/ext/pcre/php_pcre.c眺めてたら

>#define PCRE_CACHE_SIZE 4096

とか

> * If we reached cache limit, clean out the items from the head of the list;
> * these are supposedly the oldest ones (but not necessarily the least used
> * ones).

とか書いてあるんだけど、パターン使いまくりはあまりよろしくないってこと？