JSON::XS - JSON serialising/deserialising, done correctly and fast

    JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ

     use JSON::XS;

     # exported functions, they croak on error
     # and expect/generate UTF-8

     $utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
     $perl_hash_or_arrayref  = decode_json $utf8_encoded_json_text;

     # OO-interface

     $coder = JSON::XS->new->ascii->pretty->allow_nonref;
     $pretty_printed_unencoded = $coder->encode ($perl_scalar);
     $perl_scalar = $coder->decode ($unicode_json_text);

     # Note that JSON version 2.0 and above will automatically use JSON::XS
     # if available, at virtually no speed overhead either, so you should
     # be able to just:
     use JSON;

     # and do the same things, except that you have a pure-perl fallback now.

    This module converts Perl data structures to JSON and vice versa. Its
    primary goal is to be *correct* and its secondary goal is to be *fast*.
    To reach the latter goal it was written in C.

    See MAPPING, below, on how JSON::XS maps perl values to JSON values and
    vice versa.

    *   correct Unicode handling

        This module knows how to handle Unicode, documents how and when it
        does so, and even documents what "correct" means.

    *   round-trip integrity

        When you serialise a perl data structure using only data types
        supported by JSON and Perl, the deserialised data structure is
        identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
        become "2" just because it looks like a number). There *are* minor
        exceptions to this, read the MAPPING section below to learn about

    *   strict checking of JSON correctness

        There is no guessing, no generating of illegal JSON texts by
        default, and only JSON is accepted as input by default (the latter
        is a security feature).

    *   fast

        Compared to other JSON modules and other serialisers such as
        Storable, this module usually compares favourably in terms of speed,

    *   simple to use

        This module has both a simple functional interface as well as an
        object oriented interface.

    *   reasonably versatile output formats

        You can choose between the most compact guaranteed-single-line
        format possible (nice for simple line-based protocols), a pure-ASCII
        format (for when your transport is not 8-bit clean, still supports
        the whole Unicode range), or a pretty-printed format (for when you
        want to read that stuff). Or you can combine those features in
        whatever way you like.

    The following convenience methods are provided by this module. They are
    exported by default:

    $json_text = encode_json $perl_scalar
        Converts the given Perl data structure to a UTF-8 encoded, binary
        string (that is, the string contains octets only). Croaks on error.

        This function call is functionally identical to:

           $json_text = JSON::XS->new->utf8->encode ($perl_scalar)

        Except being faster.

    $perl_scalar = decode_json $json_text
        The opposite of "encode_json": expects a UTF-8 (binary) string and
        tries to parse that as a UTF-8 encoded JSON text, returning the
        resulting reference. Croaks on error.

        This function call is functionally identical to:

           $perl_scalar = JSON::XS->new->utf8->decode ($json_text)

        Except being faster.

    Since this often leads to confusion, here are a few very clear words on
    how Unicode works in Perl, modulo bugs.

    1. Perl strings can store characters with ordinal values > 255.
        This enables you to store Unicode characters as single characters in
        a Perl string - very natural.

    2. Perl does *not* associate an encoding with your strings.
        ... until you force it to, e.g. when matching it against a regex, or
        printing the scalar to a file, in which case Perl either interprets
        your string as locale-encoded text, octets/binary, or as Unicode,
        depending on various settings. In no case is an encoding stored
        together with your data, it is *use* that decides encoding, not any
        magical meta data.

    3. The internal utf-8 flag has no meaning with regards to the encoding
    of your string.
        Just ignore that flag unless you debug a Perl bug, a module written
        in XS or want to dive into the internals of perl. Otherwise it will
        only confuse you, as, despite the name, it says nothing about how
        your string is encoded. You can have Unicode strings with that flag
        set, with that flag clear, and you can have binary data with that
        flag set and that flag clear. Other possibilities exist, too.

        If you didn't know about that flag, just the better, pretend it
        doesn't exist.

    4. A "Unicode String" is simply a string where each character can be
    validly interpreted as a Unicode code point.
        If you have UTF-8 encoded data, it is no longer a Unicode string,
        but a Unicode string encoded in UTF-8, giving you a binary string.

    5. A string containing "high" (> 255) character values is *not* a UTF-8
        It's a fact. Learn to live with it.

    I hope this helps :)

    The object oriented interface lets you configure your own encoding or
    decoding style, within the limits of supported formats.

    $json = new JSON::XS
        Creates a new JSON::XS object that can be used to de/encode JSON
        strings. All boolean flags described below are by default *disabled*
        (with the exception of "allow_nonref", which defaults to *enabled*
        since version 4.0).

        The mutators for flags all return the JSON object again and thus
        calls can be chained:

           my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
           => {"a": [1, 2]}

    $json = $json->ascii ([$enable])
    $enabled = $json->get_ascii
        If $enable is true (or missing), then the "encode" method will not
        generate characters outside the code range 0..127 (which is ASCII).
        Any Unicode characters outside that range will be escaped using
        either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
        escape sequence, as per RFC4627. The resulting encoded JSON text can
        be treated as a native Unicode string, an ascii-encoded,
        latin1-encoded or UTF-8 encoded string, or any other superset of

        If $enable is false, then the "encode" method will not escape
        Unicode characters unless required by the JSON syntax or other
        flags. This results in a faster and more compact format.

        See also the section *ENCODING/CODESET FLAG NOTES* later in this

        The main use for this flag is to produce JSON texts that can be
        transmitted over a 7-bit channel, as the encoded JSON texts will not
        contain any 8 bit characters.

          JSON::XS->new->ascii (1)->encode ([chr 0x10401])
          => ["\ud801\udc01"]

    $json = $json->latin1 ([$enable])
    $enabled = $json->get_latin1
        If $enable is true (or missing), then the "encode" method will
        encode the resulting JSON text as latin1 (or iso-8859-1), escaping
        any characters outside the code range 0..255. The resulting string
        can be treated as a latin1-encoded JSON text or a native Unicode
        string. The "decode" method will not be affected in any way by this
        flag, as "decode" by default expects Unicode, which is a strict
        superset of latin1.

        If $enable is false, then the "encode" method will not escape
        Unicode characters unless required by the JSON syntax or other

        See also the section *ENCODING/CODESET FLAG NOTES* later in this

        The main use for this flag is efficiently encoding binary data as
        JSON text, as most octets will not be escaped, resulting in a
        smaller encoded size. The disadvantage is that the resulting JSON
        text is encoded in latin1 (and must correctly be treated as such
        when storing and transferring), a rare encoding for JSON. It is
        therefore most useful when you want to store data structures known
        to contain binary data efficiently in files or databases, not when
        talking to other JSON encoders/decoders.

          JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
          => ["\x{89}\\u0abc"]    # (perl syntax, U+abc escaped, U+89 not)

    $json = $json->utf8 ([$enable])
    $enabled = $json->get_utf8
        If $enable is true (or missing), then the "encode" method will
        encode the JSON result into UTF-8, as required by many protocols,
        while the "decode" method expects to be handed a UTF-8-encoded
        string. Please note that UTF-8-encoded strings do not contain any
        characters outside the range 0..255, they are thus useful for
        bytewise/binary I/O. In future versions, enabling this option might
        enable autodetection of the UTF-16 and UTF-32 encoding families, as
        described in RFC4627.

        If $enable is false, then the "encode" method will return the JSON
        string as a (non-encoded) Unicode string, while "decode" expects
        thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
        UTF-16) needs to be done yourself, e.g. using the Encode module.

        See also the section *ENCODING/CODESET FLAG NOTES* later in this

        Example, output UTF-16BE-encoded JSON:

          use Encode;
          $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);

        Example, decode UTF-32LE-encoded JSON:

          use Encode;
          $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);

    $json = $json->pretty ([$enable])
        This enables (or disables) all of the "indent", "space_before" and
        "space_after" (and in the future possibly more) flags in one call to
        generate the most readable (or most compact) form possible.

        Example, pretty-print some simple structure:

           my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
              "a" : [

    $json = $json->indent ([$enable])
    $enabled = $json->get_indent
        If $enable is true (or missing), then the "encode" method will use a
        multiline format as output, putting every array member or
        object/hash key-value pair into its own line, indenting them

        If $enable is false, no newlines or indenting will be produced, and
        the resulting JSON text is guaranteed not to contain any "newlines".

        This setting has no effect when decoding JSON texts.

    $json = $json->space_before ([$enable])
    $enabled = $json->get_space_before
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space before the ":" separating keys from values
        in JSON objects.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON texts. You will also
        most likely combine this setting with "space_after".

        Example, space_before enabled, space_after and indent disabled:

           {"key" :"value"}

    $json = $json->space_after ([$enable])
    $enabled = $json->get_space_after
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space after the ":" separating keys from values in
        JSON objects and extra whitespace after the "," separating key-value
        pairs and array members.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON texts.

        Example, space_before and indent disabled, space_after enabled:

           {"key": "value"}

    $json = $json->relaxed ([$enable])
    $enabled = $json->get_relaxed
        If $enable is true (or missing), then "decode" will accept some
        extensions to normal JSON syntax (see below). "encode" will not be
        affected in any way. *Be aware that this option makes you accept
        invalid JSON texts as if they were valid!*. I suggest only to use
        this option to parse application-specific files written by humans
        (configuration files, resource files etc.)

        If $enable is false (the default), then "decode" will only accept
        valid JSON texts.

        Currently accepted extensions are:

        *   list items can have an end-comma

            JSON *separates* array elements and key-value pairs with commas.
            This can be annoying if you write JSON texts manually and want
            to be able to quickly append elements, so this extension accepts
            comma at the end of such items not just between them:

                  2, <- this comma not normally allowed
                  "k1": "v1",
                  "k2": "v2", <- this comma not normally allowed

        *   shell-style '#'-comments

            Whenever JSON allows whitespace, shell-style comments are
            additionally allowed. They are terminated by the first
            carriage-return or line-feed character, after which more
            white-space and comments are allowed.

                 1, # this comment not allowed in JSON
                    # neither this one...

        *   literal ASCII TAB characters in strings

            Literal ASCII TAB characters are now allowed in strings (and
            treated as "\t").

                 "Hello<TAB>World", # literal <TAB> would not normally be allowed

    $json = $json->canonical ([$enable])
    $enabled = $json->get_canonical
        If $enable is true (or missing), then the "encode" method will
        output JSON objects by sorting their keys. This is adding a
        comparatively high overhead.

        If $enable is false, then the "encode" method will output key-value
        pairs in the order Perl stores them (which will likely change
        between runs of the same script, and can change even within the same
        run from 5.18 onwards).

        This option is useful if you want the same data structure to be
        encoded as the same JSON text (given the same overall settings). If
        it is disabled, the same hash might be encoded differently even if
        contains the same data, as key-value pairs have no inherent ordering
        in Perl.

        This setting has no effect when decoding JSON texts.

        This setting has currently no effect on tied hashes.

    $json = $json->allow_nonref ([$enable])
    $enabled = $json->get_allow_nonref
        Unlike other boolean options, this opotion is enabled by default
        beginning with version 4.0. See "SECURITY CONSIDERATIONS" for the
        gory details.

        If $enable is true (or missing), then the "encode" method can
        convert a non-reference into its corresponding string, number or
        null JSON value, which is an extension to RFC4627. Likewise,
        "decode" will accept those JSON values instead of croaking.

        If $enable is false, then the "encode" method will croak if it isn't
        passed an arrayref or hashref, as JSON texts must either be an
        object or array. Likewise, "decode" will croak if given something
        that is not a JSON object or array.

        Example, encode a Perl scalar as JSON value without enabled
        "allow_nonref", resulting in an error:

           JSON::XS->new->allow_nonref (0)->encode ("Hello, World!")
           => hash- or arrayref expected...

    $json = $json->allow_unknown ([$enable])
    $enabled = $json->get_allow_unknown
        If $enable is true (or missing), then "encode" will *not* throw an
        exception when it encounters values it cannot represent in JSON (for
        example, filehandles) but instead will encode a JSON "null" value.
        Note that blessed objects are not included here and are handled
        separately by c<allow_nonref>.

        If $enable is false (the default), then "encode" will throw an
        exception when it encounters anything it cannot encode as JSON.

        This option does not affect "decode" in any way, and it is
        recommended to leave it off unless you know your communications

    $json = $json->allow_blessed ([$enable])
    $enabled = $json->get_allow_blessed
        See "OBJECT SERIALISATION" for details.

        If $enable is true (or missing), then the "encode" method will not
        barf when it encounters a blessed reference that it cannot convert
        otherwise. Instead, a JSON "null" value is encoded instead of the

        If $enable is false (the default), then "encode" will throw an
        exception when it encounters a blessed object that it cannot convert

        This setting has no effect on "decode".

    $json = $json->convert_blessed ([$enable])
    $enabled = $json->get_convert_blessed
        See "OBJECT SERIALISATION" for details.

        If $enable is true (or missing), then "encode", upon encountering a
        blessed object, will check for the availability of the "TO_JSON"
        method on the object's class. If found, it will be called in scalar
        context and the resulting scalar will be encoded instead of the

        The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
        returns other blessed objects, those will be handled in the same
        way. "TO_JSON" must take care of not causing an endless recursion
        cycle (== crash) in this case. The name of "TO_JSON" was chosen
        because other methods called by the Perl core (== not by the user of
        the object) are usually in upper case letters and to avoid
        collisions with any "to_json" function or method.

        If $enable is false (the default), then "encode" will not consider
        this type of conversion.

        This setting has no effect on "decode".

    $json = $json->allow_tags ([$enable])
    $enabled = $json->get_allow_tags
        See "OBJECT SERIALISATION" for details.

        If $enable is true (or missing), then "encode", upon encountering a
        blessed object, will check for the availability of the "FREEZE"
        method on the object's class. If found, it will be used to serialise
        the object into a nonstandard tagged JSON value (that JSON decoders
        cannot decode).

        It also causes "decode" to parse such tagged JSON values and
        deserialise them via a call to the "THAW" method.

        If $enable is false (the default), then "encode" will not consider
        this type of conversion, and tagged JSON values will cause a parse
        error in "decode", as if tags were not part of the grammar.

    $json->boolean_values ([$false, $true])
    ($false, $true) = $json->get_boolean_values
        By default, JSON booleans will be decoded as overloaded
        $Types::Serialiser::false and $Types::Serialiser::true objects.

        With this method you can specify your own boolean values for
        decoding - on decode, JSON "false" will be decoded as a copy of
        $false, and JSON "true" will be decoded as $true ("copy" here is the
        same thing as assigning a value to another variable, i.e. "$copy =

        Calling this method without any arguments will reset the booleans to
        their default values.

        "get_boolean_values" will return both $false and $true values, or
        the empty list when they are set to the default.

    $json = $json->filter_json_object ([$coderef->($hashref)])
        When $coderef is specified, it will be called from "decode" each
        time it decodes a JSON object. The only argument is a reference to
        the newly-created hash. If the code reference returns a single
        scalar (which need not be a reference), this value (or rather a copy
        of it) is inserted into the deserialised data structure. If it
        returns an empty list (NOTE: *not* "undef", which is a valid
        scalar), the original deserialised hash will be inserted. This
        setting can slow down decoding considerably.

        When $coderef is omitted or undefined, any existing callback will be
        removed and "decode" will not change the deserialised hash in any

        Example, convert all JSON objects into the integer 5:

           my $js = JSON::XS->new->filter_json_object (sub { 5 });
           # returns [5]
           $js->decode ('[{}]')
           # throw an exception because allow_nonref is not enabled
           # so a lone 5 is not allowed.
           $js->decode ('{"a":1, "b":2}');

    $json = $json->filter_json_single_key_object ($key [=>
        Works remotely similar to "filter_json_object", but is only called
        for JSON objects having a single key named $key.

        This $coderef is called before the one specified via
        "filter_json_object", if any. It gets passed the single value in the
        JSON object. If it returns a single value, it will be inserted into
        the data structure. If it returns nothing (not even "undef" but the
        empty list), the callback from "filter_json_object" will be called
        next, as if no single-key callback were specified.

        If $coderef is omitted or undefined, the corresponding callback will
        be disabled. There can only ever be one callback for a given key.

        As this callback gets called less often then the
        "filter_json_object" one, decoding speed will not usually suffer as
        much. Therefore, single-key objects make excellent targets to
        serialise Perl objects into, especially as single-key JSON objects
        are as close to the type-tagged value concept as JSON gets (it's
        basically an ID/VALUE tuple). Of course, JSON does not support this
        in any way, so you need to make sure your data never looks like a
        serialised Perl hash.

        Typical names for the single object key are "__class_whatever__", or
        "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
        things like "__class_md5sum(classname)__", to reduce the risk of
        clashing with real hashes.

        Example, decode JSON objects of the form "{ "__widget__" => <id> }"
        into the corresponding $WIDGET{<id>} object:

           # return whatever is in $WIDGET{5}:
              ->filter_json_single_key_object (__widget__ => sub {
                    $WIDGET{ $_[0] }
              ->decode ('{"__widget__": 5')

           # this can be used with a TO_JSON method in some "widget" class
           # for serialisation to json:
           sub WidgetBase::TO_JSON {
              my ($self) = @_;

              unless ($self->{id}) {
                 $self->{id} =;
                 $WIDGET{$self->{id}} = $self;

              { __widget__ => $self->{id} }

    $json = $json->shrink ([$enable])
    $enabled = $json->get_shrink
        Perl usually over-allocates memory a bit when allocating space for
        strings. This flag optionally resizes strings generated by either
        "encode" or "decode" to their minimum size possible. This can save
        memory when your JSON texts are either very very long or you have
        many short strings. It will also try to downgrade any strings to
        octet-form if possible: perl stores strings internally either in an
        encoding called UTF-X or in octet-form. The latter cannot store
        everything but uses less space in general (and some buggy Perl or C
        code might even rely on that internal representation being used).

        The actual definition of what shrink does might change in future
        versions, but it will always try to save space at the expense of

        If $enable is true (or missing), the string returned by "encode"
        will be shrunk-to-fit, while all strings generated by "decode" will
        also be shrunk-to-fit.

        If $enable is false, then the normal perl allocation algorithms are
        used. If you work with your data, then this is likely to be faster.

        In the future, this setting might control other things, such as
        converting strings that look like integers or floats into integers
        or floats internally (there is no difference on the Perl level),
        saving space.

    $json = $json->max_depth ([$maximum_nesting_depth])
    $max_depth = $json->get_max_depth
        Sets the maximum nesting level (default 512) accepted while encoding
        or decoding. If a higher nesting level is detected in JSON text or a
        Perl data structure, then the encoder and decoder will stop and
        croak at that point.

        Nesting level is defined by number of hash- or arrayrefs that the
        encoder needs to traverse to reach a given point or the number of
        "{" or "[" characters without their matching closing parenthesis
        crossed to reach a given character in a string.

        Setting the maximum depth to one disallows any nesting, so that
        ensures that the object is only a single hash/object or array.

        If no argument is given, the highest possible setting will be used,
        which is rarely useful.

        Note that nesting is implemented by recursion in C. The default
        value has been chosen to be as large as typical operating systems
        allow without crashing.

        See SECURITY CONSIDERATIONS, below, for more info on why this is

    $json = $json->max_size ([$maximum_string_size])
    $max_size = $json->get_max_size
        Set the maximum length a JSON text may have (in bytes) where
        decoding is being attempted. The default is 0, meaning no limit.
        When "decode" is called on a string that is longer then this many
        bytes, it will not attempt to decode the string but throw an
        exception. This setting has no effect on "encode" (yet).

        If no argument is given, the limit check will be deactivated (same
        as when 0 is specified).

        See SECURITY CONSIDERATIONS, below, for more info on why this is

    $json_text = $json->encode ($perl_scalar)
        Converts the given Perl value or data structure to its JSON
        representation. Croaks on error.

    $perl_scalar = $json->decode ($json_text)
        The opposite of "encode": expects a JSON text and tries to parse it,
        returning the resulting simple scalar or reference. Croaks on error.

    ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
        This works like the "decode" method, but instead of raising an
        exception when there is trailing garbage after the first JSON
        object, it will silently stop parsing there and return the number of
        characters consumed so far.

        This is useful if your JSON texts are not delimited by an outer
        protocol and you need to know where the JSON text ends.

           JSON::XS->new->decode_prefix ("[1] the tail")
           => ([1], 3)

    In some cases, there is the need for incremental parsing of JSON texts.
    While this module always has to keep both JSON text and resulting Perl
    data structure in memory at one time, it does allow you to parse a JSON
    stream incrementally. It does so by accumulating text until it has a
    full JSON object, which it then can decode. This process is similar to
    using "decode_prefix" to see if a full JSON object is available, but is
    much more efficient (and can be implemented with a minimum of method

    JSON::XS will only attempt to parse the JSON text once it is sure it has
    enough text to get a decisive result, using a very simple but truly
    incremental parser. This means that it sometimes won't stop as early as
    the full parser, for example, it doesn't detect mismatched parentheses.
    The only thing it guarantees is that it starts decoding as soon as a
    syntactically valid JSON text has been seen. This means you need to set
    resource limits (e.g. "max_size") to ensure the parser will stop parsing
    in the presence if syntax errors.

    The following methods implement this incremental parser.

    [void, scalar or list context] = $json->incr_parse ([$string])
        This is the central parsing function. It can both append new text
        and extract objects from the stream accumulated so far (both of
        these functions are optional).

        If $string is given, then this string is appended to the already
        existing JSON fragment stored in the $json object.

        After that, if the function is called in void context, it will
        simply return without doing anything further. This can be used to
        add more text in as many chunks as you want.

        If the method is called in scalar context, then it will try to
        extract exactly *one* JSON object. If that is successful, it will
        return this object, otherwise it will return "undef". If there is a
        parse error, this method will croak just as "decode" would do (one
        can then use "incr_skip" to skip the erroneous part). This is the
        most common way of using the method.

        And finally, in list context, it will try to extract as many objects
        from the stream as it can find and return them, or the empty list
        otherwise. For this to work, there must be no separators (other than
        whitespace) between the JSON objects or arrays, instead they must be
        concatenated back-to-back. If an error occurs, an exception will be
        raised as in the scalar context case. Note that in this case, any
        previously-parsed JSON texts will be lost.

        Example: Parse some JSON arrays/objects in a given string and return

           my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");

    $lvalue_string = $json->incr_text
        This method returns the currently stored JSON fragment as an lvalue,
        that is, you can manipulate it. This *only* works when a preceding
        call to "incr_parse" in *scalar context* successfully returned an
        object. Under all other circumstances you must not call this
        function (I mean it. although in simple tests it might actually
        work, it *will* fail under real world conditions). As a special
        exception, you can also call this method before having parsed

        That means you can only use this function to look at or manipulate
        text before or after complete JSON objects, not while the parser is
        in the middle of parsing a JSON object.

        This function is useful in two cases: a) finding the trailing text
        after a JSON object or b) parsing multiple JSON objects separated by
        non-JSON text (such as commas).

        This will reset the state of the incremental parser and will remove
        the parsed text from the input buffer so far. This is useful after
        "incr_parse" died, in which case the input buffer and incremental
        parser state is left unchanged, to skip the text parsed so far and
        to reset the parse state.

        The difference to "incr_reset" is that only text until the parse
        error occurred is removed.

        This completely resets the incremental parser, that is, after this
        call, it will be as if the parser had never parsed anything.

        This is useful if you want to repeatedly parse JSON objects and want
        to ignore any trailing data, which means you have to reset the
        parser after each successful decode.

    The incremental parser is a non-exact parser: it works by gathering as
    much text as possible that *could* be a valid JSON text, followed by
    trying to decode it.

    That means it sometimes needs to read more data than strictly necessary
    to diagnose an invalid JSON text. For example, after parsing the
    following fragment, the parser *could* stop with an error, as this
    fragment *cannot* be the beginning of a valid JSON text:


    In reality, hopwever, the parser might continue to read data until a
    length limit is exceeded or it finds a closing bracket.

    Some examples will make all this clearer. First, a simple example that
    works similarly to "decode_prefix": We want to decode the JSON object at
    the start of a string and identify the portion after the JSON object:

       my $text = "[1,2,3] hello";

       my $json = new JSON::XS;

       my $obj = $json->incr_parse ($text)
          or die "expected JSON object or array at beginning of string";

       my $tail = $json->incr_text;
       # $tail now contains " hello"

    Easy, isn't it?

    Now for a more complicated example: Imagine a hypothetical protocol
    where you read some requests from a TCP stream, and each request is a
    JSON array, without any separation between them (in fact, it is often
    useful to use newlines as "separators", as these get interpreted as
    whitespace at the start of the JSON text, which makes it possible to
    test said protocol with "telnet"...).

    Here is how you'd do it (it is trivial to write this in an event-based

       my $json = new JSON::XS;

       # read some data from the socket
       while (sysread $socket, my $buf, 4096) {

          # split and decode as many requests as possible
          for my $request ($json->incr_parse ($buf)) {
             # act on the $request

    Another complicated example: Assume you have a string with JSON objects
    or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
    [3]"). To parse them, we have to skip the commas between the JSON texts,
    and here is where the lvalue-ness of "incr_text" comes in useful:

       my $text = "[1],[2], [3]";
       my $json = new JSON::XS;

       # void context, so no parsing done
       $json->incr_parse ($text);

       # now extract as many objects as possible. note the
       # use of scalar context so incr_text can be called.
       while (my $obj = $json->incr_parse) {
          # do something with $obj

          # now skip the optional comma
          $json->incr_text =~ s/^ \s* , //x;

    Now lets go for a very complex example: Assume that you have a gigantic
    JSON array-of-objects, many gigabytes in size, and you want to parse it,
    but you cannot load it into memory fully (this has actually happened in
    the real world :).

    Well, you lost, you have to implement your own JSON parser. But JSON::XS
    can still help you: You implement a (very simple) array parser and let
    JSON decode the array elements, which are all full JSON objects on their
    own (this wouldn't work if the array elements could be JSON numbers, for

       my $json = new JSON::XS;

       # open the monster
       open my $fh, "<bigfile.json"
          or die "bigfile: $!";

       # first parse the initial "["
       for (;;) {
          sysread $fh, my $buf, 65536
             or die "read error: $!";
          $json->incr_parse ($buf); # void context, so no parsing

          # Exit the loop once we found and removed(!) the initial "[".
          # In essence, we are (ab-)using the $json object as a simple scalar
          # we append data to.
          last if $json->incr_text =~ s/^ \s* \[ //x;

       # now we have the skipped the initial "[", so continue
       # parsing all the elements.
       for (;;) {
          # in this loop we read data until we got a single JSON object
          for (;;) {
             if (my $obj = $json->incr_parse) {
                # do something with $obj

             # add more data
             sysread $fh, my $buf, 65536
                or die "read error: $!";
             $json->incr_parse ($buf); # void context, so no parsing

          # in this loop we read data until we either found and parsed the
          # separating "," between elements, or the final "]"
          for (;;) {
             # first skip whitespace
             $json->incr_text =~ s/^\s*//;

             # if we find "]", we are done
             if ($json->incr_text =~ s/^\]//) {
                print "finished.\n";

             # if we find ",", we can continue with the next element
             if ($json->incr_text =~ s/^,//) {

             # if we find anything else, we have a parse error!
             if (length $json->incr_text) {
                die "parse error near ", $json->incr_text;

             # else add more data
             sysread $fh, my $buf, 65536
                or die "read error: $!";
             $json->incr_parse ($buf); # void context, so no parsing

    This is a complex example, but most of the complexity comes from the
    fact that we are trying to be correct (bear with me if I am wrong, I
    never ran the above example :).

    This section describes how JSON::XS maps Perl values to JSON values and
    vice versa. These mappings are designed to "do the right thing" in most
    circumstances automatically, preserving round-tripping characteristics
    (what you put in comes out as something equivalent).

    For the more enlightened: note that in the following descriptions,
    lowercase *perl* refers to the Perl interpreter, while uppercase *Perl*
    refers to the abstract Perl language itself.

        A JSON object becomes a reference to a hash in Perl. No ordering of
        object keys is preserved (JSON does not preserve object key ordering

        A JSON array becomes a reference to an array in Perl.

        A JSON string becomes a string scalar in Perl - Unicode codepoints
        in JSON are represented by the same codepoints in the Perl string,
        so no manual decoding is necessary.

        A JSON number becomes either an integer, numeric (floating point) or
        string scalar in perl, depending on its range and any fractional
        parts. On the Perl level, there is no difference between those as
        Perl handles all the conversion details, but an integer may take
        slightly less memory and might represent more values exactly than
        floating point numbers.

        If the number consists of digits only, JSON::XS will try to
        represent it as an integer value. If that fails, it will try to
        represent it as a numeric (floating point) value if that is possible
        without loss of precision. Otherwise it will preserve the number as
        a string value (in which case you lose roundtripping ability, as the
        JSON number will be re-encoded to a JSON string).

        Numbers containing a fractional or exponential part will always be
        represented as numeric (floating point) values, possibly at a loss
        of precision (in which case you might lose perfect roundtripping
        ability, but the JSON number will still be re-encoded as a JSON

        Note that precision is not accuracy - binary floating point values
        cannot represent most decimal fractions exactly, and when converting
        from and to floating point, JSON::XS only guarantees precision up to
        but not including the least significant bit.

    true, false
        These JSON atoms become "Types::Serialiser::true" and
        "Types::Serialiser::false", respectively. They are overloaded to act
        almost exactly like the numbers 1 and 0. You can check whether a
        scalar is a JSON boolean by using the "Types::Serialiser::is_bool"
        function (after "use Types::Serialier", of course).

        A JSON null atom becomes "undef" in Perl.

    shell-style comments ("# *text*")
        As a nonstandard extension to the JSON syntax that is enabled by the
        "relaxed" setting, shell-style comments are allowed. They can start
        anywhere outside strings and go till the end of the line.

    tagged values ("(*tag*)*value*").
        Another nonstandard extension to the JSON syntax, enabled with the
        "allow_tags" setting, are tagged values. In this implementation, the
        *tag* must be a perl package/class name encoded as a JSON string,
        and the *value* must be a JSON array encoding optional constructor

        See "OBJECT SERIALISATION", below, for details.

    The mapping from Perl to JSON is slightly more difficult, as Perl is a
    truly typeless language, so we can only guess which JSON type is meant
    by a Perl value.

    hash references
        Perl hash references become JSON objects. As there is no inherent
        ordering in hash keys (or JSON objects), they will usually be
        encoded in a pseudo-random order. JSON::XS can optionally sort the
        hash keys (determined by the *canonical* flag), so the same
        datastructure will serialise to the same JSON text (given same
        settings and version of JSON::XS), but this incurs a runtime
        overhead and is only rarely useful, e.g. when you want to compare
        some JSON text against another for equality.

    array references
        Perl array references become JSON arrays.

    other references
        Other unblessed references are generally not allowed and will cause
        an exception to be thrown, except for references to the integers 0
        and 1, which get turned into "false" and "true" atoms in JSON.

        Since "JSON::XS" uses the boolean model from Types::Serialiser, you
        can also "use Types::Serialiser" and then use
        "Types::Serialiser::false" and "Types::Serialiser::true" to improve

           use Types::Serialiser;
           encode_json [\0, Types::Serialiser::true]      # yields [false,true]

    Types::Serialiser::true, Types::Serialiser::false
        These special values from the Types::Serialiser module become JSON
        true and JSON false values, respectively. You can also use "\1" and
        "\0" directly if you want.

    blessed objects
        Blessed objects are not directly representable in JSON, but
        "JSON::XS" allows various ways of handling objects. See "OBJECT
        SERIALISATION", below, for details.

    simple scalars
        Simple Perl scalars (any scalar that is not a reference) are the
        most difficult objects to encode: JSON::XS will encode undefined
        scalars as JSON "null" values, scalars that have last been used in a
        string context before encoding as JSON strings, and anything else as
        number value:

           # dump as number
           encode_json [2]                      # yields [2]
           encode_json [-3.0e17]                # yields [-3e+17]
           my $value = 5; encode_json [$value]  # yields [5]

           # used as string, so dump as string
           print $value;
           encode_json [$value]                 # yields ["5"]

           # undef becomes null
           encode_json [undef]                  # yields [null]

        You can force the type to be a JSON string by stringifying it:

           my $x = 3.1; # some variable containing a number
           "$x";        # stringified
           $x .= "";    # another, more awkward way to stringify
           print $x;    # perl does it for you, too, quite often

        You can force the type to be a JSON number by numifying it:

           my $x = "3"; # some variable containing a string
           $x += 0;     # numify it, ensuring it will be dumped as a number
           $x *= 1;     # same thing, the choice is yours.

        You can not currently force the type in other, less obscure, ways.
        Tell me if you need this capability (but don't forget to explain why
        it's needed :).

        Note that numerical precision has the same meaning as under Perl (so
        binary to decimal conversion follows the same rules as in Perl,
        which can differ to other languages). Also, your perl interpreter
        might expose extensions to the floating point numbers of your
        platform, such as infinities or NaN's - these cannot be represented
        in JSON, and it is an error to pass those in.

    As JSON cannot directly represent Perl objects, you have to choose
    between a pure JSON representation (without the ability to deserialise
    the object automatically again), and a nonstandard extension to the JSON
    syntax, tagged values.

    What happens when "JSON::XS" encounters a Perl object depends on the
    "allow_blessed", "convert_blessed" and "allow_tags" settings, which are
    used in this order:

    1. "allow_tags" is enabled and the object has a "FREEZE" method.
        In this case, "JSON::XS" uses the Types::Serialiser object
        serialisation protocol to create a tagged JSON value, using a
        nonstandard extension to the JSON syntax.

        This works by invoking the "FREEZE" method on the object, with the
        first argument being the object to serialise, and the second
        argument being the constant string "JSON" to distinguish it from
        other serialisers.

        The "FREEZE" method can return any number of values (i.e. zero or
        more). These values and the paclkage/classname of the object will
        then be encoded as a tagged JSON value in the following format:

           ("classname")[FREEZE return values...]



        For example, the hypothetical "My::Object" "FREEZE" method might use
        the objects "type" and "id" members to encode the object:

           sub My::Object::FREEZE {
              my ($self, $serialiser) = @_;

              ($self->{type}, $self->{id})

    2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
        In this case, the "TO_JSON" method of the object is invoked in
        scalar context. It must return a single scalar that can be directly
        encoded into JSON. This scalar replaces the object in the JSON text.

        For example, the following "TO_JSON" method will convert all URI
        objects to JSON strings when serialised. The fatc that these values
        originally were URI objects is lost.

           sub URI::TO_JSON {
              my ($uri) = @_;

    3. "allow_blessed" is enabled.
        The object will be serialised as a JSON null value.

    4. none of the above
        If none of the settings are enabled or the respective methods are
        missing, "JSON::XS" throws an exception.

    For deserialisation there are only two cases to consider: either
    nonstandard tagging was used, in which case "allow_tags" decides, or
    objects cannot be automatically be deserialised, in which case you can
    use postprocessing or the "filter_json_object" or
    "filter_json_single_key_object" callbacks to get some real objects our
    of your JSON.

    This section only considers the tagged value case: I a tagged JSON
    object is encountered during decoding and "allow_tags" is disabled, a
    parse error will result (as if tagged values were not part of the

    If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of
    the package/classname used during serialisation (it will not attempt to
    load the package as a Perl module). If there is no such method, the
    decoding will fail with an error.

    Otherwise, the "THAW" method is invoked with the classname as first
    argument, the constant string "JSON" as second argument, and all the
    values from the JSON array (the values originally returned by the
    "FREEZE" method) as remaining arguments.

    The method must then return the object. While technically you can return
    any Perl scalar, you might have to enable the "enable_nonref" setting to
    make that work in all cases, so better return an actual blessed

    As an example, let's implement a "THAW" function that regenerates the
    "My::Object" from the "FREEZE" example earlier:

       sub My::Object::THAW {
          my ($class, $serialiser, $type, $id) = @_;

          $class->new (type => $type, id => $id)

    The interested reader might have seen a number of flags that signify
    encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
    some confusion on what these do, so here is a short comparison:

    "utf8" controls whether the JSON text created by "encode" (and expected
    by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
    control whether "encode" escapes character values outside their
    respective codeset range. Neither of these flags conflict with each
    other, although some combinations make less sense than others.

    Care has been taken to make all flags symmetrical with respect to
    "encode" and "decode", that is, texts encoded with any combination of
    these flag values will be correctly decoded when the same flags are used
    - in general, if you use different flag settings while encoding vs. when
    decoding you likely have a bug somewhere.

    Below comes a verbose discussion of these flags. Note that a "codeset"
    is simply an abstract set of character-codepoint pairs, while an
    encoding takes those codepoint numbers and *encodes* them, in our case
    into octets. Unicode is (among other things) a codeset, UTF-8 is an
    encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
    encodings at the same time, which can be confusing.

    "utf8" flag disabled
        When "utf8" is disabled (the default), then "encode"/"decode"
        generate and expect Unicode strings, that is, characters with high
        ordinal Unicode values (> 255) will be encoded as such characters,
        and likewise such characters are decoded as-is, no changes to them
        will be done, except "(re-)interpreting" them as Unicode codepoints
        or Unicode characters, respectively (to Perl, these are the same
        thing in strings unless you do funny/weird/dumb stuff).

        This is useful when you want to do the encoding yourself (e.g. when
        you want to have UTF-16 encoded JSON texts) or when some other layer
        does the encoding for you (for example, when printing to a terminal
        using a filehandle that transparently encodes to UTF-8 you certainly
        do NOT want to UTF-8 encode your data first and have Perl encode it
        another time).

    "utf8" flag enabled
        If the "utf8"-flag is enabled, "encode"/"decode" will encode all
        characters using the corresponding UTF-8 multi-byte sequence, and
        will expect your input strings to be encoded as UTF-8, that is, no
        "character" of the input string must have any value > 255, as UTF-8
        does not allow that.

        The "utf8" flag therefore switches between two modes: disabled means
        you will get a Unicode string in Perl, enabled means you get a UTF-8
        encoded octet/binary string in Perl.

    "latin1" or "ascii" flags enabled
        With "latin1" (or "ascii") enabled, "encode" will escape characters
        with ordinal values > 255 (> 127 with "ascii") and encode the
        remaining characters as specified by the "utf8" flag.

        If "utf8" is disabled, then the result is also correctly encoded in
        those character sets (as both are proper subsets of Unicode, meaning
        that a Unicode string with all character values < 256 is the same
        thing as a ISO-8859-1 string, and a Unicode string with all
        character values < 128 is the same thing as an ASCII string in

        If "utf8" is enabled, you still get a correct UTF-8-encoded string,
        regardless of these flags, just some more characters will be escaped
        using "\uXXXX" then before.

        Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
        encoding, while ASCII-encoded strings are. That is because the
        ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
        *codeset* being a subset of Unicode), while ASCII is.

        Surprisingly, "decode" will ignore these flags and so treat all
        input values as governed by the "utf8" flag. If it is disabled, this
        allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
        strict subsets of Unicode. If it is enabled, you can correctly
        decode UTF-8 encoded strings.

        So neither "latin1" nor "ascii" are incompatible with the "utf8"
        flag - they only govern when the JSON output engine escapes a
        character or not.

        The main use for "latin1" is to relatively efficiently store binary
        data as JSON, at the expense of breaking compatibility with most
        JSON decoders.

        The main use for "ascii" is to force the output to not contain
        characters with values > 127, which means you can interpret the
        resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
        any character set and 8-bit-encoding, and still get the same data
        structure back. This is useful when your channel for JSON transfer
        is not 8-bit clean or the encoding might be mangled in between (e.g.
        in mail), and works because ASCII is a proper subset of most 8-bit
        and multibyte encodings in use in the world.

  JSON and ECMAscript
    JSON syntax is based on how literals are represented in javascript (the
    not-standardised predecessor of ECMAscript) which is presumably why it
    is called "JavaScript Object Notation".

    However, JSON is not a subset (and also not a superset of course) of
    ECMAscript (the standard) or javascript (whatever browsers actually

    If you want to use javascript's "eval" function to "parse" JSON, you
    might run into parse errors for valid JSON texts, or the resulting data
    structure might not be queryable:

    One of the problems is that U+2028 and U+2029 are valid characters
    inside JSON strings, but are not allowed in ECMAscript string literals,
    so the following Perl fragment will not output something that can be
    guaranteed to be parsable by javascript's "eval":

       use JSON::XS;

       print encode_json [chr 0x2028];

    The right fix for this is to use a proper JSON parser in your javascript
    programs, and not rely on "eval" (see for example Douglas Crockford's
    json2.js parser).

    If this is not an option, you can, as a stop-gap measure, simply encode
    to ASCII-only JSON:

       use JSON::XS;

       print JSON::XS->new->ascii->encode ([chr 0x2028]);

    Note that this will enlarge the resulting JSON text quite a bit if you
    have many non-ASCII characters. You might be tempted to run some regexes
    to only escape U+2028 and U+2029, e.g.:

       # DO NOT USE THIS!
       my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
       $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
       $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
       print $json;

    Note that *this is a bad idea*: the above only works for U+2028 and
    U+2029 and thus only for fully ECMAscript-compliant parsers. Many
    existing javascript implementations, however, have issues with other
    characters as well - using "eval" naively simply *will* cause problems.

    Another problem is that some javascript implementations reserve some
    property names for their own purposes (which probably makes them
    non-ECMAscript-compliant). For example, Iceweasel reserves the
    "__proto__" property name for its own purposes.

    If that is a problem, you could parse try to filter the resulting JSON
    output for these property strings, e.g.:

       $json =~ s/"__proto__"\s*:/"__proto__renamed":/g;

    This works because "__proto__" is not valid outside of strings, so every
    occurrence of ""__proto__"\s*:" must be a string used as property name.

    If you know of other incompatibilities, please let me know.

    You often hear that JSON is a subset of YAML. This is, however, a mass
    hysteria(*) and very far from the truth (as of the time of this
    writing), so let me state it clearly: *in general, there is no way to
    configure JSON::XS to output a data structure as valid YAML* that works
    in all cases.

    If you really must use JSON::XS to generate YAML, you should use this
    algorithm (subject to change in future versions):

       my $to_yaml = JSON::XS->new->utf8->space_after (1);
       my $yaml = $to_yaml->encode ($ref) . "\n";

    This will *usually* generate JSON texts that also parse as valid YAML.
    Please note that YAML has hardcoded limits on (simple) object key
    lengths that JSON doesn't have and also has different and incompatible
    unicode character escape syntax, so you should make sure that your hash
    keys are noticeably shorter than the 1024 "stream characters" YAML
    allows and that you do not have characters with codepoint values outside
    the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
    sequences in strings (which JSON::XS does not *currently* generate, but
    other JSON generators might).

    There might be other incompatibilities that I am not aware of (or the
    YAML specification has been changed yet again - it does so quite often).
    In general you should not try to generate YAML with a JSON generator or
    vice versa, or try to parse JSON with a YAML parser or vice versa:
    chances are high that you will run into severe interoperability problems
    when you least expect it.

    (*) I have been pressured multiple times by Brian Ingerson (one of the
        authors of the YAML specification) to remove this paragraph, despite
        him acknowledging that the actual incompatibilities exist. As I was
        personally bitten by this "JSON is YAML" lie, I refused and said I
        will continue to educate people about these issues, so others do not
        run into the same problem again and again. After this, Brian called
        me a (quote)*complete and worthless idiot*(unquote).

        In my opinion, instead of pressuring and insulting people who
        actually clarify issues with YAML and the wrong statements of some
        of its proponents, I would kindly suggest reading the JSON spec
        (which is not that difficult or long) and finally make YAML
        compatible to it, and educating users about the changes, instead of
        spreading lies about the real compatibility for many *years* and
        trying to silence people who point out that it isn't true.

        Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
        even though the incompatibilities have been documented (and are
        known to Brian) for many years and the spec makes explicit claims
        that YAML is a superset of JSON. It would be so easy to fix, but
        apparently, bullying people and corrupting userdata is so much

    It seems that JSON::XS is surprisingly fast, as shown in the following
    tables. They have been generated with the help of the "eg/bench" program
    in the JSON::XS distribution, to make it easy to compare on your own

    First comes a comparison between various modules using a very short
    single-line JSON string (also available at

       {"method": "handleMessage", "params": ["user1",
       "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
       1,  0]}

    It shows the number of encodes/decodes per second (JSON::XS uses the
    functional interface, while JSON::XS/2 uses the OO interface with
    pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
    JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
    the from_json method). Higher is better:

       module        |     encode |     decode |
       JSON::DWIW/DS |  86302.551 | 102300.098 |
       JSON::DWIW/FJ |  86302.551 |  75983.768 |
       JSON::PP      |  15827.562 |   6638.658 |
       JSON::Syck    |  63358.066 |  47662.545 |
       JSON::XS      | 511500.488 | 511500.488 |
       JSON::XS/2    | 291271.111 | 388361.481 |
       JSON::XS/3    | 361577.931 | 361577.931 |
       Storable      |  66788.280 | 265462.278 |

    That is, JSON::XS is almost six times faster than JSON::DWIW on
    encoding, about five times faster on decoding, and over thirty to
    seventy times faster than JSON's pure perl implementation. It also
    compares favourably to Storable for small amounts of data.

    Using a longer test string (roughly 18KB, generated from Yahoo! Locals
    search API (<>).

       module        |     encode |     decode |
       JSON::DWIW/DS |   1647.927 |   2673.916 |
       JSON::DWIW/FJ |   1630.249 |   2596.128 |
       JSON::PP      |    400.640 |     62.311 |
       JSON::Syck    |   1481.040 |   1524.869 |
       JSON::XS      |  20661.596 |   9541.183 |
       JSON::XS/2    |  10683.403 |   9416.938 |
       JSON::XS/3    |  20661.596 |   9400.054 |
       Storable      |  19765.806 |  10000.725 |

    Again, JSON::XS leads by far (except for Storable which non-surprisingly
    decodes a bit faster).

    On large strings containing lots of high Unicode characters, some
    modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
    result will be broken due to missing (or wrong) Unicode handling. Others
    refuse to decode or encode properly, so it was impossible to prepare a
    fair comparison table for that case.

    When you are using JSON in a protocol, talking to untrusted potentially
    hostile creatures requires relatively few measures.

    First of all, your JSON decoder should be secure, that is, should not
    have any buffer overflows. Obviously, this module should ensure that and
    I am trying hard on making that true, but you never know.

    Second, you need to avoid resource-starving attacks. That means you
    should limit the size of JSON texts you accept, or make sure then when
    your resources run out, that's just fine (e.g. by using a separate
    process that can crash safely). The size of a JSON text in octets or
    characters is usually a good indication of the size of the resources
    required to decode it into a Perl structure. While JSON::XS can check
    the size of the JSON text, it might be too late when you already have it
    in memory, so you might want to check the size before you accept the

    Third, JSON::XS recurses using the C stack when decoding objects and
    arrays. The C stack is a limited resource: for instance, on my amd64
    machine with 8MB of stack size I can decode around 180k nested arrays
    but only 14k nested JSON objects (due to perl itself recursing deeply on
    croak to free the temporary). If that is exceeded, the program crashes.
    To be conservative, the default nesting limit is set to 512. If your
    process has a smaller stack, you should adjust this setting accordingly
    with the "max_depth" method.

    Something else could bomb you, too, that I forgot to think of. In that
    case, you get to keep the pieces. I am always open for hints, though...

    Also keep in mind that JSON::XS might leak contents of your Perl data
    structures in its error messages, so when you serialise sensitive
    information you might want to make sure that exceptions thrown by
    JSON::XS will not end up in front of untrusted eyes.

    If you are using JSON::XS to return packets to consumption by JavaScript
    scripts in a browser you should have a look at
    to see whether you are vulnerable to some common attack vectors (which
    really are browser design bugs, but it is still you who will have to
    deal with it, as major browser developers care only for features, not
    about getting security right).

  "OLD" VS. "NEW" JSON (RFC4627 VS. RFC7159)
    JSON originally required JSON texts to represent an array or object -
    scalar values were explicitly not allowed. This has changed, and
    versions of JSON::XS beginning with 4.0 reflect this by allowing scalar
    values by default.

    One reason why one might not want this is that this removes a
    fundamental property of JSON texts, namely that they are self-delimited
    and self-contained, or in other words, you could take any number of
    "old" JSON texts and paste them together, and the result would be
    unambiguously parseable:

       [1,3]{"k":5}[][null] # four JSON texts, without doubt

    By allowing scalars, this property is lost: in the following example, is
    this one JSON text (the number 12) or two JSON texts (the numbers 1 and

       12    # could be 12, or 1 and 2

    Another lost property of "old" JSON is that no lookahead is required to
    know the end of a JSON text, i.e. the JSON text definitely ended at the
    last "]" or "}" character, there was no need to read extra characters.

    For example, a viable network protocol with "old" JSON was to simply
    exchange JSON texts without delimiter. For "new" JSON, you have to use a
    suitable delimiter (such as a newline) after every JSON text or ensure
    you never encode/decode scalar values.

    Most protocols do work by only transferring arrays or objects, and the
    easiest way to avoid problems with the "new" JSON definition is to
    explicitly disallow scalar values in your encoder and decoder:

       $json_coder = JSON::XS->new->allow_nonref (0)

    This is a somewhat unhappy situation, and the blame can fully be put on
    JSON's inmventor, Douglas Crockford, who unilaterally changed the format
    in 2006 without consulting the IETF, forcing the IETF to either fork the
    format or go with it (as I was told, the IETF wasn't amused).

    JSON is a somewhat sloppily-defined format - it carries around obvious
    Javascript baggage, such as not really defining number range, probably
    because Javascript only has one type of numbers: IEEE 64 bit floats

    For this reaosn, RFC7493 defines "Internet JSON", which is a restricted
    subset of JSON that is supposedly more interoperable on the internet.

    While "JSON::XS" does not offer specific support for I-JSON, it of
    course accepts valid I-JSON and by default implements some of the
    limitations of I-JSON, such as parsing numbers as perl numbers, which
    are usually a superset of binary64 numbers.

    To generate I-JSON, follow these rules:

    *   always generate UTF-8

        I-JSON must be encoded in UTF-8, the default for "encode_json".

    *   numbers should be within IEEE 754 binary64 range

        Basically all existing perl installations use binary64 to represent
        floating point numbers, so all you need to do is to avoid large

    *   objects must not have duplicate keys

        This is trivially done, as "JSON::XS" does not allow duplicate keys.

    *   do not generate scalar JSON texts, use "->allow_nonref (0)"

        I-JSON strongly requests you to only encode arrays and objects into

    *   times should be strings in ISO 8601 format

        There are a myriad of modules on CPAN dealing with ISO 8601 - search
        for "ISO8601" on CPAN and use one.

    *   encode binary data as base64

        While it's tempting to just dump binary data as a string (and let
        "JSON::XS" do the escaping), for I-JSON, it's *recommended* to
        encode binary data as base64.

    There are some other considerations - read RFC7493 for the details if

    "JSON::XS" uses the Types::Serialiser module to provide boolean
    constants. That means that the JSON true and false values will be
    comaptible to true and false values of other modules that do the same,
    such as JSON::PP and CBOR::XS.

    As long as you only serialise data that can be directly expressed in
    JSON, "JSON::XS" is incapable of generating invalid JSON output (modulo
    bugs, but "JSON::XS" has found more bugs in the official JSON testsuite
    (1) than the official JSON testsuite has found in "JSON::XS" (0)).

    When you have trouble decoding JSON generated by this module using other
    decoders, then it is very likely that you have an encoding mismatch or
    the other decoder is broken.

    When decoding, "JSON::XS" is strict by default and will likely catch all
    errors. There are currently two settings that change this: "relaxed"
    makes "JSON::XS" accept (but not generate) some non-standard extensions,
    and "allow_tags" will allow you to encode and decode Perl objects, at
    the cost of not outputting valid JSON anymore.

    When you use "allow_tags" to use the extended (and also nonstandard and
    invalid) JSON syntax for serialised objects, and you still want to
    decode the generated When you want to serialise objects, you can run a
    regex to replace the tagged syntax by standard JSON arrays (it only
    works for "normal" package names without comma, newlines or single
    colons). First, the readable Perl version:

       # if your FREEZE methods return no values, you need this replace first:
       $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;

       # this works for non-empty constructor arg lists:
       $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;

    And here is a less readable version that is easy to adapt to other

       $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;

    Here is an ECMAScript version (same regex):

       json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");

    Since this syntax converts to standard JSON arrays, it might be hard to
    distinguish serialised objects from normal arrays. You can prepend a
    "magic number" as first array element to reduce chances of a collision:

       $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;

    And after decoding the JSON text, you could walk the data structure
    looking for arrays with a first element of

    The same approach can be used to create the tagged format with another
    encoder. First, you create an array with the magic string as first
    member, the classname as second, and constructor arguments last, encode
    it as part of your JSON structure, and then:

       $json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;

    Again, this has some limitations - the magic string must not be encoded
    with character escapes, and the constructor arguments must be non-empty.

    This module is *not* guaranteed to be ithread (or MULTIPLICITY-) safe
    and there are no plans to change this. Note that perl's builtin
    so-called threads/ithreads are officially deprecated and should not be

    Sometimes people avoid the Perl locale support and directly call the
    system's setlocale function with "LC_ALL".

    This breaks both perl and modules such as JSON::XS, as stringification
    of numbers no longer works correctly (e.g. "$x = 0.1; print "$x"+1"
    might print 1, and JSON::XS might output illegal JSON as JSON::XS relies
    on perl to stringify numbers).

    The solution is simple: don't call "setlocale", or use it for only those
    categories you need, such as "LC_MESSAGES" or "LC_CTYPE".

    If you need "LC_NUMERIC", you should enable it only around the code that
    actually needs it (avoiding stringification of numbers), and restore it

    At the time this module was created there already were a number of JSON
    modules available on CPAN, so what was the reason to write yet another
    JSON module? While it seems there are many JSON modules, none of them
    correctly handled all corner cases, and in most cases their maintainers
    are unresponsive, gone missing, or not listening to bug reports for
    other reasons.

    Beginning with version 2.0 of the JSON module, when both JSON and
    JSON::XS are installed, then JSON will fall back on JSON::XS (this can
    be overridden) with no overhead due to emulation (by inheriting
    constructor and methods). If JSON::XS is not available, it will fall
    back to the compatible JSON::PP module as backend, so using JSON instead
    of JSON::XS gives you a portable JSON API that can be fast when you need
    it and doesn't require a C compiler when that is a problem.

    Somewhere around version 3, this module was forked into
    "Cpanel::JSON::XS", because its maintainer had serious trouble
    understanding JSON and insisted on a fork with many bugs "fixed" that
    weren't actually bugs, while spreading FUD about this module without
    actually giving any details on his accusations. You be the judge, but in
    my personal opinion, if you want quality, you will stay away from
    dangerous forks like that.

    While the goal of this module is to be correct, that unfortunately does
    not mean it's bug-free, only that I think its design is bug-free. If you
    keep reporting bugs they will be fixed swiftly, though.

    Please refrain from using or any other bug reporting
    service. I put the contact address into my modules for a reason.

    The json_xs command line utility for quick experiments.

     Marc Lehmann <>