PERLUNITUT(1pm) Perl Programmers Reference Guide PERLUNITUT(1pm)
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 to four 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 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".
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, then
perluniintro.
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(1), perlunicode(1), perluniintro(1), Encode(3)
perl v5.24.0 2016-03-01 PERLUNITUT(1pm)
perl 5.24 - Generated Sun Dec 4 20:16:30 CST 2016