Going to Special:AbuseFilter/tools and running a "check syntax" on the following snippet:
accountname rlike "("
results in no error, but the regexp is obviously wrong. I suspect this happens because the regex is validated when the LHS is not DUNDEFINED, and accountname in this case is.
Update: This happens for invalid regexps (in (i)rlike and rcount), and invalid IP ranges passed to ip_in_range