Do not validate a backreference if the group that it refers to is not valid

regex/matching.go

@@ -319,7 +319,7 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 				if currentState.contentContains(str, idx, preferLongest) {
 					nextStates = addStateToList(str, idx+1, nextStates, *currentState.next, currentState.threadGroups, nil, preferLongest)
 				}
-			} else if currentState.isBackreference {
+			} else if currentState.isBackreference && currentState.threadGroups[currentState.referredGroup].IsValid() {
 				groupLength := currentState.threadGroups[currentState.referredGroup].EndIdx - currentState.threadGroups[currentState.referredGroup].StartIdx
 				if currentState.threadBackref == groupLength {
 					currentState.threadBackref = 0