


ext::Encode::UnicPerl:Programmerext::Encode::Unicode::Unicode(3p)


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



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ext::Encode::UnicPerl:Programmerext::Encode::Unicode::Unicode(3p)


       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



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ext::Encode::UnicPerl:Programmerext::Encode::Unicode::Unicode(3p)


           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!




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ext::Encode::UnicPerl:Programmerext::Encode::Unicode::Unicode(3p)


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



























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