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 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.
- •
- Default Byte Order
When no BOM is found, Encode 2.76 and blow croaked. Since Encode 2.77, it
falls back to BE accordingly to RFC2781 and the Unicode Standard version
8.0. This behaviour has also been backported to Encode 2.60 and later as
shipped in the Debian perl package since version 5.20.2-3+deb8u3 (see
<https://bugs.debian.org/798727>).
- •
- 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://www.ietf.org/rfc/rfc2781.txt>,
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
POD ERRORS¶
Hey!
The above document had some coding errors, which are explained
below:
- Around line 181:
- Expected '=item *'