encode::unicode(3)

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 Consor
tium (except, of course, for UTF-8, which is a native for
mat 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).
Quick Reference: Decodes from ord(N) Encodes; chr(N) to...
octet/char BOM S.P d800-dfff ord > 0xffff
bcd} ==
---------------+-----------------+-----------------------------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 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
f8d ---------------+-----------------+-----------------------------

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 tak
ing 16 bits. It does not support surrogate pairs. When a surrogate pair is encountered during decode(), its place is filled with FFD} 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 filled
with FFD} if CHECK is 0, or the routine croaks if
CHECK is 1.

UTF-16 is almost the same as UCS-2 but it supports surro_
gate pairs. When it encounters a high surrogate
(0xD800-0xDBFF), it fetches the following low surrogate
(0xDC00-0xDFFF) and "desurrogate"s them to form a charac
ter. Bogus surrogates result in death. When 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 char
acter 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 endi anness 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.

BOM as integer when fetched in network byte order: 16 32 bits/char
------------------------BE 0xFeFF 0x0000FeFF LE 0xFFeF 0xFFFe0000 ------------------------
This modules handles the BOM as follows.
� 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).
� 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.
� When BE or LE is omitted during encode(), it returns a: 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.
� "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 Dou
glas Adams in The Hitchhiker's Guide to the Galaxy Tril ogy, "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 charac
ters. Now we can store character ranges up to ff}

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: 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 800}-FFF} into a forbidden
zone but perl does not prohibit the use of characters
within this range. To perl, every one of 0000} up
to fff_ffff} (*) is a character.

(*) or fff_ffff_ffff_ffff} if your perl is compiled with

64-bit
integer support!

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