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```{index} single: YAML::XS
```

# YAML::XS

## SYNOPSIS

```perl
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

Under perl this module binds Kirill Simonov's `libyaml`, a C library written to the YAML 1.1 specification. Under pperl its compiled part, [YAML::XS::LibYAML](XS/LibYAML.md), reads YAML with [YAML::PP](PP.md) and writes it as libyaml writes it; that page lists where the two differ.

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:

    ```perl
    ---
    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](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.

    ```text
    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).

    ```perl
    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:

    ```perl
    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
$perl = Load($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:

    ```text
    # 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](https://perlpunk.github.io/yaml-test-schema/schemas.html)
    - Test data: [https://perlpunk.github.io/yaml-test-schema/data.html](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

```perl
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

```perl
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 => '',
    # cyclic_refs => 0,

    # load and dump options
    # utf8 => 0,
    # boolean => 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:

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

- cyclic\_refs

    Since version v0.907

    Default: 0

    YAML can contain data structures where an alias points to a node
    containing the current node, creating a cyclic (circular) data
    structure.
    If you load untrusted YAML documents, that can lead to memory
    leaks.
    Since it's not a very common use case, YAML::XS defaults to
    not allowing it. It will just behave as if the anchor was never
    created in this case.
    You an set it to 1 to allow it.

- boolean

    Since version v0.909

    Default: 0

    Booleans will be loaded as real booleans since perl 5.36.0, and
    as `1|''` before.

    You can set the value to [JSON::PP](../JSON/PP.md) or [boolean](https://metacpan.org/pod/boolean) to load and dump booleans as
    instances of those classes. Mainly useful for older perl versions
    where booleans could not roundtrip.

#### load

```perl
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

```perl
$yaml = $xs->dump($data);
$yaml = $xs->dump($data1, $data2, $data3);
$yaml = $xs->dump(@data);
```

## LIBYAML

`libyaml_version` (since v0.79) answers the libyaml version the module was built with. Under pperl it answers 0.2.4, the libyaml release whose behaviour pperl's engine follows:

```perl
my $libyaml_version = YAML::XS::LibYAML::libyaml_version();
```

## SEE ALSO

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

## AUTHORS

- Ingy döt Net [ingy@ingy.net](mailto:ingy@ingy.net)
- Tina Müller <tinita@cpan.org>

## COPYRIGHT AND LICENSE

Copyright 2007-2025 - Ingy döt Net

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

See [http://www.perl.com/perl/misc/Artistic.html](http://www.perl.com/perl/misc/Artistic.html)

## Modules

- [`YAML::XS::LibYAML`](XS/LibYAML.md) - the YAML engine behind pperl's YAML::XS

```{toctree}
:hidden:
:maxdepth: 1

XS/LibYAML
```