Linux Manual Pages

Free Software * Books

Source Code

Free Media

Linux

Encode::Unicode

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

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 f\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.

	•		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 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 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 :)

Opportunity

Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.

Education

Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.