m:length

<xs:simpleType name="length">
  <xs:restriction base="xs:string">
    <xs:pattern value="\s*((-?[0-9]*(\.[0-9]*)?(e[mx]|in|cm|mm|p[xtc]|%)?)|(negative)?((very){0,2}thi(n|ck)|medium)mathspace)\s*"/>
  </xs:restriction>
</xs:simpleType>