In the regex op, need clarification on what property to use - based on discussion, seems like we need to add 'code' to the list of standard properties. Also, need to clarify what type of regex the filter should use.

See discussion here: https://chat.fhir.org/#narrow/stream/179202-terminology/topic/property.20for.20op.3Dregex

Not clear if this is an R4 fix or not.