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Encode::Unicode(3pm)   Perl Programmers Reference Guide   Encode::Unicode(3pm)

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.

       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 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.

         (*) 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

perl v5.12.1                      2010-04-26              Encode::Unicode(3pm)
 

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