I have a system where a user provides a set of regular expressions and a set of replacement strings (what you put in the right side of the s/// operator). All of these 'rules' (not to be confused with Perl 6 rules) can be embedded inside each other. The problem with embedding is that it can make managing numeric capture groups references difficult to deal with. If you embed one rule in another and they both have capture groups, the numbering of capturing groups gets shifted in the final combined regex gets shifted around. I'd like to be able to not require the users to think about this problem which means that part of my preprocessing code that runs before actually creating the final regex needs to some how figure out where all the capture groups are and renumber the references and back references appropriately. This seems to be not an easy problem, but I'm hoping that there might either be a simple solution other than parsing the whole regex myself or a good regex parser around that I could use to manage this problem. Does anyone here know of good tools for finding out where all the capture groups are inside a regular expression?