YAML::XS - Perl YAML Serialization using XS and libyaml

NAME SYNOPSIS DESCRIPTION CONFIGURATION USING YAML::XS WITH UNICODE OBJECT ORIENTED INTERFACE Usage METHODS LIBYAML SEE ALSO AUTHORS COPYRIGHT AND LICENSE

NAME

YAML::XS − Perl YAML Serialization using XS and libyaml

use YAML::XS;
# Classic functional interface
my $yaml = Dump [ 1..4 ];
my $array = Load $yaml;
# EXPERIMENTAL: Object Oriented interface for YAML 1.2
# Incompatible to functional interface!
my $xs = YAML::XS−>new;
my $yaml = $xs−>dump([ 1..4 ]);
my $array = $xs−>load($yaml);

DESCRIPTION

Kirill Simonov's "libyaml" is arguably the best YAML implementation. The C library is written precisely to the YAML 1.1 specification. It was originally bound to Python and was later bound to Ruby.

This module is a Perl XS binding to libyaml which offers Perl the best YAML support to date.

This module exports the functions "Dump", "Load", "DumpFile" and "LoadFile". These functions are intended to work exactly like "YAML.pm"'s corresponding functions. Only "Load" and "Dump" are exported by default.

CONFIGURATION

The object oriented interface is described below: "OBJECT ORIENTED INTERFACE"

•

$YAML::XS::LoadBlessed (since v0.69)

Default: false.

The default was changed in version 0.81.

When set to false, it will not bless data into objects, which can be a security problem, when loading YAML from an untrusted source. It will silently ignore the tag and just load the data unblessed.

In PyYAML, this is called SafeLoad.

If set to true, it will load the following YAML as objects:

−−−
local: !Foo::Bar [a]
perl: !!perl/hash:Foo::Bar { a: 1 }
regex: !!perl/regexp:Foo::Bar pattern

You can create any kind of object with YAML. The creation itself is not the critical part. If the class has a "DESTROY" method, it will be called once the object is deleted. An example with File::Temp removing files can be found at <https://bugs.debian.org/cgi−bin/bugreport.cgi?bug=862373>.

•

$YAML::XS::ForbidDuplicateKeys (since 0.84)

Default: false

When set to true, "Load" will die when encountering a duplicate key in a hash, e.g.

key: value
key: another value

This can be useful for bigger YAML documents where it is not that obvious, and it is recommended to set it to true. That's also what a YAML loader should do by default according to the YAML specification.

	•		$YAML::XS::UseCode
	•		$YAML::XS::DumpCode
	•		$YAML::XS::LoadCode

If enabled supports deparsing and evaling of code blocks.

Note that support for loading code was added in version 0.75, although $LoadCode was documented already in earlier versions.

•

$YAML::XS::QuoteNumericStrings

When true (the default) strings that look like numbers but have not been numified will be quoted when dumping.

This ensures leading that things like leading zeros and other formatting are preserved.

•

$YAML::XS::Boolean (since v0.67)

Default: undef

Since YAML::XS 0.89: When used with perl 5.36 or later, builtin booleans will work out of the box. They will be created by "Load" and recognized by "Dump" automatically (since YAML::XS 0.89).

say Dump({ truth => builtin::true });
# truth: true

Since YAML::XS v0.902: loaded booleans are not set to readonly anymore.

For older perl versions you can use the following configuration to serialize data as YAML booleans:

When set to "JSON::PP" or "boolean", the plain (unquoted) strings "true" and "false" will be loaded as "JSON::PP::Boolean" or "boolean.pm" objects. Those objects will be dumped again as plain "true" or "false".

It will try to load [JSON::PP] or [boolean] and die if it can't be loaded.

With that it's possible to add new "real" booleans to a data structure:

local $YAML::XS::Boolean = "JSON::PP"; # or "boolean"
my $data = Load("booltrue: true");
$data−>{boolfalse} = JSON::PP::false;
my $yaml = Dump($data);
# boolfalse: false
# booltrue: true

It also lets booleans survive when loading YAML via YAML::XS and encode it in JSON via one of the various JSON encoders, which mostly support JSON::PP booleans.

Please note that JSON::PP::Boolean and boolean.pm behave a bit differently. Ideally you should only use them in boolean context.

If not set, booleans are loaded as special perl variables "PL_sv_yes" and "PL_sv_no", which have the disadvantage that they are readonly, and you can't add those to an existing data structure with pure perl.

If you simply need to load "perl booleans" that are true or false in boolean context, you will be fine with the default setting.

•

$YAML::XS::Indent (since v0.76)

Default is 2.

Sets the number of spaces for indentation for "Dump".

USING YAML::XS WITH UNICODE

Handling unicode properly in Perl can be a pain. YAML::XS only deals with streams of utf8 octets. Just remember this:

$perl = Load($utf8_octets);
$utf8_octets = Dump($perl);

There are many, many places where things can go wrong with unicode. If you are having problems, use Devel::Peek on all the possible data points.

OBJECT ORIENTED INTERFACE

Since version v0.904.0, EXPERIMENTAL

+++NOTE: This is incompatible with the functional interface and will treat YAML values in a different way.+++

This has two MAJOR differences to the old functional interface:
Object with options

This provides an interface where you create a YAML::XS object with options (instead of the old interface with global variables).

YAML 1.2 Core Schema

It implements the YAML 1.2 Core Schema.

(Note that the functional interface does not implement YAML 1.1, when it comes to loading numbers, booleans, null etc. It implements its own set of rules.)

Here is an (incomplete!) example of values that are treated differently than with the functional interface. YAML values that match a certain pattern, are not loaded as strings, but as other types:

# Functional interface: special values (not compatible to other YAML modules)
− [true, false] # booleans
− [null, ˜] # undef
− [inf, INF, iNf, iNF, InF, INf, −inf, ...] # Inf
− [100_000] # 100000
− # anything that looks_like_number() # number
# OOP YAML 1.2 special values
− [true, True, TRUE, false, False, FALSE] # booleans
− [null, Null, NULL, ˜] # undef
− [.inf, .Inf, .INF, −.inf, −.Inf, −.INF] # Inf
− [.nan, .NAN, .NaN] # nan
− [42, 0x10, 0o10] # dec 42, hex 16, oct 8

For more subtle differences regarding numbers checkout the comprehensive data here:
YAML 1.1 / 1.2 definitions:
<https://perlpunk.github.io/yaml−test−schema/schemas.html>
Test data: <https://perlpunk.github.io/yaml−test−schema/data.html>

This way the OOP interface is compatible to YAML::PP and YAML processors in other languages supporting YAML 1.2.

"load" will return the first document in scalar context

The functional interface returns the last.

Numbers don't have string flags
Booleans currently are only preserved for the new builtin booleans

Adding a JSON::PP boolean option for older perls is planned

References, objects, globs, regexes and coderefs cannot be loaded or
dumped

This is planned.

Usage

use YAML::XS ();
my $xs = YAML::XS−>new;
my $yaml = "foo: bar";
# Load single (first) document:
my $data = $xs−>load($yaml);
$yaml = $xs−>dump($data);
# Or to load all documents:
my @data = $xs−>load($yaml);
$yaml = $xs−>dump(@data);

METHODS

new

use YAML::XS;
my $xs = YAML::XS−>new(
# load options
# require_footer => 0,
# dump options
# indent => 2,
# header => 1,
# footer => 0,
# width => 80,
# anchor_prefix => '',
# load and dump options
# utf8 => 0,
);

Options:
indent

Default: 2

Sets the number of spaces for indentation for dumping.

utf8

Default: false

When false, the loader will accept unencoded character strings, and the dumper returns unencoded character strings.

When true, the loader accepts a UTF−8 encoded string of bytes, and the dumper returns a UTF−8 encoded string of bytes. This typically makes sense when you read from / write to a file directly.

header

Default: 1

Writes a "−−−" before every document

footer

Default: 0

Writes a "..." at the end of every document.

width

Set the maximum number of colums for dumping. If a value has too many characters, it will be split into multiple lines.

require_footer

Default: 0

Can be useful in a use case where you want to make sure you received the complete document from the sender.

anchor_prefix

Default: ''

If set to a string like "ANCHOR", anchors and aliases will be prefixed with this instead of just being numbers:

# my $xs = YAML::XS−>new( anchor_prefix => 'ANCHOR' );
# my $hash = { some => "mapping" };
# say $xs−>dump([$hash, $hash]);
− &ANCHOR1
some: mapping
− *ANCHOR1

load

my $yaml = <<'EOM';
−−−
− 23
−−−
some: mapping
EOM
my $array = $xs−>load($yaml);
# [23]
my @documents = $xs−>load($yaml);
# (
# [23],
# { some => "mapping" }
# )

In scalar context, if the YAML string contains more than one document, it will return the first document.

dump

$yaml = $xs−>dump($data);
$yaml = $xs−>dump($data1, $data2, $data3);
$yaml = $xs−>dump(@data);

Updated 2026-06-01 - jenkler.se | uex.se

	•		YAML.pm
	•		YAML::Syck
	•		YAML::Tiny
	•		YAML::PP
	•		YAML::PP::LibYAML