Straighten regexp postfix operator after zero-width assertion parse

The zero-width assertions \` \' \b \B were parsed in a sloppy way so
that a following postfix repetition operator could yield surprising
results.  For instance, "\\b*" would act as "\\b\\*", and "xy\\b*"
would act as "\\(?:xy\\b\\)*".

Except for \` and ^, any following postfix operator now applies to the
zero-width assertion itself only, which is predictable and consistent
with other assertions, if useless in practice.
For historical compatibility, an operator character following \` and ^
always becomes a literal. (Bug#64128)

* src/regex-emacs.c (regex_compile):
Set `laststart` appropriately for each zero-width assertion instead
of leaving it with whatever value it had before.
Remove a redundant condition.
* test/src/regex-emacs-tests.el
(regexp-tests-zero-width-assertion-repetition): New test.
* doc/lispref/searching.texi (Regexp Special):
Say that repetition operators are not special after \`,
and that they work as expected after other backslash escapes.
* etc/NEWS: Announce.

1 files changed, 13 insertions, 2 deletions

diff --git a/src/regex-emacs.c b/src/regex-emacs.c
index fea34df991b..9e298b81ebb 100644
--- a/src/regex-emacs.c
+++ b/src/regex-emacs.c
@@ -1716,7 +1716,8 @@ regex_compile (re_char *pattern, ptrdiff_t size,
 
   /* Address of start of the most recently finished expression.
      This tells, e.g., postfix * where to find the start of its
-     operand.  Reset at the beginning of groups and alternatives.  */
+     operand.  Reset at the beginning of groups and alternatives,
+     and after ^ and \` for dusty-deck compatibility.  */
   unsigned char *laststart = 0;
 
   /* Address of beginning of regexp, or inside of last group.  */
@@ -1847,12 +1848,16 @@ regex_compile (re_char *pattern, ptrdiff_t size,
         case '^':
           if (! (p == pattern + 1 || at_begline_loc_p (pattern, p)))
             goto normal_char;
+          /* Special case for compatibility: postfix ops after ^ become
+             literals.  */
+          laststart = 0;
           BUF_PUSH (begline);
           break;
 
         case '$':
           if (! (p == pend || at_endline_loc_p (p, pend)))
             goto normal_char;
+          laststart = b;
           BUF_PUSH (endline);
           break;
 
@@ -1892,7 +1897,7 @@ regex_compile (re_char *pattern, ptrdiff_t size,
 
             /* Star, etc. applied to an empty pattern is equivalent
                to an empty pattern.  */
-            if (!laststart || laststart == b)
+            if (laststart == b)
               break;
 
             /* Now we know whether or not zero matches is allowed
@@ -2544,18 +2549,24 @@ regex_compile (re_char *pattern, ptrdiff_t size,
               break;
 
             case 'b':
+              laststart = b;
               BUF_PUSH (wordbound);
               break;
 
             case 'B':
+              laststart = b;
               BUF_PUSH (notwordbound);
               break;
 
             case '`':
+              /* Special case for compatibility: postfix ops after \` become
+                 literals, as for ^ (see above).  */
+              laststart = 0;
               BUF_PUSH (begbuf);
               break;
 
             case '\'':
+              laststart = b;
               BUF_PUSH (endbuf);
               break;