The positiveInteger datatype is derived from the nonNegativeInteger type. : positiveInteger « XML Schema « XML Tutorial

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2. XML Schema
3. positiveInteger

The value space is the infinite set of integers greater than or equal to 1. 
The lexical space is a finite-length sequence of decimal digits with an optional leading +.

<!-- schema -->
<xsd:complexType name="record_sets">
  <xsd:sequence>
   <xsd:element name="record" minOccurs="0"
    maxOccurs="unbounded">
    <xsd:simpleType>
     <xsd:restriction base="xsd:positiveInteger">
      <xsd:pattern value="555\d{3}"/>
     </xsd:restriction>
    </xsd:simpleType>
   </xsd:element>
  </xsd:sequence>
</xsd:complexType>

<!-- instance document -->
<record>555158</record>
<record>555687</record>
<record>555736</record>