XML::Simple(3) User Contributed Perl Documentation XML::Simple(3)
NAME
XML::Simple - An API for simple XML files
SYNOPSIS
PLEASE DO NOT USE THIS MODULE IN NEW CODE. If you ignore this warning
and use it anyway, the "qw(:strict)" mode will save you a little pain.
use XML::Simple qw(:strict);
my $ref = XMLin([<xml file or string>] [, <options>]);
my $xml = XMLout($hashref [, <options>]);
Or the object oriented way:
require XML::Simple qw(:strict);
my $xs = XML::Simple->new([<options>]);
my $ref = $xs->XMLin([<xml file or string>] [, <options>]);
my $xml = $xs->XMLout($hashref [, <options>]);
(or see "SAX SUPPORT" for 'the SAX way').
Note, in these examples, the square brackets are used to denote
optional items not to imply items should be supplied in arrayrefs.
STATUS OF THIS MODULE
The use of this module in new code is strongly discouraged. Other
modules are available which provide more straightforward and consistent
interfaces. In particular, XML::LibXML is highly recommended and you
can refer to Perl XML::LibXML by Example <http://grantm.github.io/perl-
libxml-by-example/> for a tutorial introduction.
XML::Twig is another excellent alternative.
The major problems with this module are the large number of options
(some of which have unfortunate defaults) and the arbitrary ways in
which these options interact - often producing unexpected results.
Patches with bug fixes and documentation fixes are welcome, but new
features are unlikely to be added.
QUICK START
Say you have a script called foo and a file of configuration options
called foo.xml containing the following:
<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 qw(:strict);
my $config = XMLin(undef, KeyAttr => { server => 'name' }, ForceArray => [ 'server', 'address' ]);
will 'slurp' the configuration options into the hashref $config
(because no filename or XML string was passed as the first argument to
"XMLin()" the name and location of the XML file will be inferred from
name and location of the script). You can dump out the contents of the
hashref using Data::Dumper:
use Data::Dumper;
print Dumper($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];
Note: If the mapping between the output of Data::Dumper and the print
statements above is not obvious to you, then please refer to the
'references' tutorial (AKA: "Mark's very short tutorial about
references") at perlreftut.
In this example, the "ForceArray" option was used to list elements that
might occur multiple times and should therefore be represented as
arrayrefs (even when only one element is present).
The "KeyAttr" option was used to indicate that each "<server>" element
has a unique identifier in the "name" attribute. This allows you to
index directly to a particular server record using the name as a hash
key (as shown above).
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 an
underlying XML parsing module (either XML::Parser or one of the SAX2
parser modules). Two functions are exported: "XMLin()" and "XMLout()".
Note: you can explicitly request the lower case versions of the
function names: "xml_in()" and "xml_out()".
The simplest approach is to 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. The
object interface can also be used at either end of a SAX pipeline.
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) or in the current directory if the SearchPath
option is not defined. 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 = IO::File->new('/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 should return a data structure equivalent to the original
(see caveats below).
The "XMLout()" function can also be used to output the XML as SAX
events see the "Handler" option and "SAX SUPPORT" for more details).
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 (XML::Simple can only
usefully work with namespaces when teamed with a SAX Parser).
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, circular data structures are not supported and will cause
"XMLout()" to die.
If you wish to 'round-trip' arbitrary data structures from Perl to XML
and back to Perl, then you should probably disable array folding (using
the KeyAttr option) both with "XMLout()" and with "XMLin()". If you
still don't get the expected results, you may prefer to use XML::Dumper
which is designed for exactly that purpose.
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 increasingly tenuous). If you find yourself repeatedly having
to specify the same options, you might like to investigate "OPTIONAL OO
INTERFACE" below.
If you can't be bothered reading the documentation, refer to "STRICT
MODE" to automatically catch common mistakes.
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:
o check out "ForceArray" because you'll almost certainly want to turn
it on
o make sure you know what the "KeyAttr" option does and what its
default value is because it may surprise you otherwise (note in
particular that 'KeyAttr' affects both "XMLin" and "XMLout")
The option name headings below have a trailing 'comment' - a hash
followed by two pieces of metadata:
o Options are marked with 'in' if they are recognised by "XMLin()"
and 'out' if they are recognised by "XMLout()".
o Each option is also flagged to indicate whether it is:
'important' - don't use the module until you understand this one
'handy' - you can skip this on the first time through
'advanced' - you can skip this on the second time through
'SAX only' - don't worry about this unless you're using SAX (or
alternatively if you need this, you also need SAX)
'seldom used' - you'll probably never use this unless you were the
person that requested the feature
The options are listed alphabetically:
Note: option names are no longer case sensitive so you can use the
mixed case versions shown here; all lower case as required by versions
2.03 and earlier; or you can add underscores between the words (eg:
key_attr).
AttrIndent => 1 # out - handy
When you are using "XMLout()", enable this option to have attributes
printed one-per-line with sensible indentation rather than all on one
line.
Cache => [ cache schemes ] # in - advanced
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.
Warning! The memory-based caching schemes compare the timestamp on the
file to the time when it was last parsed. If the file is stored on an
NFS filesystem (or other network share) and the clock on the file
server is not exactly synchronised with the clock where your script is
run, updates to the source XML file may appear to be ignored.
ContentKey => 'keyname' # in+out - seldom used
When text content is parsed to a hash value, this option lets 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.
You can also prefix your selected key name with a '-' character to have
"XMLin()" try a little harder to eliminate unnecessary 'content' keys
after array folding. For example:
XMLin(
'<opt><item name="one">First</item><item name="two">Second</item></opt>',
KeyAttr => {item => 'name'},
ForceArray => [ 'item' ],
ContentKey => '-content'
)
will parse to:
{
'item' => {
'one' => 'First'
'two' => 'Second'
}
}
rather than this (without the '-'):
{
'item' => {
'one' => { 'content' => 'First' }
'two' => { 'content' => 'Second' }
}
}
DataHandler => code_ref # in - SAX only
When you use an XML::Simple object as a SAX handler, it will return a
'simple tree' data structure in the same format as "XMLin()" would
return. If this option is set (to a subroutine reference), then when
the tree is built the subroutine will be called and passed two
arguments: a reference to the XML::Simple object and a reference to the
data tree. The return value from the subroutine will be returned to
the SAX driver. (See "SAX SUPPORT" for more details).
ForceArray => 1 # in - important
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 => [ names ] # in - important
This alternative (and preferred) 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.
It is also possible (since version 2.05) to include compiled regular
expressions in the list - any element names which match the pattern
will be forced to arrays. If the list contains only a single regex,
then it is not necessary to enclose it in an arrayref. Eg:
ForceArray => qr/_list$/
ForceContent => 1 # in - seldom used
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' }
}
GroupTags => { grouping tag => grouped tag } # in+out - handy
You can use this option to eliminate extra levels of indirection in
your Perl data structure. For example this XML:
<opt>
<searchpath>
<dir>/usr/bin</dir>
<dir>/usr/local/bin</dir>
<dir>/usr/X11/bin</dir>
</searchpath>
</opt>
Would normally be read into a structure like this:
{
searchpath => {
dir => [ '/usr/bin', '/usr/local/bin', '/usr/X11/bin' ]
}
}
But when read in with the appropriate value for 'GroupTags':
my $opt = XMLin($xml, GroupTags => { searchpath => 'dir' });
It will return this simpler structure:
{
searchpath => [ '/usr/bin', '/usr/local/bin', '/usr/X11/bin' ]
}
The grouping element ("<searchpath>" in the example) must not contain
any attributes or elements other than the grouped element.
You can specify multiple 'grouping element' to 'grouped element'
mappings in the same hashref. If this option is combined with
"KeyAttr", the array folding will occur first and then the grouped
element names will be eliminated.
"XMLout" will also use the grouptag mappings to re-introduce the tags
around the grouped elements. Beware though that this will occur in all
places that the 'grouping tag' name occurs - you probably don't want to
use the same name for elements as well as attributes.
Handler => object_ref # out - SAX only
Use the 'Handler' option to have "XMLout()" generate SAX events rather
than returning a string of XML. For more details see "SAX SUPPORT"
below.
Note: the current implementation of this option generates a string of
XML and uses a SAX parser to translate it into SAX events. The normal
encoding rules apply here - your data must be UTF8 encoded unless you
specify an alternative encoding via the 'XMLDecl' option; and by the
time the data reaches the handler object, it will be in UTF8 form
regardless of the encoding you supply. A future implementation of this
option may generate the events directly.
KeepRoot => 1 # in+out - handy
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.
KeyAttr => [ list ] # in+out - important
This option controls the 'array folding' feature which translates
nested elements from an array to a hash. It also controls the
'unfolding' of hashes to arrays.
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 1: The default value for 'KeyAttr' is ['name', 'key', 'id']. If
you do not want folding on input or unfolding on output you must set
this option to an empty list to disable the feature.
Note 2: If you wish to use this option, you should also enable the
"ForceArray" option. Without 'ForceArray', a single nested element
will be rolled up into a scalar rather than an array and therefore will
not be folded (since only arrays get folded).
KeyAttr => { list } # in+out - important
This alternative (and preferred) method of specifying 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 (or a fatal error in "STRICT
MODE") 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 can be
suppressed with the lexical "no warnings;" pragma or "no warnings
'XML::Simple';".
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
'-'.
NoAttr => 1 # in+out - handy
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.
NoEscape => 1 # out - seldom used
By default, "XMLout()" will translate the characters '<', '>', '&' and
'"' to '<', '>', '&' and '"' respectively. Use this
option to suppress escaping (presumably because you've already escaped
the data in some more sophisticated manner).
NoIndent => 1 # out - seldom used
Set this option to 1 to disable "XMLout()"'s default 'pretty printing'
mode. With this option enabled, the XML output will all be on one line
(unless there are newlines in the data) - this may be easier for
downstream processing.
NoSort => 1 # out - seldom used
Newer versions of XML::Simple sort elements and attributes
alphabetically (*), by default. Enable this option to suppress the
sorting - possibly for backwards compatibility.
* Actually, sorting is alphabetical but 'key' attribute or element
names (as in 'KeyAttr') sort first. Also, when a hash of hashes is
'unfolded', the elements are sorted alphabetically by the value of the
key field.
NormaliseSpace => 0 | 1 | 2 # in - handy
This option controls how whitespace in text content is handled.
Recognised values for the option are:
o 0 = (default) whitespace is passed through unaltered (except of
course for the normalisation of whitespace in attribute values
which is mandated by the XML recommendation)
o 1 = whitespace is normalised in any value used as a hash key
(normalising means removing leading and trailing whitespace and
collapsing sequences of whitespace characters to a single space)
o 2 = whitespace is normalised in all text content
Note: you can spell this option with a 'z' if that is more natural for
you.
NSExpand => 1 # in+out handy - SAX only
This option controls namespace expansion - the translation of element
and attribute names of the form 'prefix:name' to '{uri}name'. For
example the element name 'xsl:template' might be expanded to:
'{http://www.w3.org/1999/XSL/Transform}template'.
By default, "XMLin()" will return element names and attribute names
exactly as they appear in the XML. Setting this option to 1 will cause
all element and attribute names to be expanded to include their
namespace prefix.
Note: You must be using a SAX parser for this option to work (ie: it
does not work with XML::Parser).
This option also controls whether "XMLout()" performs the reverse
translation from '{uri}name' back to 'prefix:name'. The default is no
translation. If your data contains expanded names, you should set this
option to 1 otherwise "XMLout" will emit XML which is not well formed.
Note: You must have the XML::NamespaceSupport module installed if you
want "XMLout()" to translate URIs back to prefixes.
NumericEscape => 0 | 1 | 2 # out - handy
Use this option to have 'high' (non-ASCII) characters in your Perl data
structure converted to numeric entities (eg: €) in the XML
output. Three levels are possible:
0 - default: no numeric escaping (OK if you're writing out UTF8)
1 - only characters above 0xFF are escaped (ie: characters in the
0x80-FF range are not escaped), possibly useful with ISO8859-1 output
2 - all characters above 0x7F are escaped (good for plain ASCII output)
OutputFile => <file specifier> # out - handy
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.
This option also accepts an IO handle object - especially useful in
Perl 5.8.0 and later for output using an encoding other than UTF-8, eg:
open my $fh, '>:encoding(iso-8859-1)', $path or die "open($path): $!";
XMLout($ref, OutputFile => $fh);
Note, XML::Simple does not require that the object you pass in to the
OutputFile option inherits from IO::Handle - it simply assumes the
object supports a "print" method.
ParserOpts => [ XML::Parser Options ] # in - don't use this
Note: This option is now officially deprecated. If you find it useful,
email the author with an example of what you use it for. Do not use
this option to set the ProtocolEncoding, that's just plain wrong - fix
the XML.
This option allows you to pass parameters to the constructor of the
underlying XML::Parser object (which of course assumes you're not using
SAX).
RootName => 'string' # out - handy
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.
SearchPath => [ list ] # in - handy
If you pass "XMLin()" a filename, but the filename include no directory
component, you can use this option to specify which directories should
be searched to locate the file. You might use this option to search
first in the user's home directory, then in a global directory such as
/etc.
If a filename is provided to "XMLin()" but SearchPath is not defined,
the file is assumed to be in the current directory.
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.
StrictMode => 1 | 0 # in+out seldom used
This option allows you to turn "STRICT MODE" on or off for a particular
call, regardless of whether it was enabled at the time XML::Simple was
loaded.
SuppressEmpty => 1 | '' | undef # in+out - handy
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.
The option also controls what "XMLout()" does with undefined values.
Setting the option to undef causes undefined values to be output as
empty elements (rather than empty attributes), it also suppresses the
generation of warnings about undefined values. Setting the option to a
true value (eg: 1) causes undefined values to be skipped altogether on
output.
ValueAttr => [ names ] # in - handy
Use this option to deal elements which always have a single attribute
and no content. Eg:
<opt>
<colour value="red" />
<size value="XXL" />
</opt>
Setting "ValueAttr => [ 'value' ]" will cause the above XML to parse
to:
{
colour => 'red',
size => 'XXL'
}
instead of this (the default):
{
colour => { value => 'red' },
size => { value => 'XXL' }
}
Note: This form of the ValueAttr option is not compatible with
"XMLout()" - since the attribute name is discarded at parse time, the
original XML cannot be reconstructed.
ValueAttr => { element => attribute, ... } # in+out - handy
This (preferred) form of the ValueAttr option requires you to specify
both the element and the attribute names. This is not only safer, it
also allows the original XML to be reconstructed by "XMLout()".
Note: You probably don't want to use this option and the NoAttr option
at the same time.
Variables => { name => value } # in - handy
This option allows variables in the XML to be expanded when the file is
read. (there is no facility for putting the variable names back if you
regenerate XML using "XMLout").
A 'variable' is any text of the form "${name}" which occurs in an
attribute value or in the text content of an element. If 'name'
matches a key in the supplied hashref, "${name}" will be replaced with
the corresponding value from the hashref. If no matching key is found,
the variable will not be replaced. Names must match the regex:
"[\w.]+" (ie: only 'word' characters and dots are allowed).
VarAttr => 'attr_name' # in - handy
In addition to the variables defined using "Variables", this option
allows variables to be defined in the XML. A variable definition
consists of an element with an attribute called 'attr_name' (the value
of the "VarAttr" option). The value of the attribute will be used as
the variable name and the text content of the element will be used as
the value. A variable defined in this way will override a variable
defined using the "Variables" option. For example:
XMLin( '<opt>
<dir name="prefix">/usr/local/apache</dir>
<dir name="exec_prefix">${prefix}</dir>
<dir name="bindir">${exec_prefix}/bin</dir>
</opt>',
VarAttr => 'name', ContentKey => '-content'
);
produces the following data structure:
{
dir => {
prefix => '/usr/local/apache',
exec_prefix => '/usr/local/apache',
bindir => '/usr/local/apache/bin',
}
}
XMLDecl => 1 or XMLDecl => 'string' # out - handy
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.
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:
o to define a set of default values which should be used on all
subsequent calls to "XMLin()" or "XMLout()"
o to override methods in XML::Simple to provide customised behaviour
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 = XML::Simple->new(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.
Note: when called as methods, the "XMLin()" and "XMLout()" routines may
be called as "xml_in()" or "xml_out()". The method names are aliased
so the only difference is the aesthetics.
Parsing Methods
You can explicitly call one of the following methods rather than rely
on the "xml_in()" method automatically determining whether the target
to be parsed is a string, a file or a filehandle:
parse_string(text)
Works exactly like the "xml_in()" method but assumes the first
argument is a string of XML (or a reference to a scalar containing
a string of XML).
parse_file(filename)
Works exactly like the "xml_in()" method but assumes the first
argument is the name of a file containing XML.
parse_fh(file_handle)
Works exactly like the "xml_in()" method but assumes the first
argument is a filehandle which can be read to get XML.
Hook Methods
You can make your own class which inherits from XML::Simple and
overrides certain behaviours. The following methods may provide useful
'hooks' upon which to hang your modified behaviour. You may find other
undocumented methods by examining the source, but those may be subject
to change in future releases.
new_xml_parser()
This method will be called when a new XML::Parser object must be
constructed (either because XML::SAX is not installed or
XML::Parser is preferred).
handle_options(direction, name => value ...)
This method will be called when one of the parsing methods or the
"XMLout()" method is called. The initial argument will be a string
(either 'in' or 'out') and the remaining arguments will be name
value pairs.
default_config_file()
Calculates and returns the name of the file which should be parsed
if no filename is passed to "XMLin()" (default: "$0.xml").
build_simple_tree(filename, string)
Called from "XMLin()" or any of the parsing methods. Takes either
a file name as the first argument or "undef" followed by a 'string'
as the second argument. Returns a simple tree data structure. You
could override this method to apply your own transformations before
the data structure is returned to the caller.
new_hashref()
When the 'simple tree' data structure is being built, this method
will be called to create any required anonymous hashrefs.
sorted_keys(name, hashref)
Called when "XMLout()" is translating a hashref to XML. This
routine returns a list of hash keys in the order that the
corresponding attributes/elements should appear in the output.
escape_value(string)
Called from "XMLout()", takes a string and returns a copy of the
string with XML character escaping rules applied.
escape_attr(string)
Called from "XMLout()", to handle attribute values. By default,
just calls "escape_value()", but you can override this method if
you want attributes escaped differently than text content.
numeric_escape(string)
Called from "escape_value()", to handle non-ASCII characters
(depending on the value of the NumericEscape option).
copy_hash(hashref, extra_key => value, ...)
Called from "XMLout()", when 'unfolding' a hash of hashes into an
array of hashes. You might wish to override this method if you're
using tied hashes and don't want them to get untied.
Cache Methods
XML::Simple implements three caching schemes ('storable', 'memshare'
and 'memcopy'). You can implement a custom caching scheme by
implementing two methods - one for reading from the cache and one for
writing to it.
For example, you might implement a new 'dbm' scheme that stores cached
data structures using the MLDBM module. First, you would add a
"cache_read_dbm()" method which accepted a filename for use as a lookup
key and returned a data structure on success, or undef on failure.
Then, you would implement a "cache_read_dbm()" method which accepted a
data structure and a filename.
You would use this caching scheme by specifying the option:
Cache => [ 'dbm' ]
STRICT MODE
If you import the XML::Simple routines like this:
use XML::Simple qw(:strict);
the following common mistakes will be detected and treated as fatal
errors
o Failing to explicitly set the "KeyAttr" option - if you can't be
bothered reading about this option, turn it off with: KeyAttr => [
]
o Failing to explicitly set the "ForceArray" option - if you can't be
bothered reading about this option, set it to the safest mode with:
ForceArray => 1
o Setting ForceArray to an array, but failing to list all the
elements from the KeyAttr hash.
o Data error - KeyAttr is set to say { part => 'partnum' } but the
XML contains one or more <part> elements without a 'partnum'
attribute (or nested element). Note: if strict mode is not set but
"use warnings;" is in force, this condition triggers a warning.
o Data error - as above, but non-unique values are present in the key
attribute (eg: more than one <part> element with the same partnum).
This will also trigger a warning if strict mode is not enabled.
o Data error - as above, but value of key attribute (eg: partnum) is
not a scalar string (due to nested elements etc). This will also
trigger a warning if strict mode is not enabled.
SAX SUPPORT
From version 1.08_01, XML::Simple includes support for SAX (the Simple
API for XML) - specifically SAX2.
In a typical SAX application, an XML parser (or SAX 'driver') module
generates SAX events (start of element, character data, end of element,
etc) as it parses an XML document and a 'handler' module processes the
events to extract the required data. This simple model allows for some
interesting and powerful possibilities:
o Applications written to the SAX API can extract data from huge XML
documents without the memory overheads of a DOM or tree API.
o The SAX API allows for plug and play interchange of parser modules
without having to change your code to fit a new module's API. A
number of SAX parsers are available with capabilities ranging from
extreme portability to blazing performance.
o A SAX 'filter' module can implement both a handler interface for
receiving data and a generator interface for passing modified data
on to a downstream handler. Filters can be chained together in
'pipelines'.
o One filter module might split a data stream to direct data to two
or more downstream handlers.
o Generating SAX events is not the exclusive preserve of XML parsing
modules. For example, a module might extract data from a
relational database using DBI and pass it on to a SAX pipeline for
filtering and formatting.
XML::Simple can operate at either end of a SAX pipeline. For example,
you can take a data structure in the form of a hashref and pass it into
a SAX pipeline using the 'Handler' option on "XMLout()":
use XML::Simple;
use Some::SAX::Filter;
use XML::SAX::Writer;
my $ref = {
.... # your data here
};
my $writer = XML::SAX::Writer->new();
my $filter = Some::SAX::Filter->new(Handler => $writer);
my $simple = XML::Simple->new(Handler => $filter);
$simple->XMLout($ref);
You can also put XML::Simple at the opposite end of the pipeline to
take advantage of the simple 'tree' data structure once the relevant
data has been isolated through filtering:
use XML::SAX;
use Some::SAX::Filter;
use XML::Simple;
my $simple = XML::Simple->new(ForceArray => 1, KeyAttr => ['partnum']);
my $filter = Some::SAX::Filter->new(Handler => $simple);
my $parser = XML::SAX::ParserFactory->parser(Handler => $filter);
my $ref = $parser->parse_uri('some_huge_file.xml');
print $ref->{part}->{'555-1234'};
You can build a filter by using an XML::Simple object as a handler and
setting its DataHandler option to point to a routine which takes the
resulting tree, modifies it and sends it off as SAX events to a
downstream handler:
my $writer = XML::SAX::Writer->new();
my $filter = XML::Simple->new(
DataHandler => sub {
my $simple = shift;
my $data = shift;
# Modify $data here
$simple->XMLout($data, Handler => $writer);
}
);
my $parser = XML::SAX::ParserFactory->parser(Handler => $filter);
$parser->parse_uri($filename);
Note: In this last example, the 'Handler' option was specified in the
call to "XMLout()" but it could also have been specified in the
constructor.
ENVIRONMENT
If you don't care which parser module XML::Simple uses then skip this
section entirely (it looks more complicated than it really is).
XML::Simple will default to using a SAX parser if one is available or
XML::Parser if SAX is not available.
You can dictate which parser module is used by setting either the
environment variable 'XML_SIMPLE_PREFERRED_PARSER' or the package
variable $XML::Simple::PREFERRED_PARSER to contain the module name.
The following rules are used:
o The package variable takes precedence over the environment variable
if both are defined. To force XML::Simple to ignore the
environment settings and use its default rules, you can set the
package variable to an empty string.
o If the 'preferred parser' is set to the string 'XML::Parser', then
XML::Parser will be used (or "XMLin()" will die if XML::Parser is
not installed).
o If the 'preferred parser' is set to some other value, then it is
assumed to be the name of a SAX parser module and is passed to
XML::SAX::ParserFactory. If XML::SAX is not installed, or the
requested parser module is not installed, then "XMLin()" will die.
o If the 'preferred parser' is not defined at all (the normal default
state), an attempt will be made to load XML::SAX. If XML::SAX is
installed, then a parser module will be selected according to
XML::SAX::ParserFactory's normal rules (which typically means the
last SAX parser installed).
o if the 'preferred parser' is not defined and XML::SAX is not
installed, then XML::Parser will be used. "XMLin()" will die if
XML::Parser is not installed.
Note: The XML::SAX distribution includes an XML parser written entirely
in Perl. It is very portable but it is not very fast. You should
consider installing XML::LibXML or XML::SAX::Expat if they are
available for your platform.
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 AT smith.com</email>
<email>jsmith AT yahoo.com</email>
</person>
<person firstname="Bob" lastname="Smith">
<email>bob AT smith.com</email>
</person>
</opt>
{
'person' => [
{
'email' => [
'joe AT smith.com',
'jsmith AT yahoo.com'
],
'firstname' => 'Joe',
'lastname' => 'Smith'
},
{
'email' => 'bob AT 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
(see the "KeyAttr" option):
<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 arbitrary 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 (see the "ContentKey" option):
<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 able to present a simple API because it makes some
assumptions on your behalf. These include:
o You're not interested in text content consisting only of whitespace
o You don't mind that when things get slurped into a hash the order
is lost
o You don't want fine-grained control of the formatting of generated
XML
o You would never use a hash key that was not a legal XML element
name
o You don't need help converting between different encodings
In a serious XML project, you'll probably outgrow these assumptions
fairly quickly. This section of the document used to offer some advice
on choosing a more powerful option. That advice has now grown into the
'Perl-XML FAQ' document which you can find at:
<http://perl-xml.sourceforge.net/faq/>;
The advice in the FAQ boils down to a quick explanation of tree versus
event based parsers and then recommends:
For event based parsing, use SAX (do not set out to write any new code
for XML::Parser's handler API - it is obsolete).
For tree-based parsing, you could choose between the 'Perlish' approach
of XML::Twig and more standards based DOM implementations - preferably
one with XPath support such as XML::LibXML.
SEE ALSO
XML::Simple requires either XML::Parser or XML::SAX.
To generate documents with namespaces, XML::NamespaceSupport is
required.
The optional caching functions require Storable.
Answers to Frequently Asked Questions about XML::Simple are bundled
with this distribution as: XML::Simple::FAQ
COPYRIGHT
Copyright 1999-2004 Grant McLean <grantm AT cpan.org>
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.26.3 2018-03-18 XML::Simple(3)
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