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Perl version 5.10.0 documentation - Encode::Unicode

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

N is bogus

Not Available

UCS-2LE

bogus

Not Available

UTF-16

2/4

Y is

S.P

BE/LE

UTF-16BE

2/4

S.P

0xd82a,0xdfcd

UTF-16LE

2/4

S.P

0x2ad8,0xcddf

UTF-32

- is bogus

As is

BE/LE

UTF-32BE

bogus

As is

0x0001abcd

UTF-32LE

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

desurrogates

them to form a character. Bogus surrogates result in death. When \x{10000} or above

is encountered during encode(), it

ensurrogates

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.

http://perldoc.perl.org

Page 1

Perl version 5.10.0 documentation - Encode::Unicode

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

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

32 bits/char

-------------------------

0xFeFF 0x0000FeFF

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;

http://perldoc.perl.org

Page 2

Perl version 5.10.0 documentation - Encode::Unicode

$lo = ($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 :)

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Inne pliki z tego folderu:

CJKConstants.pdf (9 KB)
Alias.pdf (22 KB)
CN.pdf (18 KB)
Byte.pdf (19 KB)
EBCDIC.pdf (16 KB)

Unicode.pdf

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