Encode - character encodings in Perl

SYNOPSIS
1. Table of Contents
DESCRIPTION
1. TERMINOLOGY
THE PERL ENCODING API
Encoding via PerlIO
Handling Malformed Data
1. List of CHECK values
2. coderef for CHECK
Defining Encodings
The UTF8 flag
1. Messing with Perl's Internals
UTF-8 vs. utf8 vs. UTF8
SEE ALSO
MAINTAINER
COPYRIGHT

SYNOPSIS

use Encode qw(decode encode);
$characters = decode('UTF-8', $octets,     Encode::FB_CROAK);
$octets     = encode('UTF-8', $characters, Encode::FB_CROAK);

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

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 interface between Perl strings and the rest of the system. Perl strings are sequences of characters.

The repertoire of characters that Perl can represent is a superset of those defined by the Unicode Consortium. On most platforms the ordinal values of a character as returned by ord(S) is the Unicode codepoint for that character. The exceptions are platforms where the legacy encoding is some variant of EBCDIC rather than a superset of ASCII; see perlebcdic.

During recent history, data is moved around a computer in 8-bit chunks, often called "bytes" but also known as "octets" in standards documents. 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: because a byte has 256 possible values, it easily fits in Perl's much larger "logical character".

This document mostly explains the how. perlunitut and perlunifaq explain the why.

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, such as a disk file, standard I/O stream, database, command-line argument, environment variable, socket etc.

THE PERL ENCODING API

Basic methods

encode

$octets  = encode(ENCODING, STRING[, CHECK])

Encodes the scalar value 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".

CAVEAT: the input scalar STRING might be modified in-place depending on what is set in CHECK. See "LEAVE_SRC" if you want your inputs to be left unchanged.

For example, to convert a string from Perl's internal format into ISO-8859-1, also known as Latin1:

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

CAVEAT: When you run $octets = encode("UTF-8", $string), then $octets might not be equal to $string. Though both contain the same data, the UTF8 flag for $octets is always off. When you encode anything, the UTF8 flag on the result is always off, even when it contains a completely valid UTF-8 string. See "The UTF8 flag" below.

If the $string is undef, then undef is returned.

str2bytes may be used as an alias for encode.

decode

$string = decode(ENCODING, OCTETS[, CHECK])

This function returns the string that results from decoding the scalar value OCTETS, assumed to be a sequence of octets in ENCODING, into Perl's internal form. As with 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".

CAVEAT: the input scalar OCTETS might be modified in-place depending on what is set in CHECK. See "LEAVE_SRC" if you want your inputs to be left unchanged.

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

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

CAVEAT: When you run $string = decode("UTF-8", $octets), then $string might not be equal to $octets. Though both contain the same data, the UTF8 flag for $string is on. See "The UTF8 flag" below.

If the $string is undef, then undef is returned.

bytes2str may be used as an alias for decode.

find_encoding

[$obj =] find_encoding(ENCODING)

Returns the encoding object corresponding to ENCODING. Returns undef if no matching ENCODING is find. The returned object is what does the actual encoding or decoding.

$string = decode($name, $bytes);

is in fact

$string = do {
    $obj = find_encoding($name);
    croak qq(encoding "$name" not found) unless ref $obj;
    $obj->decode($bytes);
};

with more error checking.

You can therefore save time by reusing this object as follows;

my $enc = find_encoding("iso-8859-1");
while(<>) {
    my $string = $enc->decode($_);
    ... # now do something with $string;
}

Besides "decode" and "encode", other methods are available as well. For instance, name() returns the canonical name of the encoding object.

find_encoding("latin1")->name; # iso-8859-1

See Encode::Encoding for details.

find_mime_encoding

[$obj =] find_mime_encoding(MIME_ENCODING)

Returns the encoding object corresponding to MIME_ENCODING. Acts same as find_encoding() but mime_name() of returned object must match to MIME_ENCODING. So as opposite of find_encoding() canonical names and aliases are not used when searching for object.

find_mime_encoding("utf8"); # returns undef because "utf8" is not valid I<MIME_ENCODING>
find_mime_encoding("utf-8"); # returns encode object "utf-8-strict"
find_mime_encoding("UTF-8"); # same as "utf-8" because I<MIME_ENCODING> is case insensitive
find_mime_encoding("utf-8-strict"); returns undef because "utf-8-strict" is not valid I<MIME_ENCODING>

from_to

[$length =] from_to($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 into Microsoft's CP1250 encoding:

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

and to convert it back:

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

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, and undef on error.

CAVEAT: The following operations may look the same, but are not:

from_to($data, "iso-8859-1", "UTF-8"); #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 the UTF8 flag on. #1 is equivalent to:

$data = encode("UTF-8", decode("iso-8859-1", $data));

See "The UTF8 flag" below.

Also note that:

from_to($octets, $from, $to, $check);

is equivalent to:

$octets = encode($to, decode($from, $octets), $check);

Yes, it does not respect the $check during decoding. It is deliberately done that way. If you need minute control, use decode followed by encode as follows:

$octets = encode($to, decode($from, $octets, $check_from), $check_to);

encode_utf8

$octets = encode_utf8($string);

WARNING: This function can produce invalid UTF-8! Do not use it for data exchange. Unless you want Perl's older "lax" mode, prefer $octets = encode("UTF-8", $string).

Equivalent to $octets = encode("utf8", $string). The characters in $string are encoded in Perl's internal format, and the result is returned as a sequence of octets. Because all possible characters in Perl have a (loose, not strict) utf8 representation, this function cannot fail.

decode_utf8

$string = decode_utf8($octets [, CHECK]);

WARNING: This function accepts invalid UTF-8! Do not use it for data exchange. Unless you want Perl's older "lax" mode, prefer $string = decode("UTF-8", $octets [, CHECK]).

Equivalent to $string = decode("utf8", $octets [, CHECK]). The sequence of octets represented by $octets is decoded from (loose, not strict) utf8 into a sequence of logical characters. Because not all sequences of octets are valid not strict utf8, it is quite possible for this function to fail. For CHECK, see "Handling Malformed Data".

CAVEAT: the input $octets might be modified in-place depending on what is set in CHECK. See "LEAVE_SRC" if you want your inputs to be left unchanged.

Listing available encodings

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

Returns a list of canonical names of available encodings that have already been loaded. To get a list of all available encodings including those that have not yet been loaded, 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 Encode::Supported.

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.

Before you do that, first make sure the alias is nonexistent using resolve_alias(), which returns the canonical name thereof. For example:

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 imported via use Encode qw(resolve_alias).

See Encode::Alias for details.

Finding IANA Character Set Registry names

The canonical name of a given encoding does not necessarily agree with IANA Character Set Registry, commonly seen as Content-Type: text/plain; charset=WHATEVER. For most cases, the canonical name works, but sometimes it does not, most notably with "utf-8-strict".

As of Encode version 2.21, a new method mime_name() is therefore added.

use Encode;
my $enc = find_encoding("UTF-8");
warn $enc->name;      # utf-8-strict
warn $enc->mime_name; # UTF-8

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 fully identical in functionality:

### Version 1 via PerlIO
  open(INPUT,  "< :encoding(shiftjis)", $infile)
      || die "Can't open < $infile for reading: $!";
  open(OUTPUT, "> :encoding(euc-jp)",  $outfile)
      || die "Can't open > $output for writing: $!";
  while (<INPUT>) {   # auto decodes $_
      print OUTPUT;   # auto encodes $_
  }
  close(INPUT)   || die "can't close $infile: $!";
  close(OUTPUT)  || die "can't close $outfile: $!";

### Version 2 via from_to()
  open(INPUT,  "< :raw", $infile)
      || die "Can't open < $infile for reading: $!";
  open(OUTPUT, "> :raw",  $outfile)
      || die "Can't open > $output for writing: $!";

  while (<INPUT>) {
      from_to($_, "shiftjis", "euc-jp", 1);  # switch encoding
      print OUTPUT;   # emit raw (but properly encoded) data
  }
  close(INPUT)   || die "can't close $infile: $!";
  close(OUTPUT)  || die "can't close $outfile: $!";

In the first version above, you let the appropriate encoding layer handle the conversion. In the second, you explicitly translate from one encoding to the other.

Unfortunately, it may be that encodings are not PerlIO-savvy. You can check to see whether your encoding is supported by PerlIO by invoking the perlio_ok method on it:

Encode::perlio_ok("hz");             # false
find_encoding("euc-cn")->perlio_ok;  # true wherever PerlIO is available

use Encode qw(perlio_ok);            # imported 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 the gory details, see Encode::Encoding and Encode::PerlIO.

Handling Malformed Data

The optional CHECK argument tells Encode what to do when encountering malformed data. Without CHECK, Encode::FB_DEFAULT (== 0) is assumed.

As of version 2.12, Encode supports coderef values for CHECK; see below.

NOTE: Not all encodings support this feature. Some encodings ignore the CHECK argument. For example, Encode::Unicode ignores CHECK and it always croaks on error.

List of CHECK values

FB_DEFAULT

I<CHECK> = Encode::FB_DEFAULT ( == 0)

If CHECK is 0, encoding and decoding replace any malformed character with a substitution character. When you encode, SUBCHAR is used. When you decode, the Unicode REPLACEMENT CHARACTER, code point U+FFFD, is used. If the data is supposed to be UTF-8, an optional lexical warning of warning category "utf8" is given.

FB_CROAK

I<CHECK> = Encode::FB_CROAK ( == 1)

If CHECK is 1, methods immediately die with an error message. Therefore, when CHECK is 1, you should trap exceptions with eval{}, unless you really want to let it die.

FB_QUIET

I<CHECK> = Encode::FB_QUIET

If CHECK is set to Encode::FB_QUIET, encoding and decoding immediately return the portion of the data that has been processed so far when an error occurs. The data argument is overwritten with everything after that point; that is, the unprocessed portion of the 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, (that is, you are reading with a fixed-width buffer). Here's some sample code to do exactly that:

my($buffer, $string) = ("", "");
while (read($fh, $buffer, 256, length($buffer))) {
    $string .= decode($encoding, $buffer, Encode::FB_QUIET);
    # $buffer now contains the unprocessed partial character
}

FB_WARN

I<CHECK> = Encode::FB_WARN

This is the same as FB_QUIET above, except that instead of being silent on errors, it issues a warning. This is handy for when you are debugging.

CAVEAT: All warnings from Encode module are reported, independently of pragma warnings settings. If you want to follow settings of lexical warnings configured by pragma warnings then append also check value ENCODE::ONLY_PRAGMA_WARNINGS. This value is available since Encode version 2.99.

FB_PERLQQ FB_HTMLCREF FB_XMLCREF

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 the Encode::XS module, CHECK == Encode::FB_PERLQQ puts encode and decode into perlqq fallback mode.

When you decode, \xHH is inserted for a malformed character, where HH is the hex representation of the octet that could not be decoded to utf8. When you encode, \x{HHHH} will be inserted, where HHHH is the Unicode code point (in any number of hex digits) of the character that cannot be found in the character repertoire of the encoding.

The HTML/XML character reference modes are about the same. In place of \x{HHHH}, HTML uses &#NNN; where NNN is a decimal number, and XML uses &#xHHHH; where HHHH is the hexadecimal number.

In Encode 2.10 or later, LEAVE_SRC is also implied.

The bitmask

These modes are all actually set via a bitmask. Here is how the FB_XXX constants are laid out. You can import the FB_XXX constants via use Encode qw(:fallbacks), and 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                                        X
PERLQQ        0x0100                                        X
HTMLCREF      0x0200
XMLCREF       0x0400

LEAVE_SRC

Encode::LEAVE_SRC

If the Encode::LEAVE_SRC bit is not set but CHECK is set, then the source string to encode() or decode() will be overwritten in place. If you're not interested in this, then bitwise-OR it with the bitmask.

coderef for CHECK

As of Encode 2.12, CHECK can also be a code reference which takes the ordinal value of the unmapped character as an argument and returns octets that represent the fallback character. For instance:

$ascii = encode("ascii", $utf8, sub{ sprintf "<U+%04X>", shift });

Acts like FB_PERLQQ but U+XXXX is used instead of \x{XXXX}.

Fallback for decode must return decoded string (sequence of characters) and takes a list of ordinal values as its arguments. So for example if you wish to decode octets as UTF-8, and use ISO-8859-15 as a fallback for bytes that are not valid UTF-8, you could write

$str = decode 'UTF-8', $octets, sub {
    my $tmp = join '', map chr, @_;
    return decode 'ISO-8859-15', $tmp;
};

Defining Encodings

To define a new encoding, use:

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

CANONICAL_NAME will be associated with $object. The object should provide the interface described in Encode::Encoding. If more than two arguments are provided, additional arguments are considered aliases for $object.

See Encode::Encoding for details.

The UTF8 flag

Before the introduction of Unicode 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 quote from 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.

When Programming Perl, 3rd ed. was written, not even Perl 5.6.0 had been born yet, many features documented in the book remained unimplemented for a long time. Perl 5.8 corrected much of this, and the introduction of the UTF8 flag is one of them. You can think of there being two fundamentally different kinds of strings and string-operations in Perl: one a byte-oriented mode for when the internal UTF8 flag is off, and the other a character-oriented mode for when the internal UTF8 flag is on.

This UTF8 flag is not visible in Perl scripts, exactly for the same reason you cannot (or rather, you don't have to) see whether a scalar contains a string, an integer, or a floating-point number. But you can still peek and poke these if you will. See the next section.

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 in a future release.

is_utf8

is_utf8(STRING [, CHECK])

[INTERNAL] Tests whether the UTF8 flag is turned on in the STRING. If CHECK is true, also checks whether STRING contains well-formed UTF-8. Returns true if successful, false otherwise.

Typically only necessary for debugging and testing. Don't use this flag as a marker to distinguish character and binary data, that should be decided for each variable when you write your code.

CAVEAT: If STRING has UTF8 flag set, it does NOT mean that STRING is UTF-8 encoded and vice-versa.

As of Perl 5.8.1, utf8 also has the utf8::is_utf8 function.

_utf8_on

_utf8_on(STRING)

[INTERNAL] Turns the STRING's internal UTF8 flag on. The STRING is not checked for containing only well-formed UTF-8. Do not use this unless you know with absolute certainty that the STRING holds only well-formed UTF-8. Returns the previous state of the UTF8 flag (so please don't treat the return value as indicating success or failure), or undef if STRING is not a string.

NOTE: For security reasons, this function does not work on tainted values.

_utf8_off

_utf8_off(STRING)

[INTERNAL] Turns the STRING's internal UTF8 flag off. Do not use frivolously. Returns the previous state of the UTF8 flag, or undef if STRING is not a string. Do not treat the return value as indicative of success or failure, because that isn't what it means: it is only the previous setting.

NOTE: For security reasons, this function does not work on tainted values.

UTF-8 vs. utf8 vs. UTF8

....We now view strings not as sequences of bytes, but as sequences
of numbers in the range 0 .. 2**32-1 (or in the case of 64-bit
computers, 0 .. 2**64-1) -- Programming Perl, 3rd ed.

That has historically been Perl's notion of UTF-8, as that is how UTF-8 was first conceived by Ken Thompson when he invented it. However, thanks to later revisions to the applicable standards, official UTF-8 is now rather stricter than that. For example, its range is much narrower (0 .. 0x10_FFFF to cover only 21 bits instead of 32 or 64 bits) and some sequences are not allowed, like those used in surrogate pairs, the 31 non-character code points 0xFDD0 .. 0xFDEF, the last two code points in any plane (0xXX_FFFE and 0xXX_FFFF), all non-shortest encodings, etc.

The former default in which Perl would always use a loose interpretation of UTF-8 has now been overruled:

From: Larry Wall <[email protected]>
Date: December 04, 2004 11:51:58 JST
To: [email protected]
Subject: Re: Make Encode.pm support the real UTF-8
Message-Id: <[email protected]>

On Fri, Dec 03, 2004 at 10:12:12PM +0000, Tim Bunce wrote:
: I've no problem with 'utf8' being perl's unrestricted uft8 encoding,
: but "UTF-8" is the name of the standard and should give the
: corresponding behaviour.

For what it's worth, that's how I've always kept them straight in my
head.

Also for what it's worth, Perl 6 will mostly default to strict but
make it easy to switch back to lax.

Larry

Got that? As of Perl 5.8.7, "UTF-8" means UTF-8 in its current sense, which is conservative and strict and security-conscious, whereas "utf8" means UTF-8 in its former sense, which was liberal and loose and lax. Encode version 2.10 or later thus groks this subtle but critically important distinction between "UTF-8" and "utf8".

encode("utf8",  "\x{FFFF_FFFF}", 1); # okay
encode("UTF-8", "\x{FFFF_FFFF}", 1); # croaks

This distinction is also important for decoding. In the following, $s stores character U+200000, which exceeds UTF-8's allowed range. $s thus stores an invalid Unicode code point:

$s = decode("utf8", "\xf8\x88\x80\x80\x80");

"UTF-8", by contrast, will either coerce the input to something valid:

$s = decode("UTF-8", "\xf8\x88\x80\x80\x80"); # U+FFFD

.. or croak:

decode("UTF-8", "\xf8\x88\x80\x80\x80", FB_CROAK|LEAVE_SRC);

In the Encode module, "UTF-8" is actually a canonical name for "utf-8-strict". That hyphen between the "UTF" and the "8" is critical; without it, Encode goes "liberal" and (perhaps overly-)permissive:

find_encoding("UTF-8")->name # is 'utf-8-strict'
find_encoding("utf-8")->name # ditto. names are case insensitive
find_encoding("utf_8")->name # ditto. "_" are treated as "-"
find_encoding("UTF8")->name  # is 'utf8'.

Perl's internal UTF8 flag is called "UTF8", without a hyphen. It indicates whether a string is internally encoded as "utf8", also without a hyphen.

MAINTAINER

This project was originated by the late Nick Ing-Simmons and later maintained by Dan Kogai <[email protected]>. See AUTHORS for a full list of people involved. For any questions, send mail to <[email protected]> so that we can all share.

While Dan Kogai retains the copyright as a maintainer, credit should go to all those involved. See AUTHORS for a list of those who submitted code to the project.

COPYRIGHT

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

Encode - character encodings in Perl

SYNOPSIS

Table of Contents

DESCRIPTION

TERMINOLOGY

character

byte

octet

THE PERL ENCODING API

Basic methods

encode

decode

find_encoding

find_mime_encoding

from_to

encode_utf8

decode_utf8

Listing available encodings

Defining Aliases

Finding IANA Character Set Registry names

Encoding via PerlIO

Handling Malformed Data

List of CHECK values

FB_DEFAULT

FB_CROAK

FB_QUIET

FB_WARN

FB_PERLQQ FB_HTMLCREF FB_XMLCREF

The bitmask

LEAVE_SRC

coderef for CHECK

Defining Encodings

The UTF8 flag

Messing with Perl's Internals

is_utf8

_utf8_on

_utf8_off

UTF-8 vs. utf8 vs. UTF8

SEE ALSO

MAINTAINER

COPYRIGHT