The rule should raise whenever a reference to a nonexistent group is made in the "replacement" argument passed to replace/replaceAll from String.

Important note: the behavior for Java is slightly different from JS/PHP: it will throw an IndexOutOfBoundsException when the group does not exist, the result string will not be erroneous (as in JS/PHP). There is therefore little chance that the program goes fine at runtime. (We do not consider the value that could be added by supporting dynamic String building since it is any way out of scope for this rule.)
We are in the situation where the rule could make sense in SL context, but if an issue reaches SQ, it is most probably an FP. We implemented a few rules with such behavior (see mockito rules), and it ends up really noisy. We should make sure to either carefully consider this rule, or not implement it at all.