bug#64128: regexp parser zero-width assertion bugs

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

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19 juni 2023 kl. 14.54 skrev Stefan Monnier <monnier@iro.umontreal.ca>:
> 
> I wish there was a way to emit warnings about oddball constructs
> (starting with the "* is literal when encountered at the beginning of
> a regexp").

I agree, but I'm more of a static analysis man. (And relint does complain about all these cases as long as the regexp is detected as such, so there probably aren't many of them left in the Emacs tree.)

Here is a reduced patch that only fixes the really silly behaviour reported earlier, by making sure that `laststart` is reset correctly for all group A assertions. This should be uncontroversial.
Maybe we should change group B assertions so that they work in the same way.


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diff --git a/src/regex-emacs.c b/src/regex-emacs.c
index fea34df991b..f2da1a2d0db 100644
--- a/src/regex-emacs.c
+++ b/src/regex-emacs.c
@@ -1716,7 +1716,9 @@ 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 zero-width assertions which should not be the target
+     of any postfix repetition operators.  */
   unsigned char *laststart = 0;
 
   /* Address of beginning of regexp, or inside of last group.  */
@@ -1847,12 +1849,14 @@ regex_compile (re_char *pattern, ptrdiff_t size,
 	case '^':
 	  if (! (p == pattern + 1 || at_begline_loc_p (pattern, p)))
 	    goto normal_char;
+	  laststart = 0;
 	  BUF_PUSH (begline);
 	  break;
 
 	case '$':
 	  if (! (p == pend || at_endline_loc_p (p, pend)))
 	    goto normal_char;
+	  laststart = 0;
 	  BUF_PUSH (endline);
 	  break;
 
@@ -1892,7 +1896,7 @@ 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 +2548,22 @@ regex_compile (re_char *pattern, ptrdiff_t size,
               break;
 
 	    case 'b':
+	      laststart = 0;
 	      BUF_PUSH (wordbound);
 	      break;
 
 	    case 'B':
+	      laststart = 0;
 	      BUF_PUSH (notwordbound);
 	      break;
 
 	    case '`':
+	      laststart = 0;
 	      BUF_PUSH (begbuf);
 	      break;
 
 	    case '\'':
+	      laststart = 0;
 	      BUF_PUSH (endbuf);
 	      break;
 
diff --git a/test/src/regex-emacs-tests.el b/test/src/regex-emacs-tests.el
index 52d43775b8e..48a487ffe15 100644
--- a/test/src/regex-emacs-tests.el
+++ b/test/src/regex-emacs-tests.el
@@ -883,4 +883,14 @@ regexp-tests-backtrack-optimization
     (should (looking-at "x*\\(=\\|:\\)*"))
     (should (looking-at "x*=*?"))))
 
+(ert-deftest regexp-tests-zero-width-assertion-repetition ()
+  ;; Check compatibility behaviour with repetition operators after
+  ;; certain zero-width assertions (bug#64128).
+  (should (equal (string-match "^*a" "*a") 0))
+  (should (equal (string-match "\\`*a" "*a") 0))
+  (should (equal (string-match "q\\b*!" "q*!") 0))
+  (should (equal (string-match "q\\b*!" "!") nil))
+  (should (equal (string-match "/\\B*z" "/*z") 0))
+  (should (equal (string-match "/\\B*z" "z") nil)))
+
 ;;; regex-emacs-tests.el ends here

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