Fix regexp_matches() handling of zero-length matches.

We'd find the same match twice if it was of zero length and not immediately
adjacent to the previous match.  replace_text_regexp() got similar cases
right, so adjust this search logic to match that.  Note that even though
the regexp_split_to_xxx() functions share this code, they did not display
equivalent misbehavior, because the second match would be considered
degenerate and ignored.

Jeevan Chalke, with some cosmetic changes by me.

src/backend/utils/adt/regexp.c

@@ -937,14 +937,13 @@ setup_regexp_matches(text *orig_str, text *pattern, text *flags,
 			break;
 
 		/*
-		 * Advance search position.  Normally we start just after the end of
-		 * the previous match, but always advance at least one character (the
-		 * special case can occur if the pattern matches zero characters just
-		 * after the prior match or at the end of the string).
+		 * Advance search position.  Normally we start the next search at the
+		 * end of the previous match; but if the match was of zero length, we
+		 * have to advance by one character, or we'd just find the same match
+		 * again.
 		 */
-		if (start_search < pmatch[0].rm_eo)
-			start_search = pmatch[0].rm_eo;
-		else
+		start_search = prev_match_end;
+		if (pmatch[0].rm_so == pmatch[0].rm_eo)
 			start_search++;
 		if (start_search > wide_len)
 			break;

src/backend/utils/adt/varlena.c

@@ -2624,7 +2624,10 @@ replace_text_regexp(text *src_text, void *regexp,
 			break;
 
 		/*
-		 * Search from next character when the matching text is zero width.
+		 * Advance search position.  Normally we start the next search at the
+		 * end of the previous match; but if the match was of zero length, we
+		 * have to advance by one character, or we'd just find the same match
+		 * again.
 		 */
 		search_start = data_pos;
 		if (pmatch[0].rm_so == pmatch[0].rm_eo)

src/test/regress/expected/strings.out

@@ -347,6 +347,64 @@ SELECT regexp_matches('foobarbequebaz', $re$barbeque$re$);
  {barbeque}
 (1 row)
 
+-- start/end-of-line matches are of zero length
+SELECT regexp_matches('foo' || chr(10) || 'bar' || chr(10) || 'bequq' || chr(10) || 'baz', '^', 'mg');
+ regexp_matches 
+----------------
+ {""}
+ {""}
+ {""}
+ {""}
+(4 rows)
+
+SELECT regexp_matches('foo' || chr(10) || 'bar' || chr(10) || 'bequq' || chr(10) || 'baz', '$', 'mg');
+ regexp_matches 
+----------------
+ {""}
+ {""}
+ {""}
+ {""}
+(4 rows)
+
+SELECT regexp_matches('1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4' || chr(10), '^.?', 'mg');
+ regexp_matches 
+----------------
+ {1}
+ {2}
+ {3}
+ {4}
+ {""}
+(5 rows)
+
+SELECT regexp_matches(chr(10) || '1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4' || chr(10), '.?$', 'mg');
+ regexp_matches 
+----------------
+ {""}
+ {1}
+ {""}
+ {2}
+ {""}
+ {3}
+ {""}
+ {4}
+ {""}
+ {""}
+(10 rows)
+
+SELECT regexp_matches(chr(10) || '1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4', '.?$', 'mg');
+ regexp_matches 
+----------------
+ {""}
+ {1}
+ {""}
+ {2}
+ {""}
+ {3}
+ {""}
+ {4}
+ {""}
+(9 rows)
+
 -- give me errors
 SELECT regexp_matches('foobarbequebaz', $re$(bar)(beque)$re$, 'gz');
 ERROR:  invalid regexp option: "z"

src/test/regress/sql/strings.sql

@@ -137,6 +137,13 @@ SELECT regexp_matches('foobarbequebaz', $re$(bar)(.+)?(beque)$re$);
 -- no capture groups
 SELECT regexp_matches('foobarbequebaz', $re$barbeque$re$);
 
+-- start/end-of-line matches are of zero length
+SELECT regexp_matches('foo' || chr(10) || 'bar' || chr(10) || 'bequq' || chr(10) || 'baz', '^', 'mg');
+SELECT regexp_matches('foo' || chr(10) || 'bar' || chr(10) || 'bequq' || chr(10) || 'baz', '$', 'mg');
+SELECT regexp_matches('1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4' || chr(10), '^.?', 'mg');
+SELECT regexp_matches(chr(10) || '1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4' || chr(10), '.?$', 'mg');
+SELECT regexp_matches(chr(10) || '1' || chr(10) || '2' || chr(10) || '3' || chr(10) || '4', '.?$', 'mg');
+
 -- give me errors
 SELECT regexp_matches('foobarbequebaz', $re$(bar)(beque)$re$, 'gz');
 SELECT regexp_matches('foobarbequebaz', $re$(barbeque$re$);