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Encode(3)
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
perlebcdic).
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 $octets is decoded from UTF-8 into a
sequence of logical characters. Not all sequences of octets form valid
UTF-8 encodings, so it is possible for this call to fail. For CHECK, see
"Handling Malformed Data".
Listing available encodings
use Encode;
@list = Encode->encodings();
Returns a list of the canonical names of the available encodings that are
loaded. To get a list of all available encodings including the ones that
are not loaded yet, say
@all_encodings = Encode->encodings(":all");
Or you can give the name of a specific module.
@with_jp = Encode->encodings("Encode::JP");
When "::" is not in the name, "Encode::" is assumed.
@ebcdic = Encode->encodings("EBCDIC");
To find out in detail which encodings are supported by this package, see
Encode::Supported.
Defining Aliases
To add a new alias to a given encoding, use:
use Encode;
use Encode::Alias;
define_alias(newName => ENCODING);
After that, newName can be used as an alias for ENCODING. ENCODING may be
either the name of an encoding or an encoding object
But before you do so, make sure the alias is nonexistent with
"resolve_alias()", which returns the canonical name thereof. i.e.
Encode::resolve_alias("latin1") eq "iso-8859-1" # true
Encode::resolve_alias("iso-8859-12") # false; nonexistent
Encode::resolve_alias($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 Encode::Alias 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 Encode::Encoding and
Encode::PerlIO.
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 Encode::Encoding. If more than two arguments
are provided then additional arguments are taken as aliases for $object.
See Encode::Encoding for more details.
The UTF-8 flag
Before the introduction of utf8 support in perl, The "eq" operator just
compared the strings represented by two scalars. Beginning with perl 5.8,
"eq" compares two strings with simultaneous consideration of the utf8 flag.
To explain why we made it so, I will quote page 402 of "Programming Perl,
3rd ed."
Goal #1:
Old byte-oriented programs should not spontaneously break on the old
byte-oriented data they used to work on.
Goal #2:
Old byte-oriented programs should magically start working on the new
character-oriented data when appropriate.
Goal #3:
Programs should run just as fast in the new character-oriented mode as in
the old byte-oriented mode.
Goal #4:
Perl should remain one language, rather than forking into a byte-oriented
Perl and a character-oriented Perl.
Back when "Programming Perl, 3rd ed." was written, not even Perl 5.6.0 was
born and many features documented in the book remained unimplemented for a
long time. Perl 5.8 corrected this and the introduction of the UTF-8 flag
is one of them. You can think of this perl notion as of a byte-oriented
mode (utf8 flag off) and a character-oriented mode (utf8 flag on).
Here is how Encode takes care of the utf8 flag.
· When you encode, the resulting utf8 flag is always off.
· When you decode, the resulting utf8 flag is on unless you can
unambiguously represent data. Here is the definition of dis-ambiguity.
After "$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, utf8 also has utf8::is_utif8().
_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
Encode::Encoding, Encode::Supported, Encode::PerlIO, encoding, perlebcdic,
"open" in perlfunc, perlunicode, utf8, 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.
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