Encode::Unicode(3p) Perl Programmers Reference Guide Encode::Unicode(3p) #
Encode::Unicode(3p) Perl Programmers Reference Guide Encode::Unicode(3p)
NNAAMMEE #
Encode::Unicode -- Various Unicode Transformation Formats
SSYYNNOOPPSSIISS #
use Encode qw/encode decode/;
$ucs2 = encode("UCS-2BE", $utf8);
$utf8 = decode("UCS-2BE", $ucs2);
AABBSSTTRRAACCTT #
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:
_C_h_a_r_a_c_t_e_r _E_n_c_o_d_i_n_g _S_c_h_e_m_e 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
---------------+-----------------+------------------------------
SSiizzee,, EEnnddiiaannnneessss,, aanndd BBOOMM You can categorize these CES by 3 criteria: size of each character, endianness, and Byte Order Mark.
bbyy ssiizzee UCS-2 is a fixed-length encoding with each character taking 16 bits. It ddooeess nnoott support _s_u_r_r_o_g_a_t_e _p_a_i_r_s. When a surrogate pair is encountered during ddeeccooddee(()), its place is filled with \x{FFFD} if _C_H_E_C_K is 0, or the routine croaks if _C_H_E_C_K is 1. When a character whose ord value is larger than 0xFFFF is encountered, its place is filled with \x{FFFD} if _C_H_E_C_K is 0, or the routine croaks if _C_H_E_C_K is 1.
UTF-16 is almost the same as UCS-2 but it supports _s_u_r_r_o_g_a_t_e _p_a_i_r_s. 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 eennccooddee(()), 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 _s_u_r_r_o_g_a_t_e _p_a_i_r_s.
bbyy eennddiiaannnneessss 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 _s_h_o_r_t or _l_o_n_g 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.
• 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).
• When BE or LE is omitted during ddeeccooddee(()), it checks if BOM is at the
beginning of the string; if one is found, the endianness is set to
what the BOM says.
• 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
• When BE or LE is omitted during eennccooddee(()), it returns a BE-encoded
string with BOM prepended. So when you want to encode a whole text
file, make sure you eennccooddee(()) 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.
SSuurrrrooggaattee PPaaiirrss To say the least, surrogate pairs were the biggest mistake of the Unicode Consortium. But according to the late Douglas Adams in _T_h_e _H_i_t_c_h_h_i_k_e_r_’_s _G_u_i_d_e _t_o _t_h_e _G_a_l_a_x_y 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 _s_u_r_r_o_g_a_t_e
_p_a_i_r 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 + ($lo - 0xDC00);
Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but perl
does not prohibit the use of characters within this range. To perl,
every one of \x{0000_0000} up to \x{ffff_ffff} (*) is _a _c_h_a_r_a_c_t_e_r.
(*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
integer support!
EErrrroorr CChheecckkiinngg Unlike most encodings which accept various ways to handle errors, Unicode encodings simply croaks.
% perl -MEncode -e'$_ = "\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 :)
SSEEEE AALLSSOO #
Encode, Encode::Unicode::UTF7, <https://www.unicode.org/glossary/>,
<https://www.unicode.org/faq/utf_bom.html>,
RFC 2781 <http://www.ietf.org/rfc/rfc2781.txt>,
The whole Unicode standard
<https://www.unicode.org/standard/standard.html>
Ch. 6 pp. 275 of "Programming Perl (3rd Edition)" by Tom Christiansen,
brian d foy & Larry Wall; O'Reilly & Associates; ISBN 978-0-596-00492-7
perl v5.36.3 2023-02-15 Encode::Unicode(3p)