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 XML::Simple - Easy API to read/write XML


NAME

XML::Simple - Easy API to read/write XML (esp config files)


SYNOPSIS

    use XML::Simple;
    my $ref = XMLin([<xml file or string>] [, <options>]);
    my $xml = XMLout($hashref [, <options>]);

Or the object oriented way:

    require XML::Simple;
    my $xs = new XML::Simple(options);
    my $ref = $xs->XMLin([<xml file or string>] [, <options>]);
    my $xml = $xs->XMLout($hashref [, <options>]);


QUICK START

Say you have a script called foo and a file of configuration options called foo.xml containing this:

  <config logdir="/var/log/foo/" debugfile="/tmp/foo.debug">
    <server name="sahara" osname="solaris" osversion="2.6">
      <address>10.0.0.101</address>
      <address>10.0.1.101</address>
    </server>
    <server name="gobi" osname="irix" osversion="6.5">
      <address>10.0.0.102</address>
    </server>
    <server name="kalahari" osname="linux" osversion="2.0.34">
      <address>10.0.0.103</address>
      <address>10.0.1.103</address>
    </server>
  </config>

The following lines of code in foo:

  use XML::Simple;
  my $config = XMLin();

will 'slurp' the configuration options into the hashref $config (because no arguments are passed to XMLin() the name and location of the XML file will be inferred from name and location of the script). You can dump out the contents of the hashref using Data::Dumper:

  use Data::Dumper;
  print Dumper($config);

which will produce something like this (formatting has been adjusted for brevity):

  {
      'logdir'        => '/var/log/foo/',
      'debugfile'     => '/tmp/foo.debug',
      'server'        => {
          'sahara'        => {
              'osversion'     => '2.6',
              'osname'        => 'solaris',
              'address'       => [ '10.0.0.101', '10.0.1.101' ]
          },
          'gobi'          => {
              'osversion'     => '6.5',
              'osname'        => 'irix',
              'address'       => '10.0.0.102'
          },
          'kalahari'      => {
              'osversion'     => '2.0.34',
              'osname'        => 'linux',
              'address'       => [ '10.0.0.103', '10.0.1.103' ]
          }
      }
  }

Your script could then access the name of the log directory like this:

  print $config->{logdir};

similarly, the second address on the server 'kalahari' could be referenced as:

  print $config->{server}->{kalahari}->{address}->[1];

What could be simpler? (Rhetorical).

For simple requirements, that's really all there is to it. If you want to store your XML in a different directory or file, or pass it in as a string or even pass it in via some derivative of an IO::Handle, you'll need to check out OPTIONS. If you want to turn off or tweak the array folding feature (that neat little transformation that produced $config->{server}) you'll find options for that as well.

If you want to generate XML (for example to write a modified version of $config back out as XML), check out XMLout().

If your needs are not so simple, this may not be the module for you. In that case, you might want to read WHERE TO FROM HERE?.


DESCRIPTION

The XML::Simple module provides a simple API layer on top of the XML::Parser module. Two functions are exported: XMLin() and XMLout().

The most common approach is to simply call these two functions directly, but an optional object oriented interface (see OPTIONAL OO INTERFACE below) allows them to be called as methods of an XML::Simple object.

XMLin()

Parses XML formatted data and returns a reference to a data structure which contains the same information in a more readily accessible form. (Skip down to EXAMPLES below, for more sample code).

XMLin() accepts an optional XML specifier followed by zero or more 'name => value' option pairs. The XML specifier can be one of the following:

A filename
If the filename contains no directory components XMLin() will look for the file in each directory in the searchpath (see OPTIONS below). eg:
  $ref = XMLin('/etc/params.xml');

Note, the filename '-' can be used to parse from STDIN.

undef
If there is no XML specifier, XMLin() will check the script directory and each of the searchpath directories for a file with the same name as the script but with the extension '.xml'. Note: if you wish to specify options, you must specify the value 'undef'. eg:
  $ref = XMLin(undef, forcearray => 1);

A string of XML
A string containing XML (recognised by the presence of '<' and '>' characters) will be parsed directly. eg:
  $ref = XMLin('<opt username="bob" password="flurp" />');

An IO::Handle object
An IO::Handle object will be read to EOF and its contents parsed. eg:
  $fh = new IO::File('/etc/params.xml');
  $ref = XMLin($fh);

XMLout()

Takes a data structure (generally a hashref) and returns an XML encoding of that structure. If the resulting XML is parsed using XMLin(), it will return a data structure equivalent to the original.

When translating hashes to XML, hash keys which have a leading '-' will be silently skipped. This is the approved method for marking elements of a data structure which should be ignored by XMLout. (Note: If these items were not skipped the key names would be emitted as element or attribute names with a leading '-' which would not be valid XML).

Caveats

Some care is required in creating data structures which will be passed to XMLout(). Hash keys from the data structure will be encoded as either XML element names or attribute names. Therefore, you should use hash key names which conform to the relatively strict XML naming rules:

Names in XML must begin with a letter. The remaining characters may be letters, digits, hyphens (-), underscores (_) or full stops (.). It is also allowable to include one colon (:) in an element name but this should only be used when working with namespaces - a facility well beyond the scope of XML::Simple.

You can use other punctuation characters in hash values (just not in hash keys) however XML::Simple does not support dumping binary data.

If you break these rules, the current implementation of XMLout() will simply emit non-compliant XML which will be rejected if you try to read it back in. (A later version of XML::Simple might take a more proactive approach).

Note also that although you can nest hashes and arrays to arbitrary levels, recursive data structures are not supported and will cause XMLout() to die.

Refer to WHERE TO FROM HERE? if XMLout() is too simple for your needs.


OPTIONS

XML::Simple supports a number of options (in fact as each release of XML::Simple adds more options, the module's claim to the name 'Simple' becomes more tenuous). If you find yourself repeatedly having to specify the same options, you might like to investigate OPTIONAL OO INTERFACE below.

Because there are so many options, it's hard for new users to know which ones are important, so here are the two you really need to know about:

Both XMLin() and XMLout() expect a single argument followed by a list of options. An option takes the form of a 'name => value' pair. The options listed below are marked with 'in' if they are recognised by XMLin() and 'out' if they are recognised by XMLout().

keyattr => [ list ] (in+out)
This option controls the 'array folding' feature which translates nested elements from an array to a hash. For example, this XML:
    <opt>
      <user login="grep" fullname="Gary R Epstein" />
      <user login="stty" fullname="Simon T Tyson" />
    </opt>

would, by default, parse to this:

    {
      'user' => [
                  {
                    'login' => 'grep',
                    'fullname' => 'Gary R Epstein'
                  },
                  {
                    'login' => 'stty',
                    'fullname' => 'Simon T Tyson'
                  }
                ]
    }

If the option 'keyattr => ``login''' were used to specify that the 'login' attribute is a key, the same XML would parse to:

    {
      'user' => {
                  'stty' => {
                              'fullname' => 'Simon T Tyson'
                            },
                  'grep' => {
                              'fullname' => 'Gary R Epstein'
                            }
                }
    }

The key attribute names should be supplied in an arrayref if there is more than one. XMLin() will attempt to match attribute names in the order supplied. XMLout() will use the first attribute name supplied when 'unfolding' a hash into an array.

Note: the keyattr option controls the folding of arrays. By default a single nested element will be rolled up into a scalar rather than an array and therefore will not be folded. Use the 'forcearray' option (below) to force nested elements to be parsed into arrays and therefore candidates for folding into hashes.

The default value for 'keyattr' is ['name', 'key', 'id']. Setting this option to an empty list will disable the array folding feature.

keyattr => { list } (in+out)
This alternative method of specifiying the key attributes allows more fine grained control over which elements are folded and on which attributes. For example the option 'keyattr => { package => 'id' } will cause any package elements to be folded on the 'id' attribute. No other elements which have an 'id' attribute will be folded at all.

Note: XMLin() will generate a warning if this syntax is used and an element which does not have the specified key attribute is encountered (eg: a 'package' element without an 'id' attribute, to use the example above). Warnings will only be generated if -w is in force.

Two further variations are made possible by prefixing a '+' or a '-' character to the attribute name:

The option 'keyattr => { user => ``+login'' }' will cause this XML:

    <opt>
      <user login="grep" fullname="Gary R Epstein" />
      <user login="stty" fullname="Simon T Tyson" />
    </opt>

to parse to this data structure:

    {
      'user' => {
                  'stty' => {
                              'fullname' => 'Simon T Tyson',
                              'login'    => 'stty'
                            },
                  'grep' => {
                              'fullname' => 'Gary R Epstein',
                              'login'    => 'grep'
                            }
                }
    }

The '+' indicates that the value of the key attribute should be copied rather than moved to the folded hash key.

A '-' prefix would produce this result:

    {
      'user' => {
                  'stty' => {
                              'fullname' => 'Simon T Tyson',
                              '-login'    => 'stty'
                            },
                  'grep' => {
                              'fullname' => 'Gary R Epstein',
                              '-login'    => 'grep'
                            }
                }
    }

As described earlier, XMLout will ignore hash keys starting with a '-'.

searchpath => [ list ] (in)
Where the XML is being read from a file, and no path to the file is specified, this attribute allows you to specify which directories should be searched.

If the first parameter to XMLin() is undefined, the default searchpath will contain only the directory in which the script itself is located. Otherwise the default searchpath will be empty.

Note: the current directory ('.') is not searched unless it is the directory containing the script.

forcearray => 1 (in)
This option should be set to '1' to force nested elements to be represented as arrays even when there is only one. Eg, with forcearray enabled, this XML:
    <opt>
      <name>value</name>
    </opt>

would parse to this:

    {
      'name' => [
                  'value'
                ]
    }

instead of this (the default):

    {
      'name' => 'value'
    }

This option is especially useful if the data structure is likely to be written back out as XML and the default behaviour of rolling single nested elements up into attributes is not desirable.

If you are using the array folding feature, you should almost certainly enable this option. If you do not, single nested elements will not be parsed to arrays and therefore will not be candidates for folding to a hash. (Given that the default value of 'keyattr' enables array folding, the default value of this option should probably also have been enabled too - sorry).

forcearray => [ name(s) ] (in)
This alternative form of the 'forcearray' option allows you to specify a list of element names which should always be forced into an array representation, rather than the 'all or nothing' approach above.

noattr => 1 (in+out)
When used with XMLout(), the generated XML will contain no attributes. All hash key/values will be represented as nested elements instead.

When used with XMLin(), any attributes in the XML will be ignored.

suppressempty => 1 | '' | undef (in)
This option controls what XMLin() should do with empty elements (no attributes and no content). The default behaviour is to represent them as empty hashes. Setting this option to a true value (eg: 1) will cause empty elements to be skipped altogether. Setting the option to 'undef' or the empty string will cause empty elements to be represented as the undefined value or the empty string respectively. The latter two alternatives are a little easier to test for in your code than a hash with no keys.

cache => [ cache scheme(s) ] (in)
Because loading the XML::Parser module and parsing an XML file can consume a significant number of CPU cycles, it is often desirable to cache the output of XMLin() for later reuse.

When parsing from a named file, XML::Simple supports a number of caching schemes. The 'cache' option may be used to specify one or more schemes (using an anonymous array). Each scheme will be tried in turn in the hope of finding a cached pre-parsed representation of the XML file. If no cached copy is found, the file will be parsed and the first cache scheme in the list will be used to save a copy of the results. The following cache schemes have been implemented:

storable
Utilises Storable.pm to read/write a cache file with the same name as the XML file but with the extension .stor

memshare
When a file is first parsed, a copy of the resulting data structure is retained in memory in the XML::Simple module's namespace. Subsequent calls to parse the same file will return a reference to this structure. This cached version will persist only for the life of the Perl interpreter (which in the case of mod_perl for example, may be some significant time).

Because each caller receives a reference to the same data structure, a change made by one caller will be visible to all. For this reason, the reference returned should be treated as read-only.

memcopy
This scheme works identically to 'memshare' (above) except that each caller receives a reference to a new data structure which is a copy of the cached version. Copying the data structure will add a little processing overhead, therefore this scheme should only be used where the caller intends to modify the data structure (or wishes to protect itself from others who might). This scheme uses Storable.pm to perform the copy.

keeproot => 1 (in+out)
In its attempt to return a data structure free of superfluous detail and unnecessary levels of indirection, XMLin() normally discards the root element name. Setting the 'keeproot' option to '1' will cause the root element name to be retained. So after executing this code:
  $config = XMLin('<config tempdir="/tmp" />', keeproot => 1)

You'll be able to reference the tempdir as $config->{config}->{tempdir} instead of the default $config->{tempdir}.

Similarly, setting the 'keeproot' option to '1' will tell XMLout() that the data structure already contains a root element name and it is not necessary to add another.

rootname => 'string' (out)
By default, when XMLout() generates XML, the root element will be named 'opt'. This option allows you to specify an alternative name.

Specifying either undef or the empty string for the rootname option will produce XML with no root elements. In most cases the resulting XML fragment will not be 'well formed' and therefore could not be read back in by XMLin(). Nevertheless, the option has been found to be useful in certain circumstances.

forcecontent (in)
When XMLin() parses elements which have text content as well as attributes, the text content must be represented as a hash value rather than a simple scalar. This option allows you to force text content to always parse to a hash value even when there are no attributes. So for example:
  XMLin('<opt><x>text1</x><y a="2">text2</y></opt>', forcecontent => 1)

will parse to:

  {
    'x' => {           'content' => 'text1' },
    'y' => { 'a' => 2, 'content' => 'text2' }
  }

instead of:

  {
    'x' => 'text1',
    'y' => { 'a' => 2, 'content' => 'text2' }
  }

contentkey => 'keyname' (in+out)
When text content is parsed to a hash value, this option let's you specify a name for the hash key to override the default 'content'. So for example:
  XMLin('<opt one="1">Text</opt>', contentkey => 'text')

will parse to:

  { 'one' => 1, 'text' => 'Text' }

instead of:

  { 'one' => 1, 'content' => 'Text' }

XMLout() will also honour the value of this option when converting a hashref to XML.

xmldecl => 1 or xmldecl => 'string' (out)
If you want the output from XMLout() to start with the optional XML declaration, simply set the option to '1'. The default XML declaration is:
        <?xml version='1.0' standalone='yes'?>

If you want some other string (for example to declare an encoding value), set the value of this option to the complete string you require.

outputfile => <file specifier> (out)
The default behaviour of XMLout() is to return the XML as a string. If you wish to write the XML to a file, simply supply the filename using the 'outputfile' option. Alternatively, you can supply an IO handle object instead of a filename.

noescape => 1 (out)
By default, XMLout() will translate the characters '<', '>', '&' and '``' to '&lt;', '&gt;', '&amp;' and '&quot' respectively. Use this option to suppress escaping (presumably because you've already escaped the data in some more sophisticated manner).

parseropts => [ XML::Parser Options ] (in)
Use this option to specify parameters that should be passed to the constructor of the underlying XML::Parser object. For example to turn on the namespace processing mode, you could say:
  XMLin($xml, parseropts => [ Namespaces => 1 ])


OPTIONAL OO INTERFACE

The procedural interface is both simple and convenient however there are a couple of reasons why you might prefer to use the object oriented (OO) interface:

The default values for the options described above are unlikely to suit everyone. The OO interface allows you to effectively override XML::Simple's defaults with your preferred values. It works like this:

First create an XML::Simple parser object with your preferred defaults:

  my $xs = new XML::Simple(forcearray => 1, keeproot => 1);

then call XMLin() or XMLout() as a method of that object:

  my $ref = $xs->XMLin($xml);
  my $xml = $xs->XMLout($ref);

You can also specify options when you make the method calls and these values will be merged with the values specified when the object was created. Values specified in a method call take precedence.

Overriding methods is a more advanced topic but might be useful if for example you wished to provide an alternative routine for escaping character data (the escape_value method) or for building the initial parse tree (the build_tree method).


ERROR HANDLING

The XML standard is very clear on the issue of non-compliant documents. An error in parsing any single element (for example a missing end tag) must cause the whole document to be rejected. XML::Simple will die with an appropriate message if it encounters a parsing error.

If dying is not appropriate for your application, you should arrange to call XMLin() in an eval block and look for errors in $@. eg:

    my $config = eval { XMLin() };
    PopUpMessage($@) if($@);

Note, there is a common misconception that use of eval will significantly slow down a script. While that may be true when the code being eval'd is in a string, it is not true of code like the sample above.


EXAMPLES

When XMLin() reads the following very simple piece of XML:

    <opt username="testuser" password="frodo"></opt>

it returns the following data structure:

    {
      'username' => 'testuser',
      'password' => 'frodo'
    }

The identical result could have been produced with this alternative XML:

    <opt username="testuser" password="frodo" />

Or this (although see 'forcearray' option for variations):

    <opt>
      <username>testuser</username>
      <password>frodo</password>
    </opt>

Repeated nested elements are represented as anonymous arrays:

    <opt>
      <person firstname="Joe" lastname="Smith">
        <email>joe@smith.com</email>
        <email>jsmith@yahoo.com</email>
      </person>
      <person firstname="Bob" lastname="Smith">
        <email>bob@smith.com</email>
      </person>
    </opt>
    {
      'person' => [
                    {
                      'email' => [
                                   'joe@smith.com',
                                   'jsmith@yahoo.com'
                                 ],
                      'firstname' => 'Joe',
                      'lastname' => 'Smith'
                    },
                    {
                      'email' => 'bob@smith.com',
                      'firstname' => 'Bob',
                      'lastname' => 'Smith'
                    }
                  ]
    }

Nested elements with a recognised key attribute are transformed (folded) from an array into a hash keyed on the value of that attribute:

    <opt>
      <person key="jsmith" firstname="Joe" lastname="Smith" />
      <person key="tsmith" firstname="Tom" lastname="Smith" />
      <person key="jbloggs" firstname="Joe" lastname="Bloggs" />
    </opt>
    {
      'person' => {
                    'jbloggs' => {
                                   'firstname' => 'Joe',
                                   'lastname' => 'Bloggs'
                                 },
                    'tsmith' => {
                                  'firstname' => 'Tom',
                                  'lastname' => 'Smith'
                                },
                    'jsmith' => {
                                  'firstname' => 'Joe',
                                  'lastname' => 'Smith'
                                }
                  }
    }

The <anon> tag can be used to form anonymous arrays:

    <opt>
      <head><anon>Col 1</anon><anon>Col 2</anon><anon>Col 3</anon></head>
      <data><anon>R1C1</anon><anon>R1C2</anon><anon>R1C3</anon></data>
      <data><anon>R2C1</anon><anon>R2C2</anon><anon>R2C3</anon></data>
      <data><anon>R3C1</anon><anon>R3C2</anon><anon>R3C3</anon></data>
    </opt>
    {
      'head' => [
                  [ 'Col 1', 'Col 2', 'Col 3' ]
                ],
      'data' => [
                  [ 'R1C1', 'R1C2', 'R1C3' ],
                  [ 'R2C1', 'R2C2', 'R2C3' ],
                  [ 'R3C1', 'R3C2', 'R3C3' ]
                ]
    }

Anonymous arrays can be nested to arbirtrary levels and as a special case, if the surrounding tags for an XML document contain only an anonymous array the arrayref will be returned directly rather than the usual hashref:

    <opt>
      <anon><anon>Col 1</anon><anon>Col 2</anon></anon>
      <anon><anon>R1C1</anon><anon>R1C2</anon></anon>
      <anon><anon>R2C1</anon><anon>R2C2</anon></anon>
    </opt>
    [
      [ 'Col 1', 'Col 2' ],
      [ 'R1C1', 'R1C2' ],
      [ 'R2C1', 'R2C2' ]
    ]

Elements which only contain text content will simply be represented as a scalar. Where an element has both attributes and text content, the element will be represented as a hashref with the text content in the 'content' key:

  <opt>
    <one>first</one>
    <two attr="value">second</two>
  </opt>
  {
    'one' => 'first',
    'two' => { 'attr' => 'value', 'content' => 'second' }
  }

Mixed content (elements which contain both text content and nested elements) will be not be represented in a useful way - element order and significant whitespace will be lost. If you need to work with mixed content, then XML::Simple is not the right tool for your job - check out the next section.


WHERE TO FROM HERE?

XML::Simple is by nature very simple.

If any of these points are a problem for you, then XML::Simple is probably not the right module for your application. The following section is intended to give pointers which might help you select a more powerful tool - it's a bit sketchy right now but submissions are welcome.

XML::Parser
XML::Simple is built on top of XML::Parser, so if you have XML::Simple working you already have XML::Parser installed. This is a comprehensive, fast, industrial strength (non-validating) parsing tool built on top of James Clark's 'expat' library. It does support converting XML into a Perl tree structure (with full support for mixed content) but for arbritrarily large documents you're probably better off defining handler routines for XML::Parser to call as each element is parsed. The distribution includes a number of sample applications.

XML::DOM
The data structure returned by XML::Simple was designed for convenience rather than standards compliance. XML::DOM is a parser built on top of XML::Parser, which returns a 'Document' object conforming to the API of the Document Object Model as described at http://www.w3.org/TR/REC-DOM-Level-1 . This Document object can then be examined, modified and written back out to a file or converted to a string.

XML::Grove
Compliance with the Document Object Model might be particularly useful when porting code to or from another language. However, if you're looking for a simpler, 'perlish' object interface, take a look at XML::Grove.

XML::Twig
XML::Twig offers a tree-oriented interface to a document while still allowing the processing of documents of any size. It allows processing chunks of documents in tree-mode which can then be flushed or purged from the memory. The XML::Twig page is at http://standards.ieee.org/resources/spasystem/twig/

libxml-perl
libxml-perl is a collection of Perl modules, scripts, and documents for working with XML in Perl. The distribution includes PerlSAX - a Perl implementation of the SAX API. It also include XML::PatAct modules for processing XML by defining patterns and associating them with actions. For more details see http://bitsko.slc.ut.us/libxml-perl/ .

XML::PYX
XML::PYX allows you to apply Unix command pipelines (using grep, sed etc) to filter or transform XML files. Ideally suited for tasks such as extracting all text content or stripping out all occurrences of a particular tag without having to write a Perl script at all. It can also be used for transforming HTML to XHTML.

XML::RAX
If you wish to process XML files containing a series of 'records', XML::RAX provides a simple purpose-designed interface. If it still hasn't made it to CPAN, try: http://www.dancentury.com/robh/

XML::Writer
XML::Writer is a helper module for Perl programs that write XML documents.

XML::Dumper
XML::Dumper dumps Perl data to a structured XML format. XML::Dumper can also read XML data that was previously dumped by the module and convert it back to Perl.

Don't forget to check out the Perl XML FAQ at: http://www.perlxml.com/faq/perl-xml-faq.html


STATUS

This version (1.06) is the current stable version.


SEE ALSO

XML::Simple requires XML::Parser and File::Spec. The optional caching functions require Storable.


COPYRIGHT

Copyright 1999-2001 Grant McLean <grantm@cpan.org>

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

 XML::Simple - Easy API to read/write XML