The SearchParameter structure definition does not constrain the search types that can have a comparator. 

The issue was identified in the DaVinci Formulary search parameters for Formulary Drug (DrugName, DrugTier, and DrugPlan) all of which include a comparator ("eq") in their structure definitions though the search types are "string" and "token".

Including a comparator for a string or a token while using the syntax described for number, date or quantity creates ambiguity in the search string (e.g., DrugName=eqPaxil vs. DrugName=Equagesic) and the logic for implementing the search type.

This issue was discussed on Zulip: https://chat.fhir.org/#narrow/stream/197730-Da-Vinci-PDex-Drug-Formulary/topic/syntax.20for.20comparator.20eq.20in.20token.20or.20string