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.