Clarify perlrebackslash.pod

[perl5.git] / pod / perlunitut.pod

=head1 NAME

perlunitut - Perl Unicode Tutorial

=head1 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
L<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.

=head2 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...

=head3 Unicode

B<Unicode> is a character set with room for lots of characters. The ordinal
value of a character is called a B<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.

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.

=head3 UTF-8

B<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.

=head3 Text strings (character strings)

B<Text strings>, or B<character strings> are made of characters. Bytes are
irrelevant here, and so are encodings. Each character is just that: the
character.

Text strings are also called B<Unicode strings>, because in Perl, every text
string is a Unicode string.

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 (C<ord>, C<chr>) is the corresponding Unicode code
point.

=head3 Binary strings (byte strings)

B<Binary strings>, or B<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;

=head3 Encoding

B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode,
you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>.
Some encodings, like the C<iso-8859> ("latin") range, do not support the full
Unicode standard; characters that can't be represented are lost in the
conversion.

=head3 Decoding

B<Decoding> is the conversion from I<binary> to I<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.

=head3 Internal format

Perl has an B<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.

=head2 Your new toolkit

Add to your standard heading the following line:

    use Encode qw(encode decode);

Or, if you're lazy, just:

    use Encode;

=head2 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
C<substr>, or C<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 C<$body> is now a byte string, C<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
C<Content-Type> header. They can also have C<Content-Length> to indicate the
number of I<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"

=head1 SUMMARY

Decode everything you receive, encode everything you send out. (If it's text
data.)

=head1 Q and A (or FAQ)

After reading this document, you ought to read L<perlunifaq> too. 

=head1 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.

=head1 AUTHOR

Juerd Waalboer <#####@juerd.nl>

=head1 SEE ALSO

L<perlunifaq>, L<perlunicode>, L<perluniintro>, L<Encode>