Here are some examples I generated from a demo with the IBM FHIR Server
Extract the references from the first subject
Bundle.entry.resource.ofType(Observation).subject[0].reference.value
Output
Patient/ebb16c62-cb06-4ff8-8ce8-ccb865e7a240
Extract the references from the first subject alternative
Bundle.entry.resource.ofType(Observation).subject.first().reference.value
Output
Patient/ebb16c62-cb06-4ff8-8ce8-ccb865e7a240
Select the first Patient reference where the Diastolic is over 90
Bundle.entry.resource.ofType(Observation).where(component.where(code.coding.code = '8462-4').value.where( value > 90.0).exists()).subject.reference.value.first()
Output
Patient/5fabfccd-254e-42af-bed2-84199a5c05f2
Select the Patient’s first name where the reference matches the logical id
Bundle.entry.resource.ofType(Patient).where(id = '" + v.split("/")[1] + "').name.given.value
Output
John