Couldn’t find it explicitly stated anywhere probably because it’s quite simple (watch \linebreaks):

<xsd:simpleType name="urnType">
 
      <xsd:annotation><xsd:documentation>
 Regular expression to validate RFC 2141 URN syntax.
"urn:[a-zA-Z0-9][a-zA-Z0-9-]{1,31}:([a-zA-Z0-9()+,.:\linebreak
=@;$_!*'-]|%[0-9A-Fa-f]{2})+"
      </xsd:documentation></xsd:annotation>
 
      <xsd:restriction base="xsd:anyURI">
          <xsd:pattern 
 
value="urn:[a-zA-Z0-9][a-zA-Z0-9-]{1,31}:\linebreak
([a-zA-Z0-9()+,.:=@;$_!*'-]|%[0-9A-Fa-f]{2})+"
 
/>
      </xsd:restriction>
</xsd:simpleType>