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
7

SYNOPSIS

9           use Encode qw/encode decode/;
10           $ucs2 = encode("UCS-2BE", $utf8);
11           $utf8 = decode("UCS-2BE", $ucs2);
12

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 seri‐
20           alization. There are Seven character encoding schemes in Unicode:
21           UTF-8, UTF-16, UTF-16BE, UTF-16LE, UTF-32 (UCS-4), UTF-32BE
22           (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 Encode::Uni‐
27           code::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   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
50       UCS-2 is a fixed-length encoding with each character taking 16 bits.
51       It does not support surrogate pairs.  When a surrogate pair is encoun‐
52       tered during decode(), its place is filled with \x{FFFD} if CHECK is 0,
53       or the routine croaks if CHECK is 1.  When a character whose ord value
54       is larger than 0xFFFF is encountered, its place is filled with \x{FFFD}
55       if CHECK is 0, or the routine croaks if CHECK is 1.
56
57       UTF-16 is almost the same as UCS-2 but it supports surrogate pairs.
58       When it encounters a high surrogate (0xD800-0xDBFF), it fetches the
59       following low surrogate (0xDC00-0xDFFF) and "desurrogate"s them to form
60       a character.  Bogus surrogates result in death.  When \x{10000} or
61       above is encountered during encode(), it "ensurrogate"s them and pushes
62       the surrogate pair to the output stream.
63
64       UTF-32 (UCS-4) is a fixed-length encoding with each character taking 32
65       bits.  Since it is 32-bit, there is no need for surrogate pairs.
66
67       by endianness
68
69       The first (and now failed) goal of Unicode was to map all character
70       repertoires into a fixed-length integer so that programmers are happy.
71       Since each character is either a short or long in C, you have to pay
72       attention to the endianness of each platform when you pass data to one
73       another.
74
75       Anything marked as BE is Big Endian (or network byte order) and LE is
76       Little Endian (aka VAX byte order).  For anything not marked either BE
77       or LE, a character called Byte Order Mark (BOM) indicating the endian‐
78       ness is prepended to the string.
79
80       CAVEAT: Though BOM in utf8 (\xEF\xBB\xBF) is valid, it is meaningless
81       and as of this writing Encode suite just leave it as is (\x{FeFF}).
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83       BOM as integer when fetched in network byte order
84                         16         32 bits/char
85             -------------------------
86             BE      0xFeFF 0x0000FeFF
87             LE      0xFFeF 0xFFFe0000
88             -------------------------
89
90       This modules handles the BOM as follows.
91
92       ·   When BE or LE is explicitly stated as the name of encoding, BOM is
93           simply treated as a normal character (ZERO WIDTH NO-BREAK SPACE).
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95       ·   When BE or LE is omitted during decode(), it checks if BOM is at
96           the beginning of the string; if one is found, the endianness is set
97           to what the BOM says.  If no BOM is found, the routine dies.
98
99       ·   When BE or LE is omitted during encode(), it returns a BE-encoded
100           string with BOM prepended.  So when you want to encode a whole text
101           file, make sure you encode() the whole text at once, not line by
102           line or each line, not file, will have a BOM prepended.
103
104       ·   "UCS-2" is an exception.  Unlike others, this is an alias of
105           UCS-2BE.  UCS-2 is already registered by IANA and others that way.
106

Surrogate Pairs

108       To say the least, surrogate pairs were the biggest mistake of the Uni‐
109       code Consortium.  But according to the late Douglas Adams in The Hitch‐
110       hiker's Guide to the Galaxy Trilogy, "In the beginning the Universe was
111       created. This has made a lot of people very angry and been widely
112       regarded as a bad move".  Their mistake was not of this magnitude so
113       let's forgive them.
114
115       (I don't dare make any comparison with Unicode Consortium and the
116       Vogons here ;)  Or, comparing Encode to Babel Fish is completely appro‐
117       priate -- if you can only stick this into your ear :)
118
119       Surrogate pairs were born when the Unicode Consortium finally admitted
120       that 16 bits were not big enough to hold all the world's character
121       repertoires.  But they already made UCS-2 16-bit.  What do we do?
122
123       Back then, the range 0xD800-0xDFFF was not allocated.  Let's split that
124       range in half and use the first half to represent the "upper half of a
125       character" and the second half to represent the "lower half of a char‐
126       acter".  That way, you can represent 1024 * 1024 = 1048576 more charac‐
127       ters.  Now we can store character ranges up to \x{10ffff} even with
128       16-bit encodings.  This pair of half-character is now called a surro‐
129       gate pair and UTF-16 is the name of the encoding that embraces them.
130
131       Here is a formula to ensurrogate a Unicode character \x{10000} and
132       above;
133
134         $hi = ($uni - 0x10000) / 0x400 + 0xD800;
135         $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
136
137       And to desurrogate;
138
139        $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
140
141       Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but
142       perl does not prohibit the use of characters within this range.  To
143       perl, every one of \x{0000_0000} up to \x{ffff_ffff} (*) is a charac‐
144       ter.
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146         (*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
147         integer support!
148

Error Checking

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

SYNOPSIS

ABSTRACT

Size, Endianness, and BOM

Surrogate Pairs

Error Checking

SEE ALSO