Encode::Unicode(3pm) Perl Programmers Reference Guide Encode::Unicode(3pm)NAMEEncode::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 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.
· 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 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 + ($lo - 0xDC00);
Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but
perl does not prohibit the use of characters within this range. To
perl, every one of \x{0000_0000} up to \x{ffff_ffff} (*) is a
character.
(*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
integer support!
Error Checking
Unlike most encodings which accept various ways to handle errors,
Unicode encodings simply croaks.
% perl -MEncode -e'$_ = "\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
perl v5.10.1 2009-04-14 Encode::Unicode(3pm)
