If you need to validate the edited document with a validation engine that is different from
the built-in engine, you can configure external validators in the Oxygen XML Editor
preferences. After a custom validation engine is properly configured, it can be
applied on the current document by selecting it from the list of custom validation engines in
the 
Validation toolbar drop-down menu. The document is validated against the
schema declared in the document.
Some validators are configured by default but there are third party processors which do not support the output message format of Oxygen XML Editor for linked messages:
- LIBXML - Included in Oxygen XML Editor (Windows edition only). It is
associated to XML Editor. It is able to validate the edited document against XML Schema,
Relax NG schema full syntax, internal DTD (included in the XML document) or a custom schema
type. XML catalogs support (the --catalogs parameter) and XInclude
processing (--xinclude) are enabled by default in the preconfigured
LIBXML validator. The --postvalid parameter is also set by default
which allows LIBXML to validate correctly the main document even if the XInclude fragments
contain IDREFS to ID's located in other fragments.
For validation against an external DTD specified by URI in the XML document, add the --dtdvalid ${ds} parameter manually to the DTD validation command line. ${ds} represents the detected DTD declaration in the XML document.
CAUTION:File paths containing spaces are not handled correctly in the LIBXML processor. For example the built-in XML catalog files of the predefined document types (DocBook, TEI, DITA, etc) are not handled by LIBXML if Oxygen XML Editor is installed in the default location on Windows (C:\Program Files) because the built-in XML catalog files are stored in the frameworks subfolder of the installation folder which in this case contains at least one space character in the file path.Attention: On OS X if the full path to the LIBXML executable file is not specified in the Executable path text field, some errors may occur during validation against a W3C XML Schema, such as:Unimplemented block at ... xmlschema.c
To avoid these errors, specify the full path to the LIBXML executable file.
- Saxon-EE - Included in Oxygen XML Editor. It is associated to XML Editor and XSD Editor. It is able to validate XML Schema schemas and XML documents against XML Schema schemas. The validation is done according to the W3C XML Schema 1.0 or 1.1. This can be configured in Preferences.
- MSXML 4.0 - Included in Oxygen XML Editor (Windows edition only). It is associated to XML Editor, XSD Editor and XSL Editor. It is able to validate the edited document against XML Schema, internal DTD (included in the XML document), external DTD or a custom schema type.
- MSXML.NET - Included in Oxygen XML Editor (Windows edition only). It is associated to XML Editor, XSD Editor and XSL Editor. It is able to validate the edited document against XML Schema, internal DTD (included in the XML document), external DTD or a custom schema type.
- XSV - Not included in Oxygen XML Editor. Windows and Linux distributions of XSV can be downloaded from http://www.cogsci.ed.ac.uk/~ht/xsv-status.html. The executable path is already configured in Oxygen XML Editor for the [OXYGEN_DIR]/xsv installation folder. If it is installed in a different folder the predefined executable path must be corrected in Preferences. It is associated to XML Editor and XSD Editor. It is able to validate the edited document against XML Schema or a custom schema type.
- SQC (Schema Quality Checker from IBM) - Not included in Oxygen XML Editor. It can be downloaded from here (it comes as a .zip file, at the time of this writing SQC2.2.1.zip is about 3 megabytes). The executable path and working directory are already configured for the SQC installation directory [OXYGEN_DIR]/sqc. If it is installed in a different folder the predefined executable path and working directory must be corrected in the Preferences page. It is associated to XSD Editor.
A custom validator cannot be applied on files loaded through an Oxygen XML Editor custom protocol plugin developed independently and added to Oxygen XML Editor after installation.