valid pattern for xsd restriction contain type float with word "exist"

Question

Hello,

i want to validate by xsd an attribute can contain a float type or a word "exist". after search in the net i found this solution(w3schools.com/xml/schema_facets.asp)  but it does not worked when i add this pattern:

<xs:restriction base="xs:Double">
      <xs:pattern value="[0-9]| 'exist'"/>
    </xs:restriction>

how to fix that ?