mbeckerle commented on a change in pull request #395:
URL: https://github.com/apache/incubator-daffodil/pull/395#discussion_r452865746



##########
File path: 
daffodil-test/src/test/resources/org/apache/daffodil/section05/facets/NulChars.tdml
##########
@@ -0,0 +1,95 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Licensed to the Apache Software Foundation (ASF) under one or more
+  contributor license agreements.  See the NOTICE file distributed with
+  this work for additional information regarding copyright ownership.
+  The ASF licenses this file to You under the Apache License, Version 2.0
+  (the "License"); you may not use this file except in compliance with
+  the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+-->
+<tdml:testSuite
+  xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-1.0/";
+  xmlns:dfdlx="http://www.ogf.org/dfdl/dfdl-1.0/extensions";
+  xmlns:tdml="http://www.ibm.com/xmlns/dfdl/testData";
+  xmlns:ex="http://example.com";
+  xmlns:xs="http://www.w3.org/2001/XMLSchema";
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
+  defaultRoundTrip="onePass"
+  defaultValidation="on">
+
+  <tdml:defineSchema name="nullTest">
+    <xs:include 
schemaLocation="org/apache/daffodil/xsd/DFDLGeneralFormat.dfdl.xsd"/>
+
+    <dfdl:format ref="ex:GeneralFormat"/>
+
+    <xs:element name="root">
+      <xs:complexType>
+        <xs:sequence>
+          <xs:element name="foo" dfdl:lengthKind="pattern" 
dfdl:lengthPattern="\x{0000}{3,3}">
+            <xs:simpleType>
+              <xs:restriction base="xs:string">
+                <xs:pattern value="&#xE000;*"/>

Review comment:
       So using both E000 and 0000 won't work because you can't say &#x0000; in 
an XML document of any kind. We would have to interpret the pattern looking for 
DFDL character entities and allow [&#xE000;%#x0000;]* for DFDL's internal 
validation, but then we'd have to remove the %#x0000; for Xerces XML 
Validation. 
   
   Our internal validation needs to take the pattern, and apply the same 
backward transformation to its characters that we use for XML data. So the 
internal validation would convert that &#xE000; back into a true NUL character 
that it is searching for in the infoset. That appears to be the bug. 
   
   Another alternative, I wasn't aware that when we turn Validation On, that 
*both* our internal validation AND Xerces run. Maybe we should have a 
Validation="xml" setting that only runs Xerces validation. This is also perhaps 
a special DFDL-language extension mode, where you can put any XML Schema 
validations at all, including key-constraints, unique constraints, patterns on 
hexBinary, all the things DFDL usually disallows, because the validations would 
be used ONLY for XML validation of an XML infoset. I suppose a few constraints 
on what you could write for facets would still apply, such as things about 
single-byte charsets when lengthUnits is bytes and lengthKind is "implicit" for 
strings. 
   
   There is another ticket for lifting DFDL's restrictions on key/unique 
constraints, again so that people can write one schema and use it for DFDL 
parse/unparse, as well as for more comprehensive XML Schema validation. 




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