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man : Encode::Unicode
ext::Encode::UnicPerl:Programmersext::Encode::Unicode::Unicode(3p)
NAME
Encode::Unicode -- Various Unicode Transformation Formats
SYNOPSIS
use Encode qw/encode decode/;
$ucs2 = encode("UCS-2BE", $utf8);
$utf8 = decode("UCS-2BE", $ucs2);
ABSTRACT
This module implements all Character Encoding Schemes of
Unicode that are officially documented by Unicode
Consortium (except, of course, for UTF-8, which is a
native format in perl).
<http://www.unicode.org/glossary/>; says:
Character Encoding Scheme A character encoding form
plus byte serialization. There are Seven character
encoding schemes in Unicode: UTF-8, UTF-16, UTF-16BE,
UTF-16LE, UTF-32 (UCS-4), UTF-32BE (UCS-4BE) and
UTF-32LE (UCS-4LE), and UTF-7.
Since UTF-7 is a 7-bit (re)encoded version of
UTF-16BE, It is not part of Unicode's Character
Encoding Scheme. It is separately implemented in
Encode::Unicode::UTF7. For details see
Encode::Unicode::UTF7.
Quick Reference
Decodes from ord(N) Encodes chr(N) to...
octet/char BOM S.P d800-dfff ord > 0xffff \x{1abcd} ==
---------------+-----------------+------------------------------
UCS-2BE 2 N N is bogus Not Available
UCS-2LE 2 N N bogus Not Available
UTF-16 2/4 Y Y is S.P S.P BE/LE
UTF-16BE 2/4 N Y S.P S.P 0xd82a,0xdfcd
UTF-16LE 2/4 N Y S.P S.P 0x2ad8,0xcddf
UTF-32 4 Y - is bogus As is BE/LE
UTF-32BE 4 N - bogus As is 0x0001abcd
UTF-32LE 4 N - bogus As is 0xcdab0100
UTF-8 1-4 - - bogus >= 4 octets \xf0\x9a\af\8d
---------------+-----------------+------------------------------
Size, Endianness, and BOM
You can categorize these CES by 3 criteria: size of each
character, endianness, and Byte Order Mark.
by size
UCS-2 is a fixed-length encoding with each character
taking 16 bits. It does not support surrogate pairs.
When a surrogate pair is encountered during decode(), its
place is filled with \x{FFFD} if CHECK is 0, or the
routine croaks if CHECK is 1. When a character whose ord
value is larger than 0xFFFF is encountered, its place is
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filled with \x{FFFD} if CHECK is 0, or the routine croaks
if CHECK is 1.
UTF-16 is almost the same as UCS-2 but it supports
surrogate pairs. When it encounters a high surrogate
(0xD800-0xDBFF), it fetches the following low surrogate
(0xDC00-0xDFFF) and "desurrogate"s them to form a
character. Bogus surrogates result in death. When
\x{10000} or above is encountered during encode(), it
"ensurrogate"s them and pushes the surrogate pair to the
output stream.
UTF-32 (UCS-4) is a fixed-length encoding with each
character taking 32 bits. Since it is 32-bit, there is no
need for surrogate pairs.
by endianness
The first (and now failed) goal of Unicode was to map all
character repertoires into a fixed-length integer so that
programmers are happy. Since each character is either a
short or long in C, you have to pay attention to the
endianness of each platform when you pass data to one
another.
Anything marked as BE is Big Endian (or network byte
order) and LE is Little Endian (aka VAX byte order). For
anything not marked either BE or LE, a character called
Byte Order Mark (BOM) indicating the endianness is
prepended to the string.
CAVEAT: Though BOM in utf8 (\xEF\xBB\xBF) is valid, it is
meaningless and as of this writing Encode suite just leave
it as is (\x{FeFF}).
BOM as integer when fetched in network byte order
16 32 bits/char
-------------------------
BE 0xFeFF 0x0000FeFF
LE 0xFFFe 0xFFFe0000
-------------------------
This modules handles the BOM as follows.
o When BE or LE is explicitly stated as the name of
encoding, BOM is simply treated as a normal character
(ZERO WIDTH NO-BREAK SPACE).
o When BE or LE is omitted during decode(), it checks if
BOM is at the beginning of the string; if one is
found, the endianness is set to what the BOM says. If
no BOM is found, the routine dies.
o When BE or LE is omitted during encode(), it returns a
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BE-encoded string with BOM prepended. So when you
want to encode a whole text file, make sure you
encode() the whole text at once, not line by line or
each line, not file, will have a BOM prepended.
o "UCS-2" is an exception. Unlike others, this is an
alias of UCS-2BE. UCS-2 is already registered by IANA
and others that way.
Surrogate Pairs
To say the least, surrogate pairs were the biggest mistake
of the Unicode Consortium. But according to the late
Douglas Adams in The Hitchhiker's Guide to the Galaxy
Trilogy, "In the beginning the Universe was created. This
has made a lot of people very angry and been widely
regarded as a bad move". Their mistake was not of this
magnitude so let's forgive them.
(I don't dare make any comparison with Unicode Consortium
and the Vogons here ;) Or, comparing Encode to Babel Fish
is completely appropriate -- if you can only stick this
into your ear :)
Surrogate pairs were born when the Unicode Consortium
finally admitted that 16 bits were not big enough to hold
all the world's character repertoires. But they already
made UCS-2 16-bit. What do we do?
Back then, the range 0xD800-0xDFFF was not allocated.
Let's split that range in half and use the first half to
represent the "upper half of a character" and the second
half to represent the "lower half of a character". That
way, you can represent 1024 * 1024 = 1048576 more
characters. Now we can store character ranges up to
\x{10ffff} even with 16-bit encodings. This pair of half-
character is now called a surrogate pair and UTF-16 is the
name of the encoding that embraces them.
Here is a formula to ensurrogate a Unicode character
\x{10000} and above;
$hi = ($uni - 0x10000) / 0x400 + 0xD800;
$lo = ($uni - 0x10000) % 0x400 + 0xDC00;
And to desurrogate;
$uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
Note this move has made \x{D800}-\x{DFFF} into a forbidden
zone but perl does not prohibit the use of characters
within this range. To perl, every one of \x{0000_0000} up
to \x{ffff_ffff} (*) is a character.
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(*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
integer support!
Error Checking
Unlike most encodings which accept various ways to handle
errors, Unicode encodings simply croaks.
% perl -MEncode -e '$_ = "\xfe\xff\xd8\xd9\xda\xdb\0\n"' \
-e 'Encode::from_to($_, "utf16","shift_jis", 0); print'
UTF-16:Malformed LO surrogate d8d9 at /path/to/Encode.pm line 184.
% perl -MEncode -e '$a = "BOM missing"' \
-e ' Encode::from_to($a, "utf16", "shift_jis", 0); print'
UTF-16:Unrecognised BOM 424f at /path/to/Encode.pm line 184.
Unlike other encodings where mappings are not one-to-one
against Unicode, UTFs are supposed to map 100% against one
another. So Encode is more strict on UTFs.
Consider that "division by zero" of Encode :)
SEE ALSO
Encode, Encode::Unicode::UTF7,
<http://www.unicode.org/glossary/>;,
<http://www.unicode.org/unicode/faq/utf_bom.html>;,
RFC 2781 <http://rfc.net/rfc2781.html>;,
The whole Unicode standard
<http://www.unicode.org/unicode/uni2book/u2.html>;
Ch. 15, pp. 403 of "Programming Perl (3rd Edition)" by
Larry Wall, Tom Christiansen, Jon Orwant; O'Reilly &
Associates; ISBN 0-596-00027-8
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