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

       Encode::Unicode -- Various Unicode Transformation Formats

           use Encode qw/encode decode/;
           $ucs2 = encode("UCS-2BE", $utf8);
           $utf8 = decode("UCS-2BE", $ucs2);

       This module implements all Character Encoding Schemes of Unicode that are officially documented by Unicode Con-
       sortium (except, of course, for UTF-8, which is a native format in perl).

       <>; 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   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 char-
       acter.  Bogus surrogates result in death.  When \x{10000} or above is encountered during encode(), it "ensurro-
       gate"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      0xFFeF 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

       ?   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 charac-
       ters 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/ 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/ 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 :)

       Encode, Encode::Unicode::UTF7, <>;, <

       RFC 2781 <>;,

       The whole Unicode standard <>;

       Ch. 15, pp. 403 of "Programming Perl (3rd Edition)" by Larry Wall, Tom Christiansen, Jon Orwant; O'Reilly & As-
       sociates; ISBN 0-596-00027-8

perl v5.8.8                       2001-09-21                Encode::Unicode(3)