[TUHS] RegExp decision for meta characters: Circumflex

[Rob Pike]

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 at 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
>
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