要在PHP PCRE函数中双重转义还是不双重转义？

I was looking for a solid article on when double escaping is necessary and when it is not, but I was not able to find anything. Perhaps I didn't look hard enough, because I'm sure there is an explanation out there somewhere, but lets just make it easy to find for the next guy that has this question!I was looking for a solid article on when doubl