Promtped by a real world example, one often-overlooked feature of most regular expressions engines is how subpatterns can useful to whip up tokenizers relatively easily. The problem? I needed to match the word any of the words "Canton", "Region" or "Group" in a string and perform a follow up action depending on which matched.

His ultimate solution used a set of preg_match generated tokens to do the replaces a bit more reliably. It also makes it easy for other scripts (like his Python example) to use them too.