Restrict the length of the code string and the characters : length « XML Schema « XML Tutorial

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2. XML Schema
3. length

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.java2s.com" 
  xmlns="http://www.java2s.com"
  elementFormDefault="qualified">

  <xs:simpleType name="iso3166_CountryCode">
    <xs:restriction base="xs:string">
      <xs:length value="3" />
      <xs:pattern value="[A-Za-z0-9]{3}" />
    </xs:restriction>
  </xs:simpleType>
</xs:schema>