PATCH: [perl #126141]: qr/]]]]][\\/ fails to raise error

This was due to the trailing \ trying to look at the next character
without verifying that one actually existed.

Author: khwilliamson

regcomp.c

@@ -15718,6 +15718,11 @@ S_regclass(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth,
         }
         else if (value == '\\') {
             /* Is a backslash; get the code point of the char after it */
+
+            if (RExC_parse >= RExC_end) {
+                vFAIL("Unmatched [");
+            }
+
 	    if (UTF && ! UTF8_IS_INVARIANT(UCHARAT(RExC_parse))) {
 		value = utf8n_to_uvchr((U8*)RExC_parse,
 				   RExC_end - RExC_parse,

t/lib/warnings/regcomp

@@ -73,3 +73,10 @@ qr/(?[[:digit: ])/
 EXPECT
 Assuming NOT a POSIX class since no blanks are allowed in one in regex; marked by <-- HERE in m/(?[[:digit: ] <-- HERE )/ at - line 2.
 syntax error in (?[...]) in regex m/(?[[:digit: ])/ at - line 2.
+########
+# NAME [perl #126141]
+# OPTION fatal
+eval {/$_/}, print "$_ ==> ", $@ || "OK!\n" for "]]]]]]]]][\\", "]]]]][\\"
+EXPECT
+]]]]]]]]][\ ==> Unmatched [ in regex; marked by <-- HERE in m/]]]]]]]]][\ <-- HERE / at - line 2.
+]]]]][\ ==> Unmatched [ in regex; marked by <-- HERE in m/]]]]][\ <-- HERE / at - line 2.