in reply to Re^3: Regex fun
in thread Regex fun

Sorry, I don't understand—not the reason for what?
Quoting myself where I am quoting you:
the variable that contains the contents of the first capture group is $1, but that's empty until the capture has completed.
You're claiming $1 is "empty" until the the capture has completed. I'm pointing that the in the case of the OP, said first capture has completed.
Do you mean ‘re-entrant’?
No, I don't. The current regexp-engine isn't re-entrant.