If the regular expression engine accepted tree structures instead of just strings, you could have first class definitions of fragments of regular expressions. Even better, you could define them as functions, so you could have parameterized fragments. So then you could just apply something like http://edicl.github.io/cl-ppcre/#create-scanner2 on the resulting expression tree without having to use the bizarre definition syntax above.
I'm working on this right now for sed :). It currently works with both GNU and BSD-style sed, either for BREs or EREs. I'm going to make it easier to install sometime soon and then hopefully expand the project to other languages.
