Re: [Bug] S-flag imposes non-greedy match where it shouldn't

[Sebastian Gniazdowski <sgniazdowski@xxxxxxxxx>]

I've attached the extended description. It includes a trick to
work-around the unintuitive behavior of S. It looks as follows:

http://psprint.blinkenshell.org/S_flag.png

I think that the way the S flag works is a bit of an inconsistency,
Because ${str%%X##**} would not stop at the first from the right
match, it would try other matches starting from the right and go on up
to the final first from the left X. I think that (S) shouldn't change
this, but on the other hand should ${(S)str%%X##} match the first
three X? Rather not, as it would resemble ## then... Intuitively,
however, it should match all the three right X.

On Thu, 19 Dec 2019 at 16:30, Daniel Shahaf <d.s@xxxxxxxxxxxxxxxxxx> wrote:
>
> Sebastian Gniazdowski wrote on Wed, 18 Dec 2019 20:44 +00:00:
> > Or rather not a bug… It seems that it's the result of how % searches
> > the substrings from the end – it stops at the first match, i.e.: after
> > finding a first X from the right.
>
> Could we improve the documentation of (S), then?



-- 
Sebastian Gniazdowski
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From 6a2d5a6f0b69cbfb8c76176e55e233a3c710a42b Mon Sep 17 00:00:00 2001
From: Sebastian Gniazdowski <sgniazdowski@xxxxxxxxx>
Date: Thu, 26 Dec 2019 19:22:41 +0100
Subject: [PATCH] Extend description of S flag

---
 Doc/Zsh/expn.yo | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Doc/Zsh/expn.yo b/Doc/Zsh/expn.yo
index d7147dbd7..90d03a91b 100644
--- a/Doc/Zsh/expn.yo
+++ b/Doc/Zsh/expn.yo
@@ -1399,6 +1399,20 @@ from the beginning and with tt(%) start from the end of the string.
 With substitution via tt(${)...tt(/)...tt(}) or
 tt(${)...tt(//)...tt(}), specifies non-greedy matching, i.e. that the
 shortest instead of the longest match should be replaced.
+The substring search means that the pattern is matched skipping the
+parts of the input string starting from the direction set by the use
+of tt(#) or tt(%). For example, to match a pattern starting from the
+end, one could use:
+
+example(str="abcXXXdefXXXghi"
+out=${(S)str%%(#b)([^X])X##}
+out=$out${match[1]}
+)
+
+The result is tt(abcXXXdefghi). It would have been tt(abcXXXdefXXghif)
+if not the tt([^X]) part, as despite the tt(%%) specifies a greedy
+match, the substring matching works by trying matches from right to
+left and stops at a first valid match.
 )
 item(tt(I:)var(expr)tt(:))(
 Search the var(expr)th match (where var(expr) evaluates to a number).
-- 
2.21.0