It sounds to me like the goal here is to gather matchtext into an output location, whether it be a variable or newly supported binvar.
If that's the suggestion, $regsub() should be the identifier to improve. $regsub() is built for that specific use case. This is in contrast with $regsubex(), which is explicitly _not_ built for that use case. Supporting &binvars as an output to $regsub() would be trivial, I'd imagine.
If the remaining issue is that $regsub() does not parse the subtext parameter in the same way, adding a switch would easily solve this.
I would simply propose:
1. Add support for &binvar in the output parameter of $regsub()
2. Add a switch (say, 'e') to $regsub() that parses subtext in "extended" mode-- i.e., the way that $regsubex() does.
Unless I'm missing something here, this should address all issues, including any issues with space preservation that may or may not be there.
