I am a US citizen doing business in China. Are there new regulations about exit and reentry ?

I can't understand you, Hong Kong and Mainland China apply different exit and enter controls, if you hold a single entry visa, use it to enter Mainland China, then go to Hong Kong, you can't use it to re-enter Mainland China because the single entyr visa has been used out. If you hold a double entry visa, you can re-enter Mainland China to use the second entry. If you hold a multiple entry visa, you can travel between Hong Kong and Mainland China for many times.