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

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.

       o   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

       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 v5.32.0			  2020-06-14		    Encode::Unicode(3)

NAME | SYNOPSIS | ABSTRACT | Size, Endianness, and BOM | Surrogate Pairs | Error Checking | SEE ALSO

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