dog eat dog 残酷无情的竞争；自相残杀

【例句】
The business world is sometimes described figuratively as dog eat dog.
有时，人们会形象地把商业界比喻成自相残杀的世界。

I'm afraid in this line of work it's a case of dog eat dog.
恐怕在这种行业中竞争是残酷无情的。