alternate-fix-zero-quantified-nested-parens.patch

diff --git a/src/backend/regex/regcomp.c b/src/backend/regex/regcomp.c
index ae3a7b6a38..d9840171a3 100644
--- a/src/backend/regex/regcomp.c
+++ b/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;
 	}
diff --git a/src/test/modules/test_regex/expected/test_regex.out b/src/test/modules/test_regex/expected/test_regex.out
index 5a6cdf47c2..96c0e6fa59 100644
--- a/src/test/modules/test_regex/expected/test_regex.out
+++ b/src/test/modules/test_regex/expected/test_regex.out
@@ -3506,6 +3506,21 @@ select * from test_regex('((.))(\2)', 'xyy', 'oRP');
  {yy,NULL,NULL,NULL}
 (2 rows)
 
+-- expectMatch	21.39 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)
+
+-- 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)
+
 -- doing 22 "multicharacter collating elements"
 -- # again ugh
 -- MCCEs are not implemented in Postgres, so we skip all these tests
diff --git a/src/test/modules/test_regex/sql/test_regex.sql b/src/test/modules/test_regex/sql/test_regex.sql
index 3419564203..29806e9417 100644
--- a/src/test/modules/test_regex/sql/test_regex.sql
+++ b/src/test/modules/test_regex/sql/test_regex.sql
@@ -1020,6 +1020,10 @@ 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');
+-- expectMatch	21.39 NPQR	{((.)){0}(\2){0}}	xyz	{}	{}	{}	{}
+select * from test_regex('((.)){0}(\2){0}', 'xyz', 'NPQR');
+-- expectNomatch	21.40 PQR	{((.)){0}(\2)}	xxx
+select * from test_regex('((.)){0}(\2)', 'xxx', 'PQR');
 
 -- doing 22 "multicharacter collating elements"
 -- # again ugh