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Manual Reference Pages  -  XML::DOM (3)

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XML::DOM - A perl module for building DOM Level 1 compliant document structures



 use XML::DOM;

 my $parser = new XML::DOM::Parser;
 my $doc = $parser->parsefile ("file.xml");

 # print all HREF attributes of all CODEBASE elements
 my $nodes = $doc->getElementsByTagName ("CODEBASE");
 my $n = $nodes->getLength;

 for (my $i = 0; $i < $n; $i++)
     my $node = $nodes->item ($i);
     my $href = $node->getAttributeNode ("HREF");
     print $href->getValue . "\n";

 # Print doc file
 $doc->printToFile ("out.xml");

 # Print to string
 print $doc->toString;

 # Avoid memory leaks - cleanup circular references for garbage collection


This module extends the XML::Parser module by Clark Cooper. The XML::Parser module is built on top of XML::Parser::Expat, which is a lower level interface to James Clark’s expat library.

XML::DOM::Parser is derived from XML::Parser. It parses XML strings or files and builds a data structure that conforms to the API of the Document Object Model as described at See the XML::Parser manpage for other available features of the XML::DOM::Parser class. Note that the ’Style’ property should not be used (it is set internally.)

The XML::Parser NoExpand option is more or less supported, in that it will generate EntityReference objects whenever an entity reference is encountered in character data. I’m not sure how useful this is. Any comments are welcome.

As described in the synopsis, when you create an XML::DOM::Parser object, the parse and parsefile methods create an XML::DOM::Document object from the specified input. This Document object can then be examined, modified and written back out to a file or converted to a string.

When using XML::DOM with XML::Parser version 2.19 and up, setting the XML::DOM::Parser option KeepCDATA to 1 will store CDATASections in CDATASection nodes, instead of converting them to Text nodes. Subsequent CDATASection nodes will be merged into one. Let me know if this is a problem.

When using XML::Parser 2.27 and above, you can suppress expansion of parameter entity references (e.g. %pent;) in the DTD, by setting ParseParamEnt to 1 and ExpandParamEnt to 0. See Hidden Nodes for details.

A Document has a tree structure consisting of Node objects. A Node may contain other nodes, depending on its type. A Document may have Element, Text, Comment, and CDATASection nodes. Element nodes may have Attr, Element, Text, Comment, and CDATASection nodes. The other nodes may not have any child nodes.

This module adds several node types that are not part of the DOM spec (yet.) These are: ElementDecl (for <!ELEMENT ...> declarations), AttlistDecl (for <!ATTLIST ...> declarations), XMLDecl (for <?xml ...?> declarations) and AttDef (for attribute definitions in an AttlistDecl.)

XML::DOM Classes

The XML::DOM module stores XML documents in a tree structure with a root node of type XML::DOM::Document. Different nodes in tree represent different parts of the XML file. The DOM Level 1 Specification defines the following node types:
o XML::DOM::Node - Super class of all node types
o XML::DOM::Document - The root of the XML document
o XML::DOM::DocumentType - Describes the document structure: <!DOCTYPE root [ ... ]>
o XML::DOM::Element - An XML element: <elem attr=val> ... </elem>
o XML::DOM::Attr - An XML element attribute: name=value
o XML::DOM::CharacterData - Super class of Text, Comment and CDATASection
o XML::DOM::Text - Text in an XML element
o XML::DOM::CDATASection - Escaped block of text: <![CDATA[ text ]]>
o XML::DOM::Comment - An XML comment: <!-- comment -->
o XML::DOM::EntityReference - Refers to an ENTITY: &ent; or %ent;
o XML::DOM::Entity - An ENTITY definition: <!ENTITY ...>
o XML::DOM::ProcessingInstruction - <?PI target>
o XML::DOM::DocumentFragment - Lightweight node for cut & paste
o XML::DOM::Notation - An NOTATION definition: <!NOTATION ...>
In addition, the XML::DOM module contains the following nodes that are not part of the DOM Level 1 Specification:
o XML::DOM::ElementDecl - Defines an element: <!ELEMENT ...>
o XML::DOM::AttlistDecl - Defines one or more attributes in an <!ATTLIST ...>
o XML::DOM::AttDef - Defines one attribute in an <!ATTLIST ...>
o XML::DOM::XMLDecl - An XML declaration: <?xml version=1.0 ...>
Other classes that are part of the DOM Level 1 Spec:
o XML::DOM::Implementation - Provides information about this implementation. Currently it doesn’t do much.
o XML::DOM::NodeList - Used internally to store a node’s child nodes. Also returned by getElementsByTagName.
o XML::DOM::NamedNodeMap - Used internally to store an element’s attributes.
Other classes that are not part of the DOM Level 1 Spec:
o XML::DOM::Parser - An non-validating XML parser that creates XML::DOM::Documents
o XML::DOM::ValParser - A validating XML parser that creates XML::DOM::Documents. It uses XML::Checker to check against the DocumentType (DTD)
o XML::Handler::BuildDOM - A PerlSAX handler that creates XML::DOM::Documents.

XML::DOM package

Constant definitions The following predefined constants indicate which type of node it is.

 UNKNOWN_NODE (0)                The node type is unknown (not part of DOM)

 ELEMENT_NODE (1)                The node is an Element.
 ATTRIBUTE_NODE (2)              The node is an Attr.
 TEXT_NODE (3)                   The node is a Text node.
 CDATA_SECTION_NODE (4)          The node is a CDATASection.
 ENTITY_REFERENCE_NODE (5)       The node is an EntityReference.
 ENTITY_NODE (6)                 The node is an Entity.
 PROCESSING_INSTRUCTION_NODE (7) The node is a ProcessingInstruction.
 COMMENT_NODE (8)                The node is a Comment.
 DOCUMENT_NODE (9)               The node is a Document.
 DOCUMENT_TYPE_NODE (10)         The node is a DocumentType.
 DOCUMENT_FRAGMENT_NODE (11)     The node is a DocumentFragment.
 NOTATION_NODE (12)              The node is a Notation.

 ELEMENT_DECL_NODE (13)          The node is an ElementDecl (not part of DOM)
 ATT_DEF_NODE (14)               The node is an AttDef (not part of DOM)
 XML_DECL_NODE (15)              The node is an XMLDecl (not part of DOM)
 ATTLIST_DECL_NODE (16)          The node is an AttlistDecl (not part of DOM)


   if ($node->getNodeType == ELEMENT_NODE)
       print "Its an Element";

<B>Not In DOM SpecB>: The DOM Spec does not mention UNKNOWN_NODE and, quite frankly, you should never encounter it. The last 4 node types were added to support the 4 added node classes.

    Global Variables

$VERSION The variable $XML::DOM::VERSION contains the version number of this implementation, e.g. 1.07.


These methods are not part of the DOM Level 1 Specification.
getIgnoreReadOnly and ignoreReadOnly (readOnly) The DOM Level 1 Spec does not allow you to edit certain sections of the document, e.g. the DocumentType, so by default this implementation throws DOMExceptions (i.e. NO_MODIFICATION_ALLOWED_ERR) when you try to edit a readonly node. These readonly checks can be disabled by (temporarily) setting the global IgnoreReadOnly flag.

The ignoreReadOnly method sets the global IgnoreReadOnly flag and returns its previous value. The getIgnoreReadOnly method simply returns its current value.

 my $oldIgnore = XML::DOM::ignoreReadOnly (1);
 eval {
 ... do whatever you want, catching any other exceptions ...
 XML::DOM::ignoreReadOnly ($oldIgnore);     # restore previous value

Another way to do it, using a local variable:

 { # start new scope
    local $XML::DOM::IgnoreReadOnly = 1;
    ... do whatever you want, dont worry about exceptions ...
 } # end of scope ($IgnoreReadOnly is set back to its previous value)

isValidName (name) Whether the specified name is a valid Name as specified in the XML spec. Characters with Unicode values > 127 are now also supported.
getAllowReservedNames and allowReservedNames (boolean) The first method returns whether reserved names are allowed. The second takes a boolean argument and sets whether reserved names are allowed. The initial value is 1 (i.e. allow reserved names.)

The XML spec states that Names starting with (X|x)(M|m)(L|l) are reserved for future use. (Amusingly enough, the XML version of the XML spec (REC-xml-19980210.xml) breaks that very rule by defining an ENTITY with the name ’xmlpio’.) A Name in this context means the Name token as found in the BNF rules in the XML spec.

XML::DOM only checks for errors when you modify the DOM tree, not when the DOM tree is built by the XML::DOM::Parser.

setTagCompression (funcref) There are 3 possible styles for printing empty Element tags:
Style 0

 <empty/> or <empty attr="val"/>

XML::DOM uses this style by default for all Elements.

Style 1

  <empty></empty> or <empty attr="val"></empty>

Style 2

  <empty /> or <empty attr="val" />

This style is sometimes desired when using XHTML. (Note the extra space before the slash /) See <> Appendix C for more details.

By default XML::DOM compresses all empty Element tags (style 0.) You can control which style is used for a particular Element by calling XML::DOM::setTagCompression with a reference to a function that takes 2 arguments. The first is the tag name of the Element, the second is the XML::DOM::Element that is being printed. The function should return 0, 1 or 2 to indicate which style should be used to print the empty tag. E.g.

 XML::DOM::setTagCompression (\&my_tag_compression);

 sub my_tag_compression
    my ($tag, $elem) = @_;

    # Print empty br, hr and img tags like this: <br />
    return 2 if $tag =~ /^(br|hr|img)$/;

    # Print other empty tags like this: <empty></empty>
    return 1;


o Perl Mappings

The value undef was used when the DOM Spec said null.

The DOM Spec says: Applications must encode DOMString using UTF-16 (defined in Appendix C.3 of [UNICODE] and Amendment 1 of [ISO-10646]). In this implementation we use plain old Perl strings encoded in UTF-8 instead of UTF-16.

o Text and CDATASection nodes

The Expat parser expands EntityReferences and CDataSection sections to raw strings and does not indicate where it was found. This implementation does therefore convert both to Text nodes at parse time. CDATASection and EntityReference nodes that are added to an existing Document (by the user) will be preserved.

Also, subsequent Text nodes are always merged at parse time. Text nodes that are added later can be merged with the normalize method. Consider using the addText method when adding Text nodes.

o Printing and toString

When printing (and converting an XML Document to a string) the strings have to encoded differently depending on where they occur. E.g. in a CDATASection all substrings are allowed except for ]]>. In regular text, certain characters are not allowed, e.g. > has to be converted to >. These routines should be verified by someone who knows the details.

o Quotes

Certain sections in XML are quoted, like attribute values in an Element. XML::Parser strips these quotes and the print methods in this implementation always uses double quotes, so when parsing and printing a document, single quotes may be converted to double quotes. The default value of an attribute definition (AttDef) in an AttlistDecl, however, will maintain its quotes.

o AttlistDecl

Attribute declarations for a certain Element are always merged into a single AttlistDecl object.

o Comments

Comments in the DOCTYPE section are not kept in the right place. They will become child nodes of the Document.

o Hidden Nodes

Previous versions of XML::DOM would expand parameter entity references (like <B>B>%pent<B>;B>), so when printing the DTD, it would print the contents of the external entity, instead of the parameter entity reference. With this release (1.27), you can prevent this by setting the XML::DOM::Parser options ParseParamEnt => 1 and ExpandParamEnt => 0.

When it is parsing the contents of the external entities, it *DOES* still add the nodes to the DocumentType, but it marks these nodes by setting the ’Hidden’ property. In addition, it adds an EntityReference node to the DocumentType node.

When printing the DocumentType node (or when using to_expat() or to_sax()), the ’Hidden’ nodes are suppressed, so you will see the parameter entity reference instead of the contents of the external entities. See test case t/dom_extent.t for an example.

The reason for adding the ’Hidden’ nodes to the DocumentType node, is that the nodes may contain <!ENTITY> definitions that are referenced further in the document. (Simply not adding the nodes to the DocumentType could cause such entity references to be expanded incorrectly.)

Note that you need XML::Parser 2.27 or higher for this to work correctly.


The Japanese version of this document by Takanori Kawai (Hippo2000) at <>

The DOM Level 1 specification at <>

The XML spec (Extensible Markup Language 1.0) at <>

The XML::Parser and XML::Parser::Expat manual pages.


The method getElementsByTagName() does not return a live NodeList. Whether this is an actual caveat is debatable, but a few people on the www-dom mailing list seemed to think so. I haven’t decided yet. It’s a pain to implement, it slows things down and the benefits seem marginal. Let me know what you think.

(To subscribe to the www-dom mailing list send an email with the subject subscribe to I only look here occasionally, so don’t send bug reports or suggestions about XML::DOM to this list, send them to instead.)


Send bug reports, hints, tips, suggestions to Enno Derksen at <>.

Thanks to Clark Cooper for his help with the initial version.

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perl v5.20.3 XML::DOM (3) 2000-02-17

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