expression
complete reversal of values or thinking over time; realizing and regretting past errors
A yojijukugo expressing the idea that what one believed in the past now seems wrong, and what one believes now is right, often with a sense of regret or awakening. Used in reflective or philosophical contexts.
彼は若い頃の過ちを振り返り、今是昨非の思いに駆られた。
Looking back on the mistakes of his youth, he was overcome by a feeling of complete reversal of values.
人生観が変わり、まさに今是昨非の心境だ。
My outlook on life has changed; it's truly a state of mind where past wrongs now seem right.
A similar proverbial expression meaning 'yesterday's wrong is today's right,' but less compact and less common as a set phrase.
A yojijukugo derived from classical Chinese, literally 'now right, past wrong.' The exact historical origin is uncertain, but it reflects a common East Asian philosophical theme of moral awakening and changing perspectives over time.