Provided by: libjson-xs-perl_3.040-1build1_amd64 bug


       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.

       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.

       As this is the n-th-something JSON module on CPAN, what was the reason to write yet
       another JSON module? While it seems there are many JSON modules, none of them correctly
       handle all corner cases, and in most cases their maintainers are unresponsive, gone
       missing, or not listening to bug reports for other reasons.

       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 those.

       ·   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, too.

       ·   simple to use

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

       ·   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

       $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 an UTF-8 (binary) string and tries to parse
           that as an UTF-8 encoded JSON text, returning the resulting reference. Croaks on

           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 string.
           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.

           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 ASCII.

           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 document.

           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 flags.

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

           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
           handled an 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

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

           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 properly.

           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

           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

           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 anyway. 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
           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 with enabled "allow_nonref", resulting in
           an invalid JSON text:

              JSON::XS->new->allow_nonref->encode ("Hello, World!")
              => "Hello, World!"

       $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 partner.

       $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 object.

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

           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 object.

           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

           This setting has no effect on "decode".

       $json = $json->allow_tags ([$enable])
       $enabled = $json->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 = $json->filter_json_object ([$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
           references returns a single scalar (which need not be a reference), this value (i.e. a
           copy of that scalar to avoid aliasing) 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

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

           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 [=> $coderef->($value)])
           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 time.

           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

           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 useful.

       $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

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

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

       $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 calls).

       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

           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 them.

              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 anything.

           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

           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

           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.

       All options that affect decoding are supported, except "allow_nonref". The reason for this
       is that it cannot be made to work sensibly: JSON objects and arrays are self-delimited,
       i.e. you can concatenate them back to back and still decode them perfectly. This does not
       hold true for JSON numbers, however.

       For example, is the string 1 a single JSON number, or is it simply the start of 12? Or is
       12 a single JSON number, or the concatenation of 1 and 2? In neither case you can tell,
       and this is why JSON::XS takes the conservative route and disallows this case.

       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 manner):

          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 example):

          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

       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 itself).

           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

           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 number).

           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 arguments.

           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 readability.

              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

              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 grammar).

       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 reference.

       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 an UTF-8 encoded octet/binary string in

       "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 Perl).

           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

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

       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

          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,

          # 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

       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.

   JSON and YAML
       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 easier.

       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 system.

       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

       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 string.

       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 (RFC 4627 VS. RFC 7159)

       TL;DR: Due to security concerns, JSON::XS will not allow scalar data in JSON texts by
       default - you need to create your own JSON::XS object and enable "allow_nonref":

          my $json = JSON::XS->new->allow_nonref;

          $text = $json->encode ($data);
          $data = $json->decode ($text);

       The long version: JSON being an important and supposedly stable format, the IETF
       standardised it as RFC 4627 in 2006. Unfortunately, the inventor of JSON, Dougles
       Crockford, unilaterally changed the definition of JSON in javascript. Rather than create a
       fork, the IETF decided to standardise the new syntax (apparently, so Iw as told, without
       finding it very amusing).

       The biggest difference between thed original JSON and the new JSON is that the new JSON
       supports scalars (anything other than arrays and objects) at the toplevel of a JSON text.
       While this is strictly backwards compatible to older versions, it breaks a number of
       protocols that relied on sending JSON back-to-back, and is a minor security concern.

       For example, imagine you have two banks communicating, and on one side, trhe JSON coder
       gets upgraded. Two messages, such as 10 and 1000 might then be confused to mean 101000,
       something that couldn't happen in the original JSON, because niether of these messages
       would be valid JSON.

       If one side accepts these messages, then an upgrade in the coder on either side could
       result in this becoming exploitable.

       This module has always allowed these messages as an optional extension, by default
       disabled. The security concerns are the reason why the default is still disabled, but
       future versions might/will likely upgrade to the newer RFC as default format, so you are
       advised to check your implementation and/or override the default with "->allow_nonref (0)"
       to ensure that future versions are safe.


       "JSON::XS" uses the Types::Serialiser module to provide boolean constants. That means that
       the JSON true and false values will be compatible 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 languages:

          $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 "XU1peReLzT4ggEllLanBYq4G9VzliwKF".

       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.


       Since this module was written, Google has written a new JSON RFC, RFC 7159 (and RFC7158).
       Unfortunately, this RFC breaks compatibility with both the original JSON specification on and RFC4627.

       As far as I can see, you can get partial compatibility when parsing by using
       "->allow_nonref". However, consider the security implications of doing so.

       I haven't decided yet when to break compatibility with RFC4627 by default (and potentially
       leave applications insecure) and change the default to follow RFC7159, but application
       authors are well advised to call "->allow_nonref(0)" even if this is the current default,
       if they cannot handle non-reference values, in preparation for the day when the default
       will change.


       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 theeads/ithreads are officially
       deprecated and should not be used.


       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 afterwards.


       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 <>