System.Xml: XSD allows patterns that should be invalid

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技术栈
csharp
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backend

调研方向

Start by tracing how System.Xml validates xs:pattern values and how its regex handling differs from the XML regex dialect described in the linked specification. The report provides two reproductions involving grouping and backreferences; use them to identify the relevant validation tests and confirm the expected schema errors. Done means those patterns are rejected while valid XML regex patterns continue to work.

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描述

area-System.Xml untriaged
Description

XSD schemas allows use of regex string that are not valid according to the XML regex dialect but are valid in .NET regex dialect. Some examples include:

  • Backtracking
  • Various grouping constructs
  • Look arounds
Reproduction Steps
using System;
using System.IO;
using System.Xml;
using System.Xml.Schema;

Check("(?&lt;x&gt;y)", "<id>y</id>"); // '?' should not be allowed to follow '('. Instead this creates a named group.
Check("(y)\\2", "<id>yy</id>"); // "\\2" should be an invalid escape sequence. Instead this creates a back reference

static void Check(string pattern, string xml)
{
    string xsd = $@"<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema'>
  <xs:element name='id'>
    <xs:simpleType>
      <xs:restriction base='xs:string'>
        <xs:pattern value='{pattern}'/>
      </xs:restriction>
    </xs:simpleType>
  </xs:element>
</xs:schema>";

    var settings = new XmlReaderSettings
    {
        ValidationType = ValidationType.Schema
    };
    settings.Schemas.Add(null, XmlReader.Create(new StringReader(xsd)));

    int errors = 0;
    settings.ValidationEventHandler += (_, e) =>
    {
        if (e.Severity == XmlSeverityType.Error)
            errors++;
        else
            throw e.Exception;
    };

    using var reader = XmlReader.Create(new StringReader(xml), settings);
    while (reader.Read()) { }

    Console.WriteLine(
        $"{pattern,-12} {xml,-36} {(errors == 0 ? "VALID" : "INVALID")}");
}
Expected behavior

Exceptions or output INVALID for all three since the patterns are not valid according to xml spec?

Actual behavior

Both checks outputs VALID.

Regression?

Not what i can find

Known Workarounds

No response

Configuration

No response

Other information

System.Xml has contribution bar "". I would expect these issues to be classified as edge-cased and the bug report to closed as won't fix, but at least it is logged.

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