NAME¶
XML::Simple - Easily read/write XML (esp config files)
SYNOPSIS¶
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 discouraged. Other modules are available
which provide more straightforward and consistent interfaces. In particular,
XML::LibXML is highly recommended.
The major problems with this module are the large number of options and the
arbitrary ways in which these options interact - often with 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 explicity 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:
- •
- check out "ForceArray" because you'll almost certainly want to
turn it on
- •
- 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:
- •
- Options are marked with 'in' if they are recognised by
"XMLin()" and ' out' if they are recognised by
"XMLout()".
- •
- 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 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.
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' }
}
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 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 (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 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
'-'.
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:
- •
- 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)
- •
- 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)
- •
- 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:
- •
- to define a set of default values which should be used on all subsequent
calls to "XMLin()" or "XMLout()"
- •
- 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.
- 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.
- 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
- •
- Failing to explicitly set the "KeyAttr" option - if you can't be
bothered reading about this option, turn it off with: KeyAttr => [
]
- •
- 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
- •
- Setting ForceArray to an array, but failing to list all the elements from
the KeyAttr hash.
- •
- 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 -w is, this
condition triggers a warning.
- •
- 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.
- •
- 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:
- •
- Applications written to the SAX API can extract data from huge XML
documents without the memory overheads of a DOM or tree API.
- •
- 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.
- •
- 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'.
- •
- One filter module might split a data stream to direct data to two or more
downstream handlers.
- •
- 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:
- •
- 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.
- •
- 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).
- •
- 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.
- •
- 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).
- •
- 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@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 (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 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 (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:
- •
- You're not interested in text content consisting only of whitespace
- •
- You don't mind that when things get slurped into a hash the order is
lost
- •
- You don't want fine-grained control of the formatting of generated
XML
- •
- You would never use a hash key that was not a legal XML element name
- •
- 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 chosing 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/
<
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 obselete).
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@cpan.org>
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.