PERLUNITUT(1) 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 than that, so you need a method to make these accessible.
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/;
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 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'));
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".
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 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>
SEE ALSO
perlunifaq, perlunicode, perluniintro, Encode
perl v5.16.3 2013-03-04 PERLUNITUT(1)
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