You'd have to ask ken why he chose the characters he did, but I can answer the second question. The beginning and end of line are the same. If you make ^ mean both beginning and end of line, what does this ed command do:

s/^/x/

Which end gets the x?

-rob

On Fri, Sep 17, 2021 at 7:00 PM markus schnalke <meillo@marmaro.de> wrote:

Hoi,

I'm interested in the early design decisions for meta characters
in REs, mainly regarding Ken's RE implementation in ed.

Two questions:

1) Circumflex

As far as I see, the circumflex (^) is the only meta character that
has two different special meanings in REs: First being the
beginning of line anchor and second inverting a character class.
Why was it chosen for the second one? Why not the exclamation mark
in that case? (Sure, C didn't exist by then, but the bang probably
was used to negate in other languages of the time, I think.)

2) Symbol for the end of line anchor

What is the reason that the beginning of line and end of line
anchors are different symbols? Is there a reason why not only one
symbol, say the circumflex, was chosen to represent both? I
currently see no disadvantages of such a design. (Circumflexes
aren't likely to end lines of text, neither.)

I would appreciate if you could help me understand these design
decisions better. Maybe there existed RE notations that were simply
copied ...


meillo