[perl #79152] super-linear cache can prevent a valid match

The super-linear cache in regexec.c can prevent a valid match
from being detected. For example:

print "yay\n" if 'xayxay' =~ /(q1|.)*(q2|.)*(x(a|bc)*y){2,}/;

This should match, but it doesn't because the cache fails to
distinguish between matching the final xay to x(a|bc)*y as the
first instance of the {2,} and matching it in the same position
as the second instance.

This seems to do the trick.

1 files changed, 10 insertions, 7 deletions

diff --git a/regcomp.c b/regcomp.c
index 4092d7957c..4b69bf74ee 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -3246,13 +3246,16 @@ S_study_chunk(pTHX_ RExC_state_t *pRExC_state, regnode **scanp,
 		    f |= SCF_DO_STCLASS_AND;
 		    f &= ~SCF_DO_STCLASS_OR;
 		}
-		/* These are the cases when once a subexpression
-		   fails at a particular position, it cannot succeed
-		   even after backtracking at the enclosing scope.
-
-		   XXXX what if minimal match and we are at the
-		        initial run of {n,m}? */
-		if ((mincount != maxcount - 1) && (maxcount != REG_INFTY))
+	        /* Exclude from super-linear cache processing any {n,m}
+		   regops for which the combination of input pos and regex
+		   pos is not enough information to determine if a match
+		   will be possible.
+
+		   For example, in the regex /foo(bar\s*){4,8}baz/ with the
+		   regex pos at the \s*, the prospects for a match depend not
+		   only on the input position but also on how many (bar\s*)
+		   repeats into the {4,8} we are. */
+               if ((mincount > 1) || (maxcount > 1 && maxcount != REG_INFTY))
 		    f &= ~SCF_WHILEM_VISITED_POS;
 
 		/* This will finish on WHILEM, setting scan, or on NULL: */