They don't usually match on something as silly as the regexp /ad/, no—but they _do_ frequently contain a regexp like /\bad\b/.

The problem being—and this is painful personal experience speaking—web frameworks will frequently also cater to directory inode limits of caching reverse-proxies in their cache-busting code by doing something like this:

    "/#{sha[0..1]}/#{sha}.#{orig_ext}"

...which means that, for a SHA that starts with "ad", you get an "/ad/" in the URL.