Encode - character encodings

- NAME
- SYNOPSIS

- Table of Contents

- DESCRIPTION

- TERMINOLOGY

- PERL ENCODING API

- Listing available encodings
- Defining Aliases

- Encoding via PerlIO
- Handling Malformed Data

- Unimplemented fallback schemes

- Defining Encodings
- The UTF-8 flag

- Messing with Perl's Internals

- SEE ALSO
- MAINTAINER

# NAME

Encode - character encodings

# SYNOPSIS

        use Encode;

## Table of Contents

Encode consists of a collection of modules whose details are too big to fit in one document. This POD itself explains the top-level APIs and general topics at a glance. For other topics and more details, see the PODs below:

      Name                          Description
      --------------------------------------------------------
      Encode::Alias         Alias definitions to encodings
      Encode::Encoding      Encode Implementation Base Class
      Encode::Supported     List of Supported Encodings
      Encode::CN            Simplified Chinese Encodings
      Encode::JP            Japanese Encodings
      Encode::KR            Korean Encodings
      Encode::TW            Traditional Chinese Encodings
      --------------------------------------------------------

# DESCRIPTION

The `Encode` module provides the interfaces between Perl's strings and the rest of the system. Perl strings are sequences of characters.

The repertoire of characters that Perl can represent is at least that defined by the Unicode Consortium. On most platforms the ordinal values of the characters (as returned by `ord(ch)`) is the ``Unicode codepoint'' for the character (the exceptions are those platforms where the legacy encoding is some variant of EBCDIC rather than a super-set of ASCII - see the perlebcdic manpage).

Traditionally, computer data has been moved around in 8-bit chunks often called ``bytes''. These chunks are also known as ``octets'' in networking standards. Perl is widely used to manipulate data of many types - not only strings of characters representing human or computer languages but also ``binary'' data being the machine's representation of numbers, pixels in an image - or just about anything.

When Perl is processing ``binary data'', the programmer wants Perl to process ``sequences of bytes''. This is not a problem for Perl - as a byte has 256 possible values, it easily fits in Perl's much larger ``logical character''.

## TERMINOLOGY

character: a character in the range 0..(2**32-1) (or more). (What Perl's strings are made of.)

byte: a character in the range 0..255 (A special case of a Perl character.)

octet: 8 bits of data, with ordinal values 0..255 (Term for bytes passed to or from a non-Perl context, e.g. a disk file.)

# PERL ENCODING API

$octets = encode(ENCODING, $string [, CHECK])
  Encodes a string from Perl's internal form into ENCODING and returns a sequence of octets. ENCODING can be either a canonical name or an alias. For encoding names and aliases, see Defining Aliases. For CHECK, see Handling Malformed Data.

  For example, to convert a string from Perl's internal format to iso-8859-1 (also known as Latin1),

        $octets = encode("iso-8859-1", $string);

  CAVEAT: When you run `$octets = encode("utf8", $string)`, then $octets may not be equal to $string. Though they both contain the same data, the utf8 flag for $octets is always off. When you encode anything, utf8 flag of the result is always off, even when it contains completely valid utf8 string. See The UTF-8 flag below.

  encode($valid_encoding, undef) is harmless but warns you for `Use of uninitialized value in subroutine entry`. encode($valid_encoding, '') is harmless and warnless.

$string = decode(ENCODING, $octets [, CHECK])
  Decodes a sequence of octets assumed to be in ENCODING into Perl's internal form and returns the resulting string. As in encode(), ENCODING can be either a canonical name or an alias. For encoding names and aliases, see Defining Aliases. For CHECK, see Handling Malformed Data.

  For example, to convert ISO-8859-1 data to a string in Perl's internal format:

        $string = decode("iso-8859-1", $octets);

  CAVEAT: When you run `$string = decode("utf8", $octets)`, then $string may not be equal to $octets. Though they both contain the same data, the utf8 flag for $string is on unless $octets entirely consists of ASCII data (or EBCDIC on EBCDIC machines). See The UTF-8 flag below.

  decode($valid_encoding, undef) is harmless but warns you for `Use of uninitialized value in subroutine entry`. decode($valid_encoding, '') is harmless and warnless.

[$length =] from_to($octets, FROM_ENC, TO_ENC [, CHECK])
  Converts in-place data between two encodings. The data in $octets must be encoded as octets and not as characters in Perl's internal format. For example, to convert ISO-8859-1 data to Microsoft's CP1250 encoding:

        from_to($octets, "iso-8859-1", "cp1250");

  and to convert it back:

        from_to($octets, "cp1250", "iso-8859-1");

  Note that because the conversion happens in place, the data to be converted cannot be a string constant; it must be a scalar variable.

  `from_to()` returns the length of the converted string in octets on success, undef otherwise.

  CAVEAT: The following operations look the same but are not quite so;

        from_to($data, "iso-8859-1", "utf8"); #1
        $data = decode("iso-8859-1", $data);  #2

  Both #1 and #2 make $data consist of a completely valid UTF-8 string but only #2 turns utf8 flag on. #1 is equivalent to

        $data = encode("utf8", decode("iso-8859-1", $data));

  See The UTF-8 flag below.

$octets = encode_utf8($string);
  Equivalent to `$octets = encode("utf8", $string);` The characters that comprise $string are encoded in Perl's internal format and the result is returned as a sequence of octets. All possible characters have a UTF-8 representation so this function cannot fail.

$string = decode_utf8($octets [, CHECK]);
  equivalent to `$string = decode("utf8", $octets [, CHECK])`. The sequence of octets represented by $octets is decoded from UTF-8 into a sequence of logical characters. Not all sequences of octets form valid UTF-8 encodings, so it is possible for this call to fail. For CHECK, see Handling Malformed Data.

## Listing available encodings

      use Encode;
      @list = Encode->encodings();

Returns a list of the canonical names of the available encodings that are loaded. To get a list of all available encodings including the ones that are not loaded yet, say

      @all_encodings = Encode->encodings(":all");

Or you can give the name of a specific module.

      @with_jp = Encode->encodings("Encode::JP");

When ``::'' is not in the name, ``Encode::'' is assumed.

      @ebcdic = Encode->encodings("EBCDIC");

To find out in detail which encodings are supported by this package, see the Encode::Supported manpage.

## Defining Aliases

To add a new alias to a given encoding, use:

      use Encode;
      use Encode::Alias;
      define_alias(newName => ENCODING);

After that, newName can be used as an alias for ENCODING. ENCODING may be either the name of an encoding or an encoding object

But before you do so, make sure the alias is nonexistent with `resolve_alias()`, which returns the canonical name thereof. i.e.

      Encode::resolve_alias("latin1") eq "iso-8859-1" # true
      Encode::resolve_alias("iso-8859-12")   # false; nonexistent
      Encode::resolve_alias($name) eq $name  # true if $name is canonical

`resolve_alias()` does not need `use Encode::Alias`; it can be exported via `use Encode qw(resolve_alias)`.

See the Encode::Alias manpage for details.

# Encoding via PerlIO

If your perl supports PerlIO (which is the default), you can use a PerlIO layer to decode and encode directly via a filehandle. The following two examples are totally identical in their functionality.

      # via PerlIO
      open my $in,  "<:encoding(shiftjis)", $infile  or die;
      open my $out, ">:encoding(euc-jp)",   $outfile or die;
      while(<$in>){ print $out $_; }

      # via from_to
      open my $in,  "<", $infile  or die;
      open my $out, ">", $outfile or die;
      while(<$in>){
        from_to($_, "shiftjis", "euc-jp", 1);
        print $out $_;
      }

Unfortunately, it may be that encodings are PerlIO-savvy. You can check if your encoding is supported by PerlIO by calling the `perlio_ok` method.

      Encode::perlio_ok("hz");             # False
      find_encoding("euc-cn")->perlio_ok;  # True where PerlIO is available

      use Encode qw(perlio_ok);            # exported upon request
      perlio_ok("euc-jp")

Fortunately, all encodings that come with Encode core are PerlIO-savvy except for hz and ISO-2022-kr. For gory details, see the Encode::Encoding manpage and the Encode::PerlIO manpage.

# Handling Malformed Data

The CHECK argument is used as follows. When you omit it, the behaviour is the same as if you had passed a value of 0 for CHECK.

CHECK = Encode::FB_DEFAULT ( == 0)
  If CHECK is 0, (en|de)code will put a substitution character in place of a malformed character. For UCM-based encodings, <subchar> will be used. For Unicode, the code point `0xFFFD` is used. If the data is supposed to be UTF-8, an optional lexical warning (category utf8) is given.

CHECK = Encode::FB_CROAK ( == 1)
  If CHECK is 1, methods will die on error immediately with an error message. Therefore, when CHECK is set to 1, you should trap the fatal error with eval{} unless you really want to let it die on error.

CHECK = Encode::FB_QUIET
  If CHECK is set to Encode::FB_QUIET, (en|de)code will immediately return the portion of the data that has been processed so far when an error occurs. The data argument will be overwritten with everything after that point (that is, the unprocessed part of data). This is handy when you have to call decode repeatedly in the case where your source data may contain partial multi-byte character sequences, for example because you are reading with a fixed-width buffer. Here is some sample code that does exactly this:

        my $data = ''; my $utf8 = '';
        while(defined(read $fh, $buffer, 256)){
          # buffer may end in a partial character so we append
          $data .= $buffer;
          $utf8 .= decode($encoding, $data, Encode::FB_QUIET);
          # $data now contains the unprocessed partial character
        }

CHECK = Encode::FB_WARN
  This is the same as above, except that it warns on error. Handy when you are debugging the mode above.

perlqq mode (CHECK = Encode::FB_PERLQQ)
HTML charref mode (CHECK = Encode::FB_HTMLCREF)
XML charref mode (CHECK = Encode::FB_XMLCREF)
  For encodings that are implemented by Encode::XS, CHECK == Encode::FB_PERLQQ turns (en|de)code into `perlqq` fallback mode.

  When you decode, `\xHH` will be inserted for a malformed character, where HH is the hex representation of the octet that could not be decoded to utf8. And when you encode, `\x{HHHH}` will be inserted, where HHHH is the Unicode ID of the character that cannot be found in the character repertoire of the encoding.

  HTML/XML character reference modes are about the same, in place of `\x{HHHH}`, HTML uses `&#NNNN`; where NNNN is a decimal digit and XML uses `&#xHHHH`; where HHHH is the hexadecimal digit.

The bitmask
  These modes are actually set via a bitmask. Here is how the FB_XX constants are laid out. You can import the FB_XX constants via `use Encode qw(:fallbacks)`; you can import the generic bitmask constants via `use Encode qw(:fallback_all)`.

                           FB_DEFAULT FB_CROAK FB_QUIET FB_WARN  FB_PERLQQ
       DIE_ON_ERR    0x0001             X
       WARN_ON_ERR   0x0002                               X
       RETURN_ON_ERR 0x0004                      X        X
       LEAVE_SRC     0x0008
       PERLQQ        0x0100                                        X
       HTMLCREF      0x0200
       XMLCREF       0x0400

## Unimplemented fallback schemes

In the future, you will be able to use a code reference to a callback function for the value of CHECK but its API is still undecided.

The fallback scheme does not work on EBCDIC platforms.

# Defining Encodings

To define a new encoding, use:

        use Encode qw(define_encoding);
        define_encoding($object, 'canonicalName' [, alias...]);

canonicalName will be associated with $object. The object should provide the interface described in the Encode::Encoding manpage. If more than two arguments are provided then additional arguments are taken as aliases for $object.

See the Encode::Encoding manpage for more details.

# The UTF-8 flag

Before the introduction of utf8 support in perl, The `eq` operator just compared the strings represented by two scalars. Beginning with perl 5.8, `eq` compares two strings with simultaneous consideration of the utf8 flag. To explain why we made it so, I will quote page 402 of `Programming Perl, 3rd ed.`

Goal #1:
  Old byte-oriented programs should not spontaneously break on the old byte-oriented data they used to work on.

Goal #2:
  Old byte-oriented programs should magically start working on the new character-oriented data when appropriate.

Goal #3:
  Programs should run just as fast in the new character-oriented mode as in the old byte-oriented mode.

Goal #4:
  Perl should remain one language, rather than forking into a byte-oriented Perl and a character-oriented Perl.

Back when `Programming Perl, 3rd ed.` was written, not even Perl 5.6.0 was born and many features documented in the book remained unimplemented for a long time. Perl 5.8 corrected this and the introduction of the UTF-8 flag is one of them. You can think of this perl notion as of a byte-oriented mode (utf8 flag off) and a character-oriented mode (utf8 flag on).

Here is how Encode takes care of the utf8 flag.

When you encode, the resulting utf8 flag is always off.

When you decode, the resulting utf8 flag is on unless you can unambiguously represent data. Here is the definition of dis-ambiguity.

After `$utf8 = decode('foo', $octet);`,

      When $octet is...   The utf8 flag in $utf8 is
      ---------------------------------------------
      In ASCII only (or EBCDIC only)            OFF
      In ISO-8859-1                              ON
      In any other Encoding                      ON
      ---------------------------------------------

As you see, there is one exception, In ASCII. That way you can assue Goal #1. And with Encode Goal #2 is assumed but you still have to be careful in such cases mentioned in CAVEAT paragraphs.

This utf8 flag is not visible in perl scripts, exactly for the same reason you cannot (or you don't have to) see if a scalar contains a string, integer, or floating point number. But you can still peek and poke these if you will. See the section below.

## Messing with Perl's Internals

The following API uses parts of Perl's internals in the current implementation. As such, they are efficient but may change.

is_utf8(STRING [, CHECK])
  [INTERNAL] Tests whether the UTF-8 flag is turned on in the STRING. If CHECK is true, also checks the data in STRING for being well-formed UTF-8. Returns true if successful, false otherwise.

  As of perl 5.8.1, the utf8 manpage also has utf8::is_utf8().

`_utf8_on(STRING)`
  [INTERNAL] Turns on the UTF-8 flag in STRING. The data in STRING is not checked for being well-formed UTF-8. Do not use unless you know that the STRING is well-formed UTF-8. Returns the previous state of the UTF-8 flag (so please don't treat the return value as indicating success or failure), or `undef` if STRING is not a string.

`_utf8_off(STRING)`
  [INTERNAL] Turns off the UTF-8 flag in STRING. Do not use frivolously. Returns the previous state of the UTF-8 flag (so please don't treat the return value as indicating success or failure), or `undef` if STRING is not a string.

# SEE ALSO

the Encode::Encoding manpage, the Encode::Supported manpage, the Encode::PerlIO manpage, the encoding manpage, the perlebcdic manpage, open in the perlfunc manpage, the perlunicode manpage, the utf8 manpage, the Perl Unicode Mailing List <perl-unicode@perl.org>

# MAINTAINER

This project was originated by Nick Ing-Simmons and later maintained by Dan Kogai <dankogai@dan.co.jp>. See AUTHORS for a full list of people involved. For any questions, use <perl-unicode@perl.org> so we can all share.

Encode - character encodings