Fix misoptimization of "{1,1}" quantifiers in regular expressions.

A bounded quantifier with m = n = 1 might be thought a no-op.  But
according to our documentation (which traces back to Henry Spencer's
original man page) it still imposes greediness, or non-greediness in the
case of the non-greedy variant "{1,1}?", on whatever it's attached to.

This turns out not to work though, because parseqatom() optimizes away
the m = n = 1 case without regard for whether it's supposed to change
the greediness of the argument RE.

We can fix this by just not applying the optimization when the greediness
needs to change; the subsequent general cases handle it fine.

The three cases in which we can still apply the optimization are
(a) no quantifier, or quantifier does not impose a preference;
(b) atom has no greediness property, implying it cannot match a
variable amount of text anyway; or
(c) quantifier's greediness is same as atom's.
Note that in most cases where one of these applies, we'd have exited
earlier in the "not a messy case" fast path.  I think it's now only
possible to get to the optimization when the atom involves capturing
parentheses or a non-top-level backref.

Back-patch to all supported branches.  I'd ordinarily be hesitant to
put a subtle behavioral change into back branches, but in this case
it's very hard to see a reason why somebody would write "{1,1}?" unless
they're trying to get the documented change-of-greediness behavior.

Discussion: https://postgr.es/m/5bb27a41-350d-37bf-901e-9d26f5592dd0@charter.net

3 files changed, 63 insertions, 1 deletions

diff --git a/src/backend/regex/regcomp.c b/src/backend/regex/regcomp.c
index 7ae9673a7d5..f72c41692bb 100644
--- a/src/backend/regex/regcomp.c
+++ b/src/backend/regex/regcomp.c
@@ -1147,7 +1147,10 @@ parseqatom(struct vars * v,
 		/* rest of branch can be strung starting from atom->end */
 		s2 = atom->end;
 	}
-	else if (m == 1 && n == 1)
+	else if (m == 1 && n == 1 &&
+			 (qprefer == 0 ||
+			  (atom->flags & (LONGER | SHORTER | MIXED)) == 0 ||
+			  qprefer == (atom->flags & (LONGER | SHORTER | MIXED))))
 	{
 		/* no/vacuous quantifier:  done */
 		EMPTYARC(s, atom->begin);		/* empty prefix */
diff --git a/src/test/regress/expected/regex.out b/src/test/regress/expected/regex.out
index 2b4f2ec2522..e5a4777f1bb 100644
--- a/src/test/regress/expected/regex.out
+++ b/src/test/regress/expected/regex.out
@@ -295,6 +295,55 @@ select regexp_matches('foo/bar/baz',
  {foo,bar,baz}
 (1 row)
 
+-- Test that greediness can be overridden by outer quantifier
+select regexp_matches('llmmmfff', '^(l*)(.*)(f*)$');
+ regexp_matches 
+----------------
+ {ll,mmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*){1,1}(.*)(f*)$');
+ regexp_matches 
+----------------
+ {ll,mmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*){1,1}?(.*)(f*)$');
+  regexp_matches  
+------------------
+ {"",llmmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*){1,1}?(.*){1,1}?(f*)$');
+ regexp_matches 
+----------------
+ {"",llmmm,fff}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*?)(.*)(f*)$');
+  regexp_matches  
+------------------
+ {"",llmmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*?){1,1}(.*)(f*)$');
+ regexp_matches 
+----------------
+ {ll,mmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*?){1,1}?(.*)(f*)$');
+  regexp_matches  
+------------------
+ {"",llmmmfff,""}
+(1 row)
+
+select regexp_matches('llmmmfff', '^(l*?){1,1}?(.*){1,1}?(f*)$');
+ regexp_matches 
+----------------
+ {"",llmmm,fff}
+(1 row)
+
 -- Test for infinite loop in cfindloop with zero-length possible match
 -- but no actual match (can only happen in the presence of backrefs)
 select 'a' ~ '$()|^\1';
diff --git a/src/test/regress/sql/regex.sql b/src/test/regress/sql/regex.sql
index 635f068eae9..9c2e9c54ad8 100644
--- a/src/test/regress/sql/regex.sql
+++ b/src/test/regress/sql/regex.sql
@@ -76,6 +76,16 @@ select regexp_matches('Programmer', '(\w)(.*?\1)', 'g');
 select regexp_matches('foo/bar/baz',
                       '^([^/]+?)(?:/([^/]+?))(?:/([^/]+?))?$', '');
 
+-- Test that greediness can be overridden by outer quantifier
+select regexp_matches('llmmmfff', '^(l*)(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*){1,1}(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*){1,1}?(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*){1,1}?(.*){1,1}?(f*)$');
+select regexp_matches('llmmmfff', '^(l*?)(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*?){1,1}(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*?){1,1}?(.*)(f*)$');
+select regexp_matches('llmmmfff', '^(l*?){1,1}?(.*){1,1}?(f*)$');
+
 -- Test for infinite loop in cfindloop with zero-length possible match
 -- but no actual match (can only happen in the presence of backrefs)
 select 'a' ~ '$()|^\1';