattern is the casual spoken form of 〜てしまった, which expresses an action done completely, often with a sense of regret.

Contraction

やってしまった → やっちゃった

Register

やっちゃった is lighter than やらかした. It suits minor slip-ups and has a self-deprecating, sometimes cute quality:

• やっちゃった: "oops" — minor mistake, somewhat playful
• やらかした: "I screwed up" — bigger blunder, more serious

〜ちゃった PATTERN:
This contraction is widely used beyond やる:

• 食(た)べちゃった: I ate it (and maybe shouldn't have)
• 忘(わす)れちゃった: I forgot (oops)
• 壊(こわ)しちゃった: I broke it (accidentally)
• 言(い)っちゃった: I said it (slipped out)

The pattern always implies the action happened unintentionally or against the speaker's better judgment.