Let regexp_replace() make use of REG_NOSUB when feasible.

If the replacement string doesn't contain \1...\9, then we don't
need sub-match locations, so we can use the REG_NOSUB optimization
here too.  There's already a pre-scan of the replacement string
to look for backslashes, so extend that to check for digits, and
refactor to allow that to happen before we compile the regexp.

While at it, try to speed up the pre-scan by using memchr() instead
of a handwritten loop.  It's likely that this is lost in the noise
compared to the regexp processing proper, but maybe not.  In any
case, this coding is shorter.

Also, add some test cases to improve the poor coverage of
appendStringInfoRegexpSubstr().

Discussion: https://postgr.es/m/3534632.1628536485@sss.pgh.pa.us

2 files changed, 26 insertions, 2 deletions

diff --git a/src/test/regress/expected/strings.out b/src/test/regress/expected/strings.out
index a9efd74c7b4..0f95b9400b6 100644
--- a/src/test/regress/expected/strings.out
+++ b/src/test/regress/expected/strings.out
@@ -571,13 +571,32 @@ SELECT 'abcdefg' SIMILAR TO '_bcd%' ESCAPE NULL AS null;
 SELECT 'abcdefg' SIMILAR TO '_bcd#%' ESCAPE '##' AS error;
 ERROR:  invalid escape string
 HINT:  Escape string must be empty or one character.
--- Test back reference in regexp_replace
+-- Test backslash escapes in regexp_replace's replacement string
 SELECT regexp_replace('1112223333', E'(\\d{3})(\\d{3})(\\d{4})', E'(\\1) \\2-\\3');
  regexp_replace 
 ----------------
  (111) 222-3333
 (1 row)
 
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\&Y', 'g');
+  regexp_replace   
+-------------------
+ fXooYbaXrrYbaXzzY
+(1 row)
+
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\\\Y', 'g');
+ regexp_replace 
+----------------
+ fX\YbaX\YbaX\Y
+(1 row)
+
+-- not an error, though perhaps it should be:
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\Y\\1Z\\');
+ regexp_replace  
+-----------------
+ fX\YoZ\barrbazz
+(1 row)
+
 SELECT regexp_replace('AAA   BBB   CCC   ', E'\\s+', ' ', 'g');
  regexp_replace 
 ----------------
diff --git a/src/test/regress/sql/strings.sql b/src/test/regress/sql/strings.sql
index 6a029cc3692..8c379182cb9 100644
--- a/src/test/regress/sql/strings.sql
+++ b/src/test/regress/sql/strings.sql
@@ -187,8 +187,13 @@ SELECT 'abcd\efg' SIMILAR TO '_bcd\%' ESCAPE '' AS true;
 SELECT 'abcdefg' SIMILAR TO '_bcd%' ESCAPE NULL AS null;
 SELECT 'abcdefg' SIMILAR TO '_bcd#%' ESCAPE '##' AS error;
 
--- Test back reference in regexp_replace
+-- Test backslash escapes in regexp_replace's replacement string
 SELECT regexp_replace('1112223333', E'(\\d{3})(\\d{3})(\\d{4})', E'(\\1) \\2-\\3');
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\&Y', 'g');
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\\\Y', 'g');
+-- not an error, though perhaps it should be:
+SELECT regexp_replace('foobarrbazz', E'(.)\\1', E'X\\Y\\1Z\\');
+
 SELECT regexp_replace('AAA   BBB   CCC   ', E'\\s+', ' ', 'g');
 SELECT regexp_replace('AAA', '^|$', 'Z', 'g');
 SELECT regexp_replace('AAA aaa', 'A+', 'Z', 'gi');