<schema xmlns=“www.ascc.net/xml/schematron” >

<pattern name="Number of characters in an abbreviation">
     <rule context="BBB">
          <report test="string-length(@bbb) &lt; 2">There is not enough letters in the abbreviation</report>
          <report test="string-length(@bbb) > 3">There is too much letters in the abbreviation</report>
     </rule>
</pattern>

</schema>