Merge forwardport of #11765 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11765.patch (created by @bentideswell) based on commit(s):
  1. 4d79191

Author: magento-engcom-team

lib/internal/Magento/Framework/View/Layout/etc/elements.xsd

@@ -138,7 +138,7 @@
 
     <xs:simpleType name="blockClassType">
         <xs:restriction base="xs:string">
-            <xs:pattern value="[A-Z][a-zA-Z\d]*(\\[A-Z][a-zA-Z\d]*)*"/>
+            <xs:pattern value="[A-Z][_a-zA-Z\d]*(\\[A-Z][_a-zA-Z\d]*)*"/>
         </xs:restriction>
     </xs:simpleType>