亡wáng羊yáng补bǔ牢láo

Literal: lose sheep (and) repair the pen
Figurative: To fix the problem after suffering a loss; it is not too late to mend things once you realize the mistake.

Where does 亡羊补牢 come from?

In the “Strategies of the Warring States”, the minister Zhuang Xin warns the king of Chu that his misrule will bring ruin. The king ignores him, loses part of his realm, and then summons him back in regret. Zhuang Xin consoles him with a saying: if you only mend the pen after a sheep has escaped, it is still not too late; what matters is correcting the lapse in time.

Source (出处): 《战国策》

How is 亡羊补牢 used? Two examples

数shù据jù丢diū了le别bié太tài自zì责zé，现xiàn在zài做zuò好hǎo备bèi份fèn还hái来lái得de及jí，亡wáng羊yáng补bǔ牢láo嘛ma。
Don't blame yourself so much for losing the data; there's still time to make a proper backup. It's never too late to fix things.
发fā现xiàn漏lòu洞dòng后hòu他tā们men立lì刻kè修xiū补bǔ了le系xì统tǒng，总zǒng算suàn是shì亡wáng羊yáng补bǔ牢láo，没méi酿niàng成chéng更gèng大dà损sǔn失shī。
After finding the vulnerability they patched the system at once; at least they mended things in time and no greater loss occurred.

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