PATCH: [perl #72998] regex looping

If a character folds to multiple ones in case-insensitive matching,
it should not match just one of those, or the regular expression can
loop.  For example, \N{LATIN SMALL LIGATURE FF} folds to 'ff', and so
    "\N{LATIN SMALL LIGATURE FF}" =~ /f+/i
should match.  Prior to this patch, this function returned that there is
a match, but left the matching string  pointer at the beginning of the
"\N{LATIN SMALL LIGATURE FF}" because it doesn't make sense to match
just half a character, and at this level it doesn't know about the '+'.
This leaves things in an inconsistent state, with the reporting of a
match, but the input pointer unchanged, the result of which is a loop.

I don't know how to fix this so that it correctly matches, and there are
semantic issues with doing so.  For example, if
    "\N{LATIN SMALL LIGATURE FF}" =~ /ff/i
matches, then one would think that so should
    "\N{LATIN SMALL LIGATURE FF}" =~ /(f)(f)/i
But $1 and $2 don't really make sense here, since they both refer to the
half of the same character.

So this patch just returns failure if only a partial character is
matched.  That leaves things consistent, and solves the problem of
looping, so that Perl doesn't hang on such a construct, but leaves the
ultimate solution for another day.

2 files changed, 11 insertions, 2 deletions

diff --git a/t/re/re.t b/t/re/re.t
index 87965f20aa..249c6ddf22 100644
--- a/t/re/re.t
+++ b/t/re/re.t
@@ -51,6 +51,14 @@ if ('1234'=~/(?:(?<A>\d)|(?<C>!))(?<B>\d)(?<A>\d)(?<B>\d)/){
     }
     is(regnames_count(),3);
 }
+
+    { # Keep this test last, as whole script will be interrupted if times out
+        # Bug #72998; this can loop 
+        watchdog(2);
+        eval '"\x{100}\x{FB00}" =~ /\x{100}\N{U+66}+/i';
+        pass("Didn't loop");
+    }
+
 # New tests above this line, don't forget to update the test count below!
-BEGIN { plan tests => 18 }
+BEGIN { plan tests => 19 }
 # No tests here!
diff --git a/utf8.c b/utf8.c
index 9ed0663e19..1a6077c8d2 100644
--- a/utf8.c
+++ b/utf8.c
@@ -2609,7 +2609,8 @@ Perl_ibcmp_utf8(pTHX_ const char *s1, char **pe1, register UV l1, bool u1, const
 
      /* A match is defined by all the scans that specified
       * an explicit length reaching their final goals. */
-     match = (f1 == 0 || p1 == f1) && (f2 == 0 || p2 == f2);
+     match = (n1 == 0 && n2 == 0    /* Must not match partial char; Bug #72998 */
+	     && (f1 == 0 || p1 == f1) && (f2 == 0 || p2 == f2));
 
      if (match) {
 	  if (pe1)