Fix regexp misbehavior with capturing parens inside "{0}".

Regexps like "(.){0}...\1" drew an "invalid backreference number".
That's not unreasonable on its face, since the capture group will
never be matched if it's iterated zero times.  However, other engines
such as Perl's don't complain about this, nor do we throw an error for
related cases such as "(.)|\1", even though that backref can never
succeed either.  Also, if the zero-iterations case happens at runtime
rather than compile time --- say, "(x)*...\1" when there's no "x" to
be found --- that's not an error, we just deem the backref to not
match.  Making this even less defensible, no error was thrown for
nested cases such as "((.)){0}...\2"; and to add insult to injury,
those cases could result in assertion failures instead.  (It seems
that nothing especially bad happened in non-assert builds, though.)

Let's just fix it so that no error is thrown and instead the backref
is deemed to never match, so that compile-time detection of no
iterations behaves the same as run-time detection.

Per report from Mark Dilger.  This appears to be an aboriginal error
in Spencer's library, so back-patch to all supported versions.

Pre-v14, it turns out to also be necessary to back-patch one aspect of
commits cb76fbd7e/00116dee5, namely to create capture-node subREs with
the begin/end states of their subexpressions, not the current lp/rp
of the outer parseqatom invocation.  Otherwise delsub complains that
we're trying to disconnect a state from itself.  This is a bit scary
but code examination shows that it's safe: in the pre-v14 code, if we
want to wrap iteration around the subexpression, the first thing we do
is overwrite the atom's begin/end fields with new states.  So the
bogus values didn't survive long enough to be used for anything, except
if no iteration is required, in which case it doesn't matter.

Discussion: https://postgr.es/m/A099E4A8-4377-4C64-A98C-3DEDDC075502@enterprisedb.com

src/backend/regex/regcomp.c

@@ -1089,11 +1089,23 @@ parseqatom(struct vars *v,
 	/* annoying special case:  {0} or {0,0} cancels everything */
 	if (m == 0 && n == 0)
 	{
-		if (atom != NULL)
-			freesubre(v, atom);
-		if (atomtype == '(')
-			v->subs[subno] = NULL;
-		delsub(v->nfa, lp, rp);
+		/*
+		 * If we had capturing subexpression(s) within the atom, we don't want
+		 * to destroy them, because it's legal (if useless) to back-ref them
+		 * later.  Hence, just unlink the atom from lp/rp and then ignore it.
+		 */
+		if (atom != NULL && (atom->flags & CAP))
+		{
+			delsub(v->nfa, lp, atom->begin);
+			delsub(v->nfa, atom->end, rp);
+		}
+		else
+		{
+			/* Otherwise, we can clean up any subre infrastructure we made */
+			if (atom != NULL)
+				freesubre(v, atom);
+			delsub(v->nfa, lp, rp);
+		}
 		EMPTYARC(lp, rp);
 		return top;
 	}

src/test/modules/test_regex/expected/test_regex.out

@@ -3576,6 +3576,28 @@ select * from test_regex('((.))(\2)', 'xyy', 'oRP');
  {yy,NULL,NULL,NULL}
 (2 rows)
 
+-- expectNomatch	21.39 PQR	{(.){0}(\1)}	xxx
+select * from test_regex('(.){0}(\1)', 'xxx', 'PQR');
+                 test_regex                 
+--------------------------------------------
+ {2,REG_UBACKREF,REG_UBOUNDS,REG_UNONPOSIX}
+(1 row)
+
+-- expectNomatch	21.40 PQR	{((.)){0}(\2)}	xxx
+select * from test_regex('((.)){0}(\2)', 'xxx', 'PQR');
+                 test_regex                 
+--------------------------------------------
+ {3,REG_UBACKREF,REG_UBOUNDS,REG_UNONPOSIX}
+(1 row)
+
+-- expectMatch	21.41 NPQR	{((.)){0}(\2){0}}	xyz	{}	{}	{}	{}
+select * from test_regex('((.)){0}(\2){0}', 'xyz', 'NPQR');
+                         test_regex                         
+------------------------------------------------------------
+ {3,REG_UBACKREF,REG_UBOUNDS,REG_UNONPOSIX,REG_UEMPTYMATCH}
+ {"",NULL,NULL,NULL}
+(2 rows)
+
 -- doing 22 "multicharacter collating elements"
 -- # again ugh
 -- MCCEs are not implemented in Postgres, so we skip all these tests

src/test/modules/test_regex/sql/test_regex.sql

@@ -1036,6 +1036,12 @@ select * from test_regex('((.))(\2){0}', 'xy', 'RPQ');
 select * from test_regex('((.))(\2)', 'xyy', 'RP');
 -- expectMatch	21.38 oRP	((.))(\2)	xyy	yy	{}	{}	{}
 select * from test_regex('((.))(\2)', 'xyy', 'oRP');
+-- expectNomatch	21.39 PQR	{(.){0}(\1)}	xxx
+select * from test_regex('(.){0}(\1)', 'xxx', 'PQR');
+-- expectNomatch	21.40 PQR	{((.)){0}(\2)}	xxx
+select * from test_regex('((.)){0}(\2)', 'xxx', 'PQR');
+-- expectMatch	21.41 NPQR	{((.)){0}(\2){0}}	xyz	{}	{}	{}	{}
+select * from test_regex('((.)){0}(\2){0}', 'xyz', 'NPQR');
 
 -- doing 22 "multicharacter collating elements"
 -- # again ugh

src/test/regress/expected/regex.out

@@ -567,6 +567,25 @@ select 'a' ~ '()+\1';
  t
 (1 row)
 
+-- Test incorrect removal of capture groups within {0}
+select 'xxx' ~ '(.){0}(\1)' as f;
+ f 
+---
+ f
+(1 row)
+
+select 'xxx' ~ '((.)){0}(\2)' as f;
+ f 
+---
+ f
+(1 row)
+
+select 'xyz' ~ '((.)){0}(\2){0}' as t;
+ t 
+---
+ t
+(1 row)
+
 -- Test ancient oversight in when to apply zaptreesubs
 select 'abcdef' ~ '^(.)\1|\1.' as f;
  f 

src/test/regress/sql/regex.sql

@@ -135,6 +135,11 @@ select 'a' ~ '.. ()|\1';
 select 'a' ~ '()*\1';
 select 'a' ~ '()+\1';
 
+-- Test incorrect removal of capture groups within {0}
+select 'xxx' ~ '(.){0}(\1)' as f;
+select 'xxx' ~ '((.)){0}(\2)' as f;
+select 'xyz' ~ '((.)){0}(\2){0}' as t;
+
 -- Test ancient oversight in when to apply zaptreesubs
 select 'abcdef' ~ '^(.)\1|\1.' as f;
 select 'abadef' ~ '^((.)\2|..)\2' as f;