Encode::Unicode(3pm)

1Encode::Unicode(3pm)   Perl Programmers Reference Guide   Encode::Unicode(3pm)
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NAME

6       Encode::Unicode -- Various Unicode Transformation Formats
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SYNOPSIS

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

14       This module implements all Character Encoding Schemes of Unicode that
15       are officially documented by Unicode Consortium (except, of course, for
16       UTF-8, which is a native format in perl).
17
18       <http://www.unicode.org/glossary/> says:
19           Character Encoding Scheme A character encoding form plus byte
20           serialization. There are Seven character encoding schemes in
21           Unicode: UTF-8, UTF-16, UTF-16BE, UTF-16LE, UTF-32 (UCS-4),
22           UTF-32BE (UCS-4BE) and UTF-32LE (UCS-4LE), and UTF-7.
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24           Since UTF-7 is a 7-bit (re)encoded version of UTF-16BE, It is not
25           part of Unicode's Character Encoding Scheme.  It is separately
26           implemented in Encode::Unicode::UTF7.  For details see
27           Encode::Unicode::UTF7.
28
29       Quick Reference
30                           Decodes from ord(N)           Encodes chr(N) to...
31                  octet/char BOM S.P d800-dfff  ord > 0xffff     \x{1abcd} ==
32             ---------------+-----------------+------------------------------
33             UCS-2BE       2   N   N  is bogus                  Not Available
34             UCS-2LE       2   N   N     bogus                  Not Available
35             UTF-16      2/4   Y   Y  is   S.P           S.P            BE/LE
36             UTF-16BE    2/4   N   Y       S.P           S.P    0xd82a,0xdfcd
37             UTF-16LE    2/4   N   Y       S.P           S.P    0x2ad8,0xcddf
38             UTF-32        4   Y   -  is bogus         As is            BE/LE
39             UTF-32BE      4   N   -     bogus         As is       0x0001abcd
40             UTF-32LE      4   N   -     bogus         As is       0xcdab0100
41             UTF-8       1-4   -   -     bogus   >= 4 octets   \xf0\x9a\af\8d
42             ---------------+-----------------+------------------------------
43

Size, Endianness, and BOM

45       You can categorize these CES by 3 criteria:  size of each character,
46       endianness, and Byte Order Mark.
47
48   by size
49       UCS-2 is a fixed-length encoding with each character taking 16 bits.
50       It does not support surrogate pairs.  When a surrogate pair is
51       encountered during decode(), its place is filled with \x{FFFD} if CHECK
52       is 0, or the routine croaks if CHECK is 1.  When a character whose ord
53       value is larger than 0xFFFF is encountered, its place is filled with
54       \x{FFFD} if CHECK is 0, or the routine croaks if CHECK is 1.
55
56       UTF-16 is almost the same as UCS-2 but it supports surrogate pairs.
57       When it encounters a high surrogate (0xD800-0xDBFF), it fetches the
58       following low surrogate (0xDC00-0xDFFF) and "desurrogate"s them to form
59       a character.  Bogus surrogates result in death.  When \x{10000} or
60       above is encountered during encode(), it "ensurrogate"s them and pushes
61       the surrogate pair to the output stream.
62
63       UTF-32 (UCS-4) is a fixed-length encoding with each character taking 32
64       bits.  Since it is 32-bit, there is no need for surrogate pairs.
65
66   by endianness
67       The first (and now failed) goal of Unicode was to map all character
68       repertoires into a fixed-length integer so that programmers are happy.
69       Since each character is either a short or long in C, you have to pay
70       attention to the endianness of each platform when you pass data to one
71       another.
72
73       Anything marked as BE is Big Endian (or network byte order) and LE is
74       Little Endian (aka VAX byte order).  For anything not marked either BE
75       or LE, a character called Byte Order Mark (BOM) indicating the
76       endianness is prepended to the string.
77
78       CAVEAT: Though BOM in utf8 (\xEF\xBB\xBF) is valid, it is meaningless
79       and as of this writing Encode suite just leave it as is (\x{FeFF}).
80
81       BOM as integer when fetched in network byte order
82                         16         32 bits/char
83             -------------------------
84             BE      0xFeFF 0x0000FeFF
85             LE      0xFFFe 0xFFFe0000
86             -------------------------
87
88       This modules handles the BOM as follows.
89
90       ·   When BE or LE is explicitly stated as the name of encoding, BOM is
91           simply treated as a normal character (ZERO WIDTH NO-BREAK SPACE).
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93       ·   When BE or LE is omitted during decode(), it checks if BOM is at
94           the beginning of the string; if one is found, the endianness is set
95           to what the BOM says.  If no BOM is found, the routine dies.
96
97       ·   When BE or LE is omitted during encode(), it returns a BE-encoded
98           string with BOM prepended.  So when you want to encode a whole text
99           file, make sure you encode() the whole text at once, not line by
100           line or each line, not file, will have a BOM prepended.
101
102       ·   "UCS-2" is an exception.  Unlike others, this is an alias of
103           UCS-2BE.  UCS-2 is already registered by IANA and others that way.
104

Surrogate Pairs

106       To say the least, surrogate pairs were the biggest mistake of the
107       Unicode Consortium.  But according to the late Douglas Adams in The
108       Hitchhiker's Guide to the Galaxy Trilogy, "In the beginning the
109       Universe was created. This has made a lot of people very angry and been
110       widely regarded as a bad move".  Their mistake was not of this
111       magnitude so let's forgive them.
112
113       (I don't dare make any comparison with Unicode Consortium and the
114       Vogons here ;)  Or, comparing Encode to Babel Fish is completely
115       appropriate -- if you can only stick this into your ear :)
116
117       Surrogate pairs were born when the Unicode Consortium finally admitted
118       that 16 bits were not big enough to hold all the world's character
119       repertoires.  But they already made UCS-2 16-bit.  What do we do?
120
121       Back then, the range 0xD800-0xDFFF was not allocated.  Let's split that
122       range in half and use the first half to represent the "upper half of a
123       character" and the second half to represent the "lower half of a
124       character".  That way, you can represent 1024 * 1024 = 1048576 more
125       characters.  Now we can store character ranges up to \x{10ffff} even
126       with 16-bit encodings.  This pair of half-character is now called a
127       surrogate pair and UTF-16 is the name of the encoding that embraces
128       them.
129
130       Here is a formula to ensurrogate a Unicode character \x{10000} and
131       above;
132
133         $hi = ($uni - 0x10000) / 0x400 + 0xD800;
134         $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
135
136       And to desurrogate;
137
138        $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
139
140       Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but
141       perl does not prohibit the use of characters within this range.  To
142       perl, every one of \x{0000_0000} up to \x{ffff_ffff} (*) is a
143       character.
144
145         (*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
146         integer support!
147

Error Checking

149       Unlike most encodings which accept various ways to handle errors,
150       Unicode encodings simply croaks.
151
152         % perl -MEncode -e'$_ = "\xfe\xff\xd8\xd9\xda\xdb\0\n"' \
153                -e'Encode::from_to($_, "utf16","shift_jis", 0); print'
154         UTF-16:Malformed LO surrogate d8d9 at /path/to/Encode.pm line 184.
155         % perl -MEncode -e'$a = "BOM missing"' \
156                -e' Encode::from_to($a, "utf16", "shift_jis", 0); print'
157         UTF-16:Unrecognised BOM 424f at /path/to/Encode.pm line 184.
158
159       Unlike other encodings where mappings are not one-to-one against
160       Unicode, UTFs are supposed to map 100% against one another.  So Encode
161       is more strict on UTFs.
162
163       Consider that "division by zero" of Encode :)
164

NAME

SYNOPSIS

ABSTRACT

Size, Endianness, and BOM

Surrogate Pairs

Error Checking

SEE ALSO