欸

āi
  1. (indicating response)
  2. (sadly sighing)
  3. also pr. èi

CEDICT

Homographs

欸 ǎi
  1. (Tw) (literary) to rebuke loudly
  2. (Tw) (literary) to sigh
  3. used in 欸乃 ǎinǎi
欸 ēi hey (used to draw attention)
欸 éi eh; huh (used to express surprise)
欸 ěi hmm (used to express disapproval)
欸 èi yes (used to express agreement)