Re: [9fans] acme g/$/ funny

["Russ Cox" <rsc@swtch.com>]

> On Jul 17, 2008, at 6:03 AM, roger peppe wrote:
>> Edit ,x/.*/g/$/a/foo/
>>
>> shouldn't this append "foo" after every line?
>>
>> sam gives slightly different behaviour here
>> (but still questionable) - it appends "foo" after
>> every empty line.
>>
>> is this actually a bug, or have i misunderstood the
>> way that '$' is meant to work?
>>
>> it does seem strange that in the following edit
>> command, the guard never matches anything.
>>
>> Edit ,x/foo$/g/foo$/d
>>

pietro:
> You misunderstood how Pike regexps work
> ...
> That appends foo at the beginning of the next line. Try i/foo/.

It always brings a smile to my face when you say
things like that to people who have forgotten more
about Plan 9 than you know.  Thank you.


rog:
> Edit ,x/.*/g/$/a/foo/
>
> shouldn't this append "foo" after every line?

I would have expected it to.


pietro:
> The  pattern /./ matches everything EXCEPT a newline,
> which would be matched with $.

This is only half right.  $ matches the empty string before a newline,
not the newline itself.  Don't believe me?  Search for $$.

The real issue here is that inside an x/.*/ loop, the text being
considered has no newline, so the position at the end is no longer
an "empty string before a newline."  (The newline is outside the
search window.)

One possible fix would be to redefine $ to match the end
of the text as well as before newlines.  I've sometimes wanted
that in x loops that don't iterate over whole lines.  That would
have the unfortunate effect that if you had a four-line file like:

	abc\n
	def\n
	ghi\n
	jkl\n

and you ran ,s/$/!/g you would then have the four-and-a-half line file:

	abc!\n
	def!\n
	ghi!\n
	jkl!\n
	!

so you'd have to then define that $ matches the end of the text
unless the last character is a newline.  This is the point where
I usually give up and decide the current semantics are not worth
fiddling with.

Russ