Help with using schema to format data field

Hi,

I created the an xsd from an xml file, how do I modify it to limit 3 characters in the CTRY field and no numeric values?

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Root">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="Row">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="CTRY_ID" type="xs:unsignedByte" />
              <xs:element name="CTRY" type="xs:string" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>