Re: git diff -I<regex> does not work with empty +/- lines

[Johannes Altmanninger <aclopte@gmail.com>]

On Sun, Nov 28, 2021 at 03:26:10PM -0800, Junio C Hamano wrote:
> Johannes Altmanninger <aclopte@gmail.com> writes:
> 
> > diff -I<regex> suppresses hunks where all +/- lines match <regex>.
> > it is useful to filter away boilerplate changes.
> >
> > Unfortunately, it doesn't help if a hunk has a blank line, because the one
> > obvious way to filter out blank lines (^$) match *every* line, surprisingly.
> > This is because for a line "01\n"
> > we have a zero-width match here ^$ (offset 3).
> >
> > IOW, while we succesfully ignore deleted blank lines
> >
> > 	printf '\n' | git diff --no-index - /dev/null -I'^$'
> > 	diff --git a/- b/-
> > 	deleted file mode 100644
> >
> > we also ignore non-blank lines (very surprising)
> >
> > 	printf 'non-blank-line\n' | git diff --no-index - /dev/null -I'^$'
> > 	diff --git a/- b/-
> > 	deleted file mode 100644
> >
> > unless they don't end in a newline (special case)
> >
> > 	printf 'line without ending newline' | git diff --no-index - /dev/null -I'^$'
> > 	diff --git a/- b/-
> > 	deleted file mode 100644
> > 	--- a/-
> > 	+++ /dev/null
> > 	@@ -1 +0,0 @@
> > 	-line without ending newline
> > 	\ No newline at end of file
> >
> > This patch fixes the second example. Is this the right direction?
> 
> I do not know where in the code the breakage in the first example
> comes from.  It sounds like a bug if a pattern is not matched
> honoring the anchor, whether the beginning-of-line "^" or the
> end-of-line "$" one.

THe first example (printf '\n' | ... -I'^$') works fine AFAICT.
The regex matches the empty line, so the hunk is ignored
(but the file header is still printed).
Only the second example (printf 'non-blank-line\n' | ... -I'^$')
shows that we ignore too much, because ^$ (incorrectly) matches,
but it should only match blank lines.

Thanks