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From: "Dmitry V. Levin" <ldv@altlinux.org>
To: Egor Ignatov <egori@altlinux.org>
Cc: Paul Eggert <eggert@cs.ucla.edu>, bug-gnulib@gnu.org
Subject: Re: [PATCH] regex: fix backreference matching
Date: Mon, 5 Jul 2021 15:12:02 +0300	[thread overview]
Message-ID: <20210705121201.GA20072@altlinux.org> (raw)
In-Reply-To: <85975173-4e58-1402-00c8-8d065b967f99@altlinux.org>

On Tue, Jun 29, 2021 at 11:51:13AM +0300, Egor Ignatov wrote:
> Well, then I have a few questions about matching and capturing
> groups.
> 
> 1. "ab" -> "^(a*)*(.)"
> So, from your test case I can assume that:
> regs[0] = (0, 2]
> regs[1] = (0, 1]
> regs[2] = (1, 2]
> 
> But if we add backref at the end:
> 2. "ab" -> "^(a*)*(.)\1"
> check_matching matches the whole string "ab",
> this means that the first group accepted 'a' but in fact is empty,
> otherwise it could not match backref later on.
> What is the correct match here? Is check_matching wrong and
> should match only "a" in the 2nd group (as it would be with
> "^(a*)(.)\1")? or should set_regs check for this and shrink the
> match?

My test-regex.c entry for a similar but a bit simplified case was:
  /* Test for ** match with backreferences.  */
  { "^(a*)*\\1", "a", REG_EXTENDED, 2, { { 0, 0 }, { 0, 0 } } }

I suppose the corresponding entry for your example would be
  { "^(a*)*(.)\1", "ab", REG_EXTENDED, 3, { { 0, 1 }, { 0, 0 }, { 0, 1 } } }

> Next,
> 3. "aaba" -> "^(a*)*(.)\1"
> Again check_matching matches "aaba", then the first group
> is "a", and were the 2nd 'a' goes?

I suppose the corresponding test-regex.c entry for this case would be
  { "^(a*)*(.)\1", "aaba", REG_EXTENDED, 3, { { 0, 4 }, { 0, 1 }, { 2, 3 } } }


-- 
ldv


  reply	other threads:[~2021-07-05 12:12 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-26  9:08 [PATCH] regex: fix match with possessive quantifier Egor Ignatov
2021-06-06 21:45 ` Dmitry V. Levin
2021-06-07  1:10   ` Dmitry V. Levin
2021-06-16  9:46     ` [PATCH] regex: fix backreference matching Egor Ignatov
2021-06-16 10:13       ` Dmitry V. Levin
2021-06-29  8:51         ` Egor Ignatov
2021-07-05 12:12           ` Dmitry V. Levin [this message]
2021-07-09 12:36             ` [PATCH v2] " Egor Ignatov
2021-06-16 10:18     ` [PATCH] regex: fix match with possessive quantifier Dmitry V. Levin
2021-06-21 21:09   ` Paul Eggert
2021-06-22 15:35     ` Egor Ignatov
2021-06-22 15:35       ` [PATCH] regex: fix assertion in re_node_set_insert Egor Ignatov
2021-06-22 19:41         ` Paul Eggert

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