. expressions

Search by arbitrary elements of resources
This is non-FHIR extension, incompatible with other FHIR servers
With parameters started with . you can provide the exact path for the element, optionally provide coercing after :: using PostgreSQL types and the operator after $.
GET /Patient?
=> WHERE resource#>>'{name,0,family}' = 'Jonhnson'
GET /Patient?$contains=Joh
=> WHERE resource#>>'{name,0,family}' ilike '%John%'
GET /Encounter?.start::timestamptz$gt=2015-01-01
=> WHERE (resource#>>'{start}')::timestamptz > '2015-01-01'
GET /Patient?.contact$isnull=true
=> WHERE resource#>>'{contact}' IS NULL
expressions with typecast require user input to be correct by PostgreSQL syntax.
e.g.: for timestamptz values must be formatted as specified in PostgreSQL date/time input doc