perlunitut
(1)
Name
perlunitut - Perl Unicode Tutorial
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLUNITUT(1)
NAME
perlunitut - Perl Unicode Tutorial
DESCRIPTION
The days of just flinging strings around are over. It's well
established that modern programs need to be capable of
communicating funny accented letters, and things like euro
symbols. This means that programmers need new habits. It's
easy to program Unicode capable software, but it does
require discipline to do it right.
There's a lot to know about character sets, and text
encodings. It's probably best to spend a full day learning
all this, but the basics can be learned in minutes.
These are not the very basics, though. It is assumed that
you already know the difference between bytes and
characters, and realise (and accept!) that there are many
different character sets and encodings, and that your
program has to be explicit about them. Recommended reading
is "The Absolute Minimum Every Software Developer
Absolutely, Positively Must Know About Unicode and Character
Sets (No Excuses!)" by Joel Spolsky, at
<http://joelonsoftware.com/articles/Unicode.html>.
This tutorial speaks in rather absolute terms, and provides
only a limited view of the wealth of character string
related features that Perl has to offer. For most projects,
this information will probably suffice.
Definitions
It's important to set a few things straight first. This is
the most important part of this tutorial. This view may
conflict with other information that you may have found on
the web, but that's mostly because many sources are wrong.
You may have to re-read this entire section a few times...
Unicode
Unicode is a character set with room for lots of characters.
The ordinal value of a character is called a code point.
(But in practice, the distinction between code point and
character is blurred, so the terms often are used
interchangeably.)
There are many, many code points, but computers work with
bytes, and a byte has room for only 256 values. Unicode has
many more characters, so you need a method to make these
accessible.
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Unicode is encoded using several competing encodings, of
which UTF-8 is the most used. In a Unicode encoding,
multiple subsequent bytes can be used to store a single code
point, or simply: character.
UTF-8
UTF-8 is a Unicode encoding. Many people think that Unicode
and UTF-8 are the same thing, but they're not. There are
more Unicode encodings, but much of the world has
standardized on UTF-8.
UTF-8 treats the first 128 codepoints, 0..127, the same as
ASCII. They take only one byte per character. All other
characters are encoded as two or more (up to six) bytes
using a complex scheme. Fortunately, Perl handles this for
us, so we don't have to worry about this.
Text strings (character strings)
Text strings, or character strings are made of characters.
Bytes are irrelevant here, and so are encodings. Each
character is just that: the character.
On a text string, you would do things like:
$text =~ s/foo/bar/;
if ($string =~ /^\d+$/) { ... }
$text = ucfirst $text;
my $character_count = length $text;
The value of a character ("ord", "chr") is the corresponding
Unicode code point.
Binary strings (byte strings)
Binary strings, or byte strings are made of bytes. Here, you
don't have characters, just bytes. All communication with
the outside world (anything outside of your current Perl
process) is done in binary.
On a binary string, you would do things like:
my (@length_content) = unpack "(V/a)*", $binary;
$binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :)
print {$fh} $binary;
my $byte_count = length $binary;
Encoding
Encoding (as a verb) is the conversion from text to binary.
To encode, you have to supply the target encoding, for
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example "iso-8859-1" or "UTF-8". Some encodings, like the
"iso-8859" ("latin") range, do not support the full Unicode
standard; characters that can't be represented are lost in
the conversion.
Decoding
Decoding is the conversion from binary to text. To decode,
you have to know what encoding was used during the encoding
phase. And most of all, it must be something decodable. It
doesn't make much sense to decode a PNG image into a text
string.
Internal format
Perl has an internal format, an encoding that it uses to
encode text strings so it can store them in memory. All text
strings are in this internal format. In fact, text strings
are never in any other format!
You shouldn't worry about what this format is, because
conversion is automatically done when you decode or encode.
Your new toolkit
Add to your standard heading the following line:
use Encode qw(encode decode);
Or, if you're lazy, just:
use Encode;
I/O flow (the actual 5 minute tutorial)
The typical input/output flow of a program is:
1. Receive and decode
2. Process
3. Encode and output
If your input is binary, and is supposed to remain binary,
you shouldn't decode it to a text string, of course. But in
all other cases, you should decode it.
Decoding can't happen reliably if you don't know how the
data was encoded. If you get to choose, it's a good idea to
standardize on UTF-8.
my $foo = decode('UTF-8', get 'http://example.com/');
my $bar = decode('ISO-8859-1', readline STDIN);
my $xyzzy = decode('Windows-1251', $cgi->param('foo'));
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Processing happens as you knew before. The only difference
is that you're now using characters instead of bytes. That's
very useful if you use things like "substr", or "length".
It's important to realize that there are no bytes in a text
string. Of course, Perl has its internal encoding to store
the string in memory, but ignore that. If you have to do
anything with the number of bytes, it's probably best to
move that part to step 3, just after you've encoded the
string. Then you know exactly how many bytes it will be in
the destination string.
The syntax for encoding text strings to binary strings is as
simple as decoding:
$body = encode('UTF-8', $body);
If you needed to know the length of the string in bytes,
now's the perfect time for that. Because $body is now a byte
string, "length" will report the number of bytes, instead of
the number of characters. The number of characters is no
longer known, because characters only exist in text strings.
my $byte_count = length $body;
And if the protocol you're using supports a way of letting
the recipient know which character encoding you used, please
help the receiving end by using that feature! For example,
E-mail and HTTP support MIME headers, so you can use the
"Content-Type" header. They can also have "Content-Length"
to indicate the number of bytes, which is always a good idea
to supply if the number is known.
"Content-Type: text/plain; charset=UTF-8",
"Content-Length: $byte_count"
SUMMARY
Decode everything you receive, encode everything you send
out. (If it's text data.)
Q and A (or FAQ)
After reading this document, you ought to read perlunifaq
too.
ACKNOWLEDGEMENTS
Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8
rants during the Amsterdam Perl Mongers meetings got me
interested and determined to find out how to use character
encodings in Perl in ways that don't break easily.
Thanks to Gerard Goossen from TTY. His presentation "UTF-8
in the wild" (Dutch Perl Workshop 2006) inspired me to
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publish my thoughts and write this tutorial.
Thanks to the people who asked about this kind of stuff in
several Perl IRC channels, and have constantly reminded me
that a simpler explanation was needed.
Thanks to the people who reviewed this document for me,
before it went public. They are: Benjamin Smith, Jan-Pieter
Cornet, Johan Vromans, Lukas Mai, Nathan Gray.
AUTHOR
Juerd Waalboer <#####@juerd.nl>
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlunifaq, perlunicode, perluniintro, Encode
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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