regex_replace: access violation

Following use of regex_replace results in an obscure access violation error message (see also regex_replace: access violation).

return regex_replace(
    workflow_name(),
    '^WF_(.+)(_\[a-zA-Z0-9\]+){2}$',
    '$1', # replace everything with first group
    null
);

This error is triggered by the use of the fourth parameter (legal) with null value. This function apparently has not been programmed in robust manner. It stands to reason that other DS functions behave the same way.
Solution: Either use the empty string '' instead of null value or omit the fourth parameter all together.