This code system http://hl7.org/fhir/search-comparator defines the following codes:

CodeDisplayDefinition
eqEqualsthe value for the parameter in the resource is equal to the provided value
neNot Equalsthe value for the parameter in the resource is not equal to the provided value
gtGreater Thanthe value for the parameter in the resource is greater than the provided value
ltLess Thenthe value for the parameter in the resource is less than the provided value
geGreater or Equalsthe value for the parameter in the resource is greater or equal to the provided value
leLess of Equalthe value for the parameter in the resource is less or equal to the provided value
saStarts Afterthe value for the parameter in the resource starts after the provided value
ebEnds Beforethe value for the parameter in the resource ends before the provided value
apApproximatelythe value for the parameter in the resource is approximately the same to the provided value.